Comparing the symbiotic efficiency of auxin and ACC-deaminase producing rhizobia and rhizobacteria to improve soybean growth under axenic conditions

Potential of auxin and ACC-deaminase producing rhizobia and PGPR-strains was examined to improve soybean growth individually and in combination with L-tryptophan (L-TRP) in two different experiments conducted in a growth chamber. A consortium of selected PGPR and rhizobial strains was prepared by following the compatibility test. These bacteria were also evaluated for improving soybean growth along with different levels of L-TRP. Bacteria were isolated from the rhizosphere and nodules of soybean, and characterized in vitro for their production of ACC-deaminase and auxin. We observed that auxin production for rhizobia and rhizobacteria ranged from 14 to 39 and 2 to 48 µg IAA equivalents mL-1, respectively. Similarly, the ACC-deaminase production potential of rhizobia and rhizobacteria ranged between 0.024 to 3.252 and 0.178 to 3.188 μM α-ketobutyrate μg-1 protein h-1, respectively. In addition, among the different L-TRP levels, 10-3 M was found to be the most significant level. The soybean growth attributes such as shoot length (46 and 30%), root length (42 and 39%), fresh biomass (33 and 34%) and dry biomass (44 and 27%) were increase to the maximum by the inoculation of ‘IRS-13’ and ‘PS-2’ in the presence of 10-3 M L-TRP compared to the uninoculated control. Similarly, in the context of consortium application, the inoculation with bacterial consortium “IRS13-PS1” along with 10-4 M TRP led to maximum increment in shoot length (104%), root length (74%), shoot fresh and dry weights (179 and 138%), shoot and root dry weights (98 and 260%) and fresh weight as well as dry plant biomass (87.5 and 117%) compared to the control. We concluded that PGPR-rhizobial consortium in combination with L-TRP @ 10-4 M can effectively improve soybean growth under axenic conditions.