Auxin biosynthesis inhibitors, identified by a genomics-based approach, provide insights into auxin biosynthesis

Despite its importance in plant growth and development, the auxin biosynthetic pathway has remained elusive. In this study, we analyzed hormone series transcriptome data from AtGenExpress in Arabidopsis and found that aminoethoxyvinylglycine (AVG) had the strongest anti-auxin activity. We also identified other effective compounds such as L-amino-oxyphenylpropionic acid (AOPP) through additional screening. These inhibitors shared characteristics in that they inhibited pyridoxal enzymes and/or aminotransferases. They reduced endogenous IAA levels in both monocots and dicots. L-AOPP inhibited root development of Arabidopsis in main root elongation, gravitropism, root skewing and root hair formation. This inhibition was generally recovered after exogenous IAA treatment, and the recovery was almost completely to the level of non-inhibited seedlings. The compounds inhibited conversion from tryptophan to indole-3-pyruvic acid in enzyme extracts from Arabidopsis and wheat. Our data collectively suggest that the inhibitors directly blocked auxin biosynthesis, and that the major target site was tryptophan aminotransferase. This enzyme probably makes up one of the major biosynthesis pathways conserved among higher plants. Each inhibitor, however, demonstrated a different action spectrum in shoot and root of rice and tomato, indicating diversity in biosynthesis pathways between organs and species. Our results provide novel insights into auxin biosynthesis and action, and uncover structural characteristics of auxin biosynthesis inhibitors.