Indolepyruvate Pathway for Indole-3-Acetic Acid Biosynthesis in Bradyrhizobium elkanii

Abstract

Indole-3-acetaldehyde (IAAId) was detected in the culture supernatant of Bradyrhizobium elkanii. Deuteriumlabelled L-tryptophan (Trp) was incorporated into IAAId and indole-3-acetic acid (IAA), suggesting that B. elkanii produces IAA via IAAId from Trp. In B. elkanii cell suspension, indole-3-pyruvic acid (IPyA) was converted to IAAId, and exogenously added IAAId was rapidly converted to IAA. Furthermore, the activity of indolepyruvat e decarboxylase (IPDC), which catalyzes the decarboxylation of IPyA to produce IAAId and is a key enzyme for IPyA pathway, was detected in B. elkanii cell-free extract. The IPDC activity depended on Mg 2+ and thiamine pyrophosphate, cofactors of decarboxylati on. This mounting evidence strongly suggests that IAA synthesis occurs via IPyA pathway (Trp -»• IPyA -*• IAAId -*• IAA) in B. elkanii.