University of Missouri

BELTSVILLE, MARYLAND

USDA Plant Hormone Laboratory

Tryptophan not required for IAA biosynthesis?

--Allen D. Wright, M. G. Neuffer, Michael B. Sampson, Jerry D. Cohen, Lech Michalzcuk and Janet P. Slovin

Tryptophan is generally assumed to be the precursor to the plant auxin indole-3-acetic acid (IAA), although other pathways have been postulated. The indole-accumulating mutant, orange pericarp, lacks tryptophan synthase activity and thus should be a good system for testing the hypothesis that IAA is derived from tryptophan. Embryos were excised from developing kernels, grown on MS media for 10 days under aseptic conditions and analyzed for IAA using a procedure which involved column cleanup, HPLC, derivitization, and GC-MS using 13C6-IAA as an internal standard (Cohen, Baldi and Slovin, Plant Physiol. 80:14-19). Surprisingly, IAA levels in the mutant were about 50 times greater than those of the non-mutant, implying that tryptophan is not a necessary precursor to IAA in maize. Addition of tryptophan to the media did not alter these results. In another study, mutants grown in 30% D·2O were found (by mass spectrometry) to have incorporated deuterium into stable ring positions of IAA, indicating de novo synthesis had occurred. The evidence indicates that IAA can be made without passing through tryptophan.

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