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Expression of Bacillus subtilis proBA Genes and Reduction of Feedback Inhibition of Proline Synthesis Increases Proline Production and Confers Osmotolerance in Transgenic Arabidopsis
Abstract
Proline accumulation has been shown to correlate with tolerance to drought and salt stresses in plants. We attempt to introduce the wild-type, mutant, and fusion proBA genes derived from Bacillus subtilis into Arabidopsis thaliana under the control of a strong promoter cauliflower mosaic virus 35S (CaMV35S). The transgenic plants produced higher level of free proline than control and the overproduction of proline resulted in the increased tolerance to osmotic stress in transgenic plants. Besides, the mutation in proBA genes, which were proved to lead gamma-glutamyl kinase (gamma-GK) reduces sensitivity to the end-product inhibition and the fusion of proB and proA also result in increasing proline production and confer osmotolerance in transgenic lines.
... Furthermore, certain rhizobacteria (PGPR) increase osmolytes and activate some phytohormone pathways, which overcomes osmotic shock in Arabidopsis thaliana (Chen et al., 2007). This mechanism has been linked to increased expression of proline which confers resistance to salt stress (Chen et al., 2007). ...
... Furthermore, certain rhizobacteria (PGPR) increase osmolytes and activate some phytohormone pathways, which overcomes osmotic shock in Arabidopsis thaliana (Chen et al., 2007). This mechanism has been linked to increased expression of proline which confers resistance to salt stress (Chen et al., 2007). In vitro experiments showed that inoculation of rice (Oryza sativa) plants under hydroponic and soil conditions exposed to salinity by salt tolerant Bacillus amyloliquefaciens was very sensitive and significantly improved plant growth and stress tolerance of this plant species through the expression of at least 14 genes under these conditions (Nautiyal et al., 2013). ...
Intercropping is an adapted farming system to optimize resource-use efficiency and crop yield, particularly in low input agricultural systems. Due to the beneficial eco-agricultural effects of grain legumes, their integration in mixed cropping systems such as intercropping systems can be more beneficial to soil fertility, soil functioning, and nutrient cycling. About 16–22% of the world’s food is provided by cropping systems. On smallholder farms in Eastern and Southern Africa, the integration of legumes has the potential to increase maize (Zea mays) production up to 35% (e.g., Maize-pigeon pea (Cajanus cajan) intercropping). Legume-based intercropping systems can also promote rhizobacterial community diversity and soil health by enhancing symbiotic and non-symbiotic beneficial population. In the rhizosphere, the bacterial community is required to improve the growth and health of both intercrops due to several “direct and indirect” mechanisms involving plant growth-promoting rhizobacteria (PGPR). This review aims to highlight the importance of both legume-based intercropping and root-associated microorganisms particular emphasis on rhizobacteria; since the whole “crop-crop-microorganism” system has the potential to improve crop agro-physiological performance. This study also discusses the key role of legumes as intercrops being fully synergistic with PGPR contributing to crop yield stability under stressful conditions, notably drought and nutrient deficiency. Thus, intercropping can be used as an agroecological practice to ensure the sustainability of production.
... GA-producing Bacillus spp. positively correlated with increased macro-and micronutrients, fructose, carotenoids and gamma-aminobutyric Mondal et al. (2019) Ni, Pb, Cd ---------Chick pea Increase proline content and decrease lipid peroxidation Khan and Bano (2018) Pb toxicity PBRB3 Mung bean Higher antioxidative enzyme activity Arif et al. (2019) Pb toxicity CIK512 Radish Regulating the homeostasis of anti-oxidants Ahmad et al. (2018) Osmotic stress tolerance -----------Arabidopsis Higher proline production Chen et al. (2007) Heat stress -----------Wheat Enhanced activity of antioxidant, proline Ashraf et al. (2019) acid (Govindasamy et al., 2010;Nascimento et al., 2020). Furthermore, GAs produced by Bacillus spp. ...
... Furthermore, chromatin assembly factor-1 (CAF-1) modulates histone trimethylation of H3K4 and the expression of genes encoding PR5, PR1, WRKY53 and WRKY6 involved in defence responses against biotic stresses. The genome sequencing of B. amyloliquefaciens FZB42 identified genes for the synthesis of secondary metabolites associated with its survival in soil and competing with neighbouring rhizospheric microbial community through the action of bacillibactin, lipopeptide and polyketides (Chen et al., 2007). The inoculation with Bacillus spp. ...
The increased dependence of farmers on chemical fertilizers poses a risk to soil fertility and ecosystem stability. Plant growth-promoting rhizobacteria (PGPR) are at the forefront of sustainable agriculture, providing multiple benefits for the enhancement of crop production and soil health. Bacillus subtilis is a common PGPR in soil that plays a key role in conferring biotic and abiotic stress tolerance to plants by induced systemic resistance (ISR), biofilm formation, and lipopeptide production. As a part of bioremediating technologies, Bacillus spp. can purify metal contaminated soil. It acts as a potent denitrifying agent in agroecosystems while improving the carbon sequestration process when applied in a regulated concentration. Although it harbors several antibiotic resistance genes (ARGs), it can reduce the horizontal transfer of ARGs during manure composting by modifying the genetic makeup of existing microbiota. In some instances, it affects the beneficial microbes of the rhizosphere. External inoculation of B. subtilis has both positive and negative impacts on the endophytic and semi-synthetic microbial community. Soil texture, type, pH, and bacterial concentration play a crucial role in the regulation of all these processes. Soil amendments and microbial consortia of Bacillus produced by microbial engineering could be used to lessen the negative effect on soil microbial diversity. The complex plant-microbe interactions could be decoded using transcriptomics, proteomics, metabolomics, and epigenomics strategies which would be beneficial for both crop productivity and the well-being of soil microbiota. Bacillus subtilis has more positive attributes similar to the character of Dr. Jekyllnd some negative attributes on plant growth, soil health, and the environment akin to the character of Mr. Hyde.
... Indeed, the levels of the four analyzed metabolites were restored to those observed in the ∆K ∆ccpC mutant GP647, indicating that the duplication reversed the e ect of the darA deletion. Interestingly, the duplicated region contained not only the argC operon but also proAB and proG, and thus all the genes required for the anabolic biosynthesis of proline (48,49). This arrangement explains that the duplication in strain GP4020 results in the restoration not only of arginine but also of proline levels. ...
Cyclic di-adenosine monophosphate (c-di-AMP) is a second messenger involved in diverse metabolic processes including osmolyte uptake, cell wall homeostasis, as well as antibiotic and heat resistance. This study investigates the role of the c-di-AMP receptor protein DarA in the osmotic stress response in Bacillus subtilis . Through a series of experiments, we demonstrate that DarA plays a central role in the cellular response to osmotic fluctuations. Our findings show that DarA becomes essential under extreme potassium limitation as well as upon salt stress, highlighting its significance in mediating osmotic stress adaptation. Suppressor screens with darA mutants reveal compensatory mechanisms involving the accumulation of osmoprotectants, particularly potassium and citrulline. Mutations affecting various metabolic pathways, including the citric acid cycle as well as glutamate and arginine biosynthesis, indicate a complex interplay between the osmotic stress response and metabolic regulation. In addition, the growth defects of the darA mutant during potassium starvation and salt stress in a strain lacking the high-affinity potassium uptake systems KimA and KtrAB can be rescued by increased affinity of the remaining potassium channel KtrCD or by increased expression of ktrD, thus resulting in increased potassium uptake. Finally, the darA mutant can respond to salt stress by the increased expression of MleN , which can export sodium ions.
IMPORTANCE
Environmental bacteria are exposed to rapidly changing osmotic conditions making an effective adaptation to these changes crucial for the survival of the cells. In Gram-positive bacteria, the second messenger cyclic di-AMP plays a key role in this adaptation by controlling (i) the influx of physiologically compatible organic osmolytes and (ii) the biosynthesis of such osmolytes. In several bacteria, cyclic di-adenosine monophosphate (c-di-AMP) can bind to a signal transduction protein, called DarA, in Bacillus subtilis . So far, no function for DarA has been discovered in any organism. We have identified osmotically challenging conditions that make DarA essential and have identified suppressor mutations that help the bacteria to adapt to those conditions. Our results indicate that DarA is a central component in the integration of osmotic stress with the synthesis of compatible amino acid osmolytes and with the homeostasis of potassium, the first response to osmotic stress.
... Many microorganisms can produce proline. The proBA genes of Bacillus subtilis strain 93151 have been overexpressed in Arabidopsis thaliana by using the 35S promoter, leading to significantly higher proline content in transgenic plants than in control plants, with salt and drought stress tolerance both being enhanced in the former [42]. Moreover, inoculation of the rhizosphere bacteria Staphylococcus haemolyticus strain ST-9 and Bacillus subtilis strain RH-4 isolated from the rhizosphere of Heleochloa schocnoides significantly improved the proline content and salt tolerance of chickpea plants [43]. ...
As the global human population continues to increase, the use of saline-alkali land for food production is an important consideration for food security. In addition to breeding or cultivating salt-tolerant crop varieties, microorganisms are increasingly being evaluated for their ability to improve plant salt tolerance. Barley is one of the most important and salt-tolerant cereal crops and is a model system for investigating the roles of microorganisms in improving plant salt tolerance. However, a comprehensive review of the mechanisms by which microorganisms improve barley salt tolerance remains lacking. In this review, the mechanisms of barley salt tolerance improvement by microorganisms are summarized, along with a discussion of existing problems in current research and areas of future research directions. In particular, with the development of sequencing technology and the great reduction of prices, the use of omics can not only comprehensively evaluate the role of microorganisms but also evaluate the impact of the microbiome on plants, which will provide us with many opportunities and challenges in this research area.
... PGPR makes osmolyte accumulation and phytohormonal signaling that prevents plant from osmotic shock due to salinization. Chen et al. (2007) observed that osmolyte proline accumulation was increased in Arabidopsis thaliana on inoculation of Bacillus subtilis, which confirm plant salt tolerance. The expression of 14 genes was altered in Oryza sativa plant on inoculation of Bacillus amyloliquefaciens PGPR (Nautiyal et al. 2013). ...
... Proline accumulation in plants increases with the increase in drought and salt tolerance. Increasing the amount of proline under salinity stress conditions helps to maintain the intracellular water and nutrient balance of plants (Chen et al. 2007). In this study, the osmotic balance, which was disturbed due to the reactive oxygen species formed by the fertilizers given in saline soil conditions, helped balancing with the increased proline content. ...
Soybean is an important cash crop; however, its productivity is significantly hampered by salinity. Therefore, it is crucial to apply fertilizer correctly for increasing crop growth and yield in salt-affected soils. This study investigated the impact of mineral fertilizer [i.e., triple super phosphate (TSP), diammonium phosphate (DAP), and Urea] and manure application on growth traits, photosynthetic pigments, proline, total soluble carbohydrates, total phenol, flavonoid and anthocyanin contents of soybean plants grown under saline and non-saline environments. Fertilizers application significantly increased plant height, root and stem length, fresh root and stem weight, and dry stem weight under saline conditions. The highest increases in plant height (26.7%), fresh root weight (47.74%) and fresh stem weight (7.54%) were recorded with phosphorus (P) application. The P application increased chlorophyll a and b contents by 59.59% and 50.70% under saline conditions, while 70.99% and 64.14% increase was recorded with DAP + Urea application. The highest phenolic compound, total flavonoid and proline contents were noted with P and DAP + Urea application under saline environment. The P application increased phenol and flavonoid contents by 31.69% and 58.00%, while the increase with DAP + Urea application was 41.27% and 50.98%, respectively. The highest proline content was noted with DAP + Urea treatment. Similarly, the highest carbohydrate contents (were recorded with P and DAP + Urea application 56.14% and 54.03% higher than control). This study revealed that application of P‑fertilizer to saline soil improved growth traits (i.e., fresh root and stem weight and plant height) of soybean plants Likewise, application of P + DAP + urea increased chlorophyll a and b contents, flavonoids, carbohydrates and phenol contents. The application of DAP + Urea had a positive effect on proline contents. The results revealed that application of mineral fertilizers served as a nutrient source for soybean crop under saline soil. Therefore, application of mineral fertilizers (Urea and DAP) could reduce adverse impacts of salinity stress on soybean growth.
... The hinge region connects the lobes and regulates the molecular brake region (M. Chen et al., 2007). ...
ABSTRACT Fibroblast Growth Factor (FGF) ligands and their receptors are crucial factors driving chemoresistance in several malignancies, challenging the efficacy of currently available anti-cancer drugs. The Fibroblast growth factor/receptor (FGF/FGFR) signalling malfunctions in tumor cells, resulting in a range of molecular pathways that may impact its drug effectiveness. Deregulation of cell signalling is critical since it can enhance tumor growth and metastasis. Overexpression and mutation of FGF/FGFR induce regulatory changes in the signalling pathways. Chromosomal translocation facilitating FGFR fusion production aggravates drug resistance. Apoptosis is inhibited by FGFR-activated signalling pathways, reducing multiple anti-cancer medications' destructive impacts. Angiogenesis and epithelial-mesenchymal transition (EMT) are facilitated by FGFRs-dependent signalling, which correlates with drug resistance and enhances metastasis. Further, lysosome-mediated drug sequestration is another prominent method of resistance. Inhibition of FGF/FGFR by following a plethora of therapeutic approaches such as covalent and multitarget inhibitors, ligand traps, monoclonal antibodies, recombinant FGFs, combination therapy, and targeting lysosomes and micro RNAs would be helpful. As a result, FGF/FGFR suppression treatment options are evolving nowadays. To increase positive impacts, the processes underpinning the FGF/FGFR axis' role in developing drug resistance need to be clarified, emphasizing the need for more studies to develop novel therapeutic options to address this significant problem. Communicated by Ramaswamy H. Sarma.
... Under saline conditions, EPS can bind to cations such as Na+, rendering it unavailable to plants. Drought and salt tolerance in plants were linked by Chen et al. (2007). When proBA genes from B. subtilis were introduced into A. thaliana, the transgenic plants produced more free proline and were more resistant to osmotic stress. ...
Most crop plants are sensitive to salinity caused by high concentrations of salts in the soil, and the area of land impacted by it is expanding day by day, salinity is one of the harshest environmental variables restricting crop plant productivity. Average yields for all significant crops are only a fraction of record yields, ranging from 20% to 50%; these losses are primarily due to drought and high soil salinity, climatic circumstances that may increase in many locations as a result of global climate change. To deal with such effects, a variety of adaptations and mitigation methods are required. Salinity stress can be alleviated through efficient resource management and crop/livestock improvement to evolve healthier breeds. However, because such solutions are time-consuming and costly, there is a need to create simple and low-cost biological methods for salinity stress control that can be implemented quickly. Microorganisms with unique qualities such as salinity tolerance, genetic diversity, synthesis of suitable solutes, generation of plant growth stimulating hormones, bio-control capability, and contact with crop plants could play a significant role in this regard.
... Some important member of a genus under Bacillus includes B. licheniformis HSW-16, B. amyloliquefaciens SN13, B. megaterium A12, B. subtilis SU47, and B. pumilus HB4N3 are reported for plant growth, and stress management [61,131,133,136,137]. The PGPR, Bacillus subtilis 93,151 inoculated transgenic Arabidopsis thaliana showed enhanced proline synthesis with proBA genes that can upsurge the plant's salinity tolerance [138]. Root hydraulic conductivity of maize plants was increased by the inoculation of Bacillus megaterium compared to the uninoculated plants under 2.59 dSm −1 of salinity. ...
Naher, U.A.; Islam, A.K.M.M.; Rana, M.M.; Rashid, M.H.; Irin, I.J.; Islam, S.S.; Rim, A.A.; Hasan, A.K. The PGPR Mechanisms of Salt Stress Adaptation and Plant Growth Promotion. Agronomy 2022, 12, 2266.
... Some important member of a genus under Bacillus includes B. licheniformis HSW-16, B. amyloliquefaciens SN13, B. megaterium A12, B. subtilis SU47, and B. pumilus HB4N3 are reported for plant growth, and stress management [61,131,133,136,137]. The PGPR, Bacillus subtilis 93,151 inoculated transgenic Arabidopsis thaliana showed enhanced proline synthesis with proBA genes that can upsurge the plant's salinity tolerance [138]. Root hydraulic conductivity of maize plants was increased by the inoculation of Bacillus megaterium compared to the uninoculated plants under 2.59 dSm −1 of salinity. ...
... Some important member of a genus under Bacillus includes B. licheniformis HSW-16, B. amyloliquefaciens SN13, B. megaterium A12, B. subtilis SU47, and B. pumilus HB4N3 are reported for plant growth, and stress management [61,131,133,136,137]. The PGPR, Bacillus subtilis 93,151 inoculated transgenic Arabidopsis thaliana showed enhanced proline synthesis with proBA genes that can upsurge the plant's salinity tolerance [138]. Root hydraulic conductivity of maize plants was increased by the inoculation of Bacillus megaterium compared to the uninoculated plants under 2.59 dSm −1 of salinity. ...
Worldwide crop productivity hampers severely due to the adverse effects of salinity. Global warming causes a rapid escalation of the salt-affected area, and new agricultural land is affected through saltwater intrusion. The ever-growing human population impulses to utilize the saline area for crop cultivation to ensure food security. Salinity resistance crops could be a promising substitute but with minor success because inappropriate tactics on saline soil management resulted in unsatisfactory yield. Salt-tolerant plant growth-promoting rhizobacteria (ST-PGPR) is considered an alternate way towards enhancing crop growth in saline ecosystems. It is reported that PGPR is enabled to produce exopolysaccharides which lead to biofilm formation and generate osmoprotectants and antioxidant enzymes that can significantly contribute to stimulating plant growth in the saline ecosystem. In addition, several plant growth-promoting characteristics of PGPR such as the acquisition of essential nutrients and upsurge hormone production could enhance plant growth simultaneously. In this review, we will explore the survival mechanisms of ST-PGPR and their influence on plant growth promotion in saline ecosystems.
... Bacillus, Pseudomonas and Rhizobium strains, have been shown to boost wheat and soybean growth, associated with the ability to produce exopolysaccharides and form hydrophilic biofilms (Bezzate et al. 2000;Ashraf et al. 2004;Rossi and De Philippis 2015). Some PGPR strains produced osmoprotectants such as proline, trehalose and glycine betaine, which play multifunctional role to help plant under stress conditions (Chen et al. 2007;Krasensky and Jonak 2012;Ahmad et al. 2013;Enebe and Babalola 2018). PGPR-producing osmolytes typically aid in protein maintenance, ROS scavenging, cytosolic acidity regulation, lipid peroxidation reduction and protein and membrane integrity maintenance during salt stress (Abbas et al. 2019). ...
Main Conclusion
The responses of plants to different abiotic stresses and mechanisms involved in their mitigation are discussed. Production of osmoprotectants, antioxidants, enzymes and other metabolites by beneficial microorganisms and their bioengineering ameliorates environmental stresses to improve food production.
Abstract
Progressive intensification of global agriculture, injudicious use of agrochemicals and change in climate conditions have deteriorated soil health, diminished the microbial biodiversity and resulted in environment pollution along with increase in biotic and abiotic stresses. Extreme weather conditions and erratic rains have further imposed additional stress for the growth and development of plants. Dominant abiotic stresses comprise drought, temperature, increased salinity, acidity, metal toxicity and nutrient starvation in soil, which severely limit crop production. For promoting sustainable crop production in environmentally challenging environments, use of beneficial microbes has emerged as a safer and sustainable means for mitigation of abiotic stresses resulting in improved crop productivity. These stress-tolerant microorganisms play an effective role against abiotic stresses by enhancing the antioxidant potential, improving nutrient acquisition, regulating the production of plant hormones, ACC deaminase, siderophore and exopolysaccharides and accumulating osmoprotectants and, thus, stimulating plant biomass and crop yield. In addition, bioengineering of beneficial microorganisms provides an innovative approach to enhance stress tolerance in plants. The use of genetically engineered stress-tolerant microbes as inoculants of crop plants may facilitate their use for enhanced nutrient cycling along with amelioration of abiotic stresses to improve food production for the ever-increasing population. In this chapter, an overview is provided about the current understanding of plant–bacterial interactions that help in alleviating abiotic stress in different crop systems in the face of climate change. This review largely focuses on the importance and need of sustainable and environmentally friendly approaches using beneficial microbes for ameliorating the environmental stresses in our agricultural systems.
... They promoted osmolytes buildup and phytohormone signaling, helping plants to control preliminary osmotic shocks following salinity stress. Increased production level of proline in transgenic Arabidopsis thaliana containing probBA genes procured from Bacillus subtilis provided tolerance to salinity in plants (Chen et al., 2007). Administration of salt-tolerating Bacillus amyloliquefaciens SN13 into rice seedlings during saline stress in hydroponic or soil conditions augmented higher salt tolerance impacting the expression levels of 14 genes of which, four (SOS1, ethylene-responsive element binding proteins EREBP, somatic embryogenesis receptor-like kinase SERK1 and NADP-malic enzyme NADP-Me2) were upregulated and two [glucose insensitive growth GIG and (SNF1) serinethreonine protein kinase SAPK4] were downregulated in hydroponic conditions while only MAPK5 (Mitogen-activated protein kinase 5) was upregulated in greenhouse conditions. ...
... They are also of paramount importance in mature biofilm formation and functional nodules in legumerhizobia symbiosis (Stoodley et al., 2002;Skorupska et al., 2006;Balsanelli et al., 2014;Janczarek et al., 2015). PGPR are also involved in altering hormonal signaling in plants during stress conditions by producing indole-3-acetic acid, abscisic acid, cytokinin and gibberellic acid hormones that directly affect plant growth (Chen et al., 2007;Porcel et al., 2014;Tsukanova et al., 2017;Kudoyarova et al., 2019). However, there is still a lack of comprehensive analysis of the roles of plant hormones in the interaction of the host plant with the PGPR. ...
Microorganisms are an important component of the ecosystem and have an enormous impact on human lives. Moreover, microorganisms are considered to have desirable effects on other co-existing species in a variety of habitats, such as agriculture and industries. In this way, they also have enormous environmental applications. Hence, collections of microorganisms with specific traits are a crucial step in developing new technologies to harness the microbial potential. Microbial culture collections (MCCs) are a repository for the preservation of a large variety of microbial species distributed throughout the world. In this context, culture collections (CCs) and microbial biological resource centres (mBRCs) are vital for the safeguarding and circulation of biological resources, as well as for the progress of the life sciences. Ex situ conservation of microorganisms tagged with specific traits in the collections is the crucial step in developing new technologies to harness their potential. Type strains are mainly used in taxonomic study, whereas reference strains are used for agricultural, biotechnological, pharmaceutical research and commercial work. Despite the tremendous potential in microbiological research, little effort has been made in the true sense to harness the potential of conserved microorganisms. This review highlights (1) the importance of available global microbial collections for man and (2) the use of these resources in different research and applications in agriculture, biotechnology, and industry. In addition, an extensive literature survey was carried out on preserved microorganisms from different collection centres using the Web of Science (WoS) and SCOPUS. This review also emphasizes knowledge gaps and future perspectives. Finally, this study provides a critical analysis of the current and future roles of microorganisms available in culture collections for different sustainable agricultural and industrial applications. This work highlights target-specific potential microbial strains that have multiple important metabolic and genetic traits for future research and use.
... Bacteria living in extreme halophilic environments induce the expression of proline biosynthesis genes proH, proJ and proA that help in survival in high saline conditions (Saum and Müller 2007). In genetically modified Arabidopsis thaliana with introgression of proBA genes from Bacillus subtilis enhanced the proline synthesis in the plant helping it to overcome the salt stress (Chen et al. 2007). Likewise, Capsicum annum inoculated with Bacillus fortis SSB21 increased the biosynthesis of proline and upregulation of stress-related genes CAPIP2, CaKR1, CaOSM1 and CAChi2 to overcome saline conditions (Yasin et al. 2018). ...
There is a significant decline in crop yield, quality of crops and soil fertility worldwide due to increased biotic and abiotic stresses that are either induced naturally or due to anthropogenic factors such as rapid urbanization and industrialization. Interaction of plants with several beneficiary microbes such as Plant Growth-Promoting Microbes (PGPM) comprising of actinomycetes, rhizospheric bacteria, and fungi help the plants to survive during abiotic stress conditions such as extreme temperatures (very low or very high temperature), flood or drought conditions, high salinity stress, heavy metal stress, nutrient deficiency and oxidative damages. Abiotic stresses harm plant growth, cellular morphology and physiology through obstruction in cellular pathways or gene regulation inside the cell. These microbes employ stress tolerance mechanisms in plants, such as the production of anti-oxidants, exopolysaccharides (EPS), phytohormones, osmolytes, formation of biofilms and siderophores, through various biosynthetic pathways. Here, in this chapter, we review recent findings in successful employment of microbial inoculation that induce abiotic stress tolerance in crop plants and study the role of bioactive metabolites liberated by microbes in association with plants which may help design strategies and tools for the development of improved and efficient microbial inoculant for optimizing plant growth in crop fields under adverse abiotic stressed conditions.
... Additionally, PGPR inoculation of different plants under salt stress enhanced the several gene expressions through accumulation of osmolytes. For example, inoculation of B. megaterium in maize plants enhanced the expression of two PIP (plasma membrane aquaporin protein) and inoculation of B. subtilis in Arabidopsis plants enhanced proBA genes expression through the accumulation of proline (Chen et al. 2007;Marulanda et al. 2010). From the above discussion, it is apparent that inoculation of PGPR under salt stress accelerates the accumulation of different osmolytes and that osmolytes function as osmoprotectant and mitigate salt-induced oxidative stress and, consequently, improve plant growth and development. ...
In this recent era, several approaches have been developed to alleviate the adverse effects of salinity stress in different plants. However, some of them are not eco-friendly. In this context, evolving sustainable approaches which enhance the productivity of saline soil without harming the environment are necessary. Many recent studies showed that plant growth-promoting rhizobacteria (PGPR) are known to confer salinity tolerance to plants. Salt-stressed plants inoculated with PGPR enhance the growth and productivity of crops by reducing oxidative damage, maintaining ionic homeostasis, enhancing antioxidant machinery, and regulating gene expressions. The PGPR also regulates the photosynthetic attributes such as net photosynthetic rate, chlorophyll, and carotenoid contents and enhances the salinity tolerance to plants. Moreover, PGPR has a great role in the enhancement of phytohormones and secondary metabolites synthesis in plants under salt stress. This review summarizes the current reports of the application of PGPR in plants under salt stress and discusses the PGPR-mediated mechanisms in plants of salt tolerance. This review also discusses the potential role of PGPR in cross-talk with phytohormones and secondary metabolites to alleviate salt stress and highlights the research gaps where further research is needed.
... Many bacteria synthesize and accumulate some osmolytes, such as proline [19], glycine-betaine [20], and trehalose [21]. In particular, proline is considered the most widely distributed osmolyte accumulated under salt stress in B. subtilis [22]. In the present study, the contents of proline and ornithine (precursor of proline synthesis) increased more in BS compared to the other samples. ...
Bacillus subtilis is often exposed to diverse culture conditions with the aim of improving hygiene or food quality. This can lead to changes in the volatile metabolite profiles related to the quality of fermented foods. To comprehensively interpret the associated metabolic expressions, changes in intracellular primary and extracellular secondary volatile metabolites were investigated by exposing B. subtilis to an alkaline pH (BP, pH 8.0) and a high salt concentration (BS, 1 M). In particular, B. subtilis was cultured in a leucine-enriched medium to investigate the formation of leucine-derived volatile metabolites. This study observed metabolic changes in several metabolic pathways, including carbohydrate metabolism, amino acid metabolism, fatty acid metabolism, and leucine degradation. The formation of proline (an osmolyte), furans, pyrrole, and monosaccharide sugars (glucose, galactose, and fructose) was enhanced in BS, whereas fatty acid derivatives (ketones and alcohols) increased in BP. In the case of leucine degradation, 3-methyl-butanal and 3-methylbutanol could be salt-specific metabolites, while the contents of 3-methylbutanoic acid and 3-methylbutylacetate increased in BP. These results show culture condition-specific metabolic changes, especially secondary volatile metabolites related to the sensory property of foods, in B. subtilis.
... Exopolysaccharide reduces the saline condition in the soil by binding with the cations including Na + and thus decreased its availability to plants. Chen et al. (Chen et al. 2007) studied the accumulation of proline in relation with the drought and salt tolerance in plants. The genes proBA derived from Bacillus subtilis inserted into Arabidopsis thaliana displayed high production of free proline as a result increased the tolerance to osmotic stress in the transgenic crops. ...
Pollution with petroleum hydrocarbons is far-reaching and thus, a problem for the environment as well as human health. The pristine environment has continuously been influenced by anthropogenic activities. Due to the globalization of various industries; their waste materials are being discharged untreated or partially treated into the ecosystem and having adverse impact on different life forms. Petroleum despite being a priceless resource and central to human life on Earth today, extraction and transportation of petroleum products has a number of ecological repercussions. The most consequential effect of petroleum use leads to the environmental pollution, adversely affecting air, soil and water quality. Petroleum product’s spill and leakage are also a major threat to the environment because petroleum products can rigorously destroy the surrounding ecosystem. So, the removal of petroleum products is imperative using eco-friendly methods, and microorganisms are the cheaply available option for doing so. Biosurfactants are extracellular amphiphilic, surface-active compounds produced by microorganisms. These microbially produced multifunctional biomolecules are versatile products having vast applications in various aspects related to clean up of environmental contaminants inclusive of enhanced oil recovery (EOR), controlling oil spills, detoxification and biodegradation of oil contaminated wastewater, soil or sediments. Biosurfactant works by reducing interfacial and surface tension by collecting at the interface of immiscible liquids and thus improve the bioavailability, solubility and subsequent biodegradation of the insoluble or hydrophobic organic compounds. This chapter summarizes the role of biosurfactant-producing bacteria in the bioremediation of petroleum hydrocarbons polluted environment.
... It has a synergetic effect on the production of other computable solutes under induced salt stress. For example, proline is involved in the synthesis of trehalose, another osmolyte in plants when inoculated with PGPR (Chen et al., 2007). Similarly, an increased concentration of total soluble sugar modulated by leaf protein content has been recorded in plants treated with PGPR (Upadhyay and Singh, 2015). ...
During the last two decades the world has experienced an abrupt change in climate. Both natural and artificial factors are climate change drivers, although the effect of natural factors are lesser than the anthropogenic drivers. These factors have changed the pattern of precipitation resulting in a rise in sea levels, changes in evapotranspiration, occurrence of flood overwintering of pathogens, increased resistance of pests and parasites, and reduced productivity of plants. Although excess CO2 promotes growth of C3 plants, high temperatures reduce the yield of important agricultural crops due to high evapotranspiration. These two factors have an impact on soil salinization and agriculture production, leading to the issue of water and food security. Farmers have adopted different strategies to cope with agriculture production in saline and saline sodic soil. Recently the inoculation of halotolerant plant growth promoting rhizobacteria (PGPR) in saline fields is an environmentally friendly and sustainable approach to overcome salinity and promote crop growth and yield in saline and saline sodic soil. These halotolerant bacteria synthesize certain metabolites which help crops in adopting a saline condition and promote their growth without any negative effects. There is a complex interkingdom signaling between host and microbes for mutual interaction, which is also influenced by environmental factors. For mutual survival, nature induces a strong positive relationship between host and microbes in the rhizosphere. Commercialization of such PGPR in the form of biofertilizers, biostimulants, and biopower are needed to build climate resilience in agriculture. The production of phytohormones, particularly auxins, have been demonstrated by PGPR, even the pathogenic bacteria and fungi which also modulate the endogenous level of auxins in plants, subsequently enhancing plant resistance to various stresses. The present review focuses on plant-microbe communication and elaborates on their role in plant tolerance under changing climatic conditions.
... Plant inoculation with rhizobacteria Bacillus megaterium up-regulated two genes ZMP IP responsible for aquaporins synthesis in plasma membrane (Marulanda et al. 2010). Chen et al. (2007) observed increased expression of genes stimulating proline biosynthesis in transgenic plant Arabidopsis thaliana having pro BA genes from Bacillus subtilis and conferred tolerance. These microbes can also stimulate carbohydrate metabolism which could be seen in source sink relation increased CO 2 fixation and increased biomass reallocation with enhance growth rate. ...
Plant growth-promoting rhizobacteria (PGPR) are nonpathogenic, beneficial microbe promotes plant growth by changing the concentration of hormones, nutritional requirements, neutralizing the stress related harm. Plants faces various kinds of abiotic stresses includes nutrients deficiency, heavy metals contamination, salinity, extreme temperature and drought, reduces crop productivity significantly. These stresses could be neutralized by using fertilizers or by developing new high yielding breeds which impose high cost and environmental hazard. Climate change and soil degradation continuously challenging the food production worldwide and sustainable solution should be adopted to solve these stresses. PGPRs help largely in solving these abiotic stresses using mechanisms like production of 1-aminocyclopropane-1-carboxylate (ACC) deaminase, Abscisic acid (ABA) synthesis, Indole-3-acetic acid (IAA) production, phosphorus solubilization and mineralization (P), Potassium (K) solubilization and mobilization, nitrogen fixation and enzyme production which enhance crop plants sustainability. These provide a sustainable and environmentally friendly tool. In this chapter, abiotic stress impacts on plants and role of PGPRs in tolerating these stresses using different mechanisms are discussed. For further success in this field, physiological, biochemical and molecular mechanisms should be understood to achieve high yielding and stress tolerant plants.
... The active participation of biostimulants with salt-tolerant nature and growth-promoting characteristics harbors ample opportunities in sustainable management of salinity affected agroecosystems (Kushwaha et al., 2020). The growth-promoting effect of bacteria in salt-stressed plant is rendered by diversified pool of antioxidant defense system (Habib et al., 2016), bacterial synthesis of 1-aminocyclopropane-1-carboxylate (ACC) deaminase (del Carmen Orozco-Mosqueda et al., 2020), induced synthesis of hormones by host (Numan et al., 2018), generation of microbial volatile organic compounds, changes in expression of genes regulating stress responses (Baek et al., 2020), alterations in ethylene metabolism (Misra and Chauhan, 2020), and regulation of proline level and ionic balances (Chen et al., 2007;Bano and Fatima, 2009). The research progress in growth-enhancing microbial resources could offer a novel boulevard in agriculture sector for augmenting host tolerance to salinity stress with the resultant increase in crop growth and productivity. ...
The improvement in cultivated ecosystems for enhancement in agricultural productivity is an essential step to support the rapidly rising human population. Pest infestation and changes in climatic conditions with possible negative consequences have further fueled the orientation of agricultural practices toward maximum crop productivity in a sustainable manner at the cost of available natural resources. The crop yield can be increased by enhancing the growth of crop plants and minimizing the losses caused by devastating plant diseases. The conventional practices of employing synthetic fertilizers and pesticides for improving crop yields pose great risks to environmental integrity and human health. In contrast, the deployment of growth stimulating organisms such as plant growth–promoting rhizobacteria and their products together with the living organisms and derived metabolic products such as microbial volatiles suppressing the growth of plant pathogens have promising potential in crop protection, thereby yield enhancement of cultivated ecosystem. Currently, the potential of numbers of bacteria, fungi, cyanobacteria, and mycorrhiza has been harnessed in agroecological management. However, the application under natural environmental conditions has certain challenges and requires extensive field investigation. Moreover, the application of living organisms and their products-based strategies for application in agroecosystem needs approval from regulatory bodies, restricting easy acceptance among farmers. This chapter deals with different biostimulants and biocontrol agents, mechanism of action, application in sustainable management of cultivated ecosystem, and challenges associated with exploitation under natural field condition.
... For example, extracellular polymeric substance (EPS)-producing strains of bacteria enhance plant tolerance against drought and salinity stress by improving soil structure (Sandhya et al., 2009). In salt-affected soils, increasing population of EPS-producing bacteria can ameliorate hazardous effects of salinity on crops because they restrict influx or uptake of sodium ions due to cation bindings (Chen et al., 2007). It can be speculated from previous studies that EPS production and formation of biofilm augment the fertility of soil and significantly improve plant growth (Ashraf and Harris, 2004;Ashraf et al., 2005;Liaqat et al., 2009;Davey and Toole, 2000). ...
... The PM community composition varies as per the signaling by plant parts in response to the environmental conditions around plant [51][52][53][54][55]. The LCOs are found to be involved in plant growth stimulation [55][56][57], the elaboration of the root system [58], the acceleration of flowering and increased fruit yield [59], and stimulating early somatic embryo development [4,50,57,60]. The enhanced germination and growth of seedlings, along with the LCOs' positive effect on mitotic cell division, suggest an activated meristem activity. ...
Recent crop production studies have aimed at an increase in the biotic and abiotic tolerance of plant communities, along with increased nutrient availability and crop yields. This can be achieved in various ways, but one of the emerging approaches is to understand the phytomicrobiome structure and associated chemical communications. The phytomicrobiome was characterized with the advent of high-throughput techniques. Its composition and chemical signaling phenomena have been revealed, leading the way for "rhizosphere engineering". In addition to the above, phytomicrobiome studies have paved the way to best tackling soil contamination with various anthropogenic activities. Agricultural lands have been found to be unbalanced for crop production. Due to the intense application of agricultural chemicals such as herbicides, fungicides, insecticides, fertilizers, etc., which can only be rejuvenated efficiently through detailed studies on the phytomicrobiome component, the phytomicrobiome has recently emerged as a primary plant trait that affects crop production. The phytomicrobiome also acts as an essential modifying factor in plant root exudation and vice versa, resulting in better plant health and crop yield both in terms of quantity and quality. Not only supporting better plant growth, phytomicrobiome members are involved in the degradation of toxic materials, alleviating the stress conditions that adversely affect plant development. Thus, the present review compiles the progress in understanding phytomicrobiome relationships and their application in achieving the goal of sustainable agriculture.
... According to Marulanda et al. (2010) Maize plant inoculated with Bacillus megaterium showed increased level of hydraulic conductivity of plant root and increased the expression of the plasma membrane aquaporin protein as compared to the uninoculated plants (Marulanda et al., 2010). PGPR helps the plant in the accumulation osmoprotectant and phytohormone signaling and facilitate the plants to overcome from the salinity stress (Chen et al., 2007). ...
Abiotic stresses have negative impacts on the crop production, by altering plants mechanisms. Drought (water deficit stress), salinity, etc can be enlisted in the abiotic stresses affecting crop productivity. Sustainable agriculture maintains and improves the quality of resources of environment, satisfying changing human requirements, where use of plant growth promoting rhizobacteria (PGPR) can be approached. Bacteria residing in rhizosphere region are the rhizobacteria, which are usually attached to the soil particles, in the vicinity of roots because of abundant food sources around them. PGPR efficiently secrete miscellaneous chemical regulators in vicinity of rhizosphere by colonizing plant roots thereby promoting plant growth. Recently, there is an increasing number of researchers in investigating various aspects to maximize the efficiency of PGPR utilization in reducing inputs of chemical fertilizer in crop production. This chapter elucidate the effects of drought stress and salinity stress on crop plants, and influence of PGPR under drought and salinity stress along with their molecular mechanisms, osmotic balance, modification of phytohormonal activity, ion homeostasis was deliberated extensively.
... Plant inoculation with rhizobacteria Bacillus megaterium up-regulated two genes ZMP IP responsible for aquaporins synthesis in plasma membrane (Marulanda et al. 2010). Chen et al. (2007) observed increased expression of genes stimulating proline biosynthesis in transgenic plant Arabidopsis thaliana having pro BA genes from Bacillus subtilis and conferred tolerance. These microbes can also stimulate carbohydrate metabolism which could be seen in source sink relation increased CO 2 fixation and increased biomass reallocation with enhance growth rate. ...
... Regarding the molecular aspects of PGPR action, the expression of novel plant genes, such as SOS1, which codifies for a plasma membrane Na+/H+ antiporter [138], was found to be promoted in PGPR-applied wheat shoots and roots, resulting in improved salt stress resistance [139]. Moreover, the use of genes from halo-tolerant PGPR may confer salt tolerance to transgenic plants; for example, expression of Bacillus subtilis proline biosynthesis (ProBA) gene conferred salt tolerance in transgenic Arabidopsis through increased proline production [140]. Likewise, halotolerant properties of PGPR, such as Enterobacter ludwigii and other enterobacters rely on the role of Na+/H+ antiporters, such as NhaA. ...
Salinity is one of the main constraints for agriculture productivity worldwide. This important abiotic stress has worsened in the last 20 years due to the increase in water demands in arid and semi-arid areas. In this context, increasing tolerance of crop plants to salt stress is needed to guarantee future food supply to a growing population. This review compiles knowledge on the use of phytoprotectants of microbial origin (arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria), osmoprotectants, melatonin, phytohormones and antioxidant metabolism-related compounds as alleviators of salt stress in numerous plant species. Phytoprotectants are discussed in detail, including their nature, applicability, and role in the plant in terms of physiological and phenotype effects. As a result, increased crop yield and crop quality can be achieved, which in turn positively impact food security. Herein, efforts from academic and industrial sectors should focus on defining the treatment conditions and plant-phytoprotectant associations providing higher benefits.
Sessile plants confront the fluctuating harsh environmental conditions and react to alterations in biotic and abiotic components of environments by symbiotic association between plant and biosphere. The origins of stresses are the vicinal environment, which is composed of biotic and abiotic agents. A wide range of molecular mechanisms are opted by the plants for their self-defense. The plant faces harsh conditions due to its molecular battery. Signaling molecules engineer the plants to tolerate the stresses. Transposable elements become active due to living and nonliving agents. Physical and chemical agents cause induction in mutation. These changes are the first driving step in the evolution of plants. During evolution, environmental changes force the plants to adapt or succumb to stress. The plants respond to the ecological conditions by modulating the gene programmer.
High salinity decreases the productivity of crops worldwide. Pepper is particularly sensitive to high salt concentrations. Herein, we subjected three tolerant pepper accessions (C12, B14 and A25) to high sodium chloride concentration (70 mM NaCl). The aerial and root biomass, leaf and root osmotic potential (Ψ π ), Na ⁺ , Cl ⁻ , K ⁺ and proline concentrations and the relative expression of the putative genes CaSOS1 , CaHKT1 , three CaNHXs and CaP5CS were measured. Different salinity tolerance strategies depending on the pepper accession were identified. In C12, tolerance was attributed to the accumulation of Na ⁺ in vacuoles and endosomes by the activation of vacuolar CaNHXs genes and the reduction in Ψ π ; additionally, the activation of CaHKT1 and CaSOS1 in leaves and roots moved and accumulated Na ⁺ ions in the xylem and xylem parenchyma cells (XPC) as well as expulsed it out of the root cells. A25 accession, on the contrary, was specialized in compartmentalizing Na ⁺ ions in root and leaf vacuoles and root XPC by the up‐regulation of CaNHXs and CaHKT1 , respectively, avoiding a toxic accumulation in leaves. Finally, B14 accession moved and accumulated Na ⁺ in xylem and XPC, reducing its concentration in roots by the activation of CaSOS1 and CaHKT1 . This study shade light on different tolerance mechanisms of pepper plants to overcome salt stress.
Abiotic stresses such as drought, salinity, temperature, flooding and heavy metal toxicity reduce crop yield. About 64% of land worldwide is affected by drought. The lack of moisture in crops may lead to 50% yield loss, and increases soil salinity levels. Therefore, the use of plant growth promoting rhizobacteria for increasing plant stress tolerance appears as a sustainable strategy. Here we review abiotic stress tolerance mediated by plant growth promoting rhizobacteria in plants with focus on phytohormones, improved physiological attributes, root system architecture and regulation of the osmotic balance. Improved antioxidant activity result in reduced oxidative damage, which promotes plant growth, and nutrient and water uptake. Other extracellular secretions trap ions and moisture and improve the growth environment. Emissions and elicitors function as signaling molecules that induce genes and transcription factors belonging to the stress responsive pathways. The use of plant growth-promoting rhizobacterial bioinoculants is effective in enhancing tolerance to crop abiotic stress. The formulations and application of these microorganisms is promising for climate smart agriculture.
Microbial genes from fungi, virus, actinomycete, and bacteria have been widely explored for the improvement of crop plants. Here we review microbial genes-based transgenic plants for tolerance against abiotic stresses, improved nutrients, and disease resistance. Genetic engineering involves the transfer of desirable genes of foreign origin to plants. The expression of foreign genes induce changes at the biochemical, physio-chemical, anatomical, morphological, and physiological levels, which ultimately improve stresses tolerance and crop production. Foreign genes of microbial origin are delivered to the plant species by methods including Agrobacterium transformation, floral dip transformation, polyethylene glycol/CaCl2-mediated transformation, viral vectors, and the biolistic method. The efficiency of these methods depend on plant type, variety/cultivar, organ, cloning method, and gene size.KeywordsAbiotic stress
Bacillus subtilis
cry geneGlycine BetainHerbicidesPhytase
Trichoderma virens
Oxidative stress is common in plants growing under extreme environments, such as water shortage, salinity, heavy metals, and temperature extremes. To overcome oxidative stress, plants employ various endogenous strategies, including the oxidative defense system and osmolyte accumulation. Proline (Pro), a key organic osmolyte, generally accumulates in plants under harsh environmental stress. The over-accumulation of Pro in most plants under abiotic stress has been linked to elevated activities of vital enzymes and the expression of corresponding Pro metabolism genes or inhibition in Pro catabolism. Moreover, accumulating knowledge of Pro regulation has shown that this potential osmolyte can scavenge free radicals generated by plants under various stressors. Thus, efforts are underway to enhance plant tolerance to oxidative stress either by applying exogenous Pro or transforming the genes involved in Pro metabolism. This review discusses how and to what extent Pro is involved in relieving oxidative stress to regulate plant survival under extreme environments. It also presents an overview on adopting advanced genome-editing technologies, such as CRISPR, for editing appropriate genes of Pro metabolism aiming to develop stress-resistant crop plants. This information is crucial for developing oxidative-stress-resistant crops by tailoring Pro metabolism genes to overcome the future food security challenges under the changing climate scenario.
The transformation of rhizosphere microbiota is essentially the result of a series of events that can enhance the formation of constant and different microbial associations in the plant microbiome/holobiome based on supportive information/communications. Beneficial microbial communities act as influential identities for the elevation of ecological stresses in plants and ultimately decrease the usage of fertilizer and pesticides in order to increase the crop yield. Microbiome has the capability to stimulate the growth of plants, develop resistance to stress, and enhance the health of plants. To accomplish these objectives, it is essential to learn more about the relationship between plant, microbiome, microbial community present in soil, and their resilience to environmental changes. The information acquired will help in understanding the effect of these microorganisms on the biotic resistance, biogeochemical cycles, and productivity of the crops. A comprehensive understanding of the biological mechanisms underlying stress-induced microbiome modifications would also allow for the development of personalized DefenseBiomes and chemicals in order to combat with crop stresses.KeywordsRhizosphereDesigner microbesRhizosphere engineeringDefenseBiomesHolobiomeBiotic and abiotic stresses
Climate change is a major concern for sustainable agriculture in the twenty-first century, as it has an impact on crop production and soil fertility, which may increase the risk of famine. Of the array of issues pertaining to climate change, salt stress is one of the most significant factors affecting crop production all over the world. Plants undergo different morphological, physio-chemical, and molecular adaptations to deal with salinity stress. However, several mitigation strategies are also used to cope with the drastic effects of salt stress. Microbial-based solutions, in particular, are highly desirable in sustainable agriculture as they provide a natural, cost-effective, and environmentally safe approach for improving plant growth and yield. Endo-phytic bacteria not only preserve soil fertility but also boost plant growth under salt-stress situations. These bacteria mitigate salt stress by lowering the synthesis of reactive oxygen species (ROS) and facilitating nutrient availability. Moreover, the endophytic bacteria also regulate the expression of genes responsible for producing various phytohormones, antioxidant enzymes, siderophores, volatile organic compounds, ROS-scavenging enzymes, and other substances. The current research extensively explored the potential pathways involved in plant growth modulation and salinity stress mitigation by endophytic bacteria. Bacterial endophytes as a plant's second genome might be a natural strategy for improving a plant's growth and yield under salt-stress conditions.
Plant growth-promoting rhizobacteria (PGPR) play a vital role in alleviating biotic and abiotic stresses in plants. They enable plant growth under adverse circumstances and help improving the yield through several direct or indirect mechanisms. With the aid of PGPR, plant can fix atmospheric nitrogen, produce phytohormones, enhance water and nutrient uptake, solubilize phosphate, and improve the provision of binding iron with the help of siderophores. PGPR-induced indirect mechanisms help plant to suppress parasitism by some deleterious rhizobacteria through induced systemic resistance, antibiosis, competition for nutrients, and production of metabolites. Thus, application of PGPR is a sustainable approach to reduce the use of chemical fertilizers in crop field. Bio-fertilizers containing PGPR have great economic prospect and potential for environmental benefits. This review highlights the various roles of PGPR in promoting growth and development of plant in a stressful environment. The study also encompasses the successful application of PGPR strains for nitrogen fixation, management of nutrients, water and salt stresses, controlling plant insects, and phytopathogens. Furthermore, recent know-how on the underlying mechanisms of PGPR-induced systemic resistance in plants will be helpful to investigate their additional uses.
Change in global climate has started to show its effect in the form of extremes of temperatures and water scarcity which is bound to impact adversely the global food security in near future. In the current review we discuss the impact of drought on plants and highlight the ability of endophytes, microbes that inhabit the plants asymptomatically, to confer stress tolerance to their host. For this we first describe the symbiotic association between plant and the endophytes and then focus on the molecular and physiological strategies/mechanisms adopted by these endophytes to confer stress tolerance. These include root alteration, osmotic adjustment, ROS scavenging, detoxification, production of phytohormones, and promoting plant growth under adverse conditions. The review further elaborates on how omics-based techniques have advanced our understanding of molecular basis of endophyte mediated drought tolerance of host plant. Detailed analysis of whole genome sequences of endophytes followed by comparative genomics facilitates in identification of genes involved in endophyte-host interaction while functional genomics further unveils the microbial targets that can be exploited for enhancing the stress tolerance of the host. Thus, an amalgamation of endophytes with other sustainable agricultural practices seems to be an appeasing approach to produce climate-resilient crops.
Salinity is the key constraint that affects the crop growth, metabolism, and
yield because of high abundance of salts present in the soil. The area and agriculture
crop affected by salinity stress are increasing day by day. Salinity stress disturbs
many physiological, biochemical, and molecular parameters in crop plants. There-
fore, there is urgent need of promising candidate, which helps to mitigate salinity
stress, favors plant growth, and also has environment-friendly impact. The charac-
terization and exploitation of soil microbes (especially mycorrhizal fungi, PGPR,
endophytes such as Piriformospora indica and cyanobacteria) in agriculture open
new alternatives to overcome salinity stress. Amelioration of salinity in plant occurs
through plant growth-promoting rhizobacteria (PGPB) by applying different strate-
gies such as they introduce synthesis of antioxidative enzyme to cope with reactive
oxygen species, which is generated during salt stress in plants, and stimulates
accumulation of osmolyte in plants, and plants inoculated with PGPB have high
K+
/Na+
ratio that favors salinity tolerance. Besides these, PGPB produces various
hormones (auxin, cytokinins, and gibberellins) to mitigate salt stress. Arbuscular
mycorrhizal fungi (AMF) equipped with fascinating mechanisms are useful in
mitigating the adverse effect of salinity stress. AMF inoculation in plant increases
the mineral nutrient uptake of K, Fe, Ca, Mg, Mn, and Zn, reduces the uptake of Na+
,
and accumulation of proline and phenol increases, which also reduced the effect of
salinity in plants. Several AMF species produced various antioxidative enzymes
such as catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR),
and peroxidase (POD) that help to minimize the effect of ROS produced at the time
of salinity stress. An endophyte, P. indica, colonizes with a broad range of plant
species. P. indica root colonization helps in amelioration of salt stress by manipu-
lating hormone signaling pathways and enhanced root cell division by production ofIAA hormone, which results in better absorption of nutrient and plant growth.
Cyanobacteria can survive and live under extreme salinity and further utilized to
increase the soil fertility.
Plants are sessile organisms subject to various types of abiotic stresses. To deal with and manage stresses, plants have evolved their own morphological, physiological and biochemical pathways regulated through the expression of various genes under different stimuli. In addition to their genetic basis of tolerance to different stresses, plants have successfully developed mutualistic interactions with different microorganisms including beneficial bacteria, such as arbuscular mycorrhizal fungi (AMF), endo-mycorrhizal fungi, plant growth-promoting rhizobacteria (PGPR), etc. Such mutualistic interactions allow them to improve their tolerance levels against different biotic and abiotic stresses. This chapter reviews the role of PGPR and their interaction with plants to tolerate abiotic stresses, including those caused by temperature extremes, drought, salinity, nutrient deficiencies and toxicities due to pesticides, heavy metals, etc. Beneficial mechanisms exhibited by PGPR are the production of osmoprotective compounds (examples), increased enzymes to reduce oxidative stress (examples), and growth regulators such as ethylene, among others. Finally, we propose the preservation of the various endemic PGPR species and strains to be potentially used as region-specific bio-inoculants.
Global climate change accelerates the coincidence of a variety of abiotic stresses, viz., salinity, drought, flooding, high and low temperature, and biotic stresses, viz. phytopathogens which degrade agricultural productivity. In such circumstances, plant growth-promoting rhizobacteria (PGPR) are eco-friendly and sustainable candidates to combat these stresses. Several PGPR with the ability to support plant growth under various stressed conditions have been commercialized. The current chapter is mainly restricted to beneficial effects of PGPR on plant growth and development under environmental and biotic stresses. It begins with the description of various abiotic and biotic stress factors affecting plant growth and their tolerance achieved by both physiological and molecular mechanisms of adaptation. The use of PGPR helps ameliorate these stresses in rhizosphere by using several mechanisms and has beneficial effects on plant growth after efficiently colonizing the root surface. Plant growth stimulation through PGPR is the net result of multiple mechanisms of action that may be activated simultaneously. Such bacteria are more likely to be used for stress tolerance to fulfill the need for food production under extreme environmental conditions. The bacterial inoculants also enhance nutrient uptake and crop growth. They are also involved in biocontrol so they may be good supplements to chemical fertilizers and agrochemicals. This chapter discusses the potential and key mechanisms used by PGPR under stress conditions for sustainable agricultural productivity followed by their prospects.
Both biotic and abiotic stresses are major limits to agricultural production. Under stress conditions, plant growth is affected by a number of factors such as hormonal and nutritional imbalance, ion toxicity, physiological disorders, susceptibility to diseases, etc. Plant growth under stress conditions may be enhanced by the application of microbial inoculation including plant growth promoting rhizobacteria (PGPR) and mycorrhizal fungi. These microbes can promote plant growth by regulating nutritional and hormonal balance, phytohormones and synthesizing some compounds or enzymes that can develop plant growth, solubilizing minerals such as phosphorus and inducing resistance against stresses. This environmentally friendly bacterial population is effective in promoting crops productivity under stress conditions. The present review comprehensively discusses recent developments on the effectiveness of PGPR for enhancing plant growth under stressful environments. The key mechanisms involved in plant stress tolerance and the effectiveness of microbial inoculation for enhancing plant growth under stress conditions have been discussed at length in this review.
The salt affected soils that include saline and alkali soils are poor in organic carbon content and therefore the microbial activity and ultimately plant growth is significantly affected. Halophilic microbes are the organisms that are tolerant to salt stress. In recent past, several species of halophiles have been isolated and reported from different saline environments from various parts of the world. The mechanism of halophiles to tolerate salt stress is mainly by expressing aminocyclopropane-1-carboxylic acid (ACC) deaminase activity that removes stress, ethylene from the rhizosphere and some halophiles produce auxins that promote root growth. Halophilic plant growth promoting bacteria that live in association with plant roots alleviate salt stress for better growth and yield, through their own mechanisms for osmotolerance, osmolyte accumulation, asymbiotic nitrogen fixation, solubilization and mineralization of essential plant nutrients and production of plant growth hormones. Plant growth promoting halophilic bacteria induce plants salt stress tolerance and can help in coming out with the cost-effective solution for saline soils, improving agricultural crop yields and improving soil health. Inoculation of halophilic plant growth promoting bacteria through their formulations is known to mitigate salt stress and enhances crop growth and yields. In this chapter, the use of halophilic plant growth promoting microbial inoculants for bio-remediation of salt affected soils, crop growth enhancement and their impact on soil biochemical properties as well as their role in recent advancement for the rehabilitation of degraded lands is discussed in detail.
Plant beneficial microorganisms (PBMs) have a tendency to colonize soil and various parts of plant (especially in root) for augmenting the nutrients in soil as well as secretion of other biomolecules. In either ways PBMs help to improve plant productivity and immunity for increasing tolerance capability or adaptation towards diverse climatic conditions. Although earlier reports have exhibited significant contribution of PBMs for the development of induced systemic resistance against abiotic stresses like low/high temperature, salinity, moisture deficit and acidic conditions. PBMs play crucial role in nutrients transportation from soil to plant that leads to the reduction in application of chemical fertilizers and low accumulation of toxic elements in agricultural lands. Reducing or minimizing the use of chemical fertilizers may also help to decrease the occurrence of contamination by fertilizers and maintains ion balance in soil, as a result soil health will be improved. In addition, exopolysaccharide secretion and biofilm formation by PBMs alter the physico-chemical properties of rhizospheric soil that also impact higher plant response to abiotic stress, such as moisture deficit, metal toxicity, chilling injury, saline, and low/high temperature. PBMs mediated adaptation or tolerance in plants towards different climatic conditions might be accompanied through different mechanisms like induction of cold/heat shock proteins and osmoprotectants. Application of these microorganisms might be effective for alleviation of climate change in various crops, thus developing an emerging approach towards sustainable agriculture. These microorganisms might also be utilized as model to decipher stress tolerance mitigation and responding processes that can be established for crop plants to cope with the stress caused by climate change.
Stress mitigation strategies present in plants alleviate stress caused by biotic and abiotic factors. In the current era, multi-omics approaches involving genomics, transcriptomics, proteomics, and metabolomics have expanded the horizon of molecular events participating in response to environmental and edaphic mediated stresses. The vigilant amalgamation of these approaches have supported a high level of information generated about root-level mechanisms involved in the alleviation of different plant-stress. Different bioinformatics tools provide a way in which huge amount of data are interpreted in a better form. This combination of multi-omics and bioinformatics approaches increases the genetic knowledge of researchers to improve the plant varieties in respect to their stress tolerance potential. Here, we provide an overview of bioinformatics resources, describing collections from multi-omics approaches, ranging from raw-data to complete databases, particularly highlighting those tools which have been used for answering the long-standing questions in the field of biotic and abiotic stress research.
Abiotic and biotic stresses are both the chief constraints to crop production. Growth and development of plants are affected by numerous factors, for example, pathogen attack, disease susceptibility, ionic toxicity, poor nutrition, etc. Different kinds of chemical supplements have been approached in order to overcome these problems. Though these chemical fertilizers increased the crop productivity, on the other side, they were proved to be harmful for soil health and make it unsuitable for sustainable agriculture. To overcome these complications, soil microbes have been used as effective alternative. Plant growth-promoting rhizobacteria and arbuscular mycorrhizal fungi can uphold plant growth by various mechanisms and are considered as highly efficient biological agents for conferring stress tolerance in plants and improving soil health. Both the PGPR and AMF form the positive relationship with their host plants and helps them in acquisition of nutrition, enhances plant growth and productivity and induce defense against biotic as well as abiotic stresses. Thus, plant growth promoting bacteria and arbuscular mycorrhizal fungi proved to be vital for sustainable agriculture. Both plant growth-promoting bacteria and arbuscular mycorrhizal fungi proved to be vital for sustainable agriculture. The present work will review the role of AMF and PGPR in alleviating stress responses and inducing the defense mechanism in plants.
Rhizosphere is a hot spot where specific kinds of diverse microbial communities develop under the influence of exudates from plant roots and in turn modulate growth and development of the plant. Such communities with or without interactions perform an array of functions, including nitrogen fixation, P, Zn, Si and K-solubilization, siderophore production, ammonification, hormones production, ACC deaminase production, ethylene production, anammox, comammox, nitrification, denitrification, antagonisms, induce resistance to plant, C-sequestration, volatile production, secondary metabolites production and many others that are known to modulate soil and plant health contributing to the corresponding responses to various stresses of biotic and abiotic nature. The magnitude of resilience of plant to biotic and abiotic stresses is completely dependent on types of communities and their interactions. With enhanced knowledge and understanding about rhizosphere, researchers are evaluating various approaches to engineer rhizosphere in such way that it enables plant to enhance the productivity and sustain it while maintaining soil health. This chapter highlights detailed account of microbial interactions in the rhizosphere with associated mechanisms that contribute to resilience of plants to stress for better growth and development.
The physical and chemical properties of soil depend upon the presence of microbial diversity. Metagenomics has potential contributions towards the study of the agriculturally important microbial population in the soil such as Plant Growth Promoting Bacteria (PGPRs). PGPRs have a huge prospective in endowing the structural and functional aspects of plant-microbial interactions followed by exploring the agronomically essential genes. The soil microbial resources can be exploited for identifying these novel genes. Metagenomics is an advanced genomic tool used to access the complete soil microbial diversity irrespective of their cultivable or non-cultivable nature. Recent molecular methods exploited for identification of desired genes are direct genomic DNA extraction, preparation of metagenomics libraries, heterologous gene expression and next-generation high throughput sequencing of environmental samples. Development of sustainable agriculture demands frequent exploitation and characterization of PGPRs. Metagenomics can considerably provide paramount of the genetic information irrespective of the culturable subsets. Furthermore, it also potentially bridges the gaps in the genetic evolution of different unidentified species of microbial communities. The Recent Next Generation Sequencing (NGS) technology enables the effective combination of crop genotyping, high throughput metagenomics and detection of the diversity of plant pathogens and PGPRs. The present chapter discusses current impact of soil microbes and advanced metagenomics technologies on plant performance and sustained crop productivity.
Drought is a multidimensional stress affecting plants at various levels of their organization. The effect of and plant response to drought at the whole plant and crop level is most complex because it reflects the integration of stress effects and responses at all underlying levels of organization over space and time. This review discusses some of the major aspects of crop response to drought stress which are relevant for plant breeding. Emphasis is given to whole plant aspects which are too often disregarded when conclusions are drawn from molecular studies towards the genetic improvement of crop drought resistance. Topics discussed are seedling emergence and establishment, plant phenology, leaf area, water deficit and assimilation, osmotic adjustment, the root and the formation of yield. The discussion is concluded with the interpretation of crop adaptation to drought conditions in its agronomic sense. Conclusions are drawn regarding plant breeding for drought-prone conditions.
Many plants synthesize and accumulate proline in response to osmotic stress. Despite the importance of this pathway, however, the exact metabolic route and enzymes involved in the synthesis of proline in plants have not been unequivocally identified. We report here the isolation of a mothbean (Vigna aconitifolia) cDNA clone encoding a bifunctional enzyme, Delta^1-pyrroline-5-carboxylate synthetase (P5CS), with both gamma-glutamyl kinase and glutamic-gamma-semialdehyde dehydrogenase activities that catalyzes the first two steps in proline biosynthesis. The two enzymatic domains of P5CS correspond to the ProB and ProA proteins of Escherichia coli and contain a leucine zipper in each domain, which may facilitate inter- or intramolecular interaction of this protein. The Vigna P5CS enzyme activity is feedback regulated by proline but is less sensitive to end-product inhibition than is the E. coli gamma-glutamyl kinase. The P5CS gene is expressed at high levels in Vigna leaves and is inducible in roots subjected to salt stress, suggesting that P5CS plays a key role in proline biosynthesis, leading to osmoregulation in plants.
Fructans are polyfructose molecules produced by approximately 15% of the flowering plant species. It is possible that, in addition to being a storage carbohydrate, fructans have other physiological roles. Owing to their solubility they may help plants survive periods of osmotic stress induced by drought or cold. To investigate the possible functional significance of fructans, use was made of transgenic tobacco (Nicotiana tabacum) plants that accumulate bacterial fructans and hence possess an extra sink for carbohydrate. Biomass production was analyzed during drought stress with the use of lines differing only in the presence of fructans. Fructan-producing tobacco plants performed significantly better under polyethylene-glycol-mediated drought stress than wild-type tobacco. The growth rate of the transgenic plants was significantly higher (+55%), as were fresh weight (+33%) and dry weight (+59%) yields. The difference in weight was observed in all organs and was particularly pronounced in roots. Under unstressed control conditions the presence of fructans had no significant effect on growth rate and yield. Under all conditions the total nonstructural carbohydrate content was higher in the transgenic plants. We conclude that the introduction of fructans in this non-fructan-producing species mediates enhanced resistance to drought stress.
Drought is a multidimensional stress affecting plants at various levels of their organization. The effect of and plant response to drought at the whole plant and crop level is most complex because it reflects the integration of stress effects and responses at all underlying levels of organization over space and time. This review discusses some of the major aspects of crop response to drought stress which are relevant for plant breeding. Emphasis is given to whole plant aspects which are too often disregarded when conclusions are drawn from molecular studies towards the genetic improvement of crop drought resistance. Topics discussed are seedling emergence and establishment, plant phenology, leaf area, water deficit and assimilation, osmotic adjustment, the root and the formation of yield. The discussion is concluded with the interpretation of crop adaptation to drought conditions in its agronomic sense. Conclusions are drawn regarding plant breeding for drought-prone conditions.
gamma-Glutamyl kinase, the first enzyme of the proline biosynthetic pathway, was purified to homogeneity from an Escherichia coli strain resistant to the proline analog 3,4-dehydroproline. The enzyme had a native molecular weight of 236,000 and was apparently comprised of six identical 40,000-dalton subunits. Enzymatic activity of the protein was detectable only in assays containing highly purified gamma-glutamyl phosphate reductase, the second enzyme of the proline pathway. Plots of gamma-glutamyl kinase activity as a function of glutamate concentration were sigmoidal, with a half-saturation value for glutamate of 33 mM, whereas plots of enzyme activity as a function of ATP concentration displayed typical Michaelis-Menten kinetics with a Km for ATP of 4 X 10(-4) M. Enzyme activity was insensitive to the glutamate analog L-methionine-DL-sulfoximine, but ADP was a potent competitive inhibitor. Characteristics of the enzyme were compared with those of a gamma-glutamyl kinase partially purified from a 3,4-dehydroproline-sensitive E. coli. These results indicated that the only major difference was that the enzyme from the 3,4-dehydroproline-resistant strain was 100-fold less sensitive to feedback inhibition by proline.
delta 1-Pyrroline-5-carboxylate synthetase (P5CS) catalyzes the first two steps in proline biosynthesis in plants. The Vigna aconitifolia P5CS cDNA was expressed in Escherichia coli, and the enzyme was purified to homogeneity. The Vigna P5CS exhibited two activities, gamma-glutamyl kinase (gamma-GK) and glutamic acid-5-semialdehyde (GSA) dehydrogenase. The gamma-GK activity of the P5CS was detected by the hydroxamate assay and by a [14C]glutamate assay. The native molecular mass of the P5CS was 450 kDa with six identical subunits. The Vigna P5CS showed a Km of 3.6 mM for glutamate, while the Km for ATP was 2.7 mM. The gamma-GK activity of the P5CS was competitively inhibited by proline, while its GSA dehydrogenase activity was insensitive to proline. In addition, a protein inhibitor of the P5CS was observed in the plant cell. Western blot showed that the level of the P5CS was enhanced in Vigna root under salt stress. A single substitution of an alanine for a phenylalanine at amino acid residue 129 of the P5CS resulted in a significant reduction of proline feedback inhibition. The 50% inhibition values of gamma-GK activity of the wild type and the mutant P5CS were observed at 5 mM and 960 mM proline, respectively. The other properties of the mutant P5CS remained unchanged. These results may allow genetic manipulation of proline biosynthesis and overproduction of proline in plants for conferring water stress tolerance.
To confer abscisic acid (ABA) and/or stress-inducible gene expression, an ABA-response complex (ABRC1) from the barley (Hordeum vulgare L.) HVA22 gene was fused to four different lengths of the 5' region from the rice (Oryza sativa L.) Act1 gene. Transient assay of beta-glucuronidase (GUS) activity in barley aleurone cells shows that, coupled with ABRC1, the shortest minimal promoter (Act1-100P) gives both the greatest induction and the highest level of absolute activity following ABA treatment. Two plasmids with one or four copies of ABRC1 combined with the same Act1-100P and HVA22(I) of barley HVA22 were constructed and used for stable expression of uidA in transgenic rice plants. Three Southern blot-positive lines with the correct hybridization pattern for each construct were obtained. Northern analysis indicated that uidA expression is induced by ABA, water-deficit, and NaCl treatments. GUS activity assays in the transgenic plants confirmed that the induction of GUS activity varies from 3- to 8-fold with different treatments or in different rice tissues, and that transgenic rice plants harboring four copies of ABRC1 show 50% to 200% higher absolute GUS activity both before and after treatments than those with one copy of ABRC1.
All microorganisms possess a positive turgor, and maintenance of this outward-directed pressure is essential since it is generally considered as the driving force for cell expansion. Exposure of microorganisms to high-osmolality environments triggers rapid fluxes of cell water along the osmotic gradient out of the cell, thus causing a reduction in turgor and dehydration of the cytoplasm. To counteract the outflow of water, microorganisms increase their intracellular solute pool by amassing large amounts of organic osmolytes, the so-called compatible solutes. These osmoprotectants are highly congruous with the physiology of the cell and comprise a limited number of substances including the disaccharide trehalose, the amino acid proline, and the trimethylammonium compound glycine betaine. The intracellular amassing of compatible solutes as an adaptive strategy to high-osmolality environments is evolutionarily well-conserved in Bacteria, Archaea, and Eukarya. Furthermore, the nature of the osmolytes that are accumulated during water stress is maintained across the kingdoms, reflecting fundamental constraints on the kind of solutes that are compatible with macromolecular and cellular functions. Generally, compatible solutes can be amassed by microorganisms through uptake and synthesis. Here we summarise the molecular mechanisms of compatible solute accumulation in Escherichia coli and Bacillus subtilis, model organisms for the gram-negative and gram-positive branches of bacteria.
The Delta(1)-pyrroline-5-carboxylate synthetase (P5CS; EC not assigned) is the rate-limiting enzyme in proline (Pro) biosynthesis in plants and is subject to feedback inhibition by Pro. It has been suggested that the feedback regulation of P5CS is lost in plants under stress conditions. We compared Pro levels in transgenic tobacco (Nicotiana tabacum) plants expressing a wild-type form of Vigna aconitifolia P5CS and a mutated form of the enzyme (P5CSF129A) whose feedback inhibition by Pro was removed by site-directed mutagenesis. Transgenic plants expressing P5CSF129A accumulated about 2-fold more Pro than the plants expressing V. aconitifolia wild-type P5CS. This difference was further increased in plants treated with 200 mM NaCl. These results demonstrated that the feedback regulation of P5CS plays a role in controlling the level of Pro in plants under both normal and stress conditions. The elevated Pro also reduced free radical levels in response to osmotic stress, as measured by malondialdehyde production, and significantly improved the ability of the transgenic seedlings to grow in medium containing up to 200 mM NaCl. These findings shed new light on the regulation of Pro biosynthesis in plants and the role of Pro in reducing oxidative stress induced by osmotic stress, in addition to its accepted role as an osmolyte.
The observed sensitivity of Listeria monocytogenes to the toxic proline analogue l-azetidine-2-carboxylic acid (AZ) suggested that proline synthesis in Listeria may be regulated by feedback inhibition of γ-glutamyl kinase (GK), the first enzyme of the proline biosynthesis pathway,
encoded by theproB gene. Taking advantage of the Epicurian coli mutator strain XL1-Red, we performed random mutagenesis of the recently described proBA operon and generated three independent mutations in the listerial proB homologue, leading to proline overproduction and salt tolerance when expressed in an E. coli (ΔproBA) background. While each of the mutations (located within a conserved 26-amino-acid region of GK) was shown to confer AZ resistance
(AZr) on an L. monocytogenes proBA mutant, listerial transformants failed to exhibit the salt-tolerant phenotype observed in E. coli. Since proline accumulation has previously been linked to the virulence potential of a number of pathogenic bacteria, we
analyzed the effect of proline overproduction on Listeria pathogenesis. However, our results suggest that as previously described for proline auxotrophy, proline hyperproduction has
no apparent impact on the virulence potential of Listeria.
Osmolyte accumulation (OA) is frequently cited as a key putative mechanism for increasing yields of crops subjected to drought conditions. The hypothesis is that OA results in a number of benefits that sustain cell and tissue activity under water-deficit conditions. It has been proposed as an effective tolerance mechanism for water deficits, which could be enhanced in crops by traditional plant breeding, marker-assisted selection or genetic engineering, to generate drought-tolerant crops. However, field studies examining the association between OA and crop yield have tended to show no consistent benefit. The few, often-cited, investigations with positive associations were obtained under severe water deficits with extremely low yields or conditions with special water-supply scenarios when much of the benefit is plant survival. Under conditions where water deficits threaten crop survival, yields are so low that even large fractional yield gains offer little practical benefit to growers. Indeed, the often-cited benefit of turgor maintenance in cells is likely to result in crop behaviour that is exactly opposite to what is beneficial to crops. The one clear mechanism identified in this review for beneficial yield responses to OA is in the maintenance of root development in order to reach water that may be available deeper in the soil profile.
Synechococcus sp. PCC7942, a fresh water cyanobacterium, was transformed by a shuttle plasmid that contains a 9-kb fragment encoding the Escherichia coli bet gene cluster, i.e. betA (choline dehydrogenase), betB (betaine aldehyde dehydrogenase), betI (a putative regulatory protein), and betT (the choline transport system). The expression of these genes was demonstrated in the cyanobacterial cells (bet-containing cells) by northern blot analysis, as well as by the detection of glycine betaine by 1H nuclear magnetic resonance in cells supplemented with choline. Endogenous choline was not detected in either control or bet-containing cells. Both control and bet-containing cyanobacterial cells were found to import choline in an energy-dependent process, although this import was restricted only to bet-containing cells in conditions of salt stress. Glycine betaine was found to accumulate to a concentration of 45 mM in bet-containing cyanobacterial cells, and this resulted in a stabilization of the photosynthetic activities of photosystems I and II, higher phycobilisome contents, and general protective effects against salt stress when compared to control cells. The growth of bet-containing cells was much faster in the presence of 0.375 M NaCl than that of control cells, indicating that the transformant acquired resistance to salt stress.
Proline (Pro) accumulation has been correlated with tolerance to drought and salinity stresses in plants. Therefore, overproduction of Pro in plants may lead to increased tolerance against these abiotic stresses. To test this possibility, we overexpressed in tobacco the mothbean [delta]-pyrroline-5-carboxylate synthetase, a bifunctional enzyme able to catalyze the conversion of glutamate to [delta]-pyrroline-5-carboxylate, which is then reduced to Pro. The transgenic plants produced a high level of the enzyme and synthesized 10- to 18-fold more Pro than control plants. These results suggest that activity of the first enzyme of the pathway is the rate-limiting factor in Pro synthesis. Exogenous supply of nitrogen further enhanced Pro production. The osmotic potentials of leaf sap from transgenic plants were less decreased under water-stress conditions compared to those of control plants. Overproduction of Pro also enhanced root biomass and flower development in transgenic plants under drought-stress conditions. These data demonstrated that Pro acts as an osmoprotectant and that overproduction of Pro results in the increased tolerance to osmotic stress in plants.
Trehalose is a nonreducing disaccharide of glucose that functions as a compatible solute in the stabilization of biological structures under abiotic stress in bacteria, fungi, and invertebrates. With the notable exception of the desiccation-tolerant "resurrection plants," trehalose is not thought to accumulate to detectable levels in most plants. We report here the regulated overexpression of Escherichia coli trehalose biosynthetic genes (otsA and otsB) as a fusion gene for manipulating abiotic stress tolerance in rice. The fusion gene has the advantages of necessitating only a single transformation event and a higher net catalytic efficiency for trehalose formation. The expression of the transgene was under the control of either tissue-specific or stress-dependent promoters. Compared with nontransgenic rice, several independent transgenic lines exhibited sustained plant growth, less photo-oxidative damage, and more favorable mineral balance under salt, drought, and low-temperature stress conditions. Depending on growth conditions, the transgenic rice plants accumulate trehalose at levels 3-10 times that of the nontransgenic controls. The observation that peak trehalose levels remain well below 1 mgg fresh weight indicates that the primary effect of trehalose is not as a compatible solute. Rather, increased trehalose accumulation correlates with higher soluble carbohydrate levels and an elevated capacity for photosynthesis under both stress and nonstress conditions, consistent with a suggested role in modulating sugar sensing and carbohydrate metabolism. These findings demonstrate the feasibility of engineering rice for increased tolerance of abiotic stress and enhanced productivity through tissue-specific or stress-dependent overproduction of trehalose.
A protein determination method which involves the binding of Coomassie Brilliant Blue G-250 to protein is described. The binding of the dye to protein causes a shift in the absorption maximum of the dye from 465 to 595 nm, and it is the increase in absorption at 595 nm which is monitored. This assay is very reproducible and rapid with the dye binding process virtually complete in approximately 2 min with good color stability for 1 hr. There is little or no interference from cations such as sodium or potassium nor from carbohydrates such as sucrose. A small amount of color is developed in the presence of strongly alkaline buffering agents, but the assay may be run accurately by the use of proper buffer controls. The only components found to give excessive interfering color in the assay are relatively large amounts of detergents such as sodium dodecyl sulfate, Triton X-100, and commercial glassware detergents. Interference by small amounts of detergent may be eliminated by the use of proper controls.
To confer abscisic acid (ABA) and/or stress-inducible gene expression, an ABA-response complex (ABRC1) from the barley (Hordeum vulgare L.) HVA22gene was fused to four different lengths of the 5′ region from the rice (Oryza sativa L.) Act1 gene. Transient assay of β-glucuronidase (GUS) activity in barley aleurone cells shows that, coupled with ABRC1, the shortest minimal promoter (Act1–100P) gives both the greatest induction and the highest level of absolute activity following ABA treatment. Two plasmids with one or four copies of ABRC1 combined with the same Act1–100P and HVA22(I) of barley HVA22 were constructed and used for stable expression of uidA in transgenic rice plants. Three Southern blot-positive lines with the correct hybridization pattern for each construct were obtained. Northern analysis indicated thatuidA expression is induced by ABA, water-deficit, and NaCl treatments. GUS activity assays in the transgenic plants confirmed that the induction of GUS activity varies from 3- to 8-fold with different treatments or in different rice tissues, and that transgenic rice plants harboring four copies of ABRC1 show 50% to 200% higher absolute GUS activity both before and after treatments than those with one copy of ABRC1.
Glycinebetaine (betaine) affords osmoprotection in bacteria, plants and animals, and protects cell components against harsh conditions in vitro. This and a compelling body of other evidence have encouraged the engineering of betaine production in plants lacking it. We have installed the metabolic step for oxidation of choline, a ubiquitous substance, to betaine in three diverse species, Arabidopsis, Brassica napus, and tobacco (Nicotiana tabacum), by constitutive expression of a bacterial choline oxidase gene. The highest levels of betaine in independent transgenics were 18.6, 12.8, and 13 μmol g−1 dry weight, respectively, values 10- to 20-fold lower than the levels found in natural betaine producers. However, choline-fed transgenic plants synthesized substantially more betaine. Increasing the choline supplementation further enhanced betaine synthesis, up to 613 μmol g−1 dry weight in Arabidopsis, 250 μmol g−1 dry weight in B. napus, and 80 μmol g−1 dry weight in tobacco. These studies demonstrate the need to enhance the endogenous choline supply to support accumulation of physiologically relevant amounts of betaine. A moderate stress tolerance was noted in some but not all betaine-producing transgenic lines based on relative shoot growth. Furthermore, the responses to stresses such as salinity, drought, and freezing were variable among the three species.
Many plants accumulate organic osmolytes in response to the imposition of environmental stresses that cause cellular dehydration. Although an adaptive role for these compounds in mediating osmotic adjustment and protecting subcellular structure has become a central dogma in stress physiology, the evidence in favour of this hypothesis is largely correlative. Transgenic plants engineered to accumulate proline, mannitol, fructans, trehalose, glycine betaine or ononitol exhibit marginal improvements in salt and/or drought tolerance. While these studies do not dismiss causative relationships between osmolyte levels and stress tolerance, the absolute osmolyte concentrations in these plants are unlikely to mediate osmotic adjustment. Metabolic benefits of osmolyte accumulation may augment the classically accepted roles of these compounds. In re-assessing the functional significance of compatible solute accumulation, it is suggested that proline and glycine betaine synthesis may buffer cellular redox potential. Disturbances in hexose sensing in transgenic plants engineered to produce trehalose, fructans or mannitol may be an important contributory factor to the stress-tolerant phenotypes observed. Associated effects on photoassimilate allocation between root and shoot tissues may also be involved. Whether or not osmolyte transport between subcellular compartments or different organs represents a bottleneck that limits stress tolerance at the whole-plant level is presently unclear. None the less, if osmolyte metabolism impinges on hexose or redox signalling, then it may be important in long-range signal transmission throughout the plant.
Proline, which increases proportionately faster than other amino acids in plants under water stress, has been suggested as an evaluating parameter for irrigation scheduling and for selecting drought-resistant varieties. The necessity to analyze numerous samples from multiple replications of field grown materials prompted the development of a simple, rapid colorimetric determination of proline. The method detected proline in the 0.1 to 36.0 moles/g range of fresh weight leaf material.
Bacteria inhabit natural and artificial environments with diverse and fluctuating osmolalities, salinities and temperatures. Many maintain cytoplasmic hydration, growth and survival most effectively by accumulating kosmotropic organic solutes (compatible solutes) when medium osmolality is high or temperature is low (above freezing). They release these solutes into their environment when the medium osmolality drops. Solutes accumulate either by synthesis or by transport from the extracellular medium. Responses to growth in high osmolality medium, including biosynthetic accumulation of trehalose, also protect Salmonella typhimurium from heat shock. Osmotically regulated transporters and mechanosensitive channels modulate cytoplasmic solute levels in Bacillus subtilis, Corynebacterium glutamicum, Escherichia coli, Lactobacillus plantarum, Lactococcus lactis, Listeria monocytogenes and Salmonella typhimurium. Each organism harbours multiple osmoregulatory transporters with overlapping substrate specificities. Membrane proteins that can act as both osmosensors and osmoregulatory transporters have been identified (secondary transporters ProP of E. coli and BetP of C. glutamicum as well as ABC transporter OpuA of L. lactis). The molecular bases for the modulation of gene expression and transport activity by temperature and medium osmolality are under intensive investigation with emphasis on the role of the membrane as an antenna for osmo- and/or thermosensors.
A protein determination method which involves the binding of Coomassie Brilliant Blue G-250 to protein is described. The binding of the dye to protein causes a shift in the absorption maximum of the dye from 465 to 595 nm, and it is the increase in absorption at 595 nm which is monitored. This assay is very reproducible and rapid with the dye binding process virtually complete in approximately 2 min with good color stability for 1 hr. There is little or no interference from cations such as sodium or potassium nor from carbohydrates such as sucrose. A small amount of color is developed in the presence of strongly alkaline buffering agents, but the assay may be run accurately by the use of proper buffer controls. The only components found to give excessive interfering color in the assay are relatively large amounts of detergents such as sodium dodecyl sulfate, Triton X-100, and commercial glassware detergents. Interference by small amounts of detergent may be eliminated by the use of proper controls.
The effects of hypersaline treatment (osmotic upshock) on solute accumulation have been studied in the Gram-positive bacterium Bacillus subtilis. Natural abundance 13C NMR spectroscopy studies revealed only proline as a major organic osmoticum in cells grown in defined medium (no exogenous organic solutes) and this finding was confirmed by amino acid analysis. Intracellular concentrations of both K+ and proline rose markedly after osmotic upshock. K+ influx from the medium was rapid (less than 1 h) but proline synthesis was a slower process (5-9 h). Proline synthesis appeared to be dependent on the prior accumulation of K+ and it is possible that K+ serves in some manner as the signal for increased proline synthesis. In cells upshocked in medium enriched in glycine betaine the endogenous synthesis of proline was repressed and glycine betaine served as the sole organic osmoticum. K+ was also accumulated under these conditions.
We determined the nucleotide (nt) sequence of a mutation that confers proline overproduction and enhanced tolerance of osmotic stress on bacteria. The mutation, designated as proB74, is an allele of the Escherichia coli proB gene which results in a loss of allosteric regulation of the protein product, gamma-glutamyl kinase. Our sequencing indicated that the proB74 mutation is a substitution of an A for a G at nt position 319 of the coding strand of the gene, resulting in a change of an aspartate to an asparagine at amino acid (aa) residue 107 of the predicted protein product. Rushlow et al. [Gene 39 (1984) 109-112] determined that another proB mutation (designated as DHPR), that resulted in a loss of allosteric inhibition by proline of the E. coli gamma-glutamyl kinase, was due to a substitution of an alanine for a glutamate at aa residue 143. Therefore, even though both the DHPR and the proB74 mutations caused a loss of allosteric inhibition of gamma-glutamyl kinase, they are due to different amino acid substitutions.
A single base pair change has been found in a site corresponding to a regulatory region of the first enzyme in the proline biosynthetic pathway. This change alters feedback inhibition and is responsible for the synthesis of high levels of proline that enable Escherichia coli to withstand osmotic stress.
Mutants of S. typhimurium with enhanced osmotolerance were isolated. These mutants were obtained as strains which over-produced proline due to regulatory mutations affecting proline biosynthesis. The mutations are located on F'proBa and upon transfer to other S. typhimurium strains, they confer enhanced osmotolerance on the recipients. The osmotolerant mutants not only have higher intracellular proline levels than the osmosensitive parental strain, but the proline levels in the osmotolerant mutants are regulated such that they increase in response to osmotic stress. Possibly reasons why elevated proline levels lead to enhanced osmotolerance are discussed.
To investigate the potential role of a polyol, mannitol, in oxidative stress protection, a bacterial mannitol-1-phosphate dehydrogenase gene was targeted to chloroplasts by the addition of an amino-terminal transit peptide. Transgenic tobacco (Nicotiana tabacum) lines accumulate mannitol at concentrations ranging from 2.5 to 7 mumol/g fresh weight. Line BS1-31 accumulated approximately 100 mM mannitol in chloroplasts and was identical to the wild type in phenotype and photosynthetic performance. The presence of mannitol in chloroplasts resulted in an increased resistance to methyl viologen (MV)-induced oxidative stress, documented by the increased retention of chlorophyll in transgenic leaf tissue following MV treatment. In the presence of MV, isolated mesophyll cells of BS1-31 exhibited higher CO2 fixation than the wild type. When the hydroxyl radical probe dimethyl sulfoxide was introduced into cells, the initial formation rate of methane sulfinic acid was significantly lower in cells containing mannitol in the chloroplast compartment than in wild-type cells, indicating an increased hydroxyl radical-scavenging capacity in BS1-31 tobacco. We suggest that the chloroplast location of mannitol can supplement endogenous radical-scavenging mechanisms and reduce oxidative damage of cells by hydroxyl radicals.
Glycinebetaine (betaine) affords osmoprotection in bacteria, plants and animals, and protects cell components against harsh conditions in vitro. This and a compelling body of other evidence have encouraged the engineering of betaine production in plants lacking it. We have installed the metabolic step for oxidation of choline, a ubiquitous substance, to betaine in three diverse species, Arabidopsis, Brassica napus, and tobacco (Nicotiana tabacum), by constitutive expression of a bacterial choline oxidase gene. The highest levels of betaine in independent transgenics were 18.6, 12.8, and 13 micromol g(-1) dry weight, respectively, values 10- to 20-fold lower than the levels found in natural betaine producers. However, choline-fed transgenic plants synthesized substantially more betaine. Increasing the choline supplementation further enhanced betaine synthesis, up to 613 micromol g(-1) dry weight in Arabidopsis, 250 micromol g(-1) dry weight in B. napus, and 80 micromol g(-1) dry weight in tobacco. These studies demonstrate the need to enhance the endogenous choline supply to support accumulation of physiologically relevant amounts of betaine. A moderate stress tolerance was noted in some but not all betaine-producing transgenic lines based on relative shoot growth. Furthermore, the responses to stresses such as salinity, drought, and freezing were variable among the three species.
The proline-auxotrophic Escherichia coli K12 strain JM83 harbouring an expression vector providing the proBA gene in trans was utilized for the fermenter production of the partially humanized IN-1 antibody F(ab) fragment. Thus, plasmid-mediated complementation of the chromosomal proBA deletion was employed as a second selection mechanism, together with a chloramphenicol resistance, in order to (i) abolish plasmid loss and (ii) benefit from E. coli JM83 as an expression strain with approved periplasmic protein secretion characteristics in the presence of a minimal medium. Starting from the generic vector pASK75, which makes use of the tightly regulated and chemically inducible tet promoter for foreign gene expression, a set of new vectors carrying the entire or part of the proBA operon was constructed and compared concerning their capability of functional Delta proBA complementation as well as recombinant protein yield. As a result, the vector pMF1 was developed, where transcription of the proBA operon is controlled by its own constitutive promoter and terminator sequences, permitting the transformed JM83 strain to grow under glucose/ammonia minimal culture conditions. When pMF1 was used for the fermenter production of the IN-1 F(ab) fragment, no plasmid loss was observed during the growth and induction phases, and the yield of functionally purified recombinant protein was found to be considerably improved.
NTG was used to make chemical mutation for Bacillus subtilis 93151. An enhanced osmotolerant mutant was obtained, which could grow in minimal medium containing 14% NaCl (w/v) and was not subject to proline-mediated feedback repression. The content of the intracellular free proline from the mutant increased rapidly with the rising of NaCl concentration. A 2.3 kb DNA fragment from the mutant was amplified using PCR method. Sequence analysis indicated that three bases changed within the proB gene, compared with the wild-type strain. One of the mutations was substitution of an A for a T at nt position 781, leading to a change of a Ser to a Thr at amino acid residue 261 of the deduced protein product, while other two were silent mutations. The recombinant vector pBE2-proB could functionally complement the proline auxotrophy E. coli 1.1252. Sequence analysis of proA showed that proA and proB overlapped by 4 nt, and there was a SD sequence at nt 14 upstream of the start codon of proA. The deduced amino acid of proA gene shared a high similarity with that of Bacillus subtilis 168 (77%).
The osmoregulation proB and proA genes from Bacillus subtilis 93151 are overlapping genes, which encode two proteins ProB and ProA. A restriction enzyme site was inserted in the overlapping region of proB and proA genes from a salt-tolerant mutant of B. subtilis 93151, and a fusion gene was constructed by cloning proB and proA genes respectively. SDS-PAGE analysis showed that a novel protein with molecular mass of 85 kD was observed. When expressed in E. coli, enhanced intracellular concentrations of free proline and osmotolerance of the strain carrying the fusion gene were observed, compared with the control host cell harbouring a plasmid encoding the separate ProB and ProA.
The accumulation of sugar alcohols and other low molecular weight metabolites such as proline and glycine-betaine is a widespread
response that may protect against environmental stress that occurs in a diverse range of organisms. Transgenic tobacco plants
that synthesize and accumulate the sugar alcohol mannitol were engineered by introduction of a bacterial gene that encodes
mannitol 1 -phosphate dehydrogenase. Growth of plants from control and mannitol-containing lines in the absence and presence
of added sodium chloride was analyzed. Plants containing mannitol had an increased ability to tolerate high salinity.
proA gene cloning and function analysis of proBA gene in osmoregulation from a salt-tolerant mutant of Bacillus subtilis
- R J Liu
- J W Cao
- L X Miao
- R J Liu
- J W Cao
- L X Miao
Osmosensing and osmoregulatory compatible solute accumulation by bacteria
- J M Wood
- E Bremer
- L N Csonka
- R Karaemer
- B Poolman
- T Van Der Heide
- L T Smith