Amelioration of salinity stress and growth stimulation of mustard (Brassica juncea L.) by salt-tolerant Pseudomonas species

Indian mustard (Brassica juncea) is a winter oilseed crop grown across the Northern Indian planes. The growth and yield of the crop have been declining over the recent years due to increase in soil salinity. Applications of plant growth-promoting rhizobacterial strains have been found to enhance crop productivity under salinity conditions. Therefore, the objective of this study was to identify bacterial isolates from the mustard rhizosphere that may help in improving the mustard growth under salt stress conditions. Various plant growth-promoting activities, i.e., production of aminolevulinic acid, indole acetic acid, 1-aminocyclopropane-1-carboxylate deaminase, and solubilization of potassium and phosphorus, were analyzed in isolated salt-tolerant rhizobacterial strains, which may contribute towards salt tolerance and crop productivity in mustard. The most promising rhizobacterial isolates HMM57 and JMM15 were identified as Pseudomonas argentinensis and P. azotoformans, respectively by 16SrRNA sequencing. In primary screening, out of ninety-four rhizobacterial isolates tested, only ten isolates HMM6, HMM13, HMM34, HMM39, HMM44, HMM57, HMM65, HMM88, JMM15 and JMM42 showed salt tolerance up to 8% NaCl concentration (0.1-20.0 mm colony size). During secondary screening on agar plates, these rhizobacterial isolates were checked for their potential to enhance seed germination of mustard and growth of seedlings at the high level of salinity (20 dS m(-1)). Moreover, bacterial isolates HMM57 and JMM15, which exhibited different plant growth-promoting traits, were used as inoculants on the mustard plants grown under high salinity conditions (8 and 12 dS m(-1)) in a greenhouse. Rhizobacterial inoculation revealed 139 to 291% increase in root and shoot dry weight even after 80 days of sowing. The results indicated that application of two potential rhizobacterial isolates HMM57, and JMM15, which were able to tolerate high salt concentration (8% NaCl) and having IAA and ALA production along with ACC utilization activities, showed stimulation of the mustard growth even up to 12 dS m(-1) in plates and also under controlled greenhouse conditions. These plant growth-promoting rhizobacterial isolates may be used for the enhancement of mustard crop productivity under salinity stress and could be used as biofertilizer.