Auxin Biosynthesis Inhibitors, Identified by a Genomics-Based Approach, Provide Insights into Auxin Biosynthesis

Abstract

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.

Comparison of indole derivatives as potential intermediates of auxin biosynthesis in Arabidopsis

“…They inhibit conversion from Trp to IPyA in both Arabidopsis and wheat enzyme extracts ( Figure 1A), and the endogenous IAA level is reduced in the presence of these inhibitors (Soeno et al 2010). In previous work, we induced auxin deficiency in Arabidopsis seedlings using these inhibitors, and analyzed several intermediates in a root growth assay as well as in an auxin-marker gene expression assay.…”

“…Recently, aminoethoxyvinylglycine (AVG) and laminooxyphenylpropionic acid (AOPP) were identified as inhibitors of IAA biosynthesis (Soeno et al 2010). They inhibit conversion from Trp to IPyA in both Arabidopsis and wheat enzyme extracts ( Figure 1A), and the endogenous IAA level is reduced in the presence of these inhibitors (Soeno et al 2010).…”

“…In the auxindeficient root growth assay, we used AOPP, in which main root elongation is inhibited and roots skew to the left when grown on vertical plate, while seedlings could recover from the growth inhibition in the presence of IAA or an IAA intermediate. In the previous report, only seven conjectured intermediates (indole,Trp,TAM,IAM,, IPyA, and indole-3-acetaldehyde [IAAld]) were analyzed in a root growth assay of Arabidopsis among the IAA intermediates shown in Figure 1A (Soeno et al 2010). Among the analyzed compounds, IPyA was the most effective for root growth recovery.…”

“…The other chemicals analyzed were purchased from commercial sources, and IAAld was prepared as described in our previous report (Soeno et al 2010). Then, AOPP was used to induce auxin deficiency in Arabidopsis seedlings to analyze the 11 potential intermediates shown in Figure 1, both in short-term (1 h) and long-term (4 days) auxin-related responses.…”

“…Wild-type Arabidopsis (ecotype Colunbia-0) seeds were sterilized and sown in halfstrength Murashige-Skoog (1/2MS) liquid medium containing 1% sucrose, cultured for 7 days. The cultured seedlings were pretreated with 60 µM AOPP for 2 h, and then incubated with IAA or its putative 11 intermediate (10 µM) for 1 h. RNA isolation and qRT-PCR of Aux/IAA1 and Aux/IAA19 genes was conducted as described in our previous report (Soeno et al 2010). …”