Biosynthesis of the halogenated auxin, 4-chloroindole-3-acetic acid

Seeds of several agriculturally important legumes are rich sources of the only halogenated plant hormone, 4-chloroindole-3-acetic acid. However, the biosynthesis of this auxin is poorly understood. Here we show that in Pisum sativum (pea) seeds, 4-chloroindole-3-acetic acid is synthesized via the novel intermediate 4- chloroindole-3-pyruvic acid, which is produced from 4-chlorotryptophan by two aminotransferases, PsTAR1 and PsTAR2. We characterize a tar2 mutant, obtained by TILLING, the seeds of which contain dramatically reduced 4-chloroindole-3- acetic acid levels as they mature. We also show that the widespread auxin, indole-3- acetic acid, is synthesized by a parallel pathway in pea.