Differential inhibition of indole-3-acetic acid and tryptophan biosynthesis by indole analogues. I. Tryptophan dependent IAA biosynthesis

Maize liquid endosperm extracts contain the enzymes necessary for all of th e steps of the plant IAA biosynthetic pathway from tryptophan, and provide a means to assay the pathway in vitro. We have analyzed the reactions in th e presence of a series of indole and indole-like analogues in order to eval uate the potential of these compounds to act as inhibitors of IAA biosynthe sis. Such inhibitors will be useful to investigate the tryptophan to IAA pa thway, to determine the precursors and intermediates involved, and to selec t for mutants in this process. A number of such compounds were tested using in vitro enzyme assays for both the tryptophan dependent IAA biosynthesis pathway and for tryptophan synthase beta activity. Some compounds showed st rong inhibition of IAA biosynthesis while having only a slight effect on th e reaction rate of tryptophan synthase beta. These results: (1) show that I AA biosynthesis can be selectively inhibited relative to tryptophan biosynt hesis; (2) suggest potential ways to screen for IAA biosynthetic pathway mu tations in plants; and (3) provide additional tools for studies of IAA bios ynthesis in plants.