Content of indoleacetic acid and its metabolism in root nodules of Melilotus alba

The root nodules of Melilotus alba, a leguminous fodder herb, contain a high amount of indoleacetic acid (IAA). The tryptophan pool present in the nodule might serve as a source for the IAA production. Metabolism of IAA in the nodules was evidenced by the presence of IAA-metabolizing enzymes, IAA oxidase and peroxidase. A high amount of IAA was produced by the symbiont isolated from the nodules in culture, when supplemented with tryptophan. For IAA production, the bacteria preferred the L-isomer over the DL- or D-isomer of tryptophan. The possible role of nodular IAA production on the legume-Rhizobium symbiosis is discussed.