Root exudation of organic acids as affected by plant growth-promoting rhizobacteria Bacillus subtilis RR4 in rice

Bacillus subtilispossesses plant growth-promoting traits and enhances the level of essential metabolites in plants for improved growth. Included among the metabolites are the primary metabolite malic acid (MA) and the secondary metabolite salicylic acid (SA). The impact of plant growth-promoting rhizobacteria (PGPR) on root exudation/secretion of metabolites (like MA and SA) remains unexplored, however. In this study, we aim to analyze the impact ofB. subtilisRR4 on the root exudation pattern of MA and SA in rice (Oryza sativaL.). For this, rice plants were treated with the PGPRB. subtilisRR4 in hydroponics and the root exudates were collected at different periods to analyze the changes in the levels of MA and SA through high-performance liquid chromatography (HPLC). Analysis of HPLC chromatograms showed that progressive colonization by RR4 enhanced root exudation of MA 3-fold and of SA 7-fold at 96 h post-inoculation (hpi), as compared to the control. This study, thus, highlights the potential ofB. subtilisRR4 of inducing/enhancing root exudation of MA and SA, which are stress alleviators, plant growth-promoting agents, and a source of carbon for microbes. With this knowledge, bioformulations, comprising the PGPRs and specific metabolites as PGPR stimulants, can be developed.