Seasonal Variation in the Metabolism of Indole Acetic Acid in Root Nodules of Phaseolus mungo L.

The mature root nodules of Phaseolus mungo L., a leguminous plant, contained higher amount of indole acetic acid (IAA) and its precursor tryptophan than non-nodulated roots throughout the year, which was higher in July. The higher level of IAA in the nodules than the roots was due to more synthesis from the precursor tryptophan and simultaneously less destruction by IAA oxidase and peroxidase, the levels of the enzymes were lowest in July. The IAA oxidizing enzymes were regulated by phenol which in turn was regulated by peroxidase and polyphenol oxidase. Non-nodulated roots contained lower amounts of IAA and tryptophan and had higher IAA oxidase and peroxidase activities than the nodulated roots. The correlation of IAA, tryptophan and phenol content with levels of their metabolizing enzymes is discussed.