Yucasin is a potent inhibitor of YUCCA, a key enzyme in auxin biosynthesis

Abstract

Summary: Indole‐3–acetic acid (IAA), an auxin plant hormone, is biosynthesized from tryptophan. The indole‐3–pyruvic acid (IP y A) pathway, involving the tryptophan aminotransferase TAA 1 and YUCCA (YUC) enzymes, was recently found to be a major IAA biosynthetic pathway in Arabidopsis. TAA 1 catalyzes the conversion of tryptophan to IP y A, and YUC produces IAA from IP y A. Using a chemical biology approach with maize coleoptiles, we identified 5–(4–chlorophenyl)‐4H‐1,2,4–triazole‐3–thiol (yucasin) as a potent inhibitor of IAA biosynthesis in YUC‐expressing coleoptile tips. Enzymatic analysis of recombinant At YUC 1‐His suggested that yucasin strongly inhibited YUC 1‐His activity against the substrate IP yA in a competitive manner. Phenotypic analysis of Arabidopsis YUC 1 over‐expression lines (35S::YUC 1) demonstrated that yucasin acts in IAA biosynthesis catalyzed by YUC. In addition, 35S::YUC 1 seedlings showed resistance to yucasin in terms of root growth. A loss‐of‐function mutant of TAA 1, sav3–2, was hypersensitive to yucasin in terms of root growth and hypocotyl elongation of etiolated seedlings. Yucasin combined with the TAA 1 inhibitor l–kynurenine acted additively in Arabidopsis seedlings, producing a phenotype similar to yucasin‐treated sav3–2 seedlings, indicating the importance of IAA biosynthesis via the IP yA pathway in root growth and leaf vascular development. The present study showed that yucasin is a potent inhibitor of YUC enzymes that offers an effective tool for analyzing the contribution of IAA biosynthesis via the IP yA pathway to plant development and physiological processes.