Tolerant Plant Growth Promoting Rhizobacteria Research Articles

Pseudochrobactrum asaccharolyticum mitigates arsenic induced oxidative stress of maize plant by enhancing water status and antioxidant defense system

BackgroundOxidative stress mediated by reactive oxygen species (ROS) is a common denominator in arsenic toxicity. Arsenic stress in soil affects the water absorption, decrease stomatal conductance, reduction in osmotic, and leaf water potential, which restrict water uptake and osmotic stress in plants. Arsenic-induced osmotic stress triggers the overproduction of ROS, which causes a number of germination, physiological, biochemical, and antioxidant alterations. Antioxidants with potential to reduce ROS levels ameliorate the arsenic-induced lesions. Plant growth promoting rhizobacteria (PGPR) increase the total soluble sugars and proline, which scavenging OH radicals thereby prevent the oxidative damages cause by ROS. The main objective of this study was to evaluate the potential role of Arsenic resistant PGPR in growth of maize by mitigating arsenic stress.MethodologyArsenic tolerant PGPR strain MD3 (Pseudochrobactrum asaccharolyticum) was used to dismiss the ‘As’ induced oxidative stress in maize grown at concentrations of 50 and 100 mg/kg. Previously isolated arsenic tolerant bacterial strain MD3 “Pseudochrobactrum asaccharolyticum was used for this experiment. Further, growth promoting potential of MD3 was done by germination and physio-biochemical analysis of maize seeds. Experimental units were arranged in Completely Randomized Design (CRD). A total of 6 sets of treatments viz., control, arsenic treated (50 & 100 mg/kg), bacterial inoculated (MD3), and arsenic stress plus bacterial inoculated with three replicates were used for Petri plates and pot experiments. After treating with this MD3 strain, seeds of corn were grown in pots filled with or without 50 mg/kg and 100 mg/kg sodium arsenate.ResultsThe plants under arsenic stress (100 mg/kg) decreased the osmotic potential (0.8 MPa) as compared to control indicated the osmotic stress, which caused the reduction in growth, physiological parameters, proline accumulation, alteration in antioxidant enzymes (Superoxide dismutase-SOD, catalase-CAT, peroxidase-POD), increased MDA content, and H2O2 in maize plants. As-tolerant Pseudochrobactrum asaccharolyticum improved the plant growth by reducing the oxidation stress and antioxidant enzymes by proline accumulation. PCA analysis revealed that all six treatments scattered differently across the PC1 and PC2, having 85.51% and 9.72% data variance, respectively. This indicating the efficiency of As-tolerant strains. The heatmap supported the As-tolerant strains were positively correlated with growth parameters and physiological activities of the maize plants.ConclusionThis study concluded that Pseudochrobactrum asaccharolyticum reduced the ‘As’ toxicity in maize plant through the augmentation of the antioxidant defense system. Thus, MD3 (Pseudochrobactrum asaccharolyticum) strain can be considered as bio-fertilizer.

PLANT-GROWTH-PROMOTING AND ANTIFUNGAL ASSET OF INDIGENOUS DROUGHT-TOLERANT RHIZOBACTERIA ISOLATED FROM OLIVE (Olea europaea L.) RHIZOSPHERE

Faced with global environmental challenges, the quest for sustainable food production has gathered momentum. While abiotic stresses adversely affect plant health and productivity, Verticillium wilt causes considerable yield losses worldwide, particularly in crops such as olive. Recently, drought-tolerant bacteria have been used to alleviate both abiotic stress and pathogen pressure in crops. In this context, our work focuses on the isolation of tolerant indigenous rhizobacteria to mitigate these challenges by investigating their role in biocontrol and abiotic stress tolerance. Thus, a total of 94 rhizobacterial strains were isolated from the rhizospheres of olive trees in southeastern Morocco and characterized to identify tolerant plant growth-promoting rhizobacteria that inhibit Verticillium dahliae. 24 strains demonstrated in vitro suppression of Verticillium dahliae Klebahn, and exhibited tolerance to different abiotic stresses (drought, salinity, and high temperature). In addition, they proved xerotolerant (Aw ≤ 0.91), halotolerant (≥10% NaCl), and thermotolerant (≥ 55°C) capabilities. Beyond, these isolates showcased multifaceted plant growth-promoting traits, such as phosphate solubilization and significant synthesis of essential bioactive compounds like siderophores, indole-3-acetic acid and hydrolytic enzymes. Evaluating outcomes, three standout rhizobacterial isolates emerged due to their exceptional stress tolerance, unique plant growth-promoting qualities, and potent antagonistic potential. Molecular analysis identified them as Bacillus paranthracis (OZ-60) and Bacillus licheniformis (OZ-48 and OZ-77) through 16S rRNA sequencing. Besides enhancing plant abiotic stress resistance, these isolates hold promise in bolstering the sustainability of olive cultivation and fortifying plant defenses against pathogens.

The Plant Growth Promoting Potential of Saline Tolerant Rhizobacteria under In vitro Saline Stressed Conditions

Since the beginning of agricultural operations, soil salinity has been one of the most significant abiotic variables limiting agricultural production. Saline tolerant plant growth-promoting rhizobacteria (PGPR) enhance nutrient equitation and soil enzyme activities, which serve as indicators of soil biological health, and help to correct nutritional imbalances in plants. Potential bioinoculants for boosting crop yield in saline agriculture include saline tolerant plant growth-promoting rhizobacteria (PGPR). Therefore, in the present study the potential saline tolerant rhizobacteria isolated from saline patches of Karnataka were screened for Exopolysacchride production and Phosphate solubilization and phytohormone production potential under in vitro saline conditions. The results revealed that among the eight organisms were the highest EPS production, phosphate solubilization and phytohormone production recorded by Bacillus subtilis GAN-4 and Staphylococcus cohnii MAN-3 under both normal and saline stressed conditions compared to other organisms. Therefore,these strains show potential as plant growth-promoting rhizobacteria (PGPR) for mitigating salinity in saline agriculture.

Co-application of biochar and salt tolerant PGPR to improve soil quality and wheat production in a naturally saline soil

Soil salinity is a global problem that drastically reduces crop productivity. Therefore, there is a need to make saline soils productive. Plant growth promoting rhizobacteria (PGPR) and biochar have been shown to increase crop productivity and soil fertility when applied separately. However, their co-application in this regard has not been studied extensively. In this study, a biochar was co-applied with Bacillus thuringiensis or Bacillus tropicus, characterized as P-solubilizing and salt-tolerant PGPR, to wheat grown in a naturally saline soil (EC 8.12 dS m−1). Co-application of biochar with PGPR increased the number of tillers & spikes, straw yield, and 100-grain weight by 16–28.57%, 26.9–36.6%, 18–21.8%, and 34–37.9% respectively. This led to a 37% and 34% increase in grain yield in the case of co-application with B. thuringiensis and B. tropicus respectively. The higher growth and yield were thanks to better plant nutrition and mitigated salinity stress under co-application. Briefly, the co-application increased phosphorus content in grain, straw, and roots by up to 9–43%, 45–69%, and 5–21.5% respectively, which was higher than that in individual applications of biochar or any PGPR. In addition, co-application increased microbial biomass by 61–77% and soil available phosphorus by 37–53% across two PGPR. Evidently, the co-application contributed to increased P nutrition in two ways. Firstly, the biochar itself served as a source of available P. Secondly, the PGPR solubilized it more abundantly when biochar was there. As far the salinity stress, biochar, its co-application with B. thuringiensis, and with B. tropicus increased soil K+/Na+ by 36%, 72% and 44% respectively leading to 40%, 63%, and 55% higher K+/Na+ in grain, straw, and roots respectively. Overall, our results indicate that biochar and PGPR worked in tandem to substantially improve soil quality and wheat production compared to their sole applications. We conclude that co-application of salt tolerant PGPR and biochar is a more sustainable and better technology to produce wheat in a naturally saline soil compared to their sole applications.

Characterization and utilization of multi-trait plant growth promoting rhizobacteria from arid soils of western Rajasthan for enhancing drought resilience in an arid legume

In the study, plant growth promoting rhizobacteria (PGPR) indigenous to arid western Rajasthan were isolated and screened for plant growth promoting (PGP) capabilities. Ten isolates showing multiple PGP activities were identified as Pseudomonas sp. G6-3, Staphylococcus sp. G6-4, Bacillus badius G8-6, Agrobacterium sp. G8-7, Bacillus pseudomycoides G10-1, Staphylococcus capitis subsp. capitis G11-4, Bacillus paralicheniformis G11-5, Bacillus subtilis G12-3, Stenotrophomonas pavanii G12-4, and Solibacillus isronensis G21-9. Physiological profiling of these isolates demonstrated them to be tolerant to drought (25% PEG-6000), high temperature (40 °C), alkalinity (pH 10.0), and salinity (4.5% NaCl). These PGPR significantly improved clusterbean seedling characteristics in moisture stress-induced hydroponic flask cultures and pots. PGPR inoculation of clusterbean in drought-induced pots significantly enhanced the plant dry weight by 15–33%, plant water potential by 5–8%, and the relative water content of leaves by 4–16%. There was also a significant reduction in the levels of anti-oxidant enzymes and proline in plant tissues under drought conditions, which indicated lower levels of stress experienced by the treated plants. These were translated into yield enhancements by 13–15% under rainfed conditions. The study demonstrates the ability of stress tolerant PGPR from arid western Rajasthan in enhancing drought resilience of arid legumes besides promoting their growth, and their potential utility in sustainable arid agriculture systems.

Rhizobacterial Isolates from Prosopis limensis Promote the Growth of Raphanus sativus L. Under Salt Stress

Microbial biotechnology employes techniques that rely based on the natural interactions that occur in ecosystems. Bacteria, including rhizobacteria, play an important role in plant growth, providing agricultural crops with an alternative that can mitigate the negative effects of abiotic stress, such as those caused by saline environments. In this study, bacterial isolates were obtained from soil and roots of Prosopis limensis Bentham from the department of Lambayeque, Peru. This region has high salinity levels, therefore, the collected samples were used to isolate plant growth-promoting rhizobacteria (PGPR), which were identified through morphological, and physical-biochemical characteristics. These salt tolerant bacteria were screened phosphate solubilization, indole acetic acid, deaminase activity and molecular characterization by 16S rDNA sequencing. Eighteen samples from saline soils of the Prosopis limensis plants in the northern coastal desert of San Jose district, Lambayeque, Peru. The bacterial isolates were screened for salt tolerance ranging from 2 to 10%, a total of 78 isolates were found. Isolates 03, 13 and 31 showed maximum salt tolerance at 10%, in vitro ACC production, phosphate solubilization and IAA production. The three isolates were identified by sequencing the amplified 16S rRNA gene and were found to be Pseudomonas sp. 03 (MW604823), Pseudomonas sp. 13 (MW604824) and Bordetella sp. 31 (MW604826). These microorganisms promoted the germination of radish plants and increased the germination rates for treatments T2, T3 and T4 by 129, 124 and 118% respectively. The beneficial effects of salt tolerant PGPR isolates isolated from saline environments can be new species, used to overcome the detrimental effects of salt stress on plants. The biochemical response and inoculation of the three isolates prove the potential of using these strains as a source of products that can be employed for the development of new compounds proving their potential as biofertilizers for saline environments.

Plant growth promoting rhizobacteria and biochar production from Parthenium hysterophorus enhance seed germination and productivity in barley under drought stress.

Drought stress can significantly affect plant growth and development. Biochar (BC) and plant growth-promoting rhizobacteria (PGPR) have been found to increase plant fertility and development under drought conditions. The single effects of BC and PGPR in different plant species have been widely reported under abiotic stress. However, there have been relatively few studies on the positive role of PGPR, BC, and their combination in barley (Hordeum vulgare L.). Therefore, the current study investigated the effects of BC from Parthenium hysterophorus, drought tolerant PGPR (Serratia odorifera), and the combination of BC + PGPR on the growth, physiology, and biochemical traits of barley plants under drought stress for two weeks. A total of 15 pots were used under five treatments. Each pot of 4 kg soil comprised the control (T0, 90% water), drought stress alone (T1, 30% water), 35 mL PGPR/kg soil (T2, 30% water), 2.5%/kg soil BC (T3, 30% water), and a combination of BC and PGPR (T4, 30% water). Combined PGPR and BC strongly mitigated the negative effects of drought by improving the shoot length (37.03%), fresh biomass (52%), dry biomass (62.5%), and seed germination (40%) compared to the control. The PGPR + BC amendment treatment enhanced physiological traits, such as chlorophyll a (27.9%), chlorophyll b (35.3%), and total chlorophyll (31.1%), compared to the control. Similarly, the synergistic role of PGPR and BC significantly (p< 0.05) enhanced the antioxidant enzyme activity including peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) to alleviate the toxicity of ROS. The physicochemical properties (N, K, P, and EL) of the soils were also enhanced by (85%, 33%, 52%, and 58%) respectively, under the BC + PGPR treatment compared to the control and drought stress alone. The findings of this study have suggested that the addition of BC, PGPR, and a combination of both will improve the soil fertility, productivity, and antioxidant defense systems of barley under drought stress. Therefore, BC from the invasive plant P. hysterophorus and PGPR can be applied to water-deficient areas to improve barley crop production.

The Effect of Cadmium Tolerant Plant Growth Promoting Rhizobacteria on Plant Growth Promotion and Phytoremediation: A Review.

Cadmium (Cd) is a heavy metal of considerable toxicity with destructive impacts on plants, microbes and environments. Its toxicity is due to mishandling and manual hazards in plants and is primarily observed within the soil to cause decline of plants and microbial activity inside the rhizosphere. Cadmium accumulation in crops and the probability of Cd entering the food chain are grave for public health in the worldwide. Cadmium toxicity leads to depletion in seed germination, initial seedling growth, plant biomass, chlorosis, necrosis, hindrance of photosynthetic machinery and other physiological and biological activities in plants. Cadmium triggers the reactive oxygen species (ROS) that influences gene mutation and DNA damage that affects the cell cycle and cell division. Cd toxicity altered the levels ofphenolic compounds,carbohydrates, glycinebetaine, proline and organic acids incrops. Under stress conditions, the plant growth promoting rhizobacteria (PGPR) have various properties such as enzymatic activities, plant growth hormones production, phosphate solubilization, siderophores production and chelating agents that help the plants tolerate against Cd stress and also increase phenolic compound levels and osmolytes. Hence, this review highlights the crucial role of cadmium tolerant PGPR for crop production, declining metal phytoavailability and enhancing morphological and physiological boundaries of plants under stress conditions. It could be an environment friendly and cost effective technology under sustainable crop production.

Wheat plant –Plant Growth Promoting Rhizobacteria (PGPR) interaction to alleviate the salt stress in vitro and in vivo

The current work studied the mechanistic basis of salt tolerant PGPR bacteria in alleviating salinity stress in wheat.Two experiments were conducted at the Agricultural Research Center (ARC), 2021-2022, to evaluate the interaction between wheat var. Misr1 and two salt tolerant PGPR bacterial strains, Azospirillum (A.) brasilense NO40 and Bacillus thuringiensis (Bt) under saline conditions. The staining technique used to detect the colonization effectiveness of applied bacteria. Images of 30-day wheat seedlings were captured and subjected to the following measurements: root area, leaf area, root length, shoot length, and dark green color index (DGCI). TEM was used to look for bacterial colonization in wheat seedling. Chlorophyll, proline, peroxidase activity, rhizosphere dehydrogenase activity and electrical conductivity (EC) were all quantified. Results revealed that the wheat seedling roots inoculated with the salt tolerant bacterial strains produced a highly intense red colour. The colonized bacteria appeared on the outer surface of the root and intercellular space of root tissue. The data showed a significant rise in all measured parameters in the inoculated wheat seedlings with the salt tolerant bacterial strains compared to the non-inoculated wheat seedlings. Additionally, the wheat seedlings inoculated with a mixture of NO40 and Bt showed the best results as compared to that inoculated with only NO40 or with only Bt. Finally, EC values before and after planting with the inoculated wheat seeds revealed a decrease in the salinity of the soil rhizosphere (9.27 mmhos/cm vs. 5.1 mmhos/cm). In conclusion: Salt stress in wheat saline soils was alleviated by bacterial strains studied. Optimum results were obtained from the mixture of A. brasilense and Bt. Results must later be verified in field trials.

A transcriptomic study reveals salt stress alleviation in cotton plants upon salt tolerant PGPR inoculation

Soil salinity is a major constraint for reducing crop productivity worldwide. To combat this situation, the current project was aimed to examine the effect of plant growth-promoting rhizobacteria (PGPR) inoculation on the growth of cotton ( Gossypium hirsutum, var. Jin668) plants during salt stress and to identify salt stress-responsive genes in cotton plants. For this purpose, two bacteria, Bacillus subtilis and Bacillus pumilus were selected among the 20 strains isolated from the cotton rhizosphere under the salt stress (200 mM NaCl) and identified by 16 S rRNA sequencing. B. subtilis and B. pumilus were applied to Jin668 plants under salt stress which enhanced resistance to salt stress (leaf and root growth) compared to only salt-treated plants and control plants. Transcriptomic analysis revealed 556 differentially expressed genes (481 up-regulated and 75 down-regulated) in the B. subtilis + Salt versus Salt treatments and 943 (536 up-regulated and 407 down-regulated) genes in the B. pumilus + Salt versus Salt treatments. KEGG analysis of B. pumilus + Salt versus Salt and B. pumilus + Salt versus Salt revealed the pathways plant-pathogen interaction and plant hormone signal transduction were expressed in both treatments, while ascorbate and aldarate metabolism pathways and glyoxylate and dicarboxylate metabolism pathways were uniquely expressed in the B. pumilus + Salt versus Salt comparison, and the pentose and glucuronate interconversions pathway was uniquely expressed in the B. pumilus + Salt treatment versus Salt comparison. These data showed that B. subtilis and B. pumilus significantly enhance salt stress tolerance in cotton plants during salt stress conditions. • Two Bacteria, Bacillus subtilis and Bacillus pumilus were selected from the cotton rhizosphere under the salt stress. • B. subtilis and B. pumilus were applied to cottonplants under salt stress which enhanced resistance to salt stress. • Transcriptomic analysis revealed 556 DEGs in the B.subtilis vs Salt and 943 DEGS in the B.pumilus vs Salt treatments. • KEGG analysis of B. subtilis and B. pumilus versus Salt revealed the variouspathways were expressed in both treatments. • In conclusion, B. subtilis and B. pumilus significantly enhancetolerance in cotton plants during salt stress conditions.

Drought tolerance induction and growth promotion by indole acetic acid producing Pseudomonas aeruginosa in Vigna radiata.

Drought accompanied with reduced precipitation is one of the key manacles to global agricultural throughput and is expected to escalate further hence posing major challenges to future food safety. For a sustainable agricultural environment, drought resistant plant growth promoting rhizobacteria (PGPR) are new encouraging prospect, which are inexpensive and have no side effects, as those of synthetic fertilizers. In the present study, five strains of Pseudomonas aeruginosa, the strain MK513745, strain MK513746, strain MK513747, strain MK513748, and strain MK513749 were used as drought tolerant PGPR with multiple traits of IAA production, N fixation, P solubilization, siderophore producing capabilities. The strain MK513745 and strain MK513749 produced higher quantities of indole acetic acid (116±0.13 and 108±0.26 μg ml-1). MK513749 yielded 12 different indole compounds in GCMS analysis. The strain MK513748 yielded maximum S.I. (3.33mm) for phosphate solubilizing test. Maximum nitrogen concentration was produced (0.18 μg ml-1) by strain MK513746. Percent siderophore units ranged from 2.65% to 2.83% as all five pseudomonas strains were siderophore positive. In all growth experiments of plant microbe interaction two varieties of Vigna radiata (AZRI-06, NM-11) plants inoculated with P. aeruginosa strains under drought stress responded significantly (P<0.05) better than control stressed plants. Maximum shoot length was enhanced up-to 125%, pod/plant 172%, number of grains 65%, 100 seed weight 95%, 100 seed straw weight 124% and total yield 293% were recorded in plants inoculated with drought stress tolerant PGPR in both varieties as compared to respective stressed control plants. Photosynthetic activity, membrane stability (45%), water content (68%) and antioxidant efficacy (19%) were improved with PGPR inoculations. The variety NM-11 (V2) was more tolerant to drought stress with inoculations of Pseudomonas strains than AZRI-06 (V1). Inoculations with these indole acetic acid producing strains would be suitable for plant growth promotion in areas facing water deficiency.

The rhizosphere of Sulla spinosissima growing in abandoned mining soils is a reservoir of heavy metals tolerant plant growth-promoting rhizobacteria

Heavy metals tolerant soil bacteria are known to play essential roles in biogeochemical cycles, biotransformation of metals, bioremediation processes and, plant adaptation. The objective of this study was to isolate and characterize the bacterial strains associated with Sulla spinosissima , a native legume species growing in three abandoned mining sites situated in the Oujda region (East Morocco). Globally, more than 370 bulk and rhizospheric soil bacteria were isolated. Their identification by 16S rDNA sequencing showed that dominant phyla were Firmicutes, Actinobacteria, and Proteobacteria, while at the genus level Bacillus dominated, followed by Stenotrophomonas, Arthrobacter , and Rhodococcus . All the soils contained strains possessing plant growth-promoting traits. The best-performing strains were LMR283 for auxin production (145 μg mL −1 ), LMR291 for phosphate solubilization (67.6 mg L −1 ), LMR280 for siderophore production (92.4%) and LMR326 for ACC deaminase activity (105 nmoL α-ketobutyrate mg −1 h −1 ). Among tolerant PGP bacteria, 17 isolates showed antagonistic activity against the pathogen Fusarium oxysporum and 26 produced lytic enzymes. It was relevant that the rhizospheric soils prospected compared to bulk soils contained more interesting isolates for all the studied properties, in particular for soils sampled from Oued El Heimer and Sidi Boubker sites (respectively 79 and 63% of performing strains). Data presented here indicate that Sulla spinosissima growing in heavy metal soils is associated with multifarious active bacterial populations that probably sustain plant tolerance and growth under the prevailing stressful conditions. Superior strains identified are good candidates to be used with selected plants in rehabilitation programs in the contaminated ecosystems. • Cultivable bacteria present in Oujda abandoned mining soils were isolated and analyzed. • Bacillus genus was dominant followed by Stenotrophomonas , Arthrobacter , and Rhodococcus . • Heavy metals tolerant and phyto-beneficial bacterial strains were characterized. • Bacterial properties and distribution are influenced by Sulla spinosissima plants. • Identification of potential bio-inoculants for phytoremediation candidate plants.

Multifarious effect of ACC deaminase and EPS producing Pseudomonas sp. and Serratia marcescens to augment drought stress tolerance and nutrient status of wheat.

Drought is the prime abiotic stress that rigorously influences plant growth, yield and quality of crops. The current investigation illustrated the bio-protective characters of Serratia marcescens and Pseudomonas sp. to ameliorate drought stress tolerance, plant growth and nutrient status of wheat. The present study aimed for search of potential drought tolerant plant growth-promoting rhizobacteria (PGPR). All screened bacterial isolates exhibited potential plant growth promoting (PGP) attributes such as production of ACC deaminase, exo-polysaccharide, siderophore, ammonia, IAA, and efficiently solubilized zinc and phosphate under in vitro conditions. To assess the in situ plant growth promotion potential of PGPR, a greenhouse experiment was conducted by priming wheat seeds with screened plant PGPR. Improved water status, reactive oxygen species, osmolyte accumulation, chlorophyll and carotenoids content in plant leaves confirmed the excellent drought tolerance conferring ability of RRN II 2 and RRC I 5. Among all PGPR, RRN II 2 and RRC I 5 inoculated plants not only demonstrated greater harvest index but also exhibited more micronutrient (zinc and iron) content in wheat grains. Further, RRN II 2 and RRC I 5 were identified through 16S rDNA sequencing as S. marcescens and Pseudomonas sp., respectively. Furthermore, amplification of acdS gene (Amplified band size of acdS gene was ~ 1.8Kb) also confirmed ACC deaminase enzyme producing ability of Pseudomonas sp. Moreover, correlation coefficient, principal component analysis and cluster analysis also demonstrated that nutrient status and values of agronomical parameters of wheat primed with S. marcescens and Pseudomonas sp. were at par with the positive control. Thus, the outcome of this comparative investigation indicates that Pseudomonas sp. and S. marcescens could be utilized as bioinoculant in wheat since they can improve the physiological status, productivity and nutrient status in wheat crop under drought.

Deciphering distinct biological control and growth promoting potential of multi-stress tolerant Bacillus subtilis PM32 for potato stem canker.

The online version contains supplementary material available at 10.1007/s12298-021-01067-2.

Synergistic effects of plant growth promoting rhizobacteria and silicon dioxide nano-particles for amelioration of drought stress in wheat

Drought tolerant plant growth-promoting rhizobacteria (PGPR) can confer drought tolerance in plants, when inoculated, and this effect can be more pronounced by their combined application with silicon oxide nanoparticles (SiO2 NPs). In this research, drought-tolerant and plant growth-promoting rhizobacterial strains were isolated from the rhizospheric soil of wheat plants growing in the arid region of Pakistan. Out of 30 isolated strains, three rhizobacterial strains were selected based on their drought tolerance, higher phytohormones (indole acetic acid (IAA), abscisic acid (ABA), and cytokinin (CK), and osmolyte (proline and sugar) production ability. These strains were identified as Bacillus sp. Azospirillum lipoferum and Azospirillum brasilense by 16S rRNA sequencing and accession numbers (MT482404, MT742664, and MT 742666, respectively) were obtained. Inoculation of these strains, alone and in combination, improved the germination attributes of wheat seeds under drought stress conditions. However, the combination of all three bacterial strains gave the best results. SiO2 NPs were prepared from silicon dioxide and characterized by scanning electron microscopy (SEM), Energy dispersive X-rays pattern (EDX), and UV–visible spectrum. The effect of SiO2 NPs was also tested on wheat seeds under drought stress and it was observed that SiO2 NPs (150 mg/L) create pronounced drought ameliorative potential in wheat seedlings. In the pot experiment, the combined application of SiO2 NPs and PGPR exhibited a synergistic role and improved the growth and yield of wheat. The interaction between SiO2 NPs and bacterial combination improved biomass (fresh and dry weight), and chlorophyll-a, b content by 138.78%, 65.70%, 128.57%, and 283.33% respectively as compared to untreated but drought exposed plants. They also improved relative water content (71.66%), gas exchange attributes, increased nutrients uptake, and osmolytes production of wheat. Up-regulation of antioxidant enzymes; superoxide dismutase (60.49%), peroxidase (55.99%), and catalase (81.69%) was also observed. This research work suggested that the application of SiO2 NPs and PGPR strains induced drought tolerance in wheat by modulating different physiological and metabolic processes in plants which ultimately improved the growth and yield of wheat under drought stress.

Phytobeneficial and salt stress mitigating efficacy of IAA producing salt tolerant strains in Gossypium hirsutum

Salinity is one of the major agricultural concern that significantly limits the crop productivity. The plant growth promoting rhizobacteria (PGPR) may contribute in sustainable crop production under salt stress. The current study was designed to isolate the Indole Acetic Acid (IAA) producing salt tolerant PGPR to promote the growth of cotton (Gossypium hirsutum, FH-142) and induce its salt stress tolerance. Ten Salt Tolerant (ST) bacterial strains were screened for their PGP trait in vitro and evaluated for their beneficial effect on cotton plants growth by plant–microbe interaction assay in lab and under natural condition. GC–MS analysis of the metabolites of the selected bacterial strains confirmed the presence of indolic compounds like indole, indole-3-butyramide, benzylmalonic acid and 4-methyl-2-pyrrolidinone. The bacterial isolates ST4, ST5, ST6, ST15, ST16, ST17, ST18, ST20, ST22 and ST25 were identified as Bacillus sp., B. sonorensis, B. cereus, B. subtilis, Brevibacillus sp. B. safensis, B. paramycoides, Bacillus sp., B. cereus and B. tequilensis respectively on the basis of 16S rDNA sequencing. Bacteria inoculated plants had a significant (P < 0.05) increase in percentage germination up to (31%), root length (17%) and shoot length (34%) in lab while in wire house pot experiments, maximum enhancement in root length (31%) and shoot length (29%) was observed. ST bacterial strains inoculation improved the chlorophyll content index (34%), relative water content (36%), leaf area (33%), absorption of K+ (28%) and decreased the uptake of Na+ (58%) from soil in plants under salt stress over control in pot experiment. These ST PGPR have the potential to act as plant defense agents by enhancing plant growth, productivity, and tolerance in saline environment.

EFFECT OF PLANT GROWTH PROMOTING RHIZOBACTERIA ON Zea mays DEVELOPMENT AND GROWTH UNDER HEAVY METAL AND SALT STRESS CONDITION

Plants, being sessile organisms are exposed to a variety of biotic and abiotic stresses. Bacteria, living in the rhizosphere of the plants can be beneficial, exhibiting plant growth promotion effect and inducing the stress tolerance of the plant. Therefore, studies regarding the role of plant growth promoting rhizobacteria (PGPR) in the stress management of the plants have received increasing attention. In the present study, we studied the stress tolerance and beneficial trait maintenance of 30 PGPR strains under stress condition. From a total of 17 heavy metal and salt stress tolerant bacterial strains, two Cd and Zn tolerant PGPR strains Viridibacillus sp. (BP13) and Defltia acidovorans (BP12) as well as two salt tolerant strains Pantoea agglomerans (8G/3) and Serratia fonticola (BB17) were selected for plant experiments. When heavy metal and salt stress were applied, the beneficial effect of PGP bacterial inoculation on maize plant growth and development was confirmed only sporadically. Stress mitigation was observed in the case of Viridibacillus sp. (BP13) and Defltia acidovorans (BP12) strains (0.1 mM Cd treatment), Serratia fonticola (BB17) (until 3 g/L NaCl) and Pantoea agglomerans (8G/3) (5 g/L NaCl). Despite the sporadically observed beneficial effect of the PGP bacterial inoculation on plant growth and development; a higher guaiacol peroxidase (GPOX) activity observed under Viridibacillus sp. (BP13) bacterial inoculation in the presence of heavy metal stress, as well as Pantoea agglomerans (8G/3) and Serratia fonticola (BB17) inoculation in the case of salt stress revealed that PGP bacterial strains increased the plant tolerance to abiotic stressors.

Potential of Salt Tolerant PGPR in Growth and Yield Augmentation of Wheat (Triticum aestivum L.) Under Saline Conditions.

Soil salinity has emerged as a major obstacle to meet world food demands. Halo-tolerant plant growth promoting rhizobacteria (PGPR) are potential bioinoculants to enhance crop productivity in saline agriculture. Current work was aimed at studying individual or synergetic impact of salt tolerant PGPR on wheat growth and yield under saline conditions. A pot experiment was conducted on two wheat genotypes (Aas-11; salt tolerant and Galaxy-13; salt sensitive) inoculated with Pseudomonas fluorescence, Bacillus pumilus, and Exiguobacterium aurantiacum alone and in consortium. The salt tolerant variety (Aas-11) exhibited maximum root fresh (665.2%) and dry biomass (865%), free proline (138.12%) and total soluble proteins (155.9%) contents, CAT (41.7%) activity and shoot potassium uptake (81.08%) upon inoculation with B. pumilus, while improved shoot dry weight (70.39%), water (23.49%) and osmotic (29.65%) potential, POD (60.51%) activity, enhanced root potassium (286.36%) and shoot calcium (400%) were manifested by E. aurantiacum. Highest shoot length (14.38%), fresh weight (72.73%), potassium (29.7%) and calcium (400%) acquisition as well as glycinebetaine (270.31%) content were found in plants treated with PGPR consortium. On the other hand, in the salt sensitive variety (Galaxy-13), P. fluorescens treated plants showed significantly improved leaf-water relations, glycinebetaine (10.78%) content, shoot potassium (23.07%), root calcium (50%) uptake, and yield parameters, respectively. Plant root length (71.72%) and potassium content (113.39%), root and shoot fresh and dry biomass, turgor potential (231.02%) and free proline (317.2%) content were maximum upon PGPR inoculation in consortium. Overall, Aas-11 (salt tolerant variety) showed significantly better performance than Galaxy-13 (salt sensitive variety). This study recommends B. pumilus and E. aurantiacum for the salt tolerant (Aas-11) and P. fluorescens for the salt sensitive (Galaxy-13) varieties, as potential bioinoculants to augment their growth and yield through modulation of morpho-physiological and biochemical attributes under saline conditions.

Isolation, characterization and screening of PGPR capable of providing relief in salinity stress

Environmental stresses such as drought, temperature, salinity, air pollution, heavy metals, pesticides, and soil pH are major limiting factors in crop production because they affect almost all plant functions. Soil salinization is a serious stress condition causing major problem for crop productivity. To combat this salinity stress, Plant growth promoting rhizobacteria (PGPR) is considered as innovative, effective and ecofriendly approach. Plant growth promoting rhizobacteria (PGPR) have various direct and indirect mechanisms which can be correlated with their ability to form biofilms, chemotaxis, and the production of exopolysaccharide, indole-3-acetic acids (IAA) and aminocyclopropane-1- carboxylate (ACC) deaminase Investigations on the interaction of PGPR with other microbes and their effect on the physiological response of crop plants under different soil salinity regimes are still at an incipient stage. An experiment was conducted to investigate the effect of PGPR on lowering down the salt stress. Treatments were control (T 1 ), Salt tolerant isolate KH-1 (T 2 ), Salt tolerant isolate KH-2 (T 3 ), Salt tolerant isolate KH-3 (T 4 ), PGPR-I (Pseudimonas) (T 5 ), PGPR-II (Azotobacter) (T 6 ). Rice was sown under saline conditions at Soil Salinity Research Institute, Pindi Bhattian. With the inoculation of salt tolerant PGPR, plant growth and yield was improved. Result showed significant increase in plant height, biomass and yield over control. Inoculation of salt tolerant isolate KH-2 produced maximum grain yield in rice (4267 kg/ha) followed by PGPR-II and it was statistically significant from all other treatments along with control. It is concluded that with the application of salt tolerant isolate (KH-2), there is significant increase in rice production.

Functionally dissimilar soil organisms improve growth and Pb/Zn uptake by Stachys inflata grown in a calcareous soil highly polluted with mining activities

Micro- and macro-organisms are key components of sustainable soil-plant systems; and are involved in plant growth stimulation and accumulation of heavy metals in the plant, with great contribution to phytoremediation of heavy metal-contaminated soils. However, the combined effect of arbuscular mycorrhizal fungi (AMF), plant growth-promoting rhizobacteria (PGPR) and earthworms on plant growth and metal uptake is not yet clear. The main objective of this study was to examine the combined influences of these soil organisms on the growth and metal uptake by a native plant species (Stachys inflata), and subsequently on potential phytoremediation in a soil highly polluted with Pb/Zn mining activities. Metal tolerant AMF, PGPR and earthworms were used either alone or in combination in a factorial pot experiment under greenhouse conditions for 4 months. Inoculation of AMF, PGPR and earthworm led to an increase of soil Pb/Zn availability with the greatest Zn availability (about 2 times) with triple inoculation of all the organisms and the highest Pb availability (about 3 times) with co-inoculation of PGPR and earthworms. Triple inoculation of soil organisms increased the total plant biomass 3 times, total Pb uptake 6 times and total Zn uptake 5 times as compared with the uninoculated plants. The growth-stimulating effect of combined soil organisms was much greater than that of individual or dually-inoculated organisms. These results showed that it is possible to use the combination of metal-tolerant soil organisms as a potential bioaugmentation tool to accelerate metal phytoremediation rate in calcareous soils polluted by Pb/Zn mining activity under arid conditions. This could occur as a consequence of the synergistic effects of AMF, PGPR and earthworms on metal availability in the soil, S. inflata growth, metal tolerance and uptake. However, it would practically require more than hundreds of cropping cycles (360 for Pb and 250 for Zn) to achieve metal maximum permissible limits under the conditions of this pot experiment.