Efficient Plant Growth Promoting Rhizobacteria Research Articles

Enhancing saffron (Crocus sativus L.) growth in the Kashmir valley with resilient and widely effective Plant Growth-Promoting Rhizobacteria (PGPR) under field conditions

This study provides a thorough examination of stress-tolerant plant growth-promoting rhizobacteria (PGPR) and their significance in enhancing plant growth and biocontrol activities, and influence on saffron cultivation in the Kashmir Valley. Quantitative analysis of 35 efficient PGPR revealed significant variability in plant growth promotion (PGP) attributes ranging from 60 to 653 µg/ml tri-calcium phosphate (TCP) solubilization, 1548–4311 nM α-ketobutyrate h/mg/protein ACC-deaminase activity, 1.4–74.8 µg/ml IAA-like auxins production, and 94–214 % siderophore production. Molecular fingerprinting disclosed 52 ERIC types that belonged to 25 genera, mainly Bacillus. Bacillus species including B. aryabhattai, B. halotolerans, B. methylotrophicus, B. safensis, B. siamensis and B. simplex dominated the PGPR population, along with Pseudomonas spp., such as P. azotoformans and P. chlororaphis, indicating a selective buildup of PGPR in the saffron rhizosphere. In-vivo and In-vitro dual inoculation test of these widespread PGPR and stress-tolerant also inhibited the mycelial growth of Fusarium species including F. oxysporum, F. oxysporum f. sp. ciceris, and F. proliferatum. The field application of seven selected PGPR strains resulted in significant improvements in saffron productivity, including increased sprouted corms, plant height, stigma length, and stigma fresh and dry weight, which ranged from 0.6-1.8 %, 0.4–0.7 %, 0.3–0.5 %, and 6–19 %, respectively. These strains have the potential to stimulate cormlet growth and ultimately increase saffron production. Therefore, the PGP traits of these stress-tolerant rhizobacteria contributing to nutrients solubilization, auxin biosynthesis, and fungicidal attributes can ameliorate the saffron agricultural practices.

Isolation, identification, and mechanism analysis of plant growth-promoting rhizobacteria in tobacco.

Plant growth, crop yield, and pest and disease control are enhanced by PGPR (Plant growth promoting rhizobacteria), which are beneficial microorganisms found in a close symbiosis with plant roots. Phytohormones are secreted, nutrient uptake is improved, and soil properties along with the microbiological environment are regulated by these microorganisms, making them a significant focus in agricultural research. In this study, the efficient PGPR strain T1 was isolated and screened from tobacco inter-root soil, and identified and confirmed by ITS sequencing technology. Tobacco growth indicators and soil property changes were observed and recorded through potting experiments. The activities of key enzymes (e.g., sucrase, catalase, urease) in soil were further determined. High-throughput sequencing technology was utilized to sequence the soil microbial community, and combined with macro-genomics analysis, the effects of T1 strain on soil microbial diversity and metabolic pathways were explored. Following the application of T1, significant improvements were observed in the height, leaf length, and width of tobacco plants. Furthermore, the physical and chemical properties of the soil were notably enhanced, including a 26.26% increase in phosphorus availability. Additionally, the activities of key soil enzymes such as sucrase, catalase, and urease were significantly increased, indicating improved soil health and fertility. Comprehensive joint microbiomics and macrogenomics analyses revealed a substantial rise in the populations of beneficial soil microorganisms and an enhancement in metabolic pathways, including amino acid metabolism, synthesis, and production of secondary metabolites. These increase in beneficial microorganisms and the enhancement of their metabolic functions are crucial for plant growth and soil fertility. This study provides valuable references for the development of innovative microbial fertilizers and offers programs for the sustainable development of modern agriculture.

Screening of high-efficiency nitrogen-fixing bacteria from the traditional Chinese medicine plant Astragalus mongolicus and its effect on plant growth promotion and bacterial communities in the rhizosphere

BackgroundAstragalus mongolicus Bunge is used in traditional Chinese medicine and is thus cultivated in bulk. The cultivation of A. mongolicus requires a large amount of nitrogen fertilizer, increasing the planting cost of medicinal materials and polluting the environment. Isolation and screening of plant growth-promoting rhizobacteria (PGPR) and exploring the nitrogen fixation potential of A. mongolicus rhizosphere microorganisms would effectively reduce the production cost of A. mongolicus.ResultsThis study used A. mongolicus roots and rhizosphere soil samples from Longxi County of Gansu Province, Jingle County, and Hunyuan County of Shanxi Province, China, to isolate and identify nitrogen-fixing bacteria. Through nitrogen fixation efficiency test, single strain inoculation test, and plant growth-promoting characteristics, three strains, Bacillus sp. J1, Arthrobacter sp. J2, and Bacillus sp. G4 were selected from 86 strains of potential nitrogen-fixing bacteria, which were the most effective in promoting the A. mongolicus growth and increasing the nitrogen, phosphorus, and potassium content in plants. The antagonistic test showed that these bacteria could grow smoothly under the co-culture conditions. The J1, J2, and G4 strains were used in a mixed inoculum and found to enhance the biomass of A. mongolicus plants and the accumulation of the main medicinal components in the field experiment. Mixed bacterial agent inoculation also increased bacterial diversity and changed the structure of the bacterial community in rhizosphere soil. Meanwhile, the relative abundance of Proteobacteria increased significantly after inoculation, suggesting that Proteobacteria play an important role in plant growth promotion.ConclusionsThese findings indicate that specific and efficient PGPRs have a significant promoting effect on the growth of A. mongolicus, while also having a positive impact on the structure of the host rhizosphere bacteria community. This study provides a basis for developing a nitrogen-fixing bacterial fertilizer and improving the ecological planting efficiency of A. mongolicus.

Characterization and performance evaluation of plant growth promoting bacteria in tomato rhizosphere

An acute and compelling need to replace chemical fertilizers has shifted our focus towards plant growth-promoting rhizobacteria (PGPR) that rapidly colonize the rhizosphere. In the present investigation, two isolates from tomato rhizosphere, TS4 and TS6, were screened for various PGPR properties. Both isolates expressed different properties such as enzyme production and hydrogen sulfide (H2S) production. Production of Indole-3-Acetic Acid (IAA) was 13.33 μg mL−1 for both TS4 and TS6. Ammonia excretion was 75.0 μg mL−1 for TS4 and 82.5 μg mL−1 for TS6. Both isolates tolerated salt stress up to 10% sodium chloride (NaCl), drought stress tolerated by isolate TS4 was up to -0.30 MPa and TS6 survived up to -0.49 MPa osmotic pressure. To assess the colonization potential of the isolates, their biofilm potential and chemotactic behavior towards the tomato root exudates were studied. Colonization of the tomato rhizosphere by TS4 and TS6 was observed quantitatively and found that root tips were more profusely colonized as compared to the root base. Nethouse studies to test their influence on tomato revealed a positive influence in some plant aspects. All the results led to the conclusion that isolates TS6 identified as Micrococcus luteus and TS4 belonging to genera Lactobacillus were efficient PGPR. This paper highlights the efficacy of PGPR as a component of sustainable agriculture for improved productivity. However, the need of risk assessment and rigorous surveillance of bacteria before developing them as a bioformulation is also emphasized.

A newly isolated Bacillus megaterium OQ560352 promotes maize growth in saline soils by altering rhizosphere microbial communities and organic phosphorus utilization

Limited activity of the rhizosphere microbiome and reduced soil nutrient cycling, in particular phosphorus (P), in saline soils, leads to a loss in crop productivity. Using plant growth-promoting rhizobacteria (PGPR) as microbiological fertilizer to regulate rhizosphere ecology and increase nutrient uptake is a potential approach to improve plant growth in saline conditions. In present study, a novel Bacillus megaterium strain isolated from saline soil was found to promote crop growth. The results showed that a B. megaterium OQ560352 strain altered the composition of fungal and bacterial communities in the rhizosphere of maize and increased the relative abundance of genes involved in P cycling. Actinobacteria and Bacteroidetes were identified as phyla because they had a negative correlation with soil P concentration and a positive correlation with alkaline phosphatase (ALP) activity. Compared to chemical fertilizer applied alone on saline soil, NPK fertilizer mixed with B. megaterium improved shoot biomass, shoot P concentration, soil P concentration, and ALP activity by 4%, 6%, 10%, and 13%, respectively. Collectively, the present study suggests that the novel isolated B. megaterium is an efficient PGPR for regulating the rhizosphere microbiome in saline soils to promote plant resistance to salinity.

Wild Wheat Rhizosphere-Associated Plant Growth-Promoting Bacteria Exudates: Effect on Root Development in Modern Wheat and Composition

Diazotrophic bacteria isolated from the rhizosphere of a wild wheat ancestor, grown from its refuge area in the Fertile Crescent, were found to be efficient Plant Growth-Promoting Rhizobacteria (PGPR), upon interaction with an elite wheat cultivar. In nitrogen-starved plants, they increased the amount of nitrogen in the seed crop (per plant) by about twofold. A bacterial growth medium was developed to investigate the effects of bacterial exudates on root development in the elite cultivar, and to analyze the exo-metabolomes and exo-proteomes. Altered root development was observed, with distinct responses depending on the strain, for instance, with respect to root hair development. A first conclusion from these results is that the ability of wheat to establish effective beneficial interactions with PGPRs does not appear to have undergone systematic deep reprogramming during domestication. Exo-metabolome analysis revealed a complex set of secondary metabolites, including nutrient ion chelators, cyclopeptides that could act as phytohormone mimetics, and quorum sensing molecules having inter-kingdom signaling properties. The exo-proteome-comprised strain-specific enzymes, and structural proteins belonging to outer-membrane vesicles, are likely to sequester metabolites in their lumen. Thus, the methodological processes we have developed to collect and analyze bacterial exudates have revealed that PGPRs constitutively exude a highly complex set of metabolites; this is likely to allow numerous mechanisms to simultaneously contribute to plant growth promotion, and thereby to also broaden the spectra of plant genotypes (species and accessions/cultivars) with which beneficial interactions can occur.

Isolation of soybean-specific plant growth-promoting rhizobacteria using soybean agglutin and evaluation of their effects to improve soybean growth, yield, and soil nutritional status

The basic requirements of plant growth-promoting rhizobacteria (PGPR) for field applications are that they have an affinity for the host plant and that they can colonize the rhizosphere. Here, a new technique was established using soybean agglutin (SBA) as a tool to isolate soybean-specific PGPR. Thirty-three PGPR strains with an affinity for soybean were obtained via the screening method with SBA. All 33 isolates were able to produce indole acetic acid and solubilize inorganic phosphate and potassium. Most isolates (93.94%) were able to solubilize organic phosphate and almost half (45.45%) were able to produce siderophores. More than 40% of the isolates exhibited all five plant growth-promoting traits. The isolate Enterobacter sp. strain DN9 was selected for further analyses of its rhizosphere colonization and soybean growth-promoting effects because of its excellent activity in phosphate and potassium solubilization. The luciferase luxAB gene was electrotransformed into DN9, and the labelled DN9 (DN9-L) was able to survive in the soybean rhizosphere and colonize new spaces as the soybean roots elongated. This strain positively affected root system development and soybean seedling growth. In pot and field experiments, the isolates DN9, DW1, and DW13 significantly increased the nutrient contents in rhizosphere soil and soybean leaves. On average, the seed number per plant and the seed weight per plant were increased by 20% and 24% respectively, in plants inoculated with these PGPR strains in the pot experiment. In a field experiment, compared with uninoculated plants, those inoculated with DW1 showed 46.78% higher pod number per plant and 5.23% higher seed oil content; those inoculated with DW13 showed 79.82% higher seed number per plant and 65.10% higher seed weight per plant; and those inoculated with DN9 showed 9.13% higher 100-seed weight. These results show that SBA can be used as a tool to isolate efficient PGPR to enhance soybean production.

Soil environment influences plant growth-promotion traits of isolated rhizobacteria

Plant growth-promoting rhizobacteria (PGPR) occur naturally in the rhizosphere, but our understanding of the soil and climatic variables that influence the occurrence of beneficial bacteria is poor. Here, we examined the effects of environmental factors on the occurrence of PGPR in the chickpea rhizosphere soils. A total of 743 rhizobacteria were isolated from the chickpea rhizosphere collected from 74 sampling sites across Australia. Results revealed that the ability of isolates to produce indole acetic acid (IAA) and 1-aminocyclopropane-1-carboxylate (ACC) deaminase were positively correlated with the potential of rhizobacteria to promote root growth. The amount of IAA production was positively correlated with soil copper, manganese and zinc concentrations, electric conductivity and the aridity index, but negatively correlated with soil carbon (C), nitrogen (N) and phosphorus (P) in the soils from which the PGPR were isolated. Additionally, the ability of PGPR to tolerate metal ion toxicity and water stress was related to their ability to produce IAA. The potential of isolates to solubilise P was negatively correlated with the C/N ratio, N, P, C and magnesium content of the soils but none of the soil environmental factors was correlated with the ACC deaminase activity of the isolates. In a principal component analysis, these environmental variables explained 53 % of the variation in the ability of the rhizobacteria to produce IAA and ACC deaminase and/or to solubilise P. These findings could be applied to the design of an efficient PGPR development strategy that incorporates ecological traits and plant growth-promoting mechanisms to increase chickpea production.

Large-scale screening of rhizobacteria to enhance the chickpea-Mesorhizobium symbiosis using a plant-based strategy

Rhizobacteria able to express multiple plant growth-promoting (PGP) traits has been frequently failed to enhance the chickpea-Mesorhizobium symbiosis possibly because a relatively large amount of organic acids exudated by chickpea root to form an acid rhizosphere reduced the survival and colonisation of plant growth-promoting rhizobacteria (PGPR). Therefore, this study was initiated to screen rhizobacteria that would enhance the chickpea-Mesorhizobium symbiosis using a plant-basedstrategyA total of 743 Bacillus-, Burkholderia- and Pseudomonas-like bacteria were isolated from 74 agricultural soil samples collected across Australia. Only 167 isolates were selected, based upon their individual performance in a seedling growth bioassay, to test their effect on nodulation and growth of chickpea in sterile growth pouches. Also, their ability to produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase and indole acetic acid (IAA) was investigated in relation to their mode of action. The diversity of the 167 isolates was investigated using 16S rDNA sequencing. Of 743 isolates, 697 (83%) were able to produce IAA in the presence of L-tryptophan. ACC deaminase production was detected in 57 isolates (7.7%). Commercial inoculant strain Mesorhizobium ciceri CC1192 produced 7 μg mL−1 IAA and lacked ACC deaminase activity. In further investigation, the promotion of seedling root elongation by rhizobacteria predicted their effect on nodulation and growth of chickpea after dual inoculation in aseptic conditions. The effect of rhizobacteria on seedling root growth as well as shoot growth and nodulation was associated with the capacity of the isolates to produce IAA and ACC deaminase. These isolates were superior in plant growth promotion than those PGPR unable to produce PGP traits. The synthesis of IAA along with ACC deaminase activity by rhizobacteria gave an added advantage in promoting the ability of rhizobia to form an efficient chickpea-Mesorhizobium symbiosis as measured by biomass production and nodulation. Most of these rhizobacteria belonged to the genus Burkholderia. The use of a plant-based first-stage screening strategy in combination with assays for in vitro production of IAA and ACC deaminase enabled the identification of efficient PGPR that were able to enhance the legume-rhizobia symbiosis.

Plant growth promoting and antifungal asset of indigenous rhizobacteria secluded from saffron (Crocus sativus L.) rhizosphere

Saffron (Crocus sativus L.) is an important plant in medicine. The Kashmir Valley (J&K, India) is one of the world's largest and finest saffron producing regions. However, over the past decade, there has been a strong declining trend in saffron production in this area. Plant Growth Promoting Rhizobacteria (PGPR) are free living soil bacteria that have ability to colonize the surfaces of the roots and ability to boost plant growth and development either directly or indirectly. Using the efficient PGPR as a bio-inoculant is another sustainable agricultural practice to improve soil health, grain yield quality, and biodiversity conservation. In the present study, a total of 13 bacterial strains were isolated from rhizospheric soil of saffron during the flowering stage of the tubers and were evaluated for various plant growth promoting characteristics under in vitro conditions such as the solubilization of phosphate, production of indole acetic acid, siderophore, hydrocyanic acid, and ammonia production and antagonism by dual culture test against Sclerotium rolfsii and Fusarium oxysporum. All the isolates were further tested for the production of hydrolytic enzymes such as protease, lipase, amylase, cellulase, and chitinase. The maximum proportions of bacterial isolates were gram-negative bacilli. About 77% of the bacterial isolates showed IAA production, 46% exhibited phosphate solubilization, 46% siderophore, 61% HCN, 100% ammonia production, 69% isolates showed protease activity, 62% lipase, 46% amylase, 85% cellulase, and 39% showed chitinase activity. Three isolates viz., AIS-3, AIS-8 and AIS-10 were found to have the most plant growth properties and effectively control the growth of Sclerotium rolfsii and Fusarium oxysporum. The bacterial isolates were identified as Brevibacterium frigoritolerans (AIS-3), Alcaligenes faecalis subsp. Phenolicus (AIS-8) and Bacillus aryabhattai (AIS-10) respectively by 16S rRNA sequence analysis. Therefore, these isolated rhizobacterial strains could be a promising source of plant growth stimulants to increase cormlets growth and increase saffron production.

Inoculation of Pseudomonas mutant strains can improve growth of soybean and corn plants in soils under salt stress

In the last decades, the applications of Plant Growth Promoting Rhizobacteria (PGPR) as inoculants for enhancing crops yield under different stress conditions have reached enormous interest. Pseudomonas putida KT2440 is a well-known rhizobacteria capable of colonizing roots of agronomically important crops. This work aims to evaluate three P. putida KT2440 mutants (mus-20, mus-42, and EU206), which have non-biofilm producing and exopolysaccharide overproducing phenotypes, for their potential as PGPR on soybean and corn plants under saline stress. Experimental studies showed that KT2440 and its mutants were effective in three main PGPR properties: solubilization of phosphates and synthesis of indolic compounds and siderophores. Notably, indolic compounds synthesis increased in mus-20 and EU206 strains. Furthermore, all evaluated strains were tolerant to saline stress. Bacterial inoculation of soybean and corn seeds was effective both in vitro and in vivo tests, even at high salt concentrations (80 and 120 mM NaCl). Among mutant bacteria, EU206 strain seems to be the most efficient PGPR by significantly increasing seed germination, shoot and root lengths, and fresh and dry weights of plants compared to non-inoculated controls, in in vivo tests under saline condition, mainly in both corn plants. These results suggest that inoculation with the P. putida KT2440 mutant strains is an appropriate strategy to enhance the growth of soybean and corn plants, so they might be employed for promoting regional crops growth at field scale using natural saline soils.

Halophile plant growth-promoting rhizobacteria induce salt tolerance traits in wheat seedlings (Triticum aestivum L.)

Salinity is one of the most important growth-limiting factors for most crops in arid and semi-arid regions; however, the use of plant growth-promoting rhizobacteria isolated from saline soils could reduce the effects of saline stress in crops. This study aimed to evaluate the efficiency of plant growth-promoting rhizobacteria (PGPRs), isolated from the rhizosphere of halophile plants, for the growth, Na+/K+ balance, ethylene emission, and gene expression of wheat seedlings (Triticum aestivum L.) grown under saline conditions (100 mmol L−1 NaCl) for 14 d. A total of 118 isolates obtained from saline soils of the deserts of Iran were tested for their capacity as PGPRs. Out of the 118 isolates, 17 could solubilize phosphate (Ca3(PO4)2), 5 could produce siderophores, and 16 could synthesize indole-3-acetic acid. Additionally, PGPRs were also evaluated for aminocyclopropane-1-carboxylate deaminase activity. A pot experiment was conducted to evaluate the ability of 28 PGPR isolates to promote growth, regulate Na+/K+ balance, and decrease ethylene emissions in plants. The most efficient PGPRs were Arthrobacter aurescens, Bacillus atrophaeus, Enterobacter asburiae, and Pseudomonas fluorescens. Gene expression analysis revealed the up-regulation of H+-PPase, HKT1, NHX7, CAT, and APX expression in roots of Enterobacter-inoculated salt-stressed plants. Salt-tolerant rhizobacteria exhibiting plant growth-promoting traits can facilitate the growth of wheat plants under saline conditions. Our results indicate that the isolation of these bacteria may be useful for formulating new inoculants to improve wheat cropping systems in saline soils.

Plant growth promoting rhizobacterium Stenotrophomonas maltophilia BJ01 augments endurance against N2 starvation by modulating physiology and biochemical activities of Arachis hypogea.

Arachis hypogea (Peanut) is one of the most important crops, and it is harvested and used for food and oil production. Being a legume crop, the fixation of atmospheric nitrogen is achieved through symbiotic association. Nitrogen deficiency is one of the major constrains for loss of crop productivity. The bacterium Stenotrophomonas maltophilia is known for interactions with plants. In this study, characteristics that promote plant growth were explored for their ability to enhance the growth of peanut plants under N2 deficit condition. In the presence of S. maltophilia, it was observed that fatty acid composition of peanut plants was influenced and increased contents of omega-7 monounsaturated fatty acid and omega-6 fatty acid (γ-Linolenic acid) were detected. Plant growth was increased in plants co-cultivated with PGPR (Plant Growth Promoting Rhizobacteria) under normal and stress (nitrogen deficient) condition. Electrolyte leakage, lipid peroxidation, and H2O2 content reduced in plants, co-cultivated with PGPR under normal (grown in a media supplemented with N2 source; C+) or stress (nitrogen deficient N+) conditions compared to the corresponding control plants (i.e. not co-cultivated with PGPR; C–or N–). The growth hormone auxin, osmoprotectants (proline, total soluble sugars and total amino acids), total phenolic-compounds and total flavonoid content were enhanced in plants co-cultivated with PGPR. Additionally, antioxidant and free radical scavenging (DPPH, hydroxyl and H2O2) activities were increased in plants that were treated with PGPR under both normal and N2 deficit condition. Overall, these results indicate that plants co-cultivated with PGPR, S. maltophilia, increase plant growth, antioxidant levels, scavenging, and stress tolerance under N2 deficit condition. The beneficial use of bacterium S. maltophilia could be explored further as an efficient PGPR for growing agricultural crops under N2 deficit conditions. However, a detail agronomic study would be prerequisite to confirm its commercial role.

A multifaceted rhizobacterium Paenibacillus lentimorbus alleviates nutrient deficiency-induced stress in Cicer arietinum L.

Nutrient deficiency in soil is one of the limiting factors responsible for stunted growth and poor flowering/fruiting of crops which result in decline in overall agricultural productivity. However, one important strategy to overcome the problem of nutrient deficiency and to avoid use of chemical fertilizers is the use of plant growth promoting rhizobacteria (PGPR). Paenibacillus lentimorbus NRRL B-30488 (hereafter B-30488), an efficient PGPR has been reported to have various plant growth promoting traits that help crops to mitigate various environmental stresses. Therefore, the present work was designed to examine the application of B-30488 on chickpea growth under nutrient stress condition. Plants inoculated with B-30488 showed positive modulation in physio-biochemical behaviour and mineral nutrient uptake for better growth and development. Alteration in gene expression and metabolic profile under nutrient stress condition in chickpea also supported the stress amelioration capability of B-30488. Principal component analysis statistically proved that improved growth performance of chickpea plants under nutrient stress was mainly due to B-30488 induced modulation of metabolic pathways. To the best of our knowledge, this is the first study for analysis of growth promotion and stress alleviation in chickpea plants subjected to nutrient stress in presence of PGPR B-30488.

Optimization of indole acetic acid production by isolated bacteria from Stevia rebaudiana rhizosphere and its effects on plant growth.

The ability to synthesize Indole-3-acetic acid (IAA) is widely associated with the plant growth promoting rhizobacteria (PGPR). The present work deals with isolation and characterization of such bacteria from the rhizosphere of medicinal plant Stevia rebaudiana and optimization of IAA production from its isolates. The optimization of IAA production was carried out at different pH and temperature with varied carbon and nitrogen sources of culture media. Out of different isolates obtained, three of them were screened as efficient PGPRs on the basis of different plant growth promoting attributes. Isolates CA1001 and CA2004 showed better production of IAA at pH 9 (91.7 µg ml−1) and at temperature 37 °C (81.7 µg ml−1). Dextrose (1%) was found to be the best carbon source for isolate CA1001 with 104 µg ml−1 IAA production. Isolate CA 2004 showed best production of IAA 36 µg ml−1 and 34 µg ml−1 at 1.5% and 1% Beef extract as nitrogen source respectively. Isolate CA 1001 showed 32 µg ml−1 IAA production at 0.5% nicotinic acid concentration. From the current study, CA1001 and CA2004 emerged as noble alternatives for IAA production further which also resulted in root and shoot biomass generation in crop plants, hence can be further used as bio-inoculants for plant growth promotion.

Partner-triggered proteome changes in the cell wall of Bacillus sonorensis and roots of groundnut benefit each other

Plant growth promoting rhizobacteria (PGPR) promote plant growth and activate defense response against phytopathogens. At the subcellular level plant-PGPR interaction is less understood, which would be essential for future improvement(s) of PGPR formulations. In a rigorous screening process, that also involved efficient PGPR strains, Bacillus sonorensis RS4 was selected to study partner-triggered interactions. The potential of B. sonorensis RS4 to improve growth of groundnut, efficiency to colonize roots, and influence on root topology was assessed. Twenty four cell wall proteins of B. sonorensis RS4 [in presence of groundnut root exudates (REs)], and 22 groundnut root proteins (in RS4-bacterized plants) were differentially expressed. The alterations in cell wall proteins of B. sonorensis RS4 were primarily related to the amino acids synthesis, chemotaxis, antioxidant-metabolism, carbohydrate metabolism, transporters, and antibiosis-related secondary metabolites. Root proteins that were differentially expressed during the interaction may be involved in plant growth, defense responses, and in transportation. The changes in B. sonorensis RS4 cell wall proteome and groundnut root proteome, suggest that at least a part of the proteome changes triggered by each of the partners appear to play a significant role in helping each other akin to symbiosis.

Potential of efficient and resistant plant growth-promoting rhizobacteria in lead uptake and plant defence stimulation in Lathyrus sativus under lead stress.

The ability of plant growth-promoting rhizobacteria (PGPR) to enhance Lathyrus sativus tolerance to lead (Pb) stress was investigated. Ten consortia formed by mixing four efficient and Pb-resistant PGPR strains were assessed for their beneficial effect in improving Pb (0.5mM) uptake and in inducing the host defence system of L.sativus under hydroponic conditions based on various physiological and biochemical parameters. Lead stress significantly decreased shoot (SDW) and root (RDW) dry weight, but PGPR inoculation improved both dry weights, withhighest increases in SDW and RDW of plants inoculated with I5 (R.leguminosarum (M5) + P.fluorescens (K23) + Luteibacter sp. + Variovorax sp.) and I9 (R.leguminosarum (M5)+Variovorax sp.+Luteibacter sp.+S.meliloti) by 151% and 94%, respectively. Additionally, inoculation significantly enhanced both chlorophyll and soluble sugar content, mainly in I5 inoculated leaves by 238% and 71%, respectively, despite the fact that Pb decreased these parameters. We also found that PGPR inoculation helps to reduce oxidative damage and enhances antioxidant enzyme activity, phenolic compound biosynthesis, carotenoids and proline content. PGPR inoculation increased Pb uptake in L. sativus, with highest increase in shoots of plants inoculated with I5 and I7, and in roots and nodules of plants inoculated with I1. Moreover, PGPR inoculation enhanced mineral homeostasis for Ca, Cu and Zn under Pb stress, mainly in plants inoculated with I1, I5, I7 and I9. Results of our study suggest the potential of efficient and Pb-resistant PGPR in alleviating harmful effects of metal stress via activation of various defence mechanisms and enhancing Pb uptake that promotes tolerance of L.sativus to Pb stress.

Effect of Pb-resistant plant growth-promoting rhizobacteria inoculation on growth and lead uptake by Lathyrus sativus.

In search of efficient and resistant plant growth-promoting rhizobacteria (PGPR) strains with multiple activities, a total of twelve bacterial belonging to R. leguminosarum, S. meliloti, Pseudomonas sp., P. fluorescens, Luteibacter sp., Variovorax sp., B. simplex, and B. megaterium were isolated from root nodules of grass pea (Lathyrus sativus L.) grown in contaminated soils. Upon screening, all test strains were able to synthesize indoleacetic acid; more than 90% were siderophore producers and 75% showed varying levels of phosphate solubilizing ability. The gaseous metabolite biosynthesis showed that 42% of strains were cyanogenic. The lead (Pb) bioaccumulation differs with incubation times between cell wall and cytoplasm. Indeed, the most part of Pb was adsorbed to cell surface. A pot experiment was conducted for investigating the capability of combined bacteria to promote plant growth of Lathyrus sativus under controlled conditions. Subsequently, the performance of symbiosis Lathyrus sativus-PGPR (I4: R. leguminosarum (M5) + B. simplex + Luteibacter sp. + Variovorax sp.) was investigated under lead stress using hydroponic culture to elucidate the effect of bacterial inoculation on Pb uptake as well as plant growth. Results showed that under 0.5 mM Pb, inoculation with I4 significantly increased shoots and roots biomass by 59% and 56%, respectively, and improved Pb uptake in both shoots and roots by 39% and 47%, respectively, as compared to uninoculated plants. The inoculation of Lathyrus sativus with efficient and Pb resistant PGPR is a promising symbiosis that having significant potential to improve phytoremediation of Pb-polluted soils.

Genetic diversity and antagonistic activity of plant growth promoting bacteria, isolated from tea-rhizosphere: a culture dependent study

Microbial activity in root environment is responsible for plant nutrition, growth and defence. The objective of this study is to analyse the plant growth promoting and antagonistic activity rendered by the rhizospheric bacteria of tea plant. In this study, 292 bacterial isolates were screened for indole acetic acid (IAA) production, phosphate solubilisation, ammonia production, chitinase production and protease production. Among all the bacterial isolates, 58 isolates were able to elicit minimum four plant growth promoting rhizobacteria (PGPR) traits, which were further analysed quantitatively for hydrogen cyanide (HCN), siderophore production and antagonistic activity. Based on the plant growth promoting potential scores and principle component analysis, 12 samples were further screened for the study of salt tolerance and antifungal activity profile against tea fungal pathogens (Rhizoctonia solani, Fomes lamenensis, Corticium invisum). The identity the isolates was revealed by 16s rDNA sequence analysis. Six isolates (n63, k32, n61, 31k, n57, n56) showing efficient PGPR traits were evaluated for growth promotion studies on rice seedlings. Isolate n61 (Bacillus cereus) induced significant increase in root length (9.85 mm), shoot length (7.86 mm), germination percentage (92.44) and vigour index (1566.42) in rice seedlings. Fresh bio mass was significantly higher in 31k (Pseudomonas putida) (81.33 mg) followed by n61 (Bacillus cereus) (80.84 mg) and dry mass was higher in n63 (Bacillus pseudomycoides) (15.84 mg) followed by n61 (Bacillus cereus ) (15.82 mg) while compared with control and the other isolates selected for in vitro growth experiments. ANOVA analysis showed significant (P<0.05) increase compared with control, indicating that, bacterial isolates are potent for plant growth promotion and productivity.

Metabolite profiling reveals abiotic stress tolerance in Tn5 mutant of Pseudomonas putida.

Pseudomonas is an efficient plant growth–promoting rhizobacteria (PGPR); however, intolerance to drought and high temperature limit its application in agriculture as a bioinoculant. Transposon 5 (Tn5) mutagenesis was used to generate a stress tolerant mutant from a PGPR Pseudomonas putida NBRI1108 isolated from chickpea rhizosphere. A mutant NBRI1108T, selected after screening of nearly 10,000 transconjugants, exhibited significant tolerance towards high temperature and drought. Southern hybridization analysis of EcoRI and XhoI restricted genomic DNA of NBRI1108T confirmed that it had a single Tn5 insertion. The metabolic changes in the polar and non-polar extracts of NBRI1108 and NBRI1108T were examined using 1H, 31P nuclear magnetic resonance (NMR) spectroscopy and gas chromatography-mass spectrometry (GC-MS). Thirty six chemically diverse metabolites consisting of amino acids, fatty acids and phospholipids were identified and quantified. Insertion of Tn5 influenced amino acid and phospholipid metabolism and resulted in significantly higher concentration of aspartic acid, glutamic acid, glycinebetaine, glycerophosphatidylcholine (GPC) and putrescine in NBRI1108T as compared to that in NBRI1108. The concentration of glutamic acid, glycinebetaine and GPC increased by 34%, 95% and 100%, respectively in the NBRI1108T as compared to that in NBRI1108. High concentration of glycerophosphatidylethanolamine (GPE) and undetected GPC in NBRI1108 indicates that biosynthesis of GPE may have taken place via the methylation pathway of phospholipid biosynthesis. However, high GPC and low GPE concentration in NBRI1108T suggest that methylation pathway and phosphatidylcholine synthase (PCS) pathway of phospholipid biosynthesis are being followed in the NBRI1108T. Application of multivariate principal component analysis (PCA) on the quantified metabolites revealed clear variations in NBRI1108 and NBRI1108T in polar and non-polar metabolites. Identification of abiotic stress tolerant metabolites from the NBRI1108T suggest that Tn5 mutagenesis enhanced tolerance towards high temperature and drought. Tolerance to drought was further confirmed in greenhouse experiments with maize as host plant, where NBRI1108T showed relatively high biomass under drought conditions.