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Molecular Determinants of Rhizosphere Colonization by Pseudomonas
Abstract and Figures
Rhizosphere colonization is one of the first steps in the pathogenesis of soilborne microorganisms. It can also be crucial for the action of microbial inoculants used as biofertilizers, biopesticides, phytostimulators, and bioremediators. Pseudomonas, one of the best root colonizers, is therefore used as a model root colonizer. This review focuses on (a) the temporal-spatial description of root-colonizing bacteria as visualized by confocal laser scanning microscopal analysis of autofluorescent microorganisms, and (b) bacterial genes and traits involved in root colonization. The results show a strong parallel between traits used for the colonization of roots and of animal tissues, indicating the general importance of such a study. Finally, we identify several noteworthy areas for future research.
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... Another adhesin, the medium adhesion protein (MapA), has been shown to play a role in biofilm formation in P. fluorescens (Collins et al., 2020). The type IV pili forms surface appendages considered part of the ECM that are also involved in biofilm formation, colonization, and adhesion to host cells, among other functions (Lugtenberg et al., 2001;Cole et al., 2017). Functional amyloids are also crucial in bacteria-plant interaction. ...
... To our knowledge, PsmE has not been linked before with biofilm formation or with host colonization. On the other hand, the role of the Flp/tad pilus, another extracellular protein involved in biofilm formation, has been described within the Pseudomonas genus with respect to its function in motility, biofilm formation, and colonization (Lugtenberg et al., 2001;Cole et al., 2017). There are two configurations of the Flp/Tad pilus in the Pseudomonas genus: type A, described for P. aeruginosa (de Bentzmann et al., 2006), which is widely distributed in the genus, and the newly described type B, which is restricted to the Pseudomonas fluorescens complex (Blanco-Romero et al., 2020). ...
Regulating the transition of bacteria from motile to sessile lifestyles is crucial for their ability to compete effectively in the rhizosphere environment. Pseudomonas are known to rely on extracellular matrix (ECM) components for microcolony and biofilm formation, allowing them to adapt to a sessile lifestyle. Pseudomonas ogarae F113 possesses eight gene clusters responsible for the production of ECM components. These gene clusters are tightly regulated by AmrZ, a major transcriptional regulator that influences the cellular levels of c-di-GMP. The AmrZ-mediated transcriptional regulation of ECM components is primarily mediated by the signaling molecule c-di-GMP and the flagella master regulator FleQ. To investigate the functional role of these ECM components in P. ogarae F113, we performed phenotypic analyses using mutants in genes encoding these ECM components. These analyses included assessments of colony morphology, dye-staining, static attachment to abiotic surfaces, dynamic biofilm formation on abiotic surfaces, swimming motility, and competitive colonization assays of the rhizosphere. Our results revealed that alginate and PNAG polysaccharides, along with PsmE and the fimbrial low molecular weight protein/tight adherence (Flp/Tad) pilus, are the major ECM components contributing to biofilm formation. Additionally, we found that the majority of these components and MapA are needed for a competitive colonization of the rhizosphere in P. ogarae F113.
... Additionally, alterations in the relationship between oxalate and malate during anthesis, as well as species involved in ammonium assimilation, underscore the heightened significance of the rhizosphere soil microenvironment. This reaffirms the crucial role of organic acids in mediating interactions between plants and microorganisms (Blakley and Simpson, 1964;Lugtenberg et al., 2001;Lebeis et al., 2015;Yuan et al., 2015). ...
This study investigated the effect of nitrogen application on the rhizosphere soil microenvironment of sunflower and clarified the relationship between ammonium assimilation and the microenvironment. In a field experiment high (HN, 190 kg/hm ² ), medium (MN, 120 kg/hm ² ) and low nitrogen (CK, 50 kg/hm ² ) treatments were made to replicate plots of sunflowers using drip irrigation. Metagenomic sequencing was used to analyze the community structure and functional genes involved in the ammonium assimilation pathway in rhizosphere soil. The findings indicated that glnA and gltB played a crucial role in the ammonium assimilation pathway in sunflower rhizosphere soil, with Actinobacteria and Proteobacteria being the primary contributors. Compared with CK treatment, the relative abundance of Actinobacteria increased by 15.57% under MN treatment, while the relative abundance decreased at flowering and maturation stages. Conversely, the relative abundance of Proteobacteria was 28.57 and 61.26% higher in the MN treatment during anthesis and maturation period, respectively, compared with the CK. Furthermore, during the bud stage and anthesis, the abundance of Actinobacteria, Proteobacteria, and their dominant species were influenced mainly by rhizosphere soil EC, ammonium nitrogen ( NH 4 + -N), and nitrate nitrogen ( NO 3 - -N), whereas, at maturity, soil pH and NO 3 - -N played a more significant role in shaping the community of ammonium-assimilating microorganisms. The MN treatment increased the root length density, surface area density, and root volume density of sunflower at the bud, flowering, and maturity stages compared to the CK. Moreover, root exudates such as oxalate and malate were positively correlated with the dominant species of Actinobacteria and Proteobacteria during anthesis and the maturation period. Under drip irrigation, applying 120 kg/hm ² of nitrogen to sunflowers effectively promoted the community structure of ammonium-assimilating microorganisms in rhizosphere soil and had a positive influence on the rhizosphere soil microenvironment and sunflower root growth.
... Despite the improvements in germination and antioxidant activity in soil, the results of salinitytolerance experiment, indicating lower germination performances in PGPB-treated samples suggest the bacterial inability to physically and physiologically colonize roots, as well as the lack of identification of bacterial genes in the phyto-signaling process of infection (Benizri et al. 2001;Lugtenberg et al. 2001;Nelson 2004;Calzavara et al. 2018). For instance, Petrillo et al. (2022) reported a reduction of 30% (B. ...
Background and aims
Reducing land degradation and safeguarding agricultural productions ensures the provision of ecosystem services and economic welfare, as highlighted by the 2030 Sustainable Development Goals. Among the promising solutions to tackle these issues, the study investigates the use of Technosol-isolated PGPB, as a novel approach for enhancing plant growth and the capability to cope with soil salinization.
Methods
Several bacteria have been isolated from a Technosol in Naples (Italy), selected to produce a consortium, based on their PGP features and tested on lettuce. The promotion of lettuce growth was evaluated both in soil (sterilized and not-sterilized Technosols) and in water agar media differing in NaCl concentrations, focusing on total polyphenols, 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, soluble proteins, and sugars, which were evaluated on seedlings.
Results
Germination kinetics differed among treatments in both soil and artificial media setups. In the Technosol trial, the PGPB treated seeds were the quickest to emerge, showing a higher radical scavenging activity and lower soluble carbohydrate content. In the salinity trial, total soluble proteins were significantly higher in primed seeds at null saline cocentration.
Conclusion
Our results indicate that the PGPB application has a beneficial outcome on the Technosol but under saline stress produces an inhibitory or a costly effect on the biochemistry and germination kinetics. However, Technosol PGPBs employement may open new research scenarios on their potential application considering Nature-Based Solutions and/or plant growth in degraded environments.
... An in-depth analysis of its PGP and HM-resistant genes provided an important theoreti cal basis for future research into the molecular mechanisms underlying the plant growth promotion and HM resistance yielded by the strain. The colonization of the rhizosphere and/or plant tissue by PGPB is crucial for plant growth (64,65). PGPB with good rhizosphere and plant tissue colonization performance are typically expected to be highly efficient in allowing for plant growth (19). ...
Codonopsis pilosula is a perennial herbaceous liana with medicinal value. It is critical to promote Codonopsis pilosula growth through effective and sustainable methods, and the use of plant growth-promoting bacteria (PGPB) is a promising candidate. In this study, we isolated a PGPB, Klebsiella michiganensis LDS17, that produced a highly active 1-aminocyclopropane-1-carboxylate deaminase from the Codonopsis pilosula rhizosphere. The strain exhibited multiple plant growth-promoting properties. The antagonistic activity of strain LDS17 against eight phytopathogenic fungi was investigated, and the results showed that strain LDS17 had obvious antagonistic effects on Rhizoctonia solani, Colletotrichum camelliae, Cytospora chrysosperma, and Phomopsis macrospore with growth inhibition rates of 54.22%, 49.41%, 48.89%, and 41.11%, respectively. Inoculation of strain LDS17 not only significantly increased the growth of Codonopsis pilosula seedlings but also increased the invertase and urease activities, the number of culturable bacteria, actinomycetes, and fungi, as well as the functional diversity of microbial communities in the rhizosphere soil of the seedlings. Heavy metal (HM) resistance tests showed that LDS17 is resistant to copper, zinc, and nickel. Whole-genome analysis of strain LDS17 revealed the genes involved in IAA production, siderophore synthesis, nitrogen fixation, P solubilization, and HM resistance. We further identified a gene (koyR) encoding a plant-responsive LuxR solo in the LDS17 genome. Klebsiella michiganensis LDS17 may therefore be useful in microbial fertilizers for Codonopsis pilosula. The identification of genes related to plant growth and HM resistance provides an important foundation for future analyses of the molecular mechanisms underlying the plant growth promotion and HM resistance of LDS17.
IMPORTANCE
We comprehensively evaluated the plant growth-promoting characteristics and heavy metal (HM) resistance ability of the LDS17 strain, as well as the effects of strain LDS17 inoculation on the Codonopsis pilosula seedling growth and the soil qualities in the Codonopsis pilosula rhizosphere. We conducted whole-genome analysis and identified lots of genes and gene clusters contributing to plant-beneficial functions and HM resistance, which is critical for further elucidating the plant growth-promoting mechanism of strain LDS17 and expanding its application in the development of plant growth-promoting agents used in the environment under HM stress.
... The mcp gene encodes multiple transmembrane receptors that are responsible for the detection of ligands and the activation of signaling cascade, leading to root-directed motility [79]. Additionally, both genomes harbor the malate dehydrogenase gene (mdh), which is essential for Pseudomonas fluorescens WCS365 growth on organic acids during tomato colonization [80]. Moreover, the xerCD recombinases annotated in the two genomes were found to be crucial for competitive root colonization [81]. ...
Background
Rahnella perminowiae S11P1 and Variovorax sp. S12S4 are two plant growth-promoting rhizobacteria that were previously isolated from the rhizosphere of Crocus sativus L. (saffron), and have demonstrated interesting PGP activities and promising results when used as inoculants in field trials. To further elucidate the molecular mechanisms underlying their beneficial effects on plant growth, comprehensive genome mining of S11P1 and S12S4 and comparative genomic analysis with closely related strains were conducted.
Results
Functional annotation of the two strains predicted a large number of genes involved in auxin and siderophore production, nitrogen fixation, sulfur metabolism, organic acid biosynthesis, pyrroloquinoline quinone production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, volatile organic compounds production, and polyamine biosynthesis. In addition, numerous genes implicated in plant-bacteria interactions, such as those involved in chemotaxis and quorum sensing, were predicted. Moreover, the two strains carried genes involved in bacterial fitness under abiotic stress conditions. Comparative genomic analysis revealed an open pan-genomic structure for the two strains. COG annotation showed that higher fractions of core and accessory genes were involved in the metabolism and transport of carbohydrates and amino acids, suggesting the metabolic versatility of the two strains as effective rhizosphere colonizers. Furthermore, this study reports the first comparison of Multilocus sequence analysis (MLSA) and core-based phylogenies of the Rahnella and Variovorax genera.
Conclusions
The present study unveils the molecular mechanisms underlying plant growth promotion and biocontrol activity of S11P1 and S12S4, and provides a basis for their further biotechnological application in agriculture.
... In other words, the ability to solubilize phosphorus depends on the genetic background of the bacterium and the environment in which it is established. Reports in the literature have shown that associations between plants and bacteria may involve specific interactions (Giassi et al., 2016), and that differences in the composition of root exudates may influence bacterial colonization of the rhizosphere (Lugtenberg et al., 2001). ...
In the farming, increasing demands for higher yields has put pressure on land resources which eventually increased demand for phosphate fertilizers (non-renewable resource), given that low availability of phosphorus in many soils is often compensated by the application high doses phosphate fertilizers. Complementary, studies have been made with a view to select phosphate-solubilizing bacteria. The purpose of his study was to: (i) isolate and characterization potential phosphate-solubilizing bacteria from sludge from the wastewater treatment plant of a poultry slaughterhouse; (ii) evaluate the ability of bacteria to solubilize phosphate rock and promoting plant growth. The bacteria were isolated in culture medium containing calcium phosphate. Morphological characterization consisted in cell shape, Gram staining and the characteristics of the colonies. To assess natural phosphate (NPh) solubilization, maize seeds were inoculated together with Araxá phosphate. The experiment consisted of 10 treatments: eight bacterial isolates + NPh, NPh and a control. The experiment was carried out in a greenhouse for 45 days, in a randomized block design, with six replications. The variables measured were the growth characteristics of the maize. Eight strains were isolated, all stained gram-positive, and 90% were rod-shaped and 10% coccoid-shaped. Inoculation with isolate LSOF-7 (Sphinghomonas sp.) combined with NPh induced increased maize dry matter by 20% compared to the treatment containing only NPh and by 67% compared to the control. It was concluded that sludge from wastewater treatment plants of poultry slaughterhouses contains bacterial strains with potential to phosphate-solubilizing and them also probably present promoting plant growth and promote maize plant growth.
Plant growth-promoting rhizobacteria (PGPRs) are the bacteria in the rhizosphere that can promote a plant’s growth through different mechanisms. Microorganisms are likely to create growth-promoting chemicals in huge numbers, which could have an indirect impact on the overall morphology of plants. These are common soil microbes that colonize plant roots and improve plant health. The use of PGPR in the growing of crops can minimize the use of agrochemicals and can promote a sustainable food industry. In the rhizosphere, rhizobacteria not only benefit from nutrients secreted by plant roots but also positively impact the plant both directly and indirectly, which results in encouraging plant growth. For a range of crop plants, PGPRs are widely employed as biofertilizers to enhance soil nutrition. The relationship between a plant and PGPR for chemical signaling molecules is not understood clearly just like other beneficial plant–microbe interactions. Metabolites are small molecules that are produced during the process of metabolism, which is the set of chemical reactions that occur within cells to maintain life. These molecules are involved in various biochemical pathways and play crucial roles in the functioning of cells, tissues, and organs. Metabolites can be classified into different groups based on their chemical nature and function.
Plant roots secrete various organic substances into the rhizosphere, which are a source of nutrition for microorganisms and largely determine the nature of plant-microbe interactions. The composition of the main fractions of root exudates in ten modern varieties of wheat was determined: amino acids, organic acids and sugars. Reliable qualitative and quantitative differences between varieties for individual components of exudates were revealed, which determined the peculiarities of cultivar clustering on this trait. Relationships between exudation and the effectiveness of plant interaction with the growth-promoting rhizobacterium Pseudomonas fluorescens SPB2137 and the phytopathogenic fungus Fusarium culmorum 30 in laboratory systems, as well as with the resistance of varieties to diseases in the field, were found. The number of P. fluorescens SPB2137 in the root zone positively correlated with the amount of many amino acids, as well as maltose, secreted by the roots. The stimulating effect of rhizobacteria on root growth positively correlated with the amount of released glucose and melibiose. The relationship between the nature of root exudation and root colonization or the susceptibility of varieties to F. culmorum 30 was not found. The analysis of correlations between the incidence of wheat varieties in the field and the intensity of exudation of certain substances, as well as with the biocomposition index of amino acid exudation, was carried out. The role of root exudate components in the formation of effective plant-microbial systems is discussed.
A total of 28 bacterial cultures were isolated from 25 composite soil samples collected from 5
Blocks of Korea district. Out of 28 bacterial cultures 4 cultures were identified as Pseudomonas
spp. and rest 24 cultures belonged to Bacillus spp. Isolation success of 100% indicates best
distribution of phosphate solubilizing bacteria but poor distribution of Pseudomonas spp. in soil
of Korea district with isolation success of 14.2%. All the four isolates of Pseudomonas (PKS10,
PKM11, PKJ25, PKB27) were characterized as Gram negative rods, non sporing, noncapsulated,
flagellated and motile and were also able to solubilize phosphate both in-vitro and in field condition
with maximum solubilization efficiency of 175 on PVK and of 200 on MPVK by isolate PKJ25.
Maximum solubilization activity was also recorded by isolate PKJ25 (20.08%) 230µg/ml after 7
days and 353.75µg/ml (30.82%) after15 days in PVK broth. The pH of the broth was found to
decline with the increase in incubation period and coincided with the increase in the phosphate
solubilization efficiency. Available P in soil recorded was found highest in case of PKJ25 at cfu
107
(7.2576 and 7.27104 gm/Kg of soil) and at cfu 109
(7.91168 and 7.73248 gm/Kg of soil)
both in pot and field experiment respectively
Microorganisms that efficiently colonize plant roots are ideal tools for the production of biocontrol agents, since many major and minor root pathogens are present at the root surface. In the past decade Pseudomonas spp. have received much attention as biocontrol agents (Kloepper et al 1980, Schippers et al 1986). The interest in this group of bacteria is mainly due to their antagonistic potentials in
vitro towards pathogens combined with their ability to colonize the roots of many plants efficiently.
Changes in the amount and composition of root exudates from young bean plants were investigated. The data were compared with those obtained from plants deprived of cotyledons or of true leaves. With intact plants, it was found that the exudation gradually decreased up to the 15th day of cultivation. Thereafter, the exudation increased. In plants deprived of cotyledons, the decrease in exudation in comparison with intact plants began on the nineteenth day after planting. The decrease in exudation from intact plants and plants without cotyledons is explained by the eventual exhaustion of reserve compounds localized in the cotyledons, which are used in the formation of true leaves. In agreements with the above explanation, it was found that the roots of plants deprived of true leaves released higher quantities of exudates from the eleventh to the twentyfourth day. The kinetics of exudation of the total amount of compounds exuded from intact plants did not correspond completely to the kinetics of exudation of particular compounds or of groups of compounds. For example, the increase exudation of α-amino and imino nitrogen on the third day of cultivation resulted primarily from an increased exudation of isoleucine and glutamic acid, while the exudation of glycine, serine, threonine, and valine remained constant for fifteen days after planting. The exudation of reducing compounds decreased after the third day of cultivation.
The resurgence of interest in the use of introduced microorganisms for biological control of plant pathogens during the past 10 years has been driven in part by trends in agriculture toward greater sustainability and increased public concern for hazards associated with the use of synthetic pesticides. Rapidly evolving technologies from molecular biology and genetics have provided new insights into the underlying mechanisms by which biocontrol agents function and have allowed evaluation of the behavior of microbial inoculants in natural environments to a degree not previously possible. The results from these advances bear directly on two fundamental sources of inconsistency in the performance of microorganisms introduced for biological control that until now have retarded their commercial development and widespread use, namely, inadequate colonization of the target site and variability in the expression or level of activity of the mechanism(s) responsible for pathogen suppression.
The major outer membrane protein (MOMP) from the rhizosphere isolate Pseudomonas fluorescens OE 28.3 adsorbs strongly and selectively to plant roots. A rapid purification procedure was developed for this protein, involving differential solubilization of the 33-kDa protein from the isolated cell envelope, and preparative SDS-PAGE followed by electro-elution. The purified protein was devoid of lipopolysaccharides, occurred in monomeric form, but was heat-modifiable.
Roots of wheat grown in unsterilized sand inoculated withGaeumannomyces graminis (Sacc.) von Arx and Olivier were examined by scanning electron microscopy. Healthy roots had a mucilaginous covering and were sparsely colonized by bacteria, but asG. graminis colonized the roots the mucilage disappeared and the numbers of bacteria on the surface increased. Lysis of the hyphae occurred, apparently caused by bacteria that colonized the hyphae. Inoculation of wheat in axenic culture with a strain ofPseudomonas fluorescens that was antagonistic toG. graminis in agar gave some protection against the pathogen; lysis of hyphae was observed where protection occurred.