Co-inoculation of auxin producing PGPR and rhizobia enhanced growth of Vigna mungo (L.) under cadmium stress

The main objective of this study was to evaluate the effect of cadmium resistant bacteria to enhance the growth of black gram (Vigna mungo) under cadmium (CdCl2) stress. For this purpose, plant growth promoting rhizobacteria (PGPR) and strains of rhizobia were isolated from the rhizosphere of different plants. 16S rRNA gene sequencing of bacteria confirmed the presence of the strains of Bacillus cereus, B. thuringiensis, Pseudomonas fluorescensand Rhizobium. L-tryptophan dependent auxin production was determined colorimetrically by growing strains in Nutrient broth. B. Cereus HG, B. thuringiensis JAF and Rhizobium sp. IBAshowed highest levels of auxin production that ranges from 140 to 146 μg ml-I. For pot trials, soil was amended with 50, 100 and 200 mg kg-1. In single bacterial inoculations, significant shoot length response was up to 65% and 39% with P. fluorescensWN-1 and B. cereusCF at 200 mg kg-I, over respective CdCl2treated control. Similarly, for fresh biomass, 2.2 and 1.3 fold increases were shown by B. cereus CF at 100 and 200 mg kg-I CdCl2 stress, respectively. For mixed culture of PGPR, combination MC-3 (CF-WN-1, A-2) significantly promoted shoot length (1 fold) at 50 mg kg-I CdCl2. In case of mixed culture of PGPR and rhizobia, up to 1 fold increases were observed for shoot length, root growth and fresh biomass with MCR-1 (IBA, HG, BS-1) at 100 mg kg-ICdCl2. Finally, it can be concluded that strains isolated from the rhizosphere of different pantsshowed promising results for plant growth promotion in cadmium amended soils.