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Twenty-five strains of plant-growth-promoting rhizobacteria (PGPR) containing 1-aminocyclopropane-1-carboxylate (ACC) deaminase and 10 strains of rhizobia were isolated from rhizosphere soil samples and nodules of mung bean. They were screened in separate trials under salt-stressed axenic conditions. The three most effective strains of PGPR (Mk1, Pseudomonas syringae ; Mk20, Pseudomonas fluorescens ; and Mk25, Pseudomonas fluorescens biotype G) and Rhizobium phaseoli strains M1, M6, and M9 were evaluated in coinoculation for their growth-promoting activity at three salinity levels (original, 4 dS·m(-1), and 6 dS·m(-1)) under axenic conditions. The results showed that salinity stress significantly reduced plant growth but inoculation with PGPR containing ACC deaminase and rhizobia enhanced plant growth, thus reducing the inhibitory effect of salinity. However, their combined application was more effective under saline conditions, and the combination Mk20 × M6 was the most efficient for improving seedling growth and nodulation. The effect of high ethylene concentrations on plant growth and the performance of these strains for reducing the negative impact of saline stress was also evaluated by conducting a classical triple-response bioassay. The intensity of the classical triple response decreased owing to inoculation with these strains, with the root and shoot lengths of inoculated mung bean seedlings increasing and stem diameter decreasing, which is a typical response to the dilution in a classical triple response bioassay. Thus, coinoculation with PGPR containing ACC deaminase and Rhizobium spp. could be a useful approach for inducing salt tolerance and thus improving growth and nodulation in mung bean under salt-affected conditions.

Ethylene synthesis is accelerated in response to various environmental stresses like salinity. Ten rhizobacterial strains isolated from wheat rhizosphere taken from different salt affected areas were screened for growth promotion of wheat under axenic conditions at 1, 5, 10 and 15 dS m(-1). Three strains, i.e., Pseudomonas putida (N21), Pseudomonas aeruginosa (N39) and Serratia proteamaculans (M35) showing promising performance under axenic conditions were selected for a pot trial at 1.63 (original), 5, 10 and 15 dS m(-1). Results showed that inoculation was effective even in the presence of higher salinity levels. P. putida was the most efficient strain compared to the other strains and significantly increased the plant height, root length, grain yield, 100-grain weight and straw yield up to 52, 60, 76, 19 and 67%, respectively, over uninoculated control at 15 dS m(-1). Similarly, chlorophyll content and K(+)/Na(+) of leaves also increased by P. putida over control. It is highly likely that under salinity stress, 1-aminocyclopropane-1-carboxylic acid-deaminase activity of these microbial strains might have caused reduction in the synthesis of stress (salt)-induced inhibitory levels of ethylene. The results suggested that these strains could be employed for salinity tolerance in wheat; however, P. putida may have better prospects in stress alleviation/reduction.

Rhizobacteria may promote plant growth by cleaving plant‐produced 1‐aminocyclopropane‐1‐carboxylic acid (ACC) through ACC‐deaminase activity. The effectiveness of plant‐growth‐promoting rhizobacteria (PGPR) for improving the growth and yield of wheat (Triticum aestivum L.) was evaluated under salinity stress. The effect of high ethylene concentrations on plant growth and performance of these strains for reducing this negative impact was also evaluated by conducting a classical triple‐response bioassay. Eighteen rhizobacterial strains containing ACC‐deaminase were screened by conducting a jar experiment under axenic conditions at 1 (original), 5, 10, and 15 dS m−1 The four most effective strains were further evaluated in a pot trial at salinity levels of 1.46 (original), 5, 10, and 15 dS m−1 In general, salinity depressed the growth of wheat, but inoculation improved the growth and yield of wheat compared with the uninoculated controls. At the high salinity level (15 dS m−1), plant height, root length, plant biomass, and grain yield increased up to 37, 70, 116, and 111%, respectively, compared with the control. Results indicated that inoculated plants had higher K+/Na+ ratios, relative water contents and chlorophyll content; however, relatively low proline contents compared with controls. The results also showed that intensity of the classical triple response decreased due to inoculation with these strains. Pseudomonas putida (W2) and P. fluorescens (W17) were the most effective strains for alleviating salt stress even at higher salinity levels. Our results suggest that the assay for ACC‐deaminase activity could be an efficient approach to screen effective PGPR for increasing the growth and yield of wheat under salt‐stressed conditions.

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Maize is a plant known for food, feed, and energy value, but being a greater biomass, it may also be utilized to extract pollutants from soil. Plant growth-promoting rhizobacteria (PGPR) may act as biofertilizer to improve plant health and indirectly may enhance metal extraction. This study focuses on five bacterial strains isolated from the vegetable (Bitter gourd) rhizosphere irrigated with industrial effluent and characterized for various plant growth-promoting activities. Based on 16S rRNA gene sequencing, bacterial strains belonging to the genera, Bacillus (CIK-517, CIK-519), Klebsiella (CIK-518), Leifsonia (CIK-521), and Enterobacter (CIK-521R), were tested for their ability to promote maize growth in axenic conditions. Results showed negative and positive regulation of maize growth by the exogenous application of Cd and PGPR, respectively. Seed germination assays revealed significant reduction in relative seedling growth of maize cultivars upon Cd exposure (0-80 mg Cd L -1 ). The tested strains showed tolerance to Cd (1.78-4.45 mmol L -1 ) and were positive for catalase, oxidase, phosphate solubilization, exopolysaccharide (EPS), and auxin production, whereas CIK-518, CIK-519, and CIK-521R were negative for EPS, phosphate solubilization, and oxidase activities, respectively. Bacterial strains significantly increased shoot/root growth and their dry biomass in normal and Cd-contaminated soil as compared to their respective controls. None of the strains showed significant effects on relative water content or membrane permeability; however, Cd uptake significantly increased in plant tissues upon bacterial inoculation. Bacterial strains CIK-518 and CIK-521R are effective colonizers and thus can be potential inoculants to promote maize growth and Cd extraction/stabilization in Cd-contaminated soil.

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One hundred rhizobacteria previously isolated from the rhizospheres of Brassica species were screened for their growth promoting activity in Brassica napus L. under gnotobiotic conditions. Results revealed that 58% of the rhizobacteria increased root length (up to 139%), 39% enhanced shoot length (up to 78%), and shoot weight (up to 72%) of Brassica napus L. Based upon growth promotion of B. napus seedlings under gnotobiotic conditions, 10 promising plant-growth-promoting rhizobacteria (PGPR) were selected and tested for their effectiveness in growth promotion, yield, and oil content of B. napus grown in pots. The pot trials revealed that inoculation with selected PGPR increased plant height, root length, number of branches per plant, stem diameter, number of pods per plant, 1000-grain weight, grain yield, and oil content over a range of 7–57% above the uninoculated control. These isolates were then assayed for their ability to produce auxins in vitro in the presence and absence of L-tryptophan. Regression analysis showed that in vitro auxin production by these bacteria was significantly related to the number of branches and oil content of B. napus. It is highly likely that improvement in growth and yield of the inoculated plants is due to an increase in the number of branches per plant, since there was a positive correlation of this growth parameter with the number of pods per plant, 1000-grain weight, grain yield, and seed oil content. Results indicated that simultaneous screening of rhizobacteria for growth promotion under gnotobiotic conditions and in vitro production of auxins could be a useful approach for selecting effective PGPR.