Metabolism of aromatic compounds in healthy and rust-infected primary leaves of wheat. III. Studies on the metabolism of tryptophan

Abstract

Extracts from healthy and rust-infected wheat leaves were assayed with Lactobacillus plantarum for their tryptophan content. Healthy leaves contained about 20 μg tryptophan per gram fresh weight immediately after detachment, and 2 to 3 times this amount 22 h after detachment.Infection with rust had no effect on the tryptophan content of leaves 3 days after inoculation. Six days after inoculation, the tryptophan content of rusted leaves had increased to about 160% of control levels in resistant leaves, and to 450% of control levels in susceptible leaves.Healthy and rust-infected leaves were allowed to metabolize D,L-tryptophan(benzene ring-U-14C), D-tryptophan(methylene-14C), and L-tryptophan(methylene-14C). Acidic ether extractives were isolated from these tissues and assayed for radioactivity. In all cases, susceptible reacting tissues incorporated more activity into this fraction than either resistant reacting leaves or healthy leaves.Both D- and L-enantiomers of radioactive tryptophan gave rise to activity in indoleacetic acid (IAA), indolepyruvic acid, and indolecarboxylic acid. Malonyltryptophan was labelled only when the radioactive D-enantiomer of tryptophan was present in the feeding solution. Evidence was obtained indicating that D-tryptophan is not normally present in wheat leaves.Infection with rust increased the incorporation of activity from D- and from L-tryptophan(methylene-14C) into IAA, indolepyruvate, and indolecarboxylic acid in susceptible leaves, but not in resistant leaves. After consideration of the effect of isotope competition in the tryptophan pools, these results indicate that infection with rust resulted in increased rates of IAA synthesis, especially in susceptible leaves.