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The mature root nodules of Phaseolus mungo (L.), a leguminous pulse, contain higher amount of indole acetic acid (IAA) than non-nodulated roots. The tryptophan pool present in the mature nodule and young roots might serve as a precursor for the IAA production. Presence of IAA metabolising enzymes - IAA oxidase and peroxidase - indicate the metabolism of IAA in the nodules and roots. In culture, the symbiont, isolated from the nodules, produced a high amount of IAA, when tryptophan was supplied in the medium as a precursor. The symbiont preferred l-isomer over the dl- or d-isomer of tryptophan for IAA production. The important physiological implication of the IAA production in the legume-Rhizobium symbiosis is discussed.

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The root nodules of Phaseolus mungo (a herbaceous leguminous pulse) contained a high amount of 3-indolylacetic acid (IAA). A tryptophan pool present in the nodule might play the role of precursor for IAA production. From the root nodule a Rhizobium sp. was isolated. The symbiont produced a large amount of IAA (142 microg/mL) from L-tryptophan-supplemented basal medium. The production of IAA by the symbiont was much increased over the control when a L-tryptophan (2 mg/mL) supplemented C-free mineral medium was enriched with mannitol (1 %), L-asparagine (0.3 %) and thiamine hydrochloride (1 microg/mL). The possible role of the rhizobial production of IAA on the rhizobia-legume symbiosis is discussed.

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