| Auxin plays an essential role in plant development. However, only a few genes related to auxin synthesis have been isolated and characterized in plants. Here, we show that the TRANSPORT INHIBITOR RESPONSE2 (TIR2) gene, required for auxin synthesis, has an important role in many growth processes. The TIR2 gene encodes a protein structurally related to aminotransferases. A tryptophan (Trp) aminotransferase has been proposed to convert Trp to indole-3-pyruvic acid (IPA), a precursor of auxin. The tir2 mutant is hypersensitive to 5-methyl-tryptophan (5-MT), an inhibitor of Trp synthesis and aminooxyacetate (AOA), an aminotransferase inhibitor. The TIR2 protein is localized at sites of auxin production. Treatment with IPA and indole-3-acetic acid (IAA) rescues the tir2 short-hypocotyl phenotype. These data suggest that the TIR2 protein functions as a Trp aminotransferase. I show that auxin negatively regulates expression of the TIR2 gene via the auxin signaling pathway suggesting that plants control auxin concentration by changing expression levels of auxin synthesis genes. Furthermore, I show that high temperature positively regulates expression of the TIR2 gene suggesting that high temperature controls auxin concentration by regulating auxin synthesis genes. This response does not appear to require the auxin-signaling pathway. Finally, I show evidence that auxin synthesized by TIR2 is essential for root meristem development. |
