PGP Traits Research Articles

Isolation and Characterization of Plant Growth Promoting Rhizospheric Bacteria From Limonium stocksii

Plant Growth Promoting Rhizospheric bacteria (PGPR) are beneficial microorganisms that can enhance plant growth against biotic and abiotic stress. The present investigation was carried out to isolate and characterize the PGPR from the rhizospheric region of Limonium stocksii, a halophyte. Salt tolerance test for the organisms was carried out up to 15% of salt concentrations. Out of 10 bacterial isolates, six were able to grow at 15% salt concentration. The isolates able to grow at 15% salt concentration were selected for further studies. Further, six isolates showed PGP traits as well as produced extracellular hydrolytic enzymes. The ongoing work would establish the usefulness of these organisms to enhance plant growth via diverse mechanisms eventually leading to the reduction and/or replacement of the synthetic fertilizers and pesticides in agriculture practices.

Screening and biochemical identification of Tef (<em>Eragrostis tef</em> Zucc.) Trotter) endophytic bacterial species with plant-growth-promoting, biotic and abiotic stress tolerance properties

Endophytic bacteria play many important beneficial roles in the metabolism and physiology of the host plants, including fixing atmospheric nitrogen, solubilizing phosphates, synthesizing phytohormones, sequestrating iron, suppression of the ethylene production, and inhibiting pathogens. The present study was conducted to screen and identify endophytic bacterial species with plant growth-promotion, biotic and abiotic stress tolerance properties to develop bio inoculants. For this purpose, root and seed samples of different cultivated tef varieties were collected. A total of 195 pure endophytic bacterial colonies were isolated and screened for PGP traits, biotic and abiotic stress tolerance factors. Fourteen endophytic bacterial species or strains were identified using the Biolog microbial identification system. Among the identified endophytes, the majority of them utilized different carbon sources tagged on microplates. Three potential endophytic bacterial species or strains were evaluated for seed germination and seedling growth ability using tef seeds. Tef seeds inoculated with potential endophytes showed 100 % germination performance on 3 to 4 days after inoculation. An increase in mean shoot and roots length of the inoculated seeds were observed up to 3 and 2.6 cm respectively. Vigor index of seedling was measured from 460 to 520 on the last day of the experiment. These promising identified tef endophytic bacterial species or strains having PGP, biotic and abiotic stress tolerance properties could be candidate organisms for the development of microbial inoculants as single strain or consortia. This strategy would work towards solving the problem of over-utilizing chemical fertilizers, and reducing environmental pollution while increasing the yield and productivity of tef crops and other agricultural plants.

Multi-trait PGP rhizobacterial endophytes alleviate drought stress in a senescent genotype of sorghum [Sorghum bicolor (L.) Moench

Root-tissue colonizing bacteria demonstrated with multiple PGP traits from sorghum plants were identified as Ochrobactrum sp. EB-165, Microbacterium sp. EB-65, Enterobacter sp. EB-14 and Enterobacter cloacae strain EB-48 on the basis of 16S rRNA gene sequencing. Here, the in vivo experiments using ½-MS media and ½-MS media + 15% PEG 8000 (for inducing drought stress) indicated stress tolerance imparting ability of these rhizobacterial endophytes in a non-stay green and senescent genotype (R-16) of sorghum. In the experiment with sterile soilrite mix base, seed bacterization with these isolates showed improved plant growth specifically the roots, in terms of root length (~ 44.2 to 50.8% over controls), root dry weight (~ 91.3 to 99.8% over controls) and root surface area (~ 1 to 1.5 fold over controls) under drought stress. Rhizobacterial endophytes were successful, not only in providing better cellular osmotic adjustment in leaves (≥ 1-fold increase in proline accumulation over controls), but favorable physiological responses like Relative Water Content (RWC) and cell Membrane Stability Index (MSI) in the inoculated plants during the drought stress induction. Up-regulation of drought responsive genes like sbP5CS2 and sbP5CS1 was observed in these endophytes-treated plants as compared to untreated control and Escherichia coli DH5α (negative control)-treated plants. Interestingly, the stress imparting traits of rhizobacterial endophytes, including up-regulation of specific genes, were observed during sorghum seedling growth only under drought stresses. The results of this study lead to the conclusion that the potential endophytic rhizobacterial interactions can contribute to plant growth promotion as well as induced stress tolerance in sorghum.

Characterization of potential plant growth-promoting rhizobacteria isolated from sunflower (Helianthus annuus L.) in Iran.

This study aims to characterize plant growth-promoting rhizobacteria (PGPR) in sunflowers growing in different locations at North West of Iran. Sunflower plants were collected from different regions of West Azarbaijan, and rhizospheric bacterial strains were isolated and screened for PGP traits. Identification and characterization of the PGPR were conducted based on 16s rDNA sequences and phenotypic analysis, the strains clustered for genetic diversity by rep-PCR method. Among the 80 bacterial isolates, 20 showed PGP traits and were selected for other potentials. All the selected isolates produced indole-3-acetic acid at the rate of 9.2-33.7 mg/ml. In addition, 13, 15, 12, and 16 were positive for phosphate solubilization, siderephore, hydrogen cyanide, and ammonia production, respectively. The results from a subsequent pot experiment indicated that PGPRs distinctly increased sun flower shoot and root length, shoot and root fresh weight, as well as shoot and root dry weight. Based on 16S rDNA sequences and biochemical and physiological characteristics, 20 PGPRs were identified as Pseudomonas fluorescens (five isolates), Pseudomonas aeruginosa (four isolates), Pseudomonas geniculata (one isolate), Bacillus subtilis (four isolates), Bacillus pumilus (two isolates), Stenotrophomonas maltophilia (two isolates), and Brevibacterium frigoritolerans (two isolates). In rep-PCR, PGPR isolates were differentiated into seven clusters (A-G) at 65% similarity level. These results demonstrated the existence of a considerable species richness and genetic diversity among PGPRs isolated from different regions of North West of Iran. To the best of our knowledge, this is first report for the identification and characterization of B. frigoritolerans as PGPR in sunflower plants.

Isolation of Cadmium and Lead Tolerant Plant Growth Promoting Rhizobacteria: Lysinibacillus varians and Pseudomonas putida from Indian Agricultural Soil

ABSTRACTCadmium and lead tolerant plant growth promoting rhizobacteria can be an eco-friendly option for reclamation of metal-contaminated soil and mitigation of heavy metal stress for phytostimulation to develop sustainable agriculture. Novel heavy metal tolerant PGPR strain isolation is prerequisite in practical scale. The present study was to isolate, characterize and identify cadmium and lead tolerant PGPR strains. Some potent multi heavy metal tolerant PGPRs were isolated from rhizospheric soil contaminated with industrial, sewage or agrochemical waste and two most promising strains were identified as Lysinibacillus varians strain KUBM17 and Pseudomonas putida strain KUBM18 on the basis of 16S rDNA sequencing. These two strains were exhibiting a high degree of Cd and Pb tolerant ability with bioaccumulatory properties to remove Cd and Pb from the medium which was evidenced by AAS study. Furthermore, L. varians strain KUBM17 and P. putida strain KUBM18 showed very promising PGP traits such as ammonia production, IAA production (20–22 µg/ml), Nitrogen fixation, phosphate solubilization (55–515 ppm) and siderophore production (14–40 nmole) capability together with biocontrol properties with survival ability under different agro-climatic conditions. These PGPR isolates improved the early radish plant growth under cadmium and lead stress condition. So, KUBM 17 and KUBM 18 might be concerned as PGPR-biofertilizer in heavy metal-contaminated agricultural soil.

Culturable endophytic bacteria of Camellia species endowed with plant growth promoting characteristics.

Tea (Camellia sinensis (L.) O. Kuntze) is an economically important caffeine-containing beverage crop with massive plantation in the Northeast corner of the agroclimatic belt of India. The main aim of the work was to isolate, identify and characterize the native plant growth promoting endophytes associated with tea for future microbe based bioformulation. A total of 129 endophytic bacteria were isolated and characterized for plant growth promoting traits such as indole-3-acetic acid (IAA), phosphate solubilization, ammonia production, biocontrol traits like siderophore and extracellular enzyme production. BOX-PCR fingerprinting was used to differentiate the various bacterial isolates obtained from six different tea species. 16S rRNA sequencing and blast analysis showed that these isolates belonged to different genera, that is, Bacillus, Brevibacterium, Paenibacillus and Lysinibacillus. Lysinibacillus sp. S24 showed the highest phosphate solubilization and IAA acid production efficiency of 268·4±14·3 and 13·5±0·5µgml-1 , respectively. Brevibacterium sp. S91 showed the highest ammonia production of 6·2±0·5µmolml-1 . Chitinase, cellulase, protease and pectinase activities were shown by 4·6, 34·1, 27·13 and 13·14% of the total isolates, respectively. Similarly, 41% of the total isolates were positive for 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity. Further, the potent PGP isolates, S24 and S91 were able to enhance the vegetative parameters such as dry/fresh weight of root and shoot of tea plants in nursery conditions. Our findings corroborate that tea endophytic bacteria possess the potential to demonstrate multiple PGP traits both, in vivo and in vitro and have the potential for further large-scale trials. The exploration of tea endophytic bacterial community is suitable for the development of bioformulations for an integrated nutrient management and thus sustainable crop production and decreasing the hazardous effects of chemical fertilizers on the environment and human health.

Screening of stress tolerant bacterial strains possessing interesting multi-plant growth promoting traits isolated from root nodules of Phaseolus vulgaris L.

Rhizobia are soil bacteria known for their ability to nodulate and fix nitrogen with legumes. Some rhizobia also show different plant growth promoting characteristics (PGP); they can therefore be used as effective biofertilizers. Phaseolus vulgaris is considered a weak nitrogen fixer; therefore, screening experiments have been designed to select stress-tolerant effective bean rhizobia that have also PGP traits. Surveys of common bean fields in Morocco lead to the isolation of 113 bacteria inhabiting root nodules. Screening tests showed that 45 isolates exhibited positive phosphate solubilization, and auxin and siderophores production as well as high salt and pH tolerance. Correspondence analysis showed that salt tolerance and phosphate solubilization were origin related. Among the screened isolates, 18 were able to renodulate common bean but only 3 were highly effective with their host plant. Interestingly, phylogenetic analysis of 16S rRNA gene showed that the nodulating isolates clustering within the Rhizobium genus were represented by only two isolates related to R. miluonense and R. yanglingense while the remaining strains belonged to the Agrobacterium genus. Our results revealed also the co-existence of several endophytic bacteria with the symbiotic strains inside the nodules. These bacteria are stress tolerant and possess potential PGP traits, which indicate that they could stimulate common bean growth and contribute to N and P plants nutrition. • 45 nodule isolates out of 113 possess PGP traits and are highly salt and pH tolerant. • 3 highly effective strains obtained among18 that renodulate common bean. • 16S rDNA sequencing revealed that nodulating strains belong to Rhizobium and Agrobacterium genera. • Stress tolerant and PGP endophytic bacteria co-exist with rhizobia in the nodules. • Good candidates for common bean biofertilisation were selected.

Characterization of cadmium-resistant rhizobacteria and their promotion effects on Brassica napus growth and cadmium uptake.

Excessive cadmium (Cd) accumulation in soil can adversely affect plants, animals, microbes, and humans; therefore, novel and uncharacterized Cd-resistant plant-growth-promoting rhizobacteria (PGPR) are required to address this issue. In the paper, 13 bacteria were screened, their partial 16S rRNA sequences determined, and the isolates, respectively, clustered into Curtobacterium (7), Chryseobacterium (4), Cupriavidus (1), and Sphingomonas (1). Evaluation of PGP traits, including indole-3-acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, siderophore secretion, and cyanhydric acid production, identified Cupriavidus necator GX_5, Sphingomonas sp. GX_15, and Curtobacterium sp. GX_31 as promising candidates for PGPR based on high IAA or ACC deaminase production. Additionally, root-elongation assays indicated that inoculating GX_5, _15, or _31 increased Brassica napus root length both in the presence and absence of Cd by 19.75-29.96% and 19.15-31.69%, respectively. Pot experiments indicated that inoculating B. napus with GX_5, _15, and _31 significantly increased the dry weight of above-ground tissues and root biomass by 40.97-85.55% and 18.99-103.13%, respectively. Moreover, these isolates significantly increased Cd uptake in the aerial parts and root tissue of B. napus by 7.38-11.98% and 48.09-79.73%, respectively. These results identified GX_5, _15, or _31 as excellent promoters of metal remediation by using microorganism-associated phytoremediation.

Assessment of Plant Growth Promoting and Abiotic Stress Tolerance Properties of Wheat Endophytic Fungi.

The aims of the present work were to isolate and characterize fungal endophytic communities associated with healthy wheat (Triticum aestivum L.) plants, collected from the North China. Segregated endophytes were screened for their PGP traits, abiotic stresses (heavy metals, salinity, drought, and temperature), and antibiotic sensitivity. A total of 16 endophytic fungi were isolated using the culture-dependent approach from different tissue parts of wheat plants. Based upon their internal transcribed spacer (ITS) rDNA gene sequencing, 15 out of 16 isolates were selected for further analysis. In the contemporary investigation, a number of the tested endophytes exhibited fairly good 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD) (0.03±0.011 to 1.43±0.01 µmol α-KB mg−1 protein hr−1), indole acetic acid (IAA) (1.125±0.04 to36.12±0.004µgml−1), and phosphate solubilizing index (PSI) (2.08±0.03to5.16±0.36) activities. More than 30% isolates gave positive result for siderophore and ammonia tests, whereas all exhibited catalase activity but only 2 (582PDA1 and 582PDA11) produced hydrogen cyanide. Trichoderma strains showed salt, heavy metals, and drought tolerance at high levels and also exhibited resistance to all the tested antibiotics. Strain 582PDA4 was found to be the most temperature (55°C) tolerant isolate. The findings of this study indicated that the microbial endophytes isolated from wheat plants possessing a crucial function to improve plant growth could be utilized as biofertilizers or bioagents to establish a sustainable crop production system.

In vitro investigation to explore the toxicity of different groups of pesticides for an agronomically important rhizosphere isolate Azotobacter vinelandii

In this work, an attempt was made to evaluate the effect of pesticides on growth pattern, surface morphology, cell viability and growth regulators of nitrogen fixing soil bacterium. Pesticide tolerant Azotobacter vinelandii strain AZ6 (Accession no. MG028654) was found to tolerate maximum level of pesticide and displayed multifarious PGP activities. At higher concentrations, pesticides triggered cellular/structural damage and reduced the cell viability as clearly shown under SEM and CLSM. With increase in concentration, pesticides exhibited a significant (p < 0.05) decrease in PGP traits of strain AZ6. Among all three groups of pesticides, herbicides glyphosate and atrazine were most toxic. Kitazin, hexaconazole, metalaxyl, glyphosate, quizalofop, atrazine, fipronil, monocrotophos and imidacloprid at 2400, 1800, 1500, 900, 1200, 900, 1800, 2100 and 2700 μg mL−1, respectively, decreased the production of IAA by 19.5 ± 1.9 (61%), 18.1 ± 1.2 (64%), 36.4 ± 3.4 (28%), 13.1 ± 0.8 (74%), 15.6 ± 1.0 (69%), 7.6 ± 0.5 (83%), 11.9 ± 0.8 (76%), 24.7 ± 1.7 (51%) and 32 ± 2.3 (37%) μg mL−1, respectively, over control (50.7 ± 3.6 μg mL−1). A maximum reduction of 8.4 ± 1.2 (46%), 5.8 ± 0.6 (62%) and 4 ± 0.2 (74%) μg mL−1 in 2, 3-DHBA at 300 (1×), 600 (2×) and 900 (3×) μg mL−1 glyphosate, respectively, While, 32.8 ± 2.7 (19%), 27.2 ± 2 (33%) and 21.5 ± 1.3 (47%) μg mL−1, respectively in the production of SA was observed at 300 (1×), 600 (2×) and 900 (3×) μg mL−1 atrazine, respectively. Likewise, with increase in concentration of pesticides, decrease in P solubilization ability and change in pH of broth was detected. The order of pesticide toxicity to PSE (percent decline over control) at highest concentration was: atrazine (45) > kitazin (44) > metalaxyl (43) > monocrotophos (43) > glyphosate (41) > hexaconazole (39) > quizalofop (33) > imidacloprid (31) > fipronil (25). The present study undoubtedly suggests that even at higher doses of pesticides, A. vinelandii maintained secreting plant growth regulators and this property makes this strain agronomically important microbe for enhancing the growth of plants.

Plant growth promoting ability of ACC deaminase producing rhizobacteria native to Sunflower (Helianthus annuus L.)

Abstract Rhizobacteria inhabiting the soil adhering to the root of the host plants are generally recognized to possess plant growth promoting properties. In the present study, a total of 120 rhizobacterial isolates from native soil samples of sunflower were isolated and screened for their ACC deaminase (ACCd) activity and plant growth promoting properties. A significant ACCd activity ranging between 322.55 and 2085.06 nmol of α-ketobutyrate mg−1 of protein h−1 was observed in 44 isolates with the maximum activity offered by Bacillus subtilis Rhizo SF 48. The rhizobacteria which produced more than 50% α-ketobutyrate were selected to evaluate their efficacy to promote the plant growth under laboratory and greenhouse conditions in sunflower. All the ten ACCd positive isolates were found to exhibit at least four different PGP traits and three were found antagonistic to Fusarium oxysporum. The seed treatment with ACCd producing PGPR also significantly enhanced both seed and vegetative growth parameters compared to control with a maximum enhancement offered by B. subtilis Rhizo SF 48. The findings of the study highlight the efficacy of ACCd producing PGPR in promoting growth parameters in sunflower plants upon seed treatment further warranting to test their ability in the suppression of biotic and abiotic stress.

Synergism of Pseudomonas aeruginosa (LSE-2) nodule endophyte with Bradyrhizobium sp. (LSBR-3) for improving plant growth, nutrient acquisition and soil health in soybean.

The present study was aimed to assess the scope of native potential endophyte Pseudomonas aeruginosa (LSE-2) strain (KX925973) with recommended Bradyrhizobium sp. (LSBR-3) (KF906140) for synergistic effect to develop as consortium biofertilizer of soybean. A total of 28 non-rhizobial endophytic bacteria were isolated from cultivated and wild sp. of soybean. All isolates were screened for multifarious PGP traits viz. Indole-3-acetic acid (IAA), phosphate (P) and zinc (Zn) solubilization, siderophore, cell wall degrading enzymes and pathogenicity. Compatible of LSBR-3 and LSE-2 enhanced IAA, P-solubilization, 1-aminocyclopropane-carboxylate deaminase and biofilm formation over the single inoculant treatment. Further, consortium was evaluated in vivo for growth, symbiotic traits, nutrient acquisition, soil quality parameters and yield attributes of soybean. Improvement in growth parameters were recorded with dual inoculant LSBR-3 + LSE-2 as compared to LSBR-3 alone and un-inoculated control treatments. Significantly (p ≥ 0.05) high symbiotic and soil quality parameters (phosphatase and soil dehydrogenase activity) was recorded with LSBR-3 + LSE-2 at vegetative and flowering stage as compared to LSBR-3 alone and un-inoculated control treatments. Single inoculation of LSBR-3 improved grain yield by 4.25% over the un-inoculated control treatment, further, enhancement in yield was recorded with consortium inoculant (LSBR-3 and LSE-2) by 3.47% over the LSBR-3 alone. Application of consortium inoculant (LSBR-3 + LSE-2) gave an additional income of Rs. 5089/ha over the un-inoculated control treatment. The results, thus strongly suggest that endophytic diazotroph LSE-2 can be used as potent bio-inoculant along with LSBR-3 as bio-enhancer for improving soybean productivity in a sustainable system.

Impact of Native ST-PGPR (Bacillus pumilus; EU927414) on PGP Traits, Antioxidants Activities, Wheat Plant Growth and Yield under Salinity

The rhizobacterial isolate ‘121’ identified as Bacillus pumilus-EU927414, which was a rich source of PGP traits like IAA, gibberellins, phosphate solubilisers and siderophore production at 8% NaCl (w/v). This isolate showed synergistic behaviour of PGP traits with its compatible ST-PGPRs like three-fold increases in IAA content in T9 combination (EU927414-B. pumilus + EU927408-B. aquimaris +EU742146-Bacillus sp.). Synergistic behaviour of B. pumilus-EU927414 was also observed in the case of ACC deaminase activities in T9 at 8% NaCl (w/v). Growth and yield of wheat crop in saline soil (ECe>3.4 dSm−1) were enhanced by the inoculation of native ST-PGPR (salt tolerantplant growth promoting rhizobacteria) under half a dose of recommended synthetic fertilisers (N: P: K) as observed in two successive cropping seasons. Salinity tolerant wheat cultivar K-7903 was used to evaluate the effect of ST-PGPRs in the saline field. Antioxidants activities like SOD, APX, CAT and GR along with proline, TSS, RS content were increased in T9 treated leaves of wheat plant at 60 days, followed by 90 days. The N, P and K contents in plant leaf were significantly increased due to inoculation of compatible ST-PGPRs including B. Pumilus-EU927414. The results showed reduced sodium content in T9 treated wheat plant's leaf at 90 days, followed by 60 days. The yield of wheat was 24% increases with the inoculation of T9 combination, followed by T3 combination (EU927414-B. pumilus + EU927412-Pseudomonas medicona + EU927410-Arthrobacter sp.), when compared with control.

Isolation, identification and in vitro characterization of grapevine rhizobacteria to control ochratoxigenic Aspergillus spp. on grapes

Aspergillus spp. are fungal pathogens that attack the grape and that are known for their secretion of mycotoxins, in particular, ochratoxin which is very toxic. A total of 39 bacterial strains isolated from the rhizosphere in Tunisian vineyards were identified using 16S rDNA and rpob gene sequencing: 35 were Bacillus spp. strains, 2 were Brevibacterium spp., 1 was Paenibacillus sp. and 1 strain was Microbacterium oxydans. Biochemical and microbiological screenings revealed that those 39 strains (i) metabolized differently carbon sources, (ii) possessed antibiotic genes and (iii) produced siderophores. Based on their PGP traits, 21 strains were selected and tested in vitro for their antagonistic effect against two fungal pathogens, Aspergillus ochraceus and A. carbonarius. All the tested antagonists were able to reduce the growth of A. ochraceus, with Bacillus amyloliquefaciens being the most efficient; and A. carbonarius, in particular Bacillus pumilus. In vitro screening using detached berries showed the potential of B. pumilus strain G3AX for inhibiting contaminations by Aspergillus spp., that are OTA-producing fungi. At the berry surface, the efficacy of the bacterial strains strongly depended on the presence or absence of wounds.

Enhancement of growth and salt tolerance of rice seedlings by ACC deaminase-producing Burkholderia sp. MTCC 12259

Increasing soil salinity is often associated with accelerated ethylene production in plants, leading to overall growth reduction. The salt-tolerant 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase-producing PGPR may alleviate salt stress by reducing the production of stress ethylene. In this study, a salt-tolerant ACC deaminase-producing strain named P50 was isolated from a coastal rice field in Odisha, India, which enhanced the growth of rice seedlings under salt stress. The P50 strain was identified as Burkholderia sp. based on phenotypic characteristics, MALDI-TOF MS data for ribosomal proteins and 16S rDNA sequence-based homology. Various PGP traits of strain P50 were characterized, among which the ACC deaminase activity was optimized at different physical conditions and confirmed by enzyme assay, as well as FTIR. The IAA, EPS and proline production of this strain were estimated under increasing NaCl concentrations essential for plant growth promotion under salt stress. Finally, the P50 strain was utilized in a gnotobiotic assay using rice seedlings (cv. Swarnamasuri) under saline stress. Seedlings treated with the P50 strain showed improvement in various morphological and biochemical characteristics, ROS scavenging antioxidant enzymatic activities, and reduced amounts of stress ethylene compared to non-inoculated strains under salinity. In addition, isolation of the ACC deaminase mutant of this strain was not found to reduce stress ethylene, confirming that the P50 strain was associated with a reduction in stress ethylene. Strain P50 was also found to colonize the root surfaces of rice seedlings associated with the plant-microbe interaction process. Thus, as an effective salt-tolerant PGPR, strain P50 can be utilized in salt-affected agricultural fields to improve plant growth in a sustainable manner.

Isolation, characterization, and selection of heavy metal-resistant and plant growth-promoting endophytic bacteria from root nodules of Robinia pseudoacacia in a Pb/Zn mining area

Multiple heavy metals (HMs) commonly coexist in mining areas, which highlights the necessity to select multiple HM-resistant plant growth-promoting bacteria for improving phytoremediation efficiency. In this study, we isolated and characterized 82 endophytic bacteria from the root nodules of black locust (Robinia pseudoacacia) grown in a Pb-Zn mining area. There were 80 isolates showing resistance to four HMs, 0.01–18.0 mM/L for Cd, 0.2–40.0 mM/L for Zn, 0.3–2.2 mM/L for Pb, and 0.2–1.4 mM/L for Cu. Indole-3-acetic acid production, siderophore production, and 1-aminocyclopropane-1-carboxylate deaminase activity were detected in 43, 50, and 17 isolates, respectively. Two symbiotic isolates selected with the highest potential for HM resistance and PGP traits, designated Mesorhizobium loti HZ76 and Agrobacterium radiobacter HZ6, were evaluated for promotion of plant growth and metal uptake by R. pseudoacacia seedlings grown in pots containing different levels of Cd, Zn, Pb, or Cu. HZ76 significantly increased plant shoot biomass, while HZ6 did not, compared with non-inoculated controls. The results indicate that inoculation with HZ76 or HZ6 relieved HM stress in the plants, depending on the type and concentration of HM in the treatment. Mesorhizobium loti HZ76 may be a better candidate for application in phytoremediation than A. radiobacter HZ6. The microsymbiosis between HM-resistant rhizobia and R. pseudoacacia is an interesting mutualistic system for phytoremediation in mining areas contaminated with multiple HMs.

Identification of Rhizosphere Bacterial Diversity with Promising Salt Tolerance, PGP Traits and Their Exploitation for Seed Germination Enhancement in Sodic Soil

The present research was carried to explore bacterial diversity in rhizosphere of salt tolerant rice and wheat genotypes cultivated in sodic soils and examined for the plant growth promoting traits, seed germination and vigor index of wheat in salt affected soils of pH > 9.30. Soil was collected and analyzed for physico-chemical properties using standard methods. Valuable bacterial population was isolated from the rhizosphere of salt tolerant rice and wheat varieties grown in salt affected sodic soils of Uttar Pradesh, India. Isolates were tested for PGP traits, NaCl uptake and tolerance at varying concentration (0.5, 5.0, 7.5, 10.0%). Bacterial isolates possessing high salt tolerance and PGP traits were selected for molecular identification and further exploited for the seed germination in wheat. On the basis of partial sequencing of 16S rRNA gene in nine potential isolates were confirmed as Pseudomonas sp., Agromyces tropicus, Lysinibacillus fusiformis, Bacillus flexus, Lysinibacillus sp., Bacillus tequilensis, Bacillus licheniformis, Pseudomonas mendocina and Bacillus licheniformis, respectively. The Lysinibacillus sp. was most effective in terms of improving seed germination percent (83%), shoot length (20.3 cm), fresh (0.41 g) and dry weight (0.057 g), and seedlings vigor index I (2056.2) and II (6.2). On the other side, root growth (length, fresh and dry weight) was found significantly higher with the inoculation of Lysinibacillus fusiformis. Salt tolerant PGPR of this study could be useful for the reclamation of saline/sodic soils concomitant with improved plant growth and yield.

The role of arsenic resistant Bacillus aryabhattai MCC3374 in promotion of rice seedlings growth and alleviation of arsenic phytotoxicity

The biological agents have been utilized as an affordable alternative to conventional costly metal remediation technologies for last few years. The present investigation introduces arsenic (As) resistant plant growth promoting rhizobacteria (PGPR) isolated from the As-contaminated agricultural field of West Bengal, India that alleviates arsenic-induced toxicity and exhibited many plant growth promoting traits (PGP). The isolated strain designated as AS6 has identified as Bacillus aryabhattai based on phenotypic characteristics, physio-biochemical tests, MALDI-TOFMS bio-typing, FAME analysis and 16S rDNA sequence homology. The strain found to exhibit five times more resistance to arsenate than arsenite with minimum inhibitory concentrations (MIC) being 100 mM and 20 mM respectively. The result showed that accumulation of As was evidenced by SEM- EDAX, TEM-EDAX studies. The intracellular accumulation of arsenic was also confirmed as in bacterial biomass by AAS, FTIR, XRD and XRF analyses. The increased rate of As (V) reduction by this strain found to be exploited for the remediation of arsenic in the contaminated agricultural field. The strain also found to exhibit important PGP traits viz., ACC deaminase activity (2022 nmol α-ketobutyrate/mg protein/h), IAA production (166 μg/ml), N2 fixation (0.32 μgN fixed/h/mg proteins) and siderophore production (72%) etc. Positive influenced of AS6 strain on rice seedlings growth promotion under As stress was observed considering the several morphological, biochemical parameters including antioxidants activities as compared with an uninoculated set. Thus this strain might be exploited for stress amelioration and plant growth enhancement of rice cultivar under arsenic spiked agricultural soil.

Characterization of a Cd2+-resistant plant growth promoting rhizobacterium (Enterobacter sp.) and its effects on rice seedling growth promotion under Cd2+-stress in vitro

Rhizosphere contamination due to heavy metals in agriculture leads to decreased production of edible crops. High water solubility and permeability of these life-threatening metals lead to increased translocation to the upper part of plants and the accumulation of these metals in food crops including cereal seeds. Apart from conventional methods, plant growth promoting rhizobacteria (PGPR)-influenced bioremediation is now an emerging, eco-friendly and inexpensive tool. The K2 isolate is one such multi-heavy-metal-resistant PGPR isolated from the metal-contaminated rice rhizosphere. The isolate was identified as Enterobacter sp. using phenotypic characterization and MALDI-TOF MS-based ribosomal protein data. The strain was able to resist a group of heavy metals/metalloids (Cd2+, Pb2+, As3+, Ni2+, Hg2+) up to certain levels. The strain possessed a collection of PGP traits such as phosphate solubilization, Indole-3-acetic acid production, 1-aminocyclopropane-1-carboxylic acid deaminase activity and nitrogen fixation. In vitro growth enhancement of a rice cultivar by the strain was investigated using Cd2+ stress, as observed in various growth parameters (seed germination, root-shoot length, root-shoot biomass, seedling vigor index, chlorophyll content). Hence, the K2 strain can be exploited regarding multi-heavy-metal/metalloid contamination for effective growth promotion under stress conditions for sustainable agriculture.

Plant growth promoting Curtobacterium albidum strain SRV4: An agriculturally important microbe to alleviate salinity stress in paddy plants

• Salt-tolerant Curtobacterium albidum SRV4 strain was isolated from saline soils. • C. albidum SRV4 showed strong plant growth promoting attributes in salinity stress. • Inoculation of C. albidum SRV4 induces salt stress tolerance in paddy plants. • C. albidum SRV4 inoculation significantly enhance paddy yields under soil salinity. Present study examined the inoculation of halo-tolerant plant-growth-promoting-rhizobacteria (PGPR) on paddy plant growth parameters and physiology under different salinity (0, 100, 200 and 300 mM) conditions. Twenty-nine promising halo-tolerant PGPRs were isolated and screened for their PGP traits from the rhizosphere of naturally growing plants of saline soils located at Sandila region of Hardoi, Uttar Pradesh, India. Out of 29 isolates, one most efficient salt-tolerant isolate having potential PGP attributes, was selected for further study. Based on 16S rRNA gene sequencing and BLASTn analysis, the isolate was identified as Curtobacterium albidum SRV4 strain. The SRV4 expressed positive attribute for nitrogen (N 2 ) fixation, exopolysaccharide production (EPS), hydrogen cyanide (HCN), Indole-3-acetic acid (IAA), and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. The higher doses of NaCl negatively affected paddy plant physiology and growth parameters. Paddy plants treated with SRV4 showed significant differences (P < 0.001) in improvement in plant growth parameters, photosynthetic pigment efficiency, membrane stabilization index and proline content. A significant variation (P < 0.001) in enhancement in antioxidative enzymatic activities catalase (CAT), superoxide dismutase (SOD), peroxidase (POX) and ascorbate peroxidase (APX) and K + uptake in paddy plants was noted due to C. albidum SRV4 inoculation. Paddy plant growth parameters (plant root and shoot length, dry weight and tiller number) and antioxidative enzymatic activities were negatively correlated. In conclusion, C. albidum SRV4 has been found as effective microbial agent to improve photosynthetic efficiency, modulation of osmolytes and antioxidative enzymes due to efficient PGP attributes, development of induced systemic tolerance and alleviating salt stress in paddy plants.