INDOLEPYRUVATE PATHWAY FOR INDOLE-3-ACETIC-ACID BIOSYNTHESIS IN BRADYRHIZOBIUM-ELKANII

Indole-3-acetaldehyde (IAAld) was detected in the culture supernatant of Bradyrhizobium elkanii. Deuterium-labelled L-tryptophan (Trp) was i ncorporated into IAAld and indole-3-acetic acid (IAA), suggesting that B. elkanii produces IAA via IAAld from Trp. In B. elkanii cell suspen sion, indole-3-pyruvic acid (IPyA) was converted to IAAld, and exogeno usly added IAAld was rapidly converted to IAA. Furthermore, the activi ty of indolepyruvate decarboxylase (IPDC), which catalyzes the decarbo xylation of IPyA to produce IAAld and is a key enzyme for IPyA pathway , was detected in B. elkanii cell-free extract. The IPDC activity depe nded on Mg2+ and thiamine pyrophosphate, cofactors of decarboxylation. This mounting evidence strongly suggests that IAA synthesis occurs vi a IPyA pathway (Trp --> IPyA --> IAAld --> TAA) in B. elkanii.