Arabidopsis mutants in the C–S lyase of glucosinolate biosynthesis establish a critical role for indole‐3‐acetaldoxime in auxin homeostasis

Abstract

Summary: We report characterization of SUPERROOT1 (SUR1) as the C–S lyase in glucosinolate biosynthesis. This is evidenced by selective metabolite profiling of sur1, which is completely devoid of aliphatic and indole glucosinolates. Furthermore, following in vivo feeding with radiolabeled p‐hydroxyphenylacetaldoxime to the sur1 mutant, the corresponding C–S lyase substrate accumulated. C–S lyase activity of recombinant SUR1 heterologously expressed in Escherichia coli was demonstrated using the C–S lyase substrate djenkolic acid. The abolishment of glucosinolates in sur1 indicates that the SUR1 function is not redundant and thus SUR1 constitutes a single gene family. This suggests that the ‘high‐auxin’ phenotype of sur1 is caused by accumulation of endogenous C–S lyase substrates as well as aldoximes, including indole‐3‐acetaldoxime (IAOx) that is channeled into the main auxin indole‐3‐acetic acid (IAA). Thereby, the cause of the ‘high‐auxin’ phenotype of sur1 mutant resembles that of two other ‘high‐auxin’ mutants, superroot2 (sur2) and yucca1. Our findings provide important insight to the critical role IAOx plays in auxin homeostasis as a key branching point between primary and secondary metabolism, and define a framework for further dissection of auxin biosynthesis.