Phenotypic characterization of rhizobacteria associated with mungbean rhizosphere

Plant growth promoting rhizobacteria have been identified in influencing the growth and yield of many plants by direct or indirect mechanisms. In search of efficient PGPR strains with multiple plant growth promoting (PGP) activities, a total of 48 isolates of rhizobacteria were isolated from 35 different samples of mungbean rhizosphere. Out of 48, 34 isolates were characterized and tentatively identified as Bacillus spp. (14), Pseudomonas spp. (11) and Azotobacter spp. (9) on the basis of their morphological and biochemical activities. Thirty five percent of these rhizobacterial isolates were able to solubilize phosphate (P) and showed solubilisation index (SI) from 1.00 to 3.83 being highest with rhizobacterial isolate B2 (3.83). These rhizobacterial isolates were selected and screened in vitro for their PGP traits (Indole acetic acid (IAA) production), biocontrol (NH₃, HCN, chitinase and protease production) and stress tolerant activity (1-amino cyclopropane-1-carboxylic acid (ACC) deaminase). Rhizobacterial isolates (B2, P10 and A3) showed maximum IAA production. Ammonia production was detected in two isolates (B2 and A3) and 5 isolates (B2, B6, P4, P10 and A3) found positive for HCN production. None of the isolates was positive for chitinase production whereas 52.9% of selected isolates were able to produce protease on skimmed milk agar. Seventy percent of the isolates indicated the growth on plates containing Dworkin and Foster (DF) minimal medium with ACC as a sole nitrogen source indicated the presence of ACC deaminase activity. Three rhizobacterial isolates (B2, P10 and A3) were found most promising for multiple activities (PGP traits, biocontrol and stress tolerant activities) and have potential to be used in future as PGP inoculants to improve mungbean crop.