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Perspective of plant growth promoting rhizobacteria (PGPR) containing ACC deaminase in stress agriculture
Abstract
Ethylene is a gaseous plant growth hormone produced endogenously by almost all plants. It is also produced in soil through a variety of biotic and abiotic mechanisms, and plays a key role in inducing multifarious physiological changes in plants at molecular level. Apart from being a plant growth regulator, ethylene has also been established as a stress hormone. Under stress conditions like those generated by salinity, drought, waterlogging, heavy metals and pathogenicity, the endogenous production of ethylene is accelerated substantially which adversely affects the root growth and consequently the growth of the plant as a whole. Certain plant growth promoting rhizobacteria (PGPR) contain a vital enzyme, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which regulates ethylene production by metabolizing ACC (an immediate precursor of ethylene biosynthesis in higher plants) into alpha-ketobutyrate and ammonia. Inoculation with PGPR containing ACC deaminase activity could be helpful in sustaining plant growth and development under stress conditions by reducing stress-induced ethylene production. Lately, efforts have been made to introduce ACC deaminase genes into plants to regulate ethylene level in the plants for optimum growth, particularly under stressed conditions. In this review, the primary focus is on giving account of all aspects of PGPR containing ACC deaminase regarding alleviation of impact of both biotic and abiotic stresses onto plants and of recent trends in terms of introduction of ACC deaminase genes into plant and microbial species.
... Furthermore, in this work, we document that strain G20-18 T also produces the plant hormone indole-3-acetic acid (IAA) and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase. Plant-associated bacteria that produce ACC deaminase have been shown to reduce the concentration of ethylene in plants and thus to mitigate ethylene stress caused by e.g., drought or water logging [6][7][8][9]. Initially, strain G20-18 was classified as a subspecies of Pseudomonas fluorescens [1,2], but with the continued rapid increase in the sequencing of bacterial genomes and expansion of genomic databases, we realize that this taxonomic placement is incorrect. ...
... Strain G20-18 T was isolated in 1986 and the other isolates from the work in [1] are not available. However, since strain G20-18 T possesses features that may be of importance to agriculture and plant biotechnology [4,5,9] and because the strain initially was misplaced in the taxonomical system, we here analyse strain G20-18 T thoroughly and report on polyphasic studies, which demonstrate that strain G20-18 T represents a novel species within the Pseudomonas mandelii subgroup. ...
... These identity percentages were lower than those between strain G20-18 T and P. migulae LMG 21608 T . Sequences were aligned with Clustal W in the CLC Main Workbench (Qiagen Aarhus A/S) and the alignments were subsequently used for reconstruction of phylogenetic trees in mega X [9]. Numbers at nodes are bootstrap values shown as percentages of 1000 replicates where only values above 50 % are shown. ...
Bacterial strain G20-18 T was previously isolated from the rhizosphere of an Arctic grass on Ellesmere Island, Canada and was characterized and described as Pseudomonas fluorescens . However, new polyphasic analyses coupled with phenotypic, phylogenetic and genomic analyses reported here demonstrate that the affiliation to the species P. fluorescens was incorrect. The strain is Gram-stain-negative, rod-shaped, aerobic and displays growth at 5–25 °C (optimum, 20–25 °C), at pH 5–9 (optimum, pH 6–7) and with 0–4 % NaCl (optimum, 2 % NaCl). The major fatty acids are C 16 : 0 (35.6 %), C 17 : 0 cyclo ω 7 c (26.3 %) and summed feature C 18 : 1 /C 18 : 1 ω7 c (13.6 %). The respiratory quinones were determined to be Q9 (93.5 %) and Q8 (6.5 %) and the major polar lipids were phosphatidylethanolamine, phosphatidylglycerol and diphosphatidylglycerol. Strain G20-18 T was shown to synthesize cytokinin and auxin plant hormones and to produce 1-aminocyclopropane-1-carboxylate deaminase. The DNA G+C content was determined to be 59.1 mol%. Phylogenetic analysis based on the 16S rRNA gene and multilocus sequence analysis (concatenated 16S rRNA, gyrB , rpoB and rpoD sequences) showed that G20-18 T was affiliated with the Pseudomonas mandelii subgroup within the genus Pseudomonas . Comparisons of the G20-18 T genome sequence and related Pseudomonas type strain sequences showed an average nucleotide identity value of ≤93.6 % and a digital DNA–DNA hybridization value of less than 54.4 % relatedness. The phenotypic, phylogenetic and genomic data support the hypothesis that strain G20-18 T represents a novel species of the genus Pseudomonas . As strain G20-18 T produces or modifies hormones, the name Pseudomonas hormoni sp. nov. is proposed. The type strain is G20-18 T (=LMG 33086 T =NCIMB 15469 T ).
... This accumulation of ET inhibits root growth, which reduces the bioavailability of water and nutrients, which leads to more stress. Therefore, in addition to being a plant growth regulator, ET also acts as a growth inhibitor or a stress hormone under adverse conditions such as salinity, heavy metal toxicity, and drought stress [102]. Under stress conditions, the endogenous amounts of ET are significantly increased which may reach levels that adversely limit plant development [102]. ...
... Therefore, in addition to being a plant growth regulator, ET also acts as a growth inhibitor or a stress hormone under adverse conditions such as salinity, heavy metal toxicity, and drought stress [102]. Under stress conditions, the endogenous amounts of ET are significantly increased which may reach levels that adversely limit plant development [102]. Plants enhance the synthesis of endogenous ET in response to stress stimuli, resulting in reduced root and shoot development, and plant growth retardation [102]. ...
... Under stress conditions, the endogenous amounts of ET are significantly increased which may reach levels that adversely limit plant development [102]. Plants enhance the synthesis of endogenous ET in response to stress stimuli, resulting in reduced root and shoot development, and plant growth retardation [102]. ET may modulate leaf function throughout the leaf life cycle and mediate drought-induced senescence. ...
Climate change has exacerbated the effects of abiotic stresses on plant growth and productivity. Drought is one of the most important abiotic stress factors that interfere with plant growth and development. Plant selection and breeding as well as genetic engineering methods used to improve crop drought tolerance are expensive and time consuming. Plants use a myriad of adaptative mechanisms to cope with the adverse effects of drought stress including the association with beneficial microorganisms such as plant growth promoting rhizobacteria (PGPR). Inoculation of plant roots with different PGPR species has been shown to promote drought tolerance through a variety of interconnected physiological, biochemical, molecular, nutritional, metabolic, and cellular processes, which include enhanced plant growth, root elongation, phytohormone production or inhibition, and production of volatile organic compounds. Therefore, plant colonization by PGPR is an eco-friendly agricultural method to improve plant growth and productivity. Notably, the processes regulated and enhanced by PGPR can promote plant growth as well as enhance drought tolerance. This review addresses the current knowledge on how drought stress affects plant growth and development and describes how PGPR can trigger plant drought stress responses at the physiological, morphological, and molecular levels.
... This hormone is produced internally by nearly all plants and is also generated by various biotic and abiotic processes in soils, playing a vital role in inducing diverse physiological changes in plants. Aside from its role as a plant growth regulator, ethylene is recognized as a stress hormone [24]. Under stressful conditions such as salinity, drought, waterlogging, heavy metals, and pathogenicity, the endogenous production of ethylene significantly increases, leading to severe impacts on overall plant growth. ...
... Under stressful conditions such as salinity, drought, waterlogging, heavy metals, and pathogenicity, the endogenous production of ethylene significantly increases, leading to severe impacts on overall plant growth. Elevated concentrations of ethylene can cause defoliation and other cellular processes that may result in decreased crop performance [7,24]. PGPR that produces the enzyme ACC deaminase contributes to plant growth and development by reducing ethylene levels. ...
Soil is an important natural resource that nurtures living microbial communities and improves plant productivity, thus ensuring food security. The chemical fertilizers used during the last few decades though improved plant productivity so rapidly; however, it is indiscriminate use results in poor soil health and less agricultural productivity, affecting food security and human health worldwide. There is an urgent need of biological agents, such as plant growth-promoting rhizobacteria (PGPR), which may serve as better alternative to solve this problem. PGPR plays an important role to increase soil fertility, plant growth promotion, and suppression of phytopathogens for the development of eco-friendly sustainable agriculture. The present study provides a critical overview on PGPR, its mechanism and function, and significance as a potential alternative tool for sustainable agriculture. An attempt has been made to propose an eco-friendly model integrating PGPR with various sectors, such as human health, agriculture, and food industry for its effective commercialization. The study might be helpful to identify the prospects and challenges of PGPR to fully integrate them into sustainable agriculture practices.
... This plant growth hormone is produced endogenously by approximately all plants and is also produced by different biotic and abiotic processes in soils and is important in inducing multifarious physiological changes in plants. Apart from being a plant growth regulator, ethylene has also been established as a stress or mone (119). Under stress conditions like those generated by salinity, drought, water logging, heavy metals and pathogenicity, the endogenous level of ethylene is significantly increased which negatively affects the overall plant growth. ...
... Under stress conditions like those generated by salinity, drought, water logging, heavy metals and pathogenicity, the endogenous level of ethylene is significantly increased which negatively affects the overall plant growth. For instance, the high concentration of ethylene induces defoliation and other cellular processes that may lead to reduced crop performance [119,120]. Plant growth promoting rhizobacteria which possess the enzyme, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, facilitate plant growth and development by decreasing ethylene levels, inducing salt tolerance and reducing drought stress in plants [121]. Currently, bacterial strains exhibiting ACC deaminase activity have been identified in a wide range of genera such as Acinetobacter, Achromobacter, Agrobacterium, Alcaligenes, Azospirillum, Bacillus, Burkholderia, Enterobacter, Pseudomonas, Ralstonia, Serratia and Rhizobium etc. [122]. ...
... Siderophores, molecules that bind metals, are produced by them [24], β-1,3 glucanase, fluorescent shades, chitinases, anti-infection agents and cyanides to safeguard plants against microorganisms [25]. They give protection from dry spells, saltiness, water-logging and oxidative pressure [26] and help in the solubilisation and mineralization of supplements [27]. They produce water-solvent nutrients like niacin, thiamine, riboflavin and biotin for plant development [28,29]. ...
Phosphorus is a crucial macronutrient in plant development, playing a vital role in metabolic activities and growth. Due to its poor availability in soil, phosphorus (P) is essential for healthy plant growth, particularly in tropical regions. P is present in nucleic acids, catalysts, coenzymes, nucleotides, and phospholipids. Optimal phosphorus availability is essential for plant reproductive structure formation during early development. Soil phosphorus content is around 0.05%, but due to insoluble phosphates, soluble forms are not readily available for plants. Chemical P fertilizers are used to increase available P levels, but these are costly and have negative environmental impacts. The limited P-source and high-quality rock P deposits may be exhausted within the next century, leading to the search for environmentally friendly alternatives. Biofertilizers with P-solubilizing properties are an environmentally friendly alternative to chemical-based Phosphorus fertilizers. PSB, beneficial microorganisms, hydrolyse insoluble phosphorus compounds into soluble P, facilitating plant uptake. This eco-friendly and economically sound approach overcomes P scarcity. Throughout the review, these PSBs are discussed in terms of how they have been applied and used to improve fruit crop growth, yield, and quality, providing promising evidence that these PSBs can be used as a viable alternative to inorganic phosphate fertilizers in the future for sustainable agriculture.
... Os principais efeitos perceptíveis da inoculação da semente/raiz como BPCV produtoras de ACC-deaminase são o alongamento da raiz da planta, aumento da nodulação de leguminosas e absorção de N, P e K, bem como a colonização micorrízica em várias safras. Por isso que em alguns casos, os efeitos de promoção do crescimento por bactérias pelo mecanismo da produção da ACC-deaminase parecem ser bem mais expressos em situações estressantes, como solos alagados, contaminados por metais pesados ou solos salinos, e em resposta para fitopatógenos (Saleem et al., 2007). ...
Os desafios relacionados ao estresse salino e ao déficit hídrico representam obstáculos significativos para a produtividade agrícola global, incluindo no Brasil, onde milhões de hectares são afetados pelo estresse salino e uma parte substancial do território enfrenta ameaças de déficit hídrico. Nesse contexto, as Bactérias Promotoras de Crescimento Vegetal (BPCV) emergem como agentes potenciais, desempenhando um papel crucial na promoção da resistência das plantas a esses estresses ambientais. A compreensão do papel das BPCV na superação do estresse salino e do déficit hídrico é crucial para práticas agrícolas sustentáveis. Essas bactérias atuam direta e indiretamente na absorção de nutrientes pelas plantas, modulam hormônios vegetais e controlam patógenos, beneficiando o crescimento das culturas. Dessa forma, foi realizada uma revisão bibliográfica, explorando artigos e livros em bases de dados científicos, sendo as informações organizadas, identificando padrões e tendências sobre as BPCV em contextos de estresse salino e déficit hídrico. Diante deste estudo, destaca-se que as BPCV apresentam mecanismos eficazes para mitigar os desafios do estresse salino e do déficit hídrico na agricultura. Bactérias promotoras como Azospirillum brasilense, Bacillus sp., e outras espécies halotolerantes, demonstram capacidade de solubilizar fósforo, sintetizar fitohormônios, produzir exopolissacarídeos e biofilmes, promovendo o crescimento radicular e aumentando a resistência das plantas. Investir em pesquisas e práticas agrícolas que promovam a simbiose entre plantas e BPCV é fundamental para a sustentabilidade agrícola, oferecendo alternativas viáveis em um cenário de mudanças climáticas e demandas crescentes por alimentos.
... One example of a beneficial practice is the inoculation of plants with PGPR, which has been demonstrated to mitigate the synthesis of ethylene triggered by stress. This reduction in ethylene levels can contribute to the maintenance of plant growth and development even in challenging environmental conditions (Saleem et al. 2007). The production of plant hormones by PGPR in response to stress is a major factor in making plants more resilient and encouraging growth in harsh environmental conditions. ...
Approximately 30% of the global agricultural area consists of calcareous soil. Horticultural plants, especially fruit trees, are sensitive to calcareous soil. People frequently use lime-resistant rootstocks and varieties in regions where they grow fruit in calcareous soils. While the use of rootstocks and varieties is not a solution in some cases, the use of plant growth-promoting rhizobacteria (PGPR) has emerged. People are increasingly using PGPR to enhance plant growth conditions, particularly when addressing suboptimal soil quality through soil remediation. This study investigated the impact of four distinct species of PGPR—Alcaligenes faecalis 637Ca, Microbacterium esteraromaticum SY48, Rhizobium radiobacter SY55, and Kocuria rhizophila SK63—on the levels of phytohormones, including abscisic acid (ABA), indole 3‑acetic acid (IAA), gibberellin (GA), jasmonic acid (JA), salicylic acid (SA), and zeatin, in the ‘Chester’ and ‘Jumbo’ blackberry cultivars. The research was conducted under high calcareous soil (41%) conditions. The research findings indicate that treatments T1 and T8 exhibited the lowest levels of ABA content, with values of 5020.24 ng g⁻¹ dry tissue (DT) and 836.84 ng g⁻¹ DT, respectively. Conversely, treatment T6 demonstrated the highest levels of GA (17.02 ng g⁻¹ DT), zeatin (5.67 ng g⁻¹ DT), and JA (62.19 ng g⁻¹ DT) content. Additionally, treatment T3 resulted in the highest levels of SA (21.26 ng g⁻¹ DT) and IAA (9.61 ng g⁻¹ DT) content. The findings suggest that a combination of the 637Ca, SY48, and SK63 bacterial strains may be a suitable recommendation for fruit cultivation in calcareous soil conditions.
... Plant growth-promoting bacteria produce ACC deaminase to efficiently defend plants against a variety of abiotic challenges, including heat, salinity, flooding and heavy metal stress. There has been evidence of ACC deaminase activity in rhizobacteria from the genera Pseudomonas, Azospirillum, Bacillus, Burkholderia, Enterobacter and Kluyvera (Saleem et al. 2007;Hassen et al. 2016). Exopolysaccharides (EPS), which are produced by some PGPR like Pseudomonas, are crucial for the creation and stabilisation of soil aggregates, the control of plant nutrients and water flow over plant roots through biofilm formation (Grover et al. 2011). ...
Soil salinity and sodicity are the major problems, especially in arid and semi-arid regions of the world. Adverse land topography, impeded drainage lines, heavy deep textured soil, toxic salt accumulation, climate change and poor resource base and mobilisation are the major constraints, which continue to degrade the soil's physical, chemical and biological environment. These make agricultural activities unsustainable and the socio-economic condition of the farming community at risk. Many conventional and non-conventional site-specific and cost-effective standard remedial measures were developed to restore the physical, chemical and biological environment of the soil for sustaining the agricultural production system and bringing the farmers’ income. Recently, the induction of scientific tools and techniques like satellite-based Remote Sensing (RS), Geographic Information System (GIS) and Global Positioning System (GPS) are employed for the systematic management of land, water and crop at the regional to local levels. These technologies’ benefits can be meaningfully translated to farmers’ fields for the planned use of natural resources and crop production functions.
... The modes of action of PGPM to improve plant growth include (but are not restricted to): (1) the synthesis of specific compounds of interest to plants (DOBBELAERE et al., 2003;ZAHIR et al., 2004;DOBBELAERE & OKON, 2007); (2) a facilitation in the absorption of nutrients from the soil (GARCIA et al., 2004a(GARCIA et al., , 2004bÇAKMAKÇI et al., 2006); and (3) a mitigation or reduction in the effects of pests and/or pathogens (JETIYANON & KLOEPPER, 2002;RAJ et al., 2003;GUO et al., 2004;SARAVANAKUMAR et al., 2008). The mechanisms by which PGPM stimulate plant development include (HAYAT et al., 2010): (1) production of vital enzymes, such as 1-aminocyclopropane-1-carboxylate (ACC) deaminase, capable of reducing the level of plant ethylene, thereby increasing root length, growth and plant development (GLICK & PENROSE, 1998;LI et al., 2000;BELIMOV et al., 2009;GAMALERO & GLICK, 2015); (2) ability to synthesize plant growth hormones such as auxins (indole acetic acid-IAA, abscisic acid-ABA), gibberellins (gibberellic acid) and cytokinins (DANGAR & BASU, 1987;PATTEN & GLICK, 2002;DOBBELAERE et al., 2003;DEY et al., 2004); (3) ability to fix nitrogen symbiotically (KENNEDY et al., 1997;; (4) capacity to antagonize phytopathogenic bacteria by the production of siderophores, ß-1,3-glucanases, chitinases, antibiotics, fluorescent pigments and cyanide (PAL et al., 2001;GLICK & PASTERNAK, 2003;ZHANG et al., 2009); (5) capability to solubilize and mineralize nutrients, especially mineral phosphates (RICHARDSON, 2001;BANERJEE & YASMIN, 2002;HAYAT et al., 2010); (6) greater resistance to drought (ALVAREZ et al., 1996), salinity, flooding (SALEEM et al., 2007;ALI et al., 2014) and oxidative stress (STAJNER et al., 1995;; and (7) the production of water-soluble B vitamins such as niacin, pantothenic acid, thiamine, riboflavin, and biotin (MARTINEZ-TOLEDO et al., 1996;SIERRA et al., 1999;REVILLAS et al., 2000). ...
Previous research has demonstrated the ability of isolate Pseudomonas thivervalensis SC5 to express the enzyme 1-aminocyclopropane-1-carboxylate deaminase (ACC), which regulates ethylene levels, one of the most important phytohormones in the regulation of plant growth and development. Thus, the present study evaluated the agronomic efficiency of a biological conditioner based on P. thivervalensis SC5 in the growth and productivity increases of corn in Brazil. It was found that corn was highly responsive to the inoculation of P. thivervalensis SC5, with increments ranging from 10.1 to 40.6% in the production of dry shoot biomass (DSB) compared to the control, while for grain yield the increments ranged from 9.0 to 27.8%. The increments are related to the levels and accumulations of N and P in the shoots of the plants. This suggested the participation of P. thivervalensis SC5 in mechanisms of soil modulation and nutrient acquisition. The inoculation of P. thivervalensis SC5 provided average increments in FDA hydrolysis ranging from 16.7 to 47.4% compared to the control, confirming the ability of this strain to increase the supply of nutrients to plants. Therefore, it is concluded that Pseudomonas thivervalensis SC5 participates in key mechanisms in the soil-plant system, with a consequent improvement in soil quality and other plant-related parameters.
... Thus the rhizosphere can be characterized as an ecosystem with a huge variety of different biologically active substances, for example, cyclic lipopeptides, siderophores, quorum-sensitive molecules, and antibiotics (Keohane et al., 2015). By mechanism of action, PGPRs can stimulate plant growth in the following ways: production of ACC deaminase to reduce ethylene levels in the roots of developing plants; production of plant growth regulators such as indoleacetic acid (Mishra et al., 2010), gibberellic acid, cytokinins (Castro et al., 2008), and ethylene (Saleem et al., 2007); asymbiotic nitrogen fixation (Ardakani et al., 2010); manifestation of antagonistic activity against phytopathogenic microorganisms by producing siderophores, b-1,3-glucanase, chitinases, antibiotics, fluorescent pigment, and cyanide (Pathma et al., 2011); solubilization of mineral phosphates and other nutrients (Hayat et al., 2010). p0255 The following metabolites for signaling in the soil microbiome and in the plant-microorganism system can be listed: o0045 1. Phytohormones: Organic compounds that are highly active at very low concentrations, including and regulating the physiological processes of plants. ...
... strengthen the host plant by providing nutrients and nutrient cycling. For instance, plants grow more rapidly when nourished by PGP endophytes (Saleem et al., 2007). Endophytes can also influence plant hormones such as auxin, cytokinin, ethylene, and gibberellin, producing other bioactive compounds (Joseph and Priya, 2011;Parthasarathi et al., 2012). ...
Abstract
The extensive use of chemicals to increase agriculture productivity has disturbed the delicate ecological balance, resulting in pathogen resistance and health risks for other living beings, including humans. A growing interest has been shown in finding eco-friendly and safe ways to increase sustainable agriculture productivity. Fungal endophytes are a significant component of plant micro-ecosystems and have been found in many plant species. They solubilize insoluble phosphates and produce plant growth-promoting hormones, including auxins, cytokinins, and gibberellins. Fungal endophytes are common in many plant species and are an important component of plant micro-ecosystems. Fungal endophytes are an important component of plant micro-ecosystems and have been found in a wide range of plant species. They dissolve insoluble phosphates and produce plant growth hormones such as auxins, cytokinins, and gibberellins. Because of the beneficial activities of fungal endophytes, research on the plant-fungus relationship has increased dramatically in recent years. Recently, genetically modified endophytes were used by researchers to improve plant productivity and defensive properties.
... Bacillus megaterium is a gram-positive soil bacterium with considerable potential for phytoremediation of metal-polluted areas (Esringüa et al., 2014). Li et al. (2017) revealed that a hybrid Pennisetum with endophytic B. megaterium H3 may be used for biomass production and Cd phytostabilization at various degrees of Cd contamination in aquatic settings (Saleem et al., 2007). And it was discovered that B. megaterium might promote Cd accumulation in plants by minimizing the detrimental impacts of heavy metals (Esringüa et al., 2014). ...
... Bacterial species found in the rhizosphere, P. fluorescens, have been found to encourage plant growth in the past. 29 Bacteria are drawn to the rhizosphere because it offers shelter, less competition from soil microorganisms, and the ability to consume plant-related exudates in exchange for its occupancy. 23 The mechanisms of nitrogen fixation, ammonia excretion, phosphate solubilization, 13 and growth hormone generation may be used to explain the rise in yield and yield characteristics brought on by the application of biofertilizer coupled with organic and commercial N fertilizer. ...
The environment and ecosystem were disrupted by the extensive use of fertilizers and pesticides which are harmful to humans and animals. Nature unfolds a biological response to overcome the different types of hazardous agrochemicals, in the form of microorganisms which have the efficiency to encourage plant growth without disturbing the environment. We conducted a biological approach to control phytopathogenic agents by plant growth-promoting rhizobacteria (PGPR), capable of restraining the devastation by phytopathogen. Pseudomonads can cling to soil particles, motile, prototrophic, and antibiotic synthesis along with the production of hydrolytic enzymes. Pseudomonas fluorescens extracted from the soils of Kerala were subjected to the identification of genes that have the phytostumillatory effect. These bacteria were immobilized using sodium alginate beads and applied to the soil where Solanum melongena (L.) was planted and the growth was compared with plants treated with cyanobacteria Spirulina platensis and NPK. The plants treated with PGPR showed high potential in growth-promoting characters when compared to cyanobacteria and NPK. P. fluorescens is an intense bio-agent to use in the field of agriculture because of its multifaceted utility.
... PGPR also employs siderophore to extract additional toxic substances from the environment and avoids heavy metals from causing toxicity in crops (P. Soil microbes trigger plants to synthesize additional ACC than they would normally require and promote ACC discharge from the roots of plants, while additionally providing microbes with an unusual supply of nitrogen (ACC), and as a result, microorganisms proliferate with ACC deaminase increases in the vicinity of the roots of plants in contrast with standard soil microbes (Saleem et al., 2007). When ACCDproducing bacterial strains colonize the plant's root surfaces of a challenged plant, the bacteria operate as reservoirs of ACC, lowering levels of ethylene and speeds up growth of roots. ...
... ACC is synthesized in the plant in response to stresses such as drought, cold, flooding, pathogen infection, and heavy metals etc by PGPR with ACC deaminase producing activity provided a better tolerance against different plant stresses [71]. Further, several biotic stresses can induce ethylene production in plants and cause a series of physiological changes for the regulation of plant growth as well as the induction of plant defense [72]. The higher level of ethylene production in the cell is sensed by different receptors, which triggers some cellular responses as well [73]. ...
The food and nutritional security of the increasing human population is one of the biggest challenges of the present century. Various kinds of plant stresses (due to different biotic and abiotic causes) are one of the major limiting factors for the increase in crop productivity. Out of the various methods tried to overcome the adverse effects of the different plant stresses, the use of plant growth-promoting bacteria has been established as a potential method to reduce the adverse effects of plant stress in an ecologically sustainable manner. The treatment of a PGPR may be combined with another PGPR species (one or more) to get a more desirable outcome. Further, the combination of PGPR with the other soil-inhabiting micro- and macro-organisms has also been found effective. These combinations include the use of mycorrhizal fungi and microalgae as microorganisms and earthworms as the macro-organisms; along with one or more PGPR species. The present chapter, therefore, is an attempt to explore the potential of these combinations of PGPR with other compatible soil micro- and macro-organisms as a better stress ameliorating treatment than the treatment with a PGPR species alone in overcoming the adverse effects of different plant stresses. Moreover, the combination approach can also be
helpful in improving the soil properties, reducing the use of agrochemicals in an ecologically sustainable manner besides improving crop productivity under stressful environments.
... They play a crucial role in enhancing plant growth through various mechanisms. They facilitate phosphorus dissolution, nitrogen fixation and an improved mineral uptake, thereby promoting efficient nutrient utilization and enhancing both shoot and root development [28,38,46]. Additionally, PGPR can enhance a plant's resistance to diseases and abiotic stresses by influencing plant secondary metabolism, detoxifying heavy metals, regulating ethylene levels and emitting volatile organic compounds [47,48]. ...
Hydroponics is a contemporary agricultural system providing precise control over growing conditions, potentially enhancing productivity. Biofertilizers are environmentally friendly, next-generation fertilizers that augment product yield and quality in hydroponic cultivation. In this study, we investigated the effect of three bio-fertilizers in a hydroponic floating system, microalgae, plant growth-promoting rhizobacteria (PGPR) and arbuscular mycorrhizal fungi (AMF), combined with a 50% reduction in mineral fertilizer, on lettuce yield and quality parameters including antioxidants: vitamin C, total phenols and flavonoids. The treatments tested were: 100% mineral fertilizer (control 1), 50% mineral fertilizer (control 2), 50% mineral fertilizer with microalgae, 50% mineral fertilizer with PGPR and 50% mineral fertilizer with AMF. The research was conducted during the winter months within a controlled environment of a glasshouse in a Mediterranean climate. The PGPR comprised three distinct bacterial strains, while the AMF comprised nine different mycorrhizal species. The microalgae consisted of only a single species, Chlorella vulgaris. AMF inoculation occurred once during seed sowing, while the introduction of PGPR and microalgae occurred at 10-day intervals into the root medium. Our findings revealed that the treatment with PGPR resulted in the highest growth parameters, including the lettuce circumference, stem diameter and fresh leaf weight. The 100% mineral fertilizer and PGPR treatments also yielded the highest lettuce production. Meanwhile, the treatment with AMF showed the highest total phenol and flavonoid content, which was statistically similar to that of the PGPR treatment. Furthermore, the PGPR recorded the maximum range of essential nutrients, including nitrogen (N), potassium (K), iron (Fe), zinc (Zn) and copper (Cu). Thus, the inclusion of PGPR holds promise for optimizing the lettuce growth and nutrient content in hydroponic systems. In conclusion, PGPR has the potential to enhance nutrient availability in a floating hydroponic system, reducing the dependence on chemical fertilizers. This mitigates environmental pollution and fosters sustainable agriculture.
... In essence, ACC deaminase converts ACC, the direct precursor of ET, into α-ketobutyric acid and ammonia (Saleem et al., 2007). PGPR can grow on limiting media using ACC as the only nitrogen source, including Alcaligenes sp., B. pumilus, Pseudomonas sp., and Variovorax paradoxus, Azoarcus, Azorhizobium caulinodans, Azospirillum sp., Gluconacetobacter diazotrophicus, Herbaspirillum (Kour et al., 2019). ...
... Gibberellins have a crucial role in controlling plant growth by influencing seed germination, stem development, flowering, and fruit setting. Additionally, it reduces ethylene production and controls ethylene levels under stressful conditions by producing 1aminocyclopropane-1-carboxylate (ACC) deaminase (Saleem et al. 2007;Glick 2014). ...
... The enzyme ACC-deaminase present in some microorganisms hydrolyzes ACC into ammonia and aketobutyrate, limiting the substrate availability for ethylene production and thereby indirectly stimulating plant growth [15]. In vitro ACC-deaminase activity of the rhizobacterial isolates used in the present study varied substantially but compared suitably with reported values [47]. Plant growth-promoting Bacillus sp. with ACC deaminase activity was first reported by Ghosh et al. [45]. ...
Beneficial plant-associated bacteria play a key role in supporting and/or promoting plant growth and health. Plant growth promoting bacteria present in the rhizosphere of crop plants can directly affect plant metabolism or modulate phytohormone production or degradation. We isolated 355 bacteria from the rhizosphere of rice plants grown in the farmers' fields in the coastal rice field soil from five different locations of the Ganjam district of Odisha, India. Six bacteria producing both ACC deaminase (ranging from 603.94 to 1350.02 nmol a-ketobutyrate mg À1 h À1) and indole acetic acid (IAA; ranging from 10.54 to 37.65 mM ml À1) in pure cultures were further identified using polyphasic taxonomy including BIOLOG (R) , FAME analysis and the 16S rRNA gene sequencing. Phylogenetic analyses of the isolates resulted into five major clusters to include members of the genera Bacillus, Microbacterium, Methylophaga, Agromyces, and Paenibacillus. Seed inoculation of rice (cv. Naveen) by the six individual PGPR isolates had a considerable impact on different growth parameters including root elongation that was positively correlated with ACC deaminase activity and IAA production. The cultures also had other plant growth attributes including ammonia production and at least two isolates produced siderophores. Study indicates that presence of diverse rhizobacteria with effective growth-promoting traits, in the rice rhizosphere, may be exploited for a sustainable crop management under field conditions.
... It is a sulfhydryl enzyme that utilizes pyridoxal 5-phosphate as an essential cofactor (Singh et al., 2015). ACC deaminase has been found in a wide range of Gram-negative bacteria (Saleem et al., 2007;Orozco-Mosqueda et al., 2019), Gram-positive bacteria (Saghafi et al., 2020), endophytes (Afridi et al., 2019), and fungi (Nascimento et al., 2014). ...
An investigation was carried out to isolate, characterize and identify the rhizobacterial isolates of crop plants grown in the arid region of the North-eastern Karnataka.Twenty-one strains out of 120 rhizobacterial isolates that grew vigorously at-0.30Mpa were selected, and strains were biochemically characterized and screened in vitro to determine their water stress tolerance activity. Among 21 isolates, APPP-1 and GPPP-1 showed ACC deaminase activity (5.13 µMmg-1/h and 2.69µMmg-1/h, respectively). The isolate BSOP-1 produced higher proline (138.87µMmg-1) and IAA (68.49 µg ml-1); while NONB-2 produced higher exopolysaccharides (23.53mg mg-1 protein) at-0.30Mpa. Our study recorded the ACC Deaminase activity of the isolates, APPP-1and GPPP-1 later identified as a novel strain of Pseudomonas sp. VG309(NCBI accession number MH664930) and Corynebacterium kroppenstedtii (NCBI accession number MH671918) through 16SrDNA molecular sequencing technique
... Ethylene, a vaporous chemical, is engaged with a wide scope of growth and developmental cycles, for example, germination of seed, root hair stretching and development, abscission of leaf and petal, fruit ripening and organ senescence; yet it is quickly integrated into the light of outside burdens and acknowledged to initiate the outflow of various stress-related genes. The discharge of ethylene because of these circumstances is usually known as pressure ethylene (Saleem et al. 2007). Soil salinity influences plants in three different ways as shown in Figure 5.2. ...
This chapter dealt with the alleviation of Salinity and Drought stress in food crops and various mechanisms adapted by microbes in degraded land in the growth of the Food crops
... However, PGPR with ACC-deaminase activity hydrolyze the precursor of ethylene biosynthesis, the 1-amino cyclopropane carboxylic (ACC), producing α-ketobutyrate and ammonia. Thus, this reduces ethylene levels and stress in plants, therefore restoring plant growth in HM-contaminated environments [153] ( Table 4). Evidence demonstrates that PGPR possessing ACC-deaminase activity improves plants' growth in the presence of HMs. ...
Heavy metal pollution is a severe concern worldwide, owing to its harmful effects on ecosystems. Phytoremediation has been applied to remove heavy metals from water, soils, and sediments by using plants and associated microorganisms to restore contaminated sites. The Typha genus is one of the most important genera used in phytoremediation strategies because of its rapid growth rate, high biomass production, and the accumulation of heavy metals in its roots. Plant growth-promoting rhizobacteria have attracted much attention because they exert biochemical activities that improve plant growth, tolerance, and the accumulation of heavy metals in plant tissues. Because of their beneficial effects on plants, some studies have identified bacterial communities associated with the roots of Typha species growing in the presence of heavy metals. This review describes in detail the phytoremediation process and highlights the application of Typha species. Then, it describes bacterial communities associated with roots of Typha growing in natural ecosystems and wetlands contaminated with heavy metals. Data indicated that bacteria from the phylum Proteobacteria are the primary colonizers of the rhizosphere and root-endosphere of Typha species growing in contaminated and non-contaminated environments. Proteobacteria include bacteria that can grow in different environments due to their ability to use various carbon sources. Some bacterial species exert biochemical activities that contribute to plant growth and tolerance to heavy metals and enhance phytoremediation.
... Direct mechanisms include nitrogen fixation, phosphate solubilization, iron absorption, and production of phytohormones namely auxin (incl. IAA), cytokinin, and gibberellins whereas indirect mechanisms refer to bacterial ability to inhibit phytopathogen growth via production of ACC deaminase, antimicrobial compounds, cell wall inhibitors, and siderophore [5][6][7][8]. Moreover, PGPR can prevent root diseases such as root withering and rotting [9]. ...
The roles of plant growth-promoting rhizobacteria in promoting plant growth and soil health, including alteration in plant metabolism and production of phytohormones such as indole-3-acetic acid (IAA) and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase, are indisputable. This study aimed to isolate and characterize beneficial bacteria isolated from the rhizosphere of pineapple from distinct stress-inducing habitats, including water excess-, herbicide-over-treated-, and pathogen-infected areas at PT Great Giant Foods located in Lampung, Indonesia. The isolated bacteria were screened based on IAA production and ACC deaminase activities. Six selected isolates produced IAA with concentrations of up to 36.93 mgL−1. The highest value belongs to Bacillus sp. NCTB5I, followed by Brevundimonas sp. CHTB 2C (13.13 mgL−1) and Pseudomonas sp. CHTB 5B (6.65 mgL−1). All isolates were detected with ACC deaminase activities with Brevundimonas sp. CHTJ 5H consuming 88% of ACC over 24 h, the highest among all. Brevundimonas sp. CHTB 2C was detected with the highest ACC deaminase activity with the value of 13,370 nm α-ketobutyrate mg−1h−1. In another experiment, it was revealed that all selected isolates promote soybean growth. These bacteria are potential to be developed in the future as bioagents to promote plant growth, especially under stressful environmental conditions.
There are tens of thousands of different kinds of bacteria that are connected to plant roots, making up a very diverse group. The second genome of the plant, or this intricate microbial population connected with the plant, is essential for plant health. A promising method of protecting plants from infections in the long run involves investigating the rhizosphere of plants, which is a region with a huge diversity and number of microorganisms. Recent research has demonstrated how crucial the soil microbiome is to the process of natural plant defence and how current management techniques might interfere with the dynamics of these microbial communities forming a protective microbiome. The role rhizospheric microorganisms of in the process of promoting plant growth, their processes, and their significance in crop production on a sustainable basis have all been covered in this chapter.
Plant growth-promoting rhizobacteria (PGPRs) are the bacteria in the rhizosphere that can promote a plant’s growth through different mechanisms. Microorganisms are likely to create growth-promoting chemicals in huge numbers, which could have an indirect impact on the overall morphology of plants. These are common soil microbes that colonize plant roots and improve plant health. The use of PGPR in the growing of crops can minimize the use of agrochemicals and can promote a sustainable food industry. In the rhizosphere, rhizobacteria not only benefit from nutrients secreted by plant roots but also positively impact the plant both directly and indirectly, which results in encouraging plant growth. For a range of crop plants, PGPRs are widely employed as biofertilizers to enhance soil nutrition. The relationship between a plant and PGPR for chemical signaling molecules is not understood clearly just like other beneficial plant–microbe interactions. Metabolites are small molecules that are produced during the process of metabolism, which is the set of chemical reactions that occur within cells to maintain life. These molecules are involved in various biochemical pathways and play crucial roles in the functioning of cells, tissues, and organs. Metabolites can be classified into different groups based on their chemical nature and function.
Endophytes are endosymbiotic microorganisms colonizing the internal tissues of healthy host plants [1] and possess the ability to improve the quality and growth rate of theirrespective hosts [2]. Their colonization does not produce any disease symptoms or morphological changes like gall formation of plant tissues [3]. Most of the plants on earth arehost to one or more types of endophytes [4]. These endophytes can be either bacteria or fungi [5,6]. Their population density in a host plant can vary from hundreds to more than 9 x 109 bacteria per gram of plant tissue [7,8, 9]. They can be either obligate or a facultative and the obligate types cannot be cultured due to their specificity of growth conditions. On the other hand, facultative endophytes can be cultured outside the plant tissue using artificial nutrient media [10, 11]. Endophytes form an important part of the micro-ecosystem inside plant tissues [12]. The most explored endophytes are non-pathogenic fungi that provide a number of useful characteristics to their host plant. However, bacterial endophytes remain an unexplored group [13]. Any bacteria which could be isolated from a surface-sterilized plant or extracted from its tissues can be called an endophyte if it does not affect the plant negatively. Bacteria can positively promote plant growth whereas studies show that plants are able to select these beneficial bacterial members in their microbiome including those inside the plant tissues [14, 15, 16]. There is no shred of evidence suggesting that these bacteria take advantage in this relationship [17], but certainly, they get protection from pathogens in adverse times. They could also communicate much better than the rhizospheric bacteria at times of stress [18,19]
Hyper soil salinity is currently one of the major concerns for global agricultural yield as it directly hinders the qualitative and quantitative aspects of agronomic outcomes. Owing to ever-increasing food requirements and a vast proportion of saline agricultural land in the world, developing salinity-resilient crops is of utmost need. To address this issue, various approaches based on conventional breeding as well as biotechnological and omics-based strategies have been explored by researchers and plant breeders. Out of them, genetic engineering-based alterations of plant genomes via inserting/overexpressing beneficial salt-responsive genes originating from different organisms have shown great potential and thus explored heavily. Interestingly, a group of halotolerant organisms, plants, algae, fungi, and bacteria, collectively referred to as halobiome, holds advantageous physical, chemical, and molecular characteristics for survival in the hypersaline environment. These characteristics include effective distribution and compartmentalization of ions, elevated production of the osmoprotectants, improved activity of antioxidant machinery, and regulated synthesis of phytohormones. There are several genes from halobiome identified and successfully used to improve the salt tolerance level of glycophytic crops. However, the gene pool from the halobiome is far from its full-potential exploration. Besides, non-coding RNAs also present a potent resource to be utilized for enhancing the salinity tolerance in crop plants. Further, the use of priming agents and biofertilizers from the halobiome sources is also turning into an effective solution for plant growth enhancement and salinity tolerance. In the current review, we present the current status and recent developments in identifying and exploring halotolerant gene pools (coding and non-coding) from the constituent members of halobiome and their exploration in engineering salt-tolerant crops. Technological advancements and challenges for their full-potential exploration in crop improvement programs have been discussed. The review also provides futuristic insights about the unexplored organisms or genes from halobiome in developing salt resilience in crops.
The main approach to bring Meloidogyne spp. populations beneath their damaging threshold level has been by rotation of crops, use of resistant genotypes and application of synthetic nematicides. However, it is difficult to get resistant seeds in developing economies and rotation is limited by the complexity of smallholder farming systems. Thus, many farmers rely on the application of synthetic nematicides for control. Such chemical handling coupled with the effects of these synthetics in the environment brought manifold problems to farmers, the environment and society. A salient environmentally safe substitute course for handling M. incognita infestation is by implementing Bacillus spp. as a standard in pest management practices. Recent breakthroughs in using Bacillus sp., as an auxiliary in M. incognita management on cardinal crops are highlighted in this chapter. Significant problems, challenges and drawbacks in these procedure are also discussed.
The current agricultural system is confronted with the challenge of excessive reliance on chemical-based fertilizers and pesticides. While these inputs have revolutionized agriculture, they also pose significant environmental risks. As a result, the utilization of agriculturally important microorganisms has become imperative to ensure sustainable agriculture in an environmentally friendly manner. These microorganisms can serve as biofertilizers, offering a wide range of plant growth-stimulating traits such as nitrogen fixation, nutrient solubilization, synthesis of siderophores and phytohormones, etc. By establishing symbiotic relationships, they enhance soil fertility, improve nutrient availability, and promote plant growth, thereby reducing the rely on synthetic fertilizers. Moreover, beneficial microorganisms act as natural adversaries to pests, providing an alternative to chemical pesticides. Microbes also enhance crop resilience to abiotic stresses such as drought and salinity through the production of stress-tolerant compounds, modulation of plant hormones, and improved nutrient uptake efficiency. Furthermore, they contribute to climate-smart agriculture by sequestering carbon in the soil, thereby mitigating greenhouse gas emissions. The use of microbial consortia further enhances plant growth, disease suppression, and stress tolerance. Additionally, microorganisms play an imperative role in biofortifying food crops, improving nutrient absorption, and addressing malnutrition. In summary, microorganisms offer diverse applications in sustainable agriculture, providing transformative solutions for crop-based food production.
The monotonous and regular use of chemical pesticides or fertilisers not only adversely affects the nutrient and food quality but also affects the health of human beings and surrounding environment. Hence nowadays utilisation of biofertilisers is paramount to support eco-friendly and sustainable agriculture. Modern agriculture should be accompanied by beneficial microorganisms to revamp soil fertility and agricultural productivity. Bio-priming with the efficient microorganisms showed multiple benefit to the plants by modulating plant growth and phytopathogen management. Plant growth-promoting bacteria or fungi (PGPB/PGPF) as priming agents facilitate the acquisition of soil nutrients, and modulate phytohormones. However, it is poorly understood how beneficial microbes (both invading and non-invading) connect successful association with plants by evading host plant immune system. Here, we aimed to discuss the basics of bio-priming agents and describe various molecular events used by beneficial microbes to avoid recognising the immune signalling of plants for their colonisation to ‘primed state’. The importance of multi-omics approaches for unravelling bio-priming is also outlined. The chapter may advance our current understanding of bio-priming-induced molecular events and defence mechanisms against biotic stresses and its importance in integrating into modern agriculture.
Background and Aims
Operation of both tryptophan-dependent and -independent pathways leading to higher IAA production by certain bacteria is known to beneficially influence plant growth and development. This study aimed to detect the operation of different pathways in bacteria for IAA production and evaluate the PGP (plant growth promoting) potential of the best IAA-producing bacteria in wheat crop.
Methods
The bacteria isolated from chickpea rhizosphere were screened for IAA production through tryptophan-dependent and independent pathways. The prominent IAA producing bacteria were identified by 16S rRNA gene sequencing and evaluated for their growth promoting, soil and plant nutrient enriching potential in wheat crop (cv. Sujata).
Results
Out of the 80 bacteria screened, three isolates, KS-14, BEMS-9-1 and BS-2 were found to produce high levels of IAA by operating both the pathways. These isolates were identified as Brevibacillus formosus, Bacillus paramycoides and Bacillus tequilensis, respectively. Evaluation of various combinations of these promising bacteria showed that the consortium of Brevibacillus formosus KS-14 and Bacillus paramycoides BEMS-9-1 along with a 50% recommended dose of fertilizer (RDF) not only significantly improved the morphological and physiological traits of wheat including yield and grain micronutrient loading, mediated through enhanced soil biological activities. To the best of our knowledge, this is the first report of the occurrence of tryptophan–independent pathway for IAA production in the three bacilli.
Conclusion
This study is a novel approach towards utilizing the bacteria producing IAA through tryptophan-dependent and -independent pathways for their promise as biostimulants in wheat.
The rhizosphere is a microhabitat around plant roots that is actively created and regulated by plants, while having a major impact on plant life itself. Plants constantly enrich the rhizosphere with organic matter from their rhizodepositions and root exudates, shaping the chemical and microbiological composition of the space surrounding their roots. In this chapter, we discuss the interactions between plants and other organisms through the processes in the rhizosphere. Plant-associated microorganisms, such as plant growth-promoting bacteria and mycorrhizal fungi, play important roles in enhancing the survival of plants, through increasing the availability of nutrients from the soil to plants, degrading and immobilizing toxic compounds, alleviating the effects of abiotic stress, acting as biocontrol agents, protecting the plants from pathogens, and bolstering the efficiency of plant responses to biotic and abiotic stress. Moreover, plants use the rhizosphere as a battlefield for mutual competition by releasing allelochemicals that are detrimental to other plant species, and to harmful soil nematodes and insects. Similarly to land plants, aquatic plants also create a rhizosphere around their roots, with important implications for rice cultivation, methane emissions from wetlands, denitrification of aquatic habitats, and wastewater treatment through constructed wetlands. We emphasize how the existing and potential rhizosphere engineering approaches take advantage of the plant-associated microorganisms and allelopathic interactions between plants to improve the health and yield of agricultural crops, and help preserve the natural environment.
Osmotic stress is a serious physiological disorder that affects water movement within the cell membranes. Osmotic stress adversely affects agricultural production and sustainability and is largely caused by soil salinity and water stress. An integrated nitrogen-fixing bacteria (NFB) soil amendment and an exogenous foliar application of Aloe vera leaf extract (ALE), and moringa leaf extract (MLE) were evaluated on roselle (Hibiscus sabdariffa L.) growth, calyx yield, secondary metabolites, and tolerance to osmotic stress in salt-affected soil. The osmotic stress markedly decreased above- and below-ground development of the roselle plant, but integrated NFB soil amendment with ALE or MLE foliar application significantly alleviated its negative impacts. Broadly, an improvement was observed in chlorophyll, carbohydrates, and protein levels following NFB and extracts foliar application, as well as a significant enhancement in antioxidant production (total phenols, ascorbic acid, and FRAP), which decreased peroxide production and increased stress tolerance in plants. Under osmotic stress, the roselle calyx revealed the highest anthocyanin levels, which declined following NFB soil amendment and foliar extract application. Additionally, an enhancement in nitrogen (N), phosphorus (P), and potassium (K) contents and the K/Na ratio, along with a depression in sodium (Na) content, was noticed. The integrated application of Azospirillum lipoferum × ALE exhibited the best results in terms of enhancing above- and below-ground growth, calyx yield, secondary metabolites, and tolerance to osmotic stress of the roselle plants cultivated in the salt-affected soil.
General plant diseases as well as soil‐borne pathogens severely reduce
agricultural yield. The rhizosphere (the region of the soil that includes and
surrounds the roots) is an important niche for microbial diversity in particular
phytobeneficial bacteria including plant growth‐promoting rhizobacteria
(PGPR) which have been used for a very long time to combat plant diseases.
Pathogen control and crop productivity can both be improved through the use
of PGPR several mechanisms, including iron‐based nutrition, antibiotics,
volatile substances, enzymes, biofilm, allelochemicals, and so on. Their modes
of action and molecular mechanisms have improved our comprehension of
how they are used to control crop disease. Therefore, there is a lot of literal
information available regarding PGPR, but this review stands out since it starts
with the fundamentals: the concept of the rhizosphere and the colonization
process of the latter, particularly because it covers the most mechanisms. A
broad figure is used to present the study's findings. The advantages of using
PGPR as bioinoculants in sustainable agriculture are also mentioned.
Microbial compost plays a crucial role in improving soil health, soil fertility, and plant biomass. These biofertilizers, based on microorganisms, offer numerous benefits such as enhanced nutrient acquisition (N, P, and K), production of hydrogen cyanide (HCN), and control of pathogens through induced systematic resistance. Additionally, they promote the production of phytohormones, siderophore, vitamins, protective enzymes, and antibiotics, further contributing to soil sustainability and optimal agricultural productivity. The escalating generation of organic waste from farm operations poses significant threats to the environment and soil fertility. Simultaneously, the excessive utilization of chemical fertilizers to achieve high crop yields results in detrimental impacts on soil structure and fertility. To address these challenges, a sustainable agriculture system that ensures enhanced soil fertility and minimal ecological impact is imperative. Microbial composts, developed by incorporating characterized plant-growth-promoting bacteria or fungal strains into compost derived from agricultural waste, offer a promising solution. These biofertilizers, with selected microbial strains capable of thriving in compost, offer an eco-friendly, cost-effective, and sustainable alternative for agricultural practices. In this review article, we explore the potential of microbial composts as a viable strategy for improving plant growth and environmental safety. By harnessing the benefits of microorganisms in compost, we can pave the way for sustainable agriculture and foster a healthier relationship between soil, plants, and the environment.
A field experiment was conducted during 2018 at Agricultural Research Station, Swami Keshwanand Rajasthan Agricultural University, Bikaner, Rajasthan 334001, India with five treatments consist General recommended dose, STCR based nutrient recommendation 20 q ha-1 and STCR based nutrient recommendation 15 q ha-1 with IPNS, STCR based nutrient recommendation 20 q ha-1 and STCR based nutrient recommendation 15 q ha-1 with fertilizer in a randomized block design (RBD). Our results demonstrated that application of STCR based nutrient recommendation 20 q ha-1 with IPNS recorded significantly highest porosity (43.96% and 42.09% 0-7.5 cm and 7.5-15 cm soil depth respectively), water storage (7.71% and 9.03% 0-7.5 cm and 7.5-15 cm soil depth respectively) lowest particle density highest microbial population (i.e. Fungi 5x 10-4 c.f.u. g-1 , Actinomycetes 5.8 x 10-4 c.f.u. g-1 and Bactria 5.78 x 10-6 c.f.u. g-1 0-15 cm soil depth).
As climate change exacerbates drought conditions, global crop production faces an escalating threat. Nevertheless, an eco-friendly solution lies in harnessing the potential of plant-associated plant growth-promoting bacteria. However, it's crucial to recognize that drought's impact extends beyond plants; it also influences the composition, abundance, and activity of bacterial communities. Amid these root-associated bacterial communities, Actinobacteria emerge as key players in preserving the well-being of plant hosts during drought stress, with research demonstrating minimal disruption to these communities under such conditions. Actinobacteria, found ubiquitously, are exceptional candidates for promoting plant growth due to their prevalence in soil and the rhizosphere, their adeptness at colonizing plant roots and surfaces, and their capability to produce diverse secondary metabolites under drought stress. With these attributes, members of the Actinobacteria phylum present themselves as the most promising candidates for microbial inoculation. They are enriched in the rhizosphere and endosphere microbiomes of crops enduring water deficit stress conditions. Notably, Actinobacteria, particularly the Streptomyces genus, employ various mechanisms, such as the modulation of phytohormone levels, reinforcement of antioxidant enzymes, enhanced water and nutrient uptake, and more, to alleviate water deficit stress in crops. This comprehensive review explores actinobacterial diversity associated with plants and delves into the impact of drought stress on the diversity of the Actinobacteria. It also examines the mechanisms through which Actinobacteria mitigate drought stress in plants. Emphasizing the role of multi-omics techniques in broadening our understanding of plant-Actinobacteria interactions, this review aims to inspire further exploration in this relatively uncharted research territory. Furthermore, it discusses future research directions for the application of Actinobacteria with plant growth-promoting traits, underlining their potential for sustainable agricultural practices.
The search for life on other planets is one of the most important scientific challenges of this century, centered mainly on Mars. The Atacama Desert is one of the places on the planet with an environment similar to the red planet, with a hyper-arid core that constitutes an extreme environment and scarce life, due to environmental factors. At the moment it has not been possible to confirm the existence of life in this planet, but it is planned to take the life to this planet by means of potato crop. Also has been research the application of microorganisms for the recovery of desert soils, with high salinity or low fertility, through the interaction of microorganisms and plants. The present review describes the similarities between the La Joya desert in Atacama and Mars, showing its importance for the search for life on that planet. Show the recent advances in the investigation of potato crops for its development on Mars or in similar conditions, in addition to the importance of the application of microorganisms that facilitate the growth and adaptability of this crop to inhospitable conditions.
There are hundreds of species of hazardous pests and disease-causing micro-organisms in the area of agricultural ecosystems, but there are also hundreds of species of helpful companions of farming insects and useful microorganisms including fungal, bacterial, and viral organisms. These clever crop pests are fed by pathogenic bacteria and, like a quiet soldier, perform a crucial role in pest control. Which can be put to good use in pest control and has the potential to be a well-rounded, long-lasting, and inexpensive tool for doing so. When microorganisms are used to suppress pest populations, the process is known as microbial control. Finding and breeding more of a pest's natural enemies could improve their efficacy in biological management, therefore it's important to keep an eye out for them. An innovative method of biological management, this strategy makes use of naturally occurring microorganisms that are spread by the targeted pests. Which is accessible from people who are competent in marking and is also extremely easy to get at, basically, we are able to tackle this issue in such a way that it may be fixed. To disseminate the word about the benefits of organic farming, the researchers must maintain their emphasis on the phrase "organic" and actively participate in outreach programs.
The production and consumption of blueberry have increased in Mexico owing to its health benefits. Symbiotic relationships have been shown to be crucial in blueberry plants. In particular, phytohormone production by Pseudomonas fluorescens is an important mechanism of plant growth promotion. However, there are only a few reports on the effects of plant growth-promoting bacteria in blueberries. Therefore, we aimed to evaluate the effects of four strains of P. fluorescens (UM16, UM240, UM256, and UM270) and two types of slow-release fertilizer (nitrophosphate and basacote) on the development of blueberry var. Biloxi under greenhouse conditions. Blueberry seedlings obtained from in vitro culture and adapted under greenhouse conditions were inoculated with 1 x 10 6 CFU with any of the four strains, depending on treatment. Plants inoculated showed increased average plant length, plant fresh weight, root length, and root fresh and dry weight, compared with those with the control treatment (non-inoculated plants). The plants inoculated and fertilized with nitrophosphate had a better development compared with those fertilized with basacote or the control plants (inoculated or fertilized). Inoculated plants fertilized with nitrophosphate also had greater plant length, higher fresh plant weight, longer roots, and greater root fresh and dry weight than the control (non-inoculated or non-fertilized plants). Our study could facilitate the sustainable propagation of blueberry plants.
The tree fruit industry in Nova Scotia, Canada, is dominated by the apple (Malus domestica) sector. However, the sector is faced with numerous challenges, including apple replant disease (ARD), which is a well-known problem in areas with intensive apple cultivation. A study was performed using 16S rRNA/18S rRNA and 16S rRNA/ITS2 amplicon sequencing to assess soil- and root-associated microbiomes, respectively, from mature apple orchards and soil microbiomes alone from uncultivated soil. The results indicated significant (p < 0.05) differences in soil microbial community structure and composition between uncultivated soil and cultivated apple orchard soil. We identified an increase in the number of potential pathogens in the orchard soil compared to uncultivated soil. At the same time, we detected a significant (p < 0.05) increase in relative abundances of several potential plant-growth-promoting or biocontrol microorganisms and non-fungal eukaryotes capable of promoting the proliferation of bacterial biocontrol agents in orchard soils. Additionally, the apple roots accumulated several potential PGP bacteria from Proteobacteria and Actinobacteria phyla, while the relative abundances of fungal taxa with the potential to contribute to ARD, such as Nectriaceae and plant pathogenic Fusarium spp., were decreased in the apple root microbiome compared to the soil microbiome. The results suggest that the health of a mature apple tree can be ascribed to a complex interaction between potential pathogenic and plant growth-promoting microorganisms in the soil and on apple roots.
Chemical inhibitors of plant ethylene production prolong the life of petals from ethylene sensitive flowers while addition of ethylene precursor, 1-aminocyclopropane-1-carboxylate, increases their rate of senescence. Plant growth promoting rhizobacteria, that have been previously shown to contain enzyme, ACC deaminase, which can lower the level of 1-aminocyclopropane-1-carboxylate and hence ethylene concentration in plants, are also effective at prolonging the life of petals from ethylene sensitive flowers.
Transgenic tomato plants with 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase gene from Enterobacter cloacae UW4 under the control of a pathogenesis-related promoter (prb-1b) from tobacco were challenged by abiotic stresses to determine the expression patterns ofthe transgene. No ACC deaminase RNA or protein was detected by RT-PCR and in western blots prepared from leaf proteins of transgenic plants after wounding or treatment with α-amino butyric acid, xylanase, ethephon, salicylic acid, jasmonic acid, ethylene, or ethylene plus jasmonic acid. However, expression of the ACC deaminase transgene was observed in leaves and roots oftransformed tomato lines exposed to UV light. The UV response required a minimum of 48 h of exposure and was specific to UV-8 light.
Soil salinity decreases plant growth and photosynthetic activity besides resulting in nutrient imbalance in plants. Plant growth promoting rhizobacteria (PGPR) can induce plant tolerance to salinity by producing various hormones and enhancing the availability of nutrients from soil matrix. A pot study was conducted to evaluate the effect of different PGPR strains on maize growth and ions uptake under salt stress conditions. Three salinity levels (4, 8 and 12 dSm-1) along with original EC were maintained in the pots using NaCl salt. Maize seeds inoculated with pre-selected strains (S5, S15 and S20) along with uninoculated control were sown in the pots. Recommended doses of NPK were applied. In general, maize growth was decreased with the increase in salinity. Results indicated that PGPR inoculation, even at higher EC (12 dS m-1), significantly increased shoot/root fresh weight, shoot/root dry weight, chlorophyll a, b and cartenoid contents upto 64/114, 102/102, 154, 102 and 58%, respectively, compared with un-inoculated control. Similarly, inoculation restricted the uptake of Na+/Cl- ions and enhanced the accumulation of N, P and K in shoot compared to control. Among the three selected strains, S20 performed better at all EC levels. The growth promotion and increased ions uptake exhibited by strain S20 might be due to its high in vitro IAA production, chitinase activity, P-solubilition and more intensive root colonization, besides ACC-deaminase activity.
It was previously observed that transgenic tomato plants that express the Enterobacter cloacae UW4 1-aminocyclopropane-1-carboxylate (ACC) deaminase (EC 4.1.99.4) gene, and thereby produce lower levels of ethylene, were partially protected from the deleterious effects of six different metals. However, since tomato plants are unlikely to be utilized in the phytoremediation of contaminated terrestrial sites, transgenic canola (Brassica napus) plants that constitutively express the same gene were generated and tested for their ability to proliferate in the presence of high levels of arsenate in the soil and to accumulate it in plant tissues. The ability of the plant growth-promoting bacterium E. cloacae CAL2 to facilitate the growth of both non-transformed and ACC deaminase-expressing canola plants was also tested. In the presence of arsenate, in both the presence and absence of the added plant growth-promoting bacterium, transgenic canola plants grew to a significantly greater extent than non-transformed canola plants.
The ways in which plant growth promoting rhizobacteria facilitate the growth of plants are considered and discussed. Both indirect and direct mechanisms of plant growth promotion are dealt with. The possibility of improving plant growth promoting rhizobacteria by specific genetic manipulation is critically examined.Key words: plant growth promoting rhizobacteria, PGPR, bacterial fertilizer, soil bacteria.The ways in which plant growth promoting rhizobacteria facilitate the growth of plants are considered and discussed. Both indirect and direct mechanisms of plant growth promotion are dealt with. The possibility of improving plant growth promoting rhizobacteria by specific genetic manipulation is critically examined.Key words: plant growth promoting rhizobacteria, PGPR, bacterial fertilizer, soil bacteria.
Summary Canola, Brassica napus cv. Westar, was transformed to express a bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase (EC 4.1.99.4) gene under
the transcriptional control of (a) the constitutive and strong 35S promoter from cauliflower mosaic virus, (b) the root-specific promoter of the rolD gene within the T-DNA from the Ri plasmid of Agrobacterium rhizogenes, and (c) the promoter for the pathogenesis-related prb-1b gene from tobacco. Following the growth of transformed and non-transformed canola plants in the presence of 0–200 mM NaCl,
the fresh and dry weights of plants, leaf protein concentration, and leaf chlorophyll contents were measured. The data suggest
that the presence of ACC deaminase provides the transgenic canola lines with tolerance to the inhibitory effects of salt stress,
compared to the non-transformed canola plants, with the rolD transformants being the most effective. The improved salt tolerance of these transgenic plants is likely the consequence
of the decreased synthesis of stress ethylene. This data is consistent with previous studies with transgenic tomato plants
expressing bacterial ACC deaminase which showed that lowering ethylene levels partially protected plants against growth inhibition
by metals, phytopathogens and flooding.
Bacteria carrying 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity lower stress induced ethylene levels and may be effective to improve plant growth under salt stress conditions. Twenty strains of rhizobacteria isolated from soil samples taken from different salt affected aras were screened for plant growth promotion and ACC- deaminase enzyme activity under axenic conditions at 6 dS rn'. Three strains (S5, S15 and S20) that promoted growth to the greatest extent under axenic conditions were selected for further study in pot trial at 0, 5, 10 dS rn'. Results of pot trial showed that the increase in salinity level decreased the growth of the maize seedlings. However, inoculation of maize seeds with these three rhizobacterial strains performed well at all salinity levels, and the strain S20 at 5 dS rn' significantly increased root/shoot length, root fresh/dry weight and shoot fresh/dry weight up to 56/62, 51/71, 52/61 %, respectively, over uninoculated control. At 10 dS rn' increase was 120/63, 52/71, 59/118%, respectively, over uninoculated control. Similarly, increase in chlorophyll a, band carotenoid contents of fresh leaves increased up to 86% at 5 dS rn' and up to 84% at 10 dS rn' by strain S20 over its respective control. Results revealed that it is highly likely that these rhizobacterial isolates deaminated endogenous ACC. Therefore, negative effects of stress induced ethylene could be partially eliminated through inoculation with ACC-deaminase containing rhizobacteria.
Ethylene production by flowers, petals and leaves of rose was correlated with severity of grey mould. However, when the host became completely macerated, ethylene production diminished. Ethylene production by Botrytis cinerea grown on autoclaved flowers which were supplemented with methionine was negligible. Methionine spray, incubation with ethylene, or precooling of flowers at 4°C increased disease incidence considerably. Ethylene also induced susceptibility of carnation flowers to attack by B. cinerea. On the other hand, sprays of silver thiosulphate (STS) aminooxyacetic acid (AOA) and aminoethoxyvinylglycine (AVG) decreased disease severity in rose petals and leaves inoculated with mycelial plugs or conidia. Treatment of cut rose flowers with STS (by dipping) or AOA (by spraying) significantly decreased disease incidence during subsequent incubation at 20 and 10°C. This suggests a treatment for reducing grey mould damage in flowers transported overseas.
Die Angriffsstelle finden: Strukturstudien an 1-Aminocyclopropan-1-carboxylat(ACC)-Desaminase im Komplex mit dem fest bindenden Inhibitor 1-Aminocyclopropanphosphonat (ACP, siehe Bild) ergaben das Auftreten eines Aminyladdukts als Intermediat. Die Struktur des Enzyms im Kristall und Mutationsstudien sprechen dafür, dass ein nucleophiler Angriff durch Tyr 294 die ACC-Ringöffnung auslöst.
The breadth and depth of knowledge concerning ethylene synthesis and action, coupled with the rapid pace of new progress makes a survey of the field a daunting task. Therefore, experts who were actively engaged in different aspects of ethylene research from different countries, spanning four continents were enlisted to complete this monograph. This book discusses a historical perspective as well as future trends and possibilities in this field.
The aim of this book is to give an overview of the most important aspects of physiological and biochemical basis for metal toxicity and tolerance in plants. The book is expected to serve as a reference to university and college teachers, students of plant sciences, environmental biology, environmental biotechnology, agriculture, horticulture, forestry, plant molecular biology, and genetics.
* APPROACHES TO RISK AND DISASTER: Introduction: Danger and ModernityRisk and Damaging Events * The Geographicalness of Disaster * Natural Hazards * Technological Hazards * Social Hazards: Violence and the Disasters of War * Vulnerability Perspectives: The Human Ecology of Endangerment * Active Perspectives: Responses to Disaster and Adjustments to Risks * COMMUNITIES AT RISK, PLACES OF DISASTER: 'Unnatural' Disasters: The Case of Earthquake Hazards * Contexts of Risk: Mountain Land Hazards and Vulnerabilities * Risks in the City * Place Annihilation: Air War and the Vulnerability of Cities * Holocaust: Genocide and Geographical Calamity Concluding Remarks: the perspective of ideas
The production of ethylene by six mycorrhizal fungi of pine (Pinus sylvestris L.) grown in media with and without methionine at temperatures of 20° C and 26° C and at pH 4.0, 6.0 or 7.0 was studied. The fungi produced more ethylene at 26° C than at 20° C, and more ethylene in media containing methionine than in media without this precursor. The fungi studied synthesized the highest amounts of ethylene at 26° C and pH 6.O.
Ethylene as well as other plant growth regulators (PGRs) are important chemicals in agricultural production. Plant growth regulators are now used worldwide on a diversity of crops each year (Thomas, 1982). The plant hormone, C2H4 strongly influences nearly every development stage in plant growth, from germination to fruit ripening and senescence. Moreover, its critical role in post-harvest physiology of agricultural products has also been well documented. Obviously, a compound with so many different effects may be useful in many ways to modify plant growth and development as required by growers. However, many factors including its gaseous nature and some negative effects on plant growth, restrict the extensive practical usefulness of C2H4. Furthermore, the consistency of results observed under controlled experimental conditions may not always be achieved under conditions of practical applications (i.e., under natural field conditions).
Endophytic bacteria reside within plant tissues and have often been reported to promote plant growth. Rhizobia are particularly known for their symbiotic relationship with legumes. A bacterial strain MSSP was isolated from surface-sterilized root nodules of Mimosa pudica. MSSP was Gram-negative, capsulated, motile, non-endospore forming rod with free nitrogen (N) fixation ability. Unlike N-fixing bacteria forming symbiotic relationship with legumes that largely exist in α-subclass of proteobacteria, MSSP belongs to β-class of proteobacteria. Phylogenetic analysis of 16 S rDNA demonstrated that MSSP belongs to the genus Burkholderia. This isolate secretes phytohormone, ACC deaminase, solubilizes phosphate and is antagonistic against phytopathogens.
Petunia hybrida pollen accumulates significant levels of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC) late in development. This pollen ACC is thought to play a role in the rapid burst of ethylene produced by pollinated pistils. To investigate this further, we have expressed the ACC deaminase gene product from Pseudomonas in transgenic petunias under the control of three different promoters including CaMV-35S, LAT52, and TA29 directing construction expression, pollen-specific expression and tapetum-specific expression, respectively. Several transgenic plants expressing the LAT52-ACC deaminase gene exhibited significant reduction of ACC in pollen. Two independent transformants contained only trace amounts of ACC in pollen. In contrast, the other promoters did not lead to reduced ACC in pollen. Pollination of wild-type pistils with pollen from LAT52-ACC deaminase plants elicited increased ethylene similar to wild-type pollen. Fecundity was unaffected by the reduction in pollen ACC content. Taken together, we conclude pollen-borne ACC is not the elicitor of pollination-induced ethylene production by pistils.
Legumes form a mutualistic symbiosis with bacteria collectively referred to as rhizobia. The bacteria induce the formation of nodules on the roots of the appropriate host plant, and this process requires the bacterial signaling molecule Nod factor. Although the interaction is beneficial to the plant, the number of nodules is tightly regulated. The gaseous plant hormone ethylene has been shown to be involved in the regulation of nodule number. The mechanism of the ethylene inhibition on nodulation is unclear, and the position at which ethylene acts in this complex developmental process is unknown. Here, we used direct and indirect ethylene application and inhibition of ethylene biosynthesis, together with comparison of wild-type plants and an ethylene-insensitive supernodulating mutant, to assess the effect of ethylene at multiple stages of this interaction in the model legume Medicago truncatula. We show that ethylene inhibited all of the early plant responses tested, including the initiation of calcium spiking. This finding suggests that ethylene acts upstream or at the point of calcium spiking in the Nod factor signal transduction pathway, either directly or through feedback from ethylene effects on downstream events. Furthermore, ethylene appears to regulate the frequency of calcium spiking, suggesting that it can modulate both the degree and the nature of Nod factor pathway activation.
Several microorganisms capable of utilizing 1-aminocyclopropane-l-carboxylate (ACPC) were isolated from soil. A bacterium which belongs to Pseudomonas accumulated cellular a-aminobutyrate with consumption of ACPC and cells incubated with ACPC medium had the activity deaminating the substrate to form α-ketobutyrate. An enzyme, ACPC deaminase, was highly purified and its molecular weight, substrate specificity and absorption spectrum were investigated. These results suggested that this enzyme was a pyridoxal 5'-phosphate enzyme which has the molecular weight of 104000 and high specificity for ACPC, Km=1.5mM. A yeast, Hansenula saturnus, is also capable of forming ACPC deaminase, which has a lower molecular weight, 69000, and higher Km value, 2.6mM.
1 -Aminocyclopropane-1 -carboxylate deaminase was inhibited by several sulfhydryl-modifying reagents. Inhibition by 5,5′-dithiobis(2-nitrobenzoic acid) were reversible. The chemically sensitive sulfhydryl groups were about 3 mol per mol of enzyme. A competitive inhibitor, l-serine, reduced modification of the sensitive sulfhydryl groups. The mechanism-based inhibition by β-chloro-D-alanine almost completely prevented the modification of sulfhydryl groups. The inhibition by monoiodoacetamide and N-ethylmaleimide was strengthened at pH values above 8.5, and in this range of pH, the maximum velocity of the enzyme reaction increased.The enzyme had 2.8 mol of tightly bound pyridoxal 5′-phosphate per mol (110,000 g) and was dissociated to a single subunit of molecular weight 36,500 by sodium dodecyl sulfate. The purified enzyme showed pH-dependent absorption maxima at 326 and 416 nm. The absorbance at 416 nm decreased as the pH of the enzyme solution increased from 7 to 9. The decrease in the absorbance corresp...
Stress ethylene emission is positively correlated with ozone sensitivity in various plant species, indicating that ethylene may be involved in the control of ozone damage. This study shows that ozone exposure of tomato plants for 5 h at 85 nl l−1 and above leads to leaf injury within 24 h. 1-aminocyclopropane-1-carboxylic acid (ACC) content and ACC synthase activity were accordingly elevated within 1–2 h. Pre-treatment of leaves with inhibitors of ACC synthase and ACC oxidase significantly inhibited the evolution of ethylene and reduced ozone-induced visible damage. Transcript levels for only one out of three S-adenosyl-l-methionine (SAM) synthetase genes (SAM3), and one out of four ACC synthase genes (LE-ACS2) were induced by ozone (maximum at 2 h). Treatment with protein kinase (K-252a) and phosphatase inhibitors (calyculin A) revealed that ACC synthase activity was additionally regulated by protein phosphorylation/dephosphorylation. Transcripts of ACC oxidase (pTOM13 cDNA probe) displayed the fastest response of the parameters tested (maximum at 30 min), suggesting a regulatory role for ACC oxidase in ethylene formation of ozone-exposed plants. The results demonstrate a highly selective ozone response by ethylene biosynthetic genes which resembles that of plant—pathogen interactions.
Studies were conducted to (1) compare stress ethylene production from roots and shoots; (2) determine the association between stress ethylene production and tissue Cd levels; and (3) investigate the time course of stress ethylene production following the rhizosphere application of cadmium chloride solutions. The shoots and/or roots of bush bean plants (Phaseolus vulgaris L. cv. Bush Blue Lake 290) were encapsulated at specific time intervals following CdCl2-application and kept in the dark for 2 h prior to determining ethylene production. Cadmium accumulated more in the roots than in the shoots and ethylene production was likewise higher in the roots. Cadmium chloride (10 mM)-induced stress ethylene production increased rapidly, peaked within 5 to 10 h and declined to 0 h levels within 24 h after treatment. A subsequent application of 10 mM CdCl2 at 24 h elicited a similar ethylene response, indicating that the plants retained functional ethylene metabolism. Consecutive, daily applications (12 days) of CdCl2 (0.1 or 0.5 mM) induced only small increases in ethylene production. In all studies, however, tissues accumulated large amounts of Cd. Significant increases in ethylene production after a single CdCl2 application were associated with Cd concentrations of ≧ 6 μ-gg-1 dry shoot tissue. The association between stress ethylene production and tissue Cd levels was lost as stress ethylene production declined and also in the consecutive application study. The decline in stress ethylene production was attributed to Cd-sequestering which removed the Cd stress.
One of the major factors affecting ethylene (C2H4) accumulation in soil is the availability of organic substrates. Various organic compounds were screened for their stimulatory effect on C2H4 accumulation in soil. Since L-methionine (L-met), a sole precursor of C2H4 synthesis in plants, is also an excellent precursor for microbial biosynthesis of C2H4, all the compounds tested were added on a C-equivalent basis with respect to 1 g L-met kg-1 soil. Incubation was carried out under ambient conditions (24 ± 3 °C) for 14 d and the soil was maintained at field capacity (-33 kPa.). Among the 63 compounds tested, the majority of amino acids, organic acids, carbohydrates, proteins, alcohols, and MET analogs promoted C2H4 generation to a greater degree than did the unamended soil.
Drought is the most damaging environmental phenomenon. During 1967-91,
droughts affected 50% of the 2.8 billion people who suffered from
weather-related disasters. Since droughts cover large areas, it is
difficult to monitor them using conventional systems. In recent years
the National Oceanic and Atmospheric Administration has designed a new
Advanced Very High Resolution Radiometer- (AVHRR) based Vegetation
Condition Index (VCI) and Temperature Condition Index (TCI), which have
been useful in detecting and monitoring large area, drought-related
vegetation stress. The VCI was derived from the Normalized Difference
Vegetation Index (NDVI), which is the ratio of the difference between
AVHRR-measured near-infrared and visible reflectance to their sum. The
TCI was derived from the 10.3-11.3-mm AVHRR-measured radiances,
converted to brightness temperature (BT). Algorithms were developed to
reduce the noise and to adjust NDVI and BT for land surface
nonhomogeneity. The VCI and TCI are used to determine the water- and
temperature-related vegetation stress occuring during drought. This
paper provides the principles of these indices, describes data
processing, and gives examples of VCI-TCI applications in different
ecological environments of the world. The results presented here are the
first attempt to use both NDVI and thermal channels on a large area with
very diversified ecological resources. The application of VCI and TCI
are illustrated and validated by in situ measurements. These indices
were also used for assessment of drought impact on regional agricultural
production in South America, Africa, Asia, North America, and Europe.
For this purpose, the average VCI-TCI values for a given region and for
each week of the growing season were calculated and compared with yields
of agricultural crops. The results showed a very strong correlation
between these indices and yield, particularly during the critical
periods of crop growth.
Petal senescence in mature flowers was studied in 93 species from 22 families. The initial symptom of senescence was either wilting or abscission, but in some species the time span between wilting and abscission was very short.
There was no apparent relationship between corolla form (choripetalous or sympetalous), ovary position (inferior or superior with respect to the corolla) and type of senescence (initial wilting or initial abscission). In monocots no initial abscission was found, while in dicots the difference between the wilting type and the abscission type was generally at the family level. With respect to petal senescence, sensitivity to exogenous ethylene (C2H4) was also related to the family level.
Except for a few families (all tested Campanulaceae, Caryophyllaceae and Malvaceae, and most Orchidaceae), most of the flowers investigated that showed initial wilting were not sensitive to exogenous ethylene, e.g. all tested Compositae, Iridaceae, and Liliaceae. Most of the flowers showing initial abscission were sensitive to exogenous ethylene (Geraniaceae, Labiatae, Ranunculaceae, Rosaceae, Scrophulariaceae).
Experiments with silver thiosulphate (STS) confirmed the effects of exogenous ethylene, both in flowers showing initial wilting and in flowers showing initial abscission. The data indicate, therefore, that ethylene is involved in the natural senescence of only a minority of the wilting type of flowers and in a majority (if not all) of the abscising type of flowers.
Because of their ability to transform atmospheric N2 into ammonia that can be used by the plant, researchers were originally very optimistic about the potential of associative diazotrophic bacteria to promote the growth of many cereals and grasses. However, multiple inoculation experiments during recent decades failed to show a substantial contribution of Biological Nitrogen Fixation (BNF) to plant growth in most cases. It is now clear that associative diazotrophs exert their positive effects on plant growth directly or indirectly through (a combination of) different mechanisms. Apart from fixing N2, diazotrophs can affect plant growth directly by the synthesis of phytohormones and vitamins, inhibition of plant ethylene synthesis, improved nutrient uptake, enhanced stress resistance, solubilization of inorganic phosphate and mineralization of organic phosphate. Indirectly, diazotrophs are able to decrease or prevent the deleterious effects of pathogenic microorganisms, mostly through the synthesis of antibiotics and/or fungicidal compounds, through competition for nutrients (for instance, by siderophore production) or by the induction of systemic resistance to pathogens. In addition, they can affect the plant indirectly by interacting with other beneficial microorganisms, for example, Azospirillum increasing nodulation of legumes by rhizobia. The further elucidation of the different mechanisms involved will help to make associative diazotrophs a valuable partner in future agriculture.
Two preselected plant growth promoting rhizobacteria (PGPR) containing 1-aminocyclopropane-1-carboxylate (ACC)-deaminase (EC 4.1.99.4) were used to investigate their potential to ameliorate the effects of drought stress on growth, yield, and ripening of pea (Pisum sativum L.). Inoculated and uninoculated (control) seeds of pea cultivar 2000 were sown in pots (four seeds pot−1) and placed in a wire house. The plants were exposed to drought stress at different stages of growth (vegetative, flowering, and pod formation) by skipping the respective irrigation. Results revealed that inoculation of peas with PGPR containing ACC-deaminase significantly decreased the “drought stress imposed effects” on the growth and yield of peas. Exposure of plants to drought stress at vegetative growth stage significantly decreased shoot growth by 41% in the case of uninoculated plants, whereas, by only 18% in the case of inoculated plants compared to nonstressed uninoculated control.Grain yield was decreased when plants were exposed to drought stress at the flowering and pod formation stage, but inoculation resulted in better grain yield (up to 62% and 40% higher, respectively) than the respective uninoculated nonstressed control. Ripening of pods was also delayed in plants inoculated with PGPR, which may imply decreased endogenous ethylene production in inoculated plants. This premise is further supported by the observation that inoculation with PGPR reduced the intensity of classical “triple” response in etiolated pea seedlings, caused by externally applied ACC. It is very probable that the drought stress induced inhibitory effects of ethylene could be partially or completely eliminated by inoculation with PGPR containing ACC-deaminase.
The enzyme ACC oxidase catalyses the last step of ethylene biosynthesis in plants, converting 1-aminocyclopropane-l-carboxytic acid (ACC) to ethylene. We have previously described the isolation and characterization of a cDNA clone (pMEL1) encoding an ACC oxidase homolog from melon (Cucumis melo L.). Here we report the isolation and characterization of three genomic clones, corresponding to three putative members of the ACC oxidase gene family in melon. All are transcriptionally active. The sequences of these genes have been determined. One genomic clone (CM-AC01), corresponding to the cDNA previously isolated, presents a coding region interrupted by three introns. Its transcription initiation site has been defined with RNA from ripe fruit and ethylene-treated leaves. The other two genes (CM-AC02, CM-AC03) have only two introns, at positions identical to their counterparts in CM-ACO]. The degree of DNA homology in the coding regions of CM-AC02 and CM-AC03 relative to CM=ACO1 is 59% and 75%, respectively. CM-AC02 and CM-AC03 are 59% homologous in their coding regions. These three genes have close homology to PH-AC03, a member of the ACC oxidase multigene family of petunia. The predicted amino acid sequences of CM-ACO1 and CM-ACO3 are 77% to 1 These authors contributed equally to this work 81% identical to those encoded by the tomato and petunia genes, while the deduced amino acid sequence of CM-AC02 shows only 42% to 45% homology. RT-PCR analysis using gene-specific primers shows that the three genes are differentially expressed during development , ethylene treatment and wounding. CM-AC01 is induced in ripe fruit and in response to wounding and to ethylene treatment in leaves. CM-AC02 is detectable at low level in etiolated hypocotyls. CM-AC03 is expressed in flowers and is not induced by any of the stimuli tested.
The effects of salinity on tomato plant growth and fruit production, the cultural techniques which can be applied to alleviate the deleterious effects of salt, and the possibilities of breeding salt- tolerant tomatoes are reviewed. Salinity reduces tomato seed germination and lengthens the time needed for germination to such an extent that the establishment of a competitive crop by direct seeding would be difficult in soils where the electrical conductivity (EC) of a saturated extract was equal to or above 8 dS m ˇ1 . Priming seeds primed with 1 M NaCl for 36 h seems advisable to establish a crop by direct sowing in saline soils, and seedling conditioning, either by exposure to moderately saline water exposure or by withholding watering until seedlings wilt for 20-24 h, can be recommended for crops that are to be established by transplanting. Yields are reduced when plants are grown with a nutrient solution of 2.5 dS m ˇ1 or higher and above 3.0 dS m ˇ1 an increase of 1 dS m ˇ1 results in a yield reduction of about 9-10%. At low ECs, yield reduction is caused mainly by reduction in the average fruit weight, whilst the declining number of fruits explains the main portion of yield reduction at high ECs. Since the smaller the fruit, the less important the reduction in fruit weight caused by salt, small size tomatoes are recommended to be grown at moderate salinity. Short cycle crops, in which only 4-6 trusses are harvested, are also recommended - especially since upper inflorescences are particularly sensitive to salt. Root growth, which slows when salinity reaches 4-6 dS m ˇ1 , appears to be less affected by salt than shoot growth. Salinity raises Na á concentration in roots and leaves of tomato plants. A higher Na á concentration in the leaves lowers the osmotic potential and promotes water uptake, but it is the ability to regulate Na á in older leaves while maintaining a low Na á concentration in young leaves which seems to be related to salinity tolerance. Ca 2á and K á concentrations in roots of salinised tomato plants change little under salinity whilst they are greatly reduced in leaves; those plants taking up more Ca 2á and K á from the salinised medium will have lower Na á /K á and Na á /Ca 2á ratios and an equilibrium of nutrients more similar to the non-salinised plants. Increasing Ca 2á and K á concentrations in the nutrient solution is, consequently, advisable. Root NO ˇ 3 concentration is maintained for longer periods after salinisation or under higher salinity levels than leaf NO ˇ 3 concentration. Salinity enhances tomato fruit taste by increasing both sugars and acids, fruit shelf life and firmness are unchanged or slightly lowered, but the incidence of blossom end rot is much higher. Breeding of tomato cultivars tolerant to moderate salinity will only occur after pyramiding in a single genotype
The chapter presents a discussion on plant growth-regulating (PGR) substances in the rhizosphere, with emphasis on microbial production and functions. The chapter discusses the rhizosphere as a site of plant-microbe interactions; plant growth-regulating substances and their sources; biochemistry of microbial production of PGRs; production of PGRs by rhizosphere microorganisms; metabolism of PGRs in soil; and ecological significance of PGRs produced in the rhizosphere. The chapter provides a better understanding of the mechanisms of actions of microbially derived PGRs and their interactions with plants. Moreover, development of hormone-deficient plant mutants can provide opportunities to clearly define the role of microbially produced PGRs in plan-microbe interactions. An understanding of these aspects can aid in the utilization of microbial PGRs for the betterment and benefit of sustainable agriculture.
Foliar wilting, epinasty, abscission, chlorosis, and necrosis are common symptoms in plants affected by water and salinity stresses. Ethylene evolution and ammonium accumulation frequently accompany the expression of the symptoms of stresses from various origins. These symptoms and physiological phenomena have been associated with other environmental stresses, such as ammonium toxicity. Intact and excised tomato plants (Lycopersicon esculentum Mill. ‘Heinz 1350’ and neglecta‐1) were subjected to stresses of waterlogging, water‐deficit, or saline conditions (NaCl or CaCl2). In soil culture in the greenhouse, tomato plants subjected to waterlogging developed epinasty and chlorosis and had increased ethylene evolution and ammonium accumulation. The application of aminooxyacetic acid (AOA) ameliorated the symptoms and reduced ethylene evolution and ammonium accumulation. Tomato subjected to drought developed chlorosis and had enhanced ammonium accumulation, but no increased ethylene evolution was observed. The chlorotic and necrotic symptoms were observed for plants receiving NaCl or CaCl2. Application of ammonium nutrition or water stress aggravated the development of toxic symptoms. Ammonium accumulation and ethylene evolution were enhanced with intact plants or excised seedlings under these stresses. Application of AOA through stems of excised seedlings suppressed the enhancement. ‘Heinz 1350’ receiving CaCl2 accumulated more Ca and had higher ethylene evolution than those receiving NaCl or the neglecta‐1 receiving CaCl2. Neglecta‐1 accumulated more Na with the NaCl treatment and had higher ethylene evolution than ‘Heinz 1350’. The results indicate that environmental stresses stimulate ammonium accumulation and initiate ethylene evolution, which may function in development of stress induced symptoms.
Diclofop-methyl (DM) and haloxyfop-ethoxyethyl (HE) are effective inducers of ethylene in susceptible species. Ethylene is a collateral product resulting from oxidative stress due to the production of reactive oxygen species (ROS). Lipoxygenase (LOX) inhibitors, free radical scavengers, and 2,4-D reverse the phytotoxicity of HE and DM, respectively, by interrupting the formation or action of ROS as indicated by the inhibition of ethylene induced by HE and DM. DM caused irreversible damage to apical meristems in susceptible biotypes ofLolium rigidum,Alopecurus myosuroides, andLolium multiflorumat 10 mM DM within 22–24 h after treatment (HAT). Damage to apical meristems was indicated by the extent of new shoot regrowth from excised stems of the monocots. The apical meristems of all resistant biotypes of the three species were relatively unaffected. DM increased ethylene evolution within 22–24 HAT in susceptible biotypes ofL. rigidum,A. myosuroides, andL. multiflorum(177, 224, and 155;pc of control, at 5, 10, and 15 mM DM, respectively). Little or no increase in ethylene formation above their controls was induced by DM in any of the resistant biotypes. The membrane potentials (E;zm) of susceptibleL. rigidumandL. multiflorumwere depolarized by 10 μM diclofop whereas depolarization of susceptibleA. myosuroidesrequired 25 μM diclofop. Repolarization ofEmdid not occur in all susceptible biotypes upon removal of diclofop. TheEmof all resistant biotypes was unaffected at 10 μM diclofop but, except forL. rigidum(R1), depolarization ofEmoccurred at 25 μM diclofop. However, in contrast to the susceptible biotypes, repolarization ofEμm occurred in the resistant biotypes upon removal of exogenous diclofop. The correlation among injury to apical meristem tissues, ethylene induction, and response ofEmto diclofop was consistent with the resistance or susceptibility of the biotypes to DM. Oxidative stress resulting in the formation of ROS is the most likely lethal mechanism of action of DM.
two of these traits. We selected 116 isolates from bulk soil and the ganisms. rhizosphere of soybean (Glycine max (L.) Merr.) and examined them There are some cases where PGPR may promote for a wide array of traits that might increase early soybean growth plant growth in nonsterile soil by controlling fungal dis- in nonsterile soil (PGPR traits). A subsample of 23 isolates, all but eases. The addition of a siderophore-producing Pseu- one of which tested positive for one or more of these PGPR traits, was domonas putida converted a Fusarium-conducive soil further screened for traits associated with biocontrol, (brady)rhizobial into a Fusarium-suppressive soil for the growth of three inhibition, and rhizosphere competence. Six of eight isolates positive different plants (Scher and Baker, 1982). An isolate for 1-aminocyclopropane-1-carboxylate (ACC, a precursor of ethyl- of Pseudomonas cepacia, positive for b-1,3-glucanase ene) deaminase production, four of seven isolates positive for sidero-
The interaction between the cotton leaf pathogens Alternaria macrospora and Alternaria alternata was studied using dual inoculation at dosages (≈ 103 spores/(mL ∙ pathogen)) that did not produce symptoms with either pathogen alone. This dual inoculation produced the typical disease symptoms (spots and shedding) and disease severity similar to inoculation with 104 spores/mL of A. macrospora alone. Neither pathogen produced ethylene in culture; however, they induced production of ethylene concentrations by diseased tissue that were correlated to both disease severity and leaf shedding. Plants infected by both pathogens produced the highest concentration of ethylene. Leaf discs either from leaves exhibiting symptoms or from symptomless infected leaves produced similarly high concentrations of ethylene. Inoculation of any site of the leaf with A. macrospora alone or with both pathogens resulted in shedding of the leaf. Pretreating inoculated plants with several ethylene inhibitors or an auxin decreased ethylene production, disease severity, and leaf shedding. Alternaria alternata apparently triggers symptom expression by A. macrospora in leaf blight disease of Pima cotton, and disease is manifested by the production of ethylene that leads to the typical leaf shedding symptom. Key words: Alternaria macrospora, Alternaria alternata, cotton leaf blight, defoliation, ethylene, fungus – fungus interaction, leaf spot of cotton, symptomless infections, virulence.
Ethylene, the simplest olefine, is present in nature at trace amounts. It is produced either chemically through the incomplete
combustion of hydrocarbons and biologically by almost all living organisms (2). Low levels of ethylene have been found in
the expired gases of animals that are generated through a lipid peroxidation pathway. Many microbes among bacteria and fungi
produce ethylene from two possible pathways: (i) a methionine and 2-oxo-4-methylthiobutyric acid (KMBA1) pathway in which ethylene is formed from KMBA by chemical reaction, and (ii) an α- ketoglutaric acid (KGA) pathway in which
KGA is generated from glucose and many other substrates and an ethylene-forming enzyme having very divergent sequence with
the enzyme responsible for the last step of ethylene biosynthesis in higher plants (4).
Plant-growth promoting rhizobacteria (PGPR), in conjuction with efficient Rhizobium, can affect the growth and nitrogen fixation in pigeonpea by inducing the occupancy of introduced Rhizobium in the nodules of the legume. This study assessed the effect of different plant-growth promoting rhizobacteria (Azotobacter chroococcum, Azospirillum brasilense, Pseudomonas fluorescens, Pseudomonas putida and Bacillus cereus) on pigeonpea (Cajanus cajan (L) Milsp.) cv. P-921 inoculated with Rhizobium sp. (AR-2–2 k). A glasshouse experiment was carried out with a sandy-loam soil in which the seeds were treated with Rhizobium alone or in combination with several PGPR isolates. It was monitored on the basis of nodulation, N 2 fixation, shoot biomass, total N content in shoot and legume grain yield. The competitive ability of the introduced Rhizobium strain was assessed by calculating nodule occupancy. The PGPR isolates used did not antagonize the introduced Rhizobium strain and the dual inoculation with either Pseudomonas putida, P. fluorescens or Bacillus cereus resulted in a significant increase in plant growth, nodulation and enzyme activity over Rhizobium-inoculated and uninoculated control plants. The nodule occupancy of the introduced Rhizobium strain increased from 50% (with Rhizobium alone) to 85% in the presence of Pseudomonas putida. This study enabled us to select an ideal combination of efficient Rhizobium strain and PGPR for pigeonpea grown in the semiarid tropics.
Ozone has long been known to cause reductions in the yield of crops1 although the precise mechanism of how this occurs is still unclear. Recently, ozone has also been suggested as being involved in the forest decline that has occurred in central Europe2—perhaps in combination with other atmospheric pollutants or with various environmental stresses such as chilling, mineral leaching, insect predation or disease. Consequently, it is desirable that the cellular mechanisms by which ozone is toxic to both crops and conifers, and how they might be modulated by other atmospheric pollutants or environmental stresses, are fully understood. Ethylene is normally produced by all plants and in trace amounts it may interact with other plant growth substances to coordinate a wide variety of developmental processes. However, when plants experience environmental stresses they respond by liberating larger amounts of ethylene—often called stress ethylene3. We have been able to demonstrate, using pea seedlings in an experimental fumigation system4, that the formation of stress ethylene determines the sensivity of plants to atmospheric levels of ozone and to suggest how other air pollutants may enhance ozone-mediated leaf injury.
Soils vary widely in their capacity to produce ethylene (C2H4). This laboratory study was conducted to assess various California soils for their potential to produce C2H4 and to evaluate the effects of trace elements on C2H4 evolution in soil. Soils unamended and amended with L-methio-nine and/or D-glucose were monitored for C2H4 production over 7 days at ambient laboratory conditions (24 +/- 3[degrees]C). Unamended soils varied in their potential to generate C2H4 ranging from 1.1 to 348.4 nmol C2H4 kg-1 soil. In most cases, organic amendments (L-methionine and/or D-glucose) significantly promoted C2H4 production in soils. There was a significant negative linear correlation between soil pH and C2H4 production in the unamended soils (r = 0.54*) and in L-methionine-amended soils (r = 0.53*). The effects of various trace elements on L-methionine-dependent C2H4 generation in soil was highly concentration-dependent. Seven trace elements [Ag(I), Cu(II), Fe(II), Mn(II), Ni(II), Zn(II), Al(III)] significantly inhibited C2H4 production when applied at concentrations >100 mg[middle dot]kg-1 soil. The most effective trace elements in promoting synthesis of C2H4 were Co(II) and As(III) when added at 100 mg[middle dot]kg-1 soil. Ethylene generation was inhibited in the presence of Hg(II), Fe(III), and Mo(VI) at >=10 mg[middle dot]kg-1 soil. Application of Fe(II) at >=100 mg[middle dot]kg-1 promoted the abiotic production of C2H4 in soil. The influence of trace elements on C2H4 production in soil may be related to their effects on Eh the catalytic activity (cofactors versus inhibitors), and toxicity to the microflora.
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Tomato plants that are delayed in fruit ripening have been developed by Agrobacterium tumefaciens-mediated transfer of a gene encoding 1-aminocyclopropane-1-carboxylic acid deaminase (ACCd) into the tomato genome. Two delayed ripening (DR) tomato lines are characterized in this paper. Line 8338, transformed by a double border plasmid vector, contains a single copy of the accd gene, and DNA outside the plasmid border sequences was not transferred to the plant genome. Line 5673, transformed by a single border plasmid vector, contains a single, complete copy of the accd gene plus tandem, incomplete copies of the gene. The mean expression levels of ACCd in fruit collected from four field trials were 39.4 and 20.6 μg/g of fresh weight for lines 8338 and 5673, respectively. Fruit ethylene synthesis was significantly reduced, and time for fruit to ripen was extended for both DR tomato lines relative to the parental control line. Introduction of the accd gene and the delayed ripening trait into appropriate tomato varieties will potentially allow production of tomato fruit with superior taste quality.
Stress ethylene emission is positively correlated with ozone sensitivity in various plant species, indicating that ethylene may be involved in the control of ozone damage. This study shows that ozone exposure of tomato plants for 5 h at 85 nl l−1 and above leads to leaf injury within 24 h. 1-aminocyclopropane-1-carboxylic acid (ACC) content and ACC synthase activity were accordingly elevated within 1–2 h. Pre-treatment of leaves with inhibitors of ACC synthase and ACC oxidase significantly inhibited the evolution of ethylene and reduced ozone-induced visible damage. Transcript levels for only one out of three S-adenosyl-l-methionine (SAM) synthetase genes (SAM3), and one out of four ACC synthase genes (LE-ACS2) were induced by ozone (maximum at 2 h). Treatment with protein kinase (K-252a) and phosphatase inhibitors (calyculin A) revealed that ACC synthase activity was additionally regulated by protein phosphorylation/dephosphorylation. Transcripts of ACC oxidase (pTOM13 cDNA probe) displayed the fastest response of the parameters tested (maximum at 30 min), suggesting a regulatory role for ACC oxidase in ethylene formation of ozone-exposed plants. The results demonstrate a highly selective ozone response by ethylene biosynthetic genes which resembles that of plant—pathogen interactions.
Plant-growth promoting rhizobacteria (PGPR), in conjuction with efficient Rhizobium, can affect the growth and nitrogen fixation in pigeonpea by inducing the occupancy of introduced Rhizobium in the nodules of the legume. This study assessed the effect of different plant-growth promoting rhizobacteria (Azotobacter chroococcum,Azospirillum brasilense, Pseudomonas fluorescens, Pseudomonas putida and Bacillus cereus) on pigeonpea (Cajanus cajan (L) Milsp.) cv. P-921 inoculated with Rhizobium sp. (AR-2–2 k). A glasshouse experiment was carried out with a sandy-loam soil in which the seeds were treated with Rhizobium alone or in combination with several PGPR isolates. It was monitored on the basis of nodulation, N2 fixation, shoot biomass, total N content in shoot and legume grain yield. The competitive ability of the introduced Rhizobium strain was assessed by calculating nodule occupancy. The PGPR isolates used did not antagonize the introduced Rhizobium strain and the dual inoculation with either Pseudomonas putida, P. fluorescens or Bacillus cereus resulted in a significant increase in plant growth, nodulation and enzyme activity over Rhizobium-inoculated and uninoculated control plants. The nodule occupancy of the introduced Rhizobium strain increased from 50% (with Rhizobium alone) to 85% in the presence of Pseudomonas putida. This study enabled us to select an ideal combination of efficient Rhizobium strain and PGPR for pigeonpea grown in the semiarid tropics.