Indole-3-acetic acid biosynthesis in Lemna gibba studied using stable isotope labeled anthranilate and tryptophan

IAA biosynthesis in many plants, including Lemna gibba, has been shown to involve at least two different pathways, one from tryptophan and a tryptophan-independent route. To study the kinetics of IAA biosynthesis in Lemna, we simultaneously measured the incorporation of label from [N-15]-anthranilate and [H-2(5)]-tryptophan into IAA by Lemna plants in short term feeding studies. The data show that label from anthranilate rapidly goes into IAA and tryptophan. Labeling of the IAA pool by [N-15]-anthranilate slightly precedes labeling of the tryptophan pool, confirming that more than one route to IAA exists in these plants. Longer term feeding studies (5-25 h) suggest that exogenous tryptophan is used preferentially to label IAA as compared to tryptophan made by the plant. This is indicated by the fact that the IAA pool was more enriched than the tryptophan pool in [H-2(5)]-label, but less enriched than the tryptophan pool in [N-15] (which comes about by de novo synthesis of tryptophan from [N-15]-anthranilate by the plant).