Biochemical analyses of a species-specific auxin biosynthetic pathway in Arabidopsis

Abstract

A main plant auxin, indole-3-acetic acid (IAA), regulates the most aspects of plant development. We studied the IAA biosynthetic pathway via indole-3-acetaldoxyme (IAOx) in plants by analyzing IAA intermediates with LC-ESI-MS/MS. We found that indole-3-aceamide (IAM) and indole-3-acetonitrile (IAN) are likely intermediates in the IAOx-dependent IAA biosynthesis in Arabidopsis. The level of IAM was dramatically reduced in the cyp79b2cyp79b3double knockout mutants. IAOx and IAN were not detected in cyp79b2cyp79b3mutants. We demonstrated that application of IAOx restores the levels of IAM and IAN in cyp79b2cyp79b3mutants. Furthermore, IAM and IAN were efficiently labeled with ¹³C when ¹³C₆-IAOx was fed tocyp79b2cyp79b3mutants. IAOx was not detected from rice, maize and tobacco. These results indicated that the IAA biosynthesis via IAOx is not a common but a species-specific pathway in plants.