Isolation and characterization of plant associated rhizobacteria for plant growth promoting traits

Abstract

The use of plant growth promoting rhizobacteria (PGPR) in sustainable and eco-friendly management of plant growth promoting is gaining importance over the previous decades around the world. In the current research work, the isolation of the rhizobacteria were done using nutrient agar media fol- lowing standard protocol for isolation of bacteria. We have isolated and characterized 32 rhizobacterial isolates from five different plant species and subjected to N₂-fixation, phosphorus solubilization and indole-3-acetic acid assay to identify potential PGPR. All the 32 rhizobacterial isolates showed at least one of the three major functionalities; viz. phosphorus solubilization, in- dole acetic acid production and N₂ fixation; considered for selection of PGPR when tested in vitro. Among the 32 isolates, 15 produced clear halo zones surrounding their colonies indicating phosphate solubilization with variable intensities. Among the fifteen, six bacterial isolates having high phosphate solubilization index (PSI) proved to be efficient phosphorus solubilizer in liquid medium. The bacterial isolate MQ2 solubilized maximum (0.697 μg mL⁻¹) phosphorus in liquid medium, followed by MQ3 and MQ1. Ten of the bacterial isolates were able to synthesize indole-3-acetic acid (IAA) in L-tryptophan supplemented media at varying capacity. The isolate OSn8 pro- duced highest amount (6.204 μg mL⁻¹) of IAA followed by MQ5 and OSbr6, while the lowest amount of IAA (1.268 μg mL⁻¹) was produced by MQ1. All the isolated bacteria were tested positive for putative N2-fixing ability with variation among the isolates as indicated by their growth in N₂-free medium. Considering the all three tested functionalities, the isolate MQ1 proved to be the best candidate as potential biofertilizer development. A consortium of isolated rhizobacteria comprising the best isolates from each category could be constructed to provide the best benefit to crops for maintaining yield and quality while decreasing agrochemical inputs.