Plant growth promoting rhizobacteria Bacillus subtilis RR4 isolated from rice rhizosphere induces malic acid biosynthesis in rice roots

Malic acid, one of the major organic acid exudates from roots, plays a significant role in the chemotaxis of beneficial bacteria to the plantâ s rhizosphere. In this study, the effect of a plant growth promoting rhizobacterium, Bacillus subtilis RR4, on the synthesis and exudation of malic acid (MA) from roots is demonstrated in rice. To test the chemotactic ability of RR4 towards MA, capillary chemotaxis assay was performed which revealed a positive response (relative chemotactic ratio of 6.15 with 10 ÂľM MA) and with increasing concentrations of MA, an elevated chemotactic response was observed. qPCR performed to analyze the influence of RR4 on the MA biosynthetic gene, malate synthase (OsMS) and the transporter, aluminium-activated malate transporter (OsALMT) demonstrated significant differential expression with 1.8 and -0.58 fold changes, respectively, in RR4 treated roots. The gene expression pattern of OsMS confirmed with that of HPLC data which showed elevated MA levels in roots (1.52 fold) while the levels of MA in root exudates were not altered significantly although expression of OsALMT was reduced. Our results demonstrate that B. subtilis RR4 is chemotactic to malic acid and can induce biosynthesis of MA in rice roots.

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