Stimulation of indoleacetic acid production in a <i>Rhizobium</i> isolate of <i>Vigna mungo</i> by root nodule phenolic acids

The influence of endogenous root nodules phenolic acids on indoleacetic acid (IAA) production by its symbiont ( Rhizobium ) was examined. The root nodules contain higher amount of IAA and phenolic acids than non-nodulated roots. Presence of IAA metabolizing enzymes, IAA oxidase, peroxidase, and polyphenol oxidase indicate the metabolism of IAA in the nodules and roots. Three most abundant endogenous root nodule phenolic acids (protocatechuic acid, 4-hydroxybenzaldehyde and p -coumaric acid) have been identified and their effects on IAA production by the symbiont have been studied in l -tryptophan supplemented yeast extract basal medium. Protocatechuic acid (1.5 μg ml −1 ) showed maximum stimulation (2.15-fold over control) of IAA production in rhizobial culture. These results indicate that the phenolic acids present in the nodule might serve as a stimulator for IAA production by the symbiont ( Rhizobium ).