Arabidopsis cytochrome P450 cyp83B1 mutations activate the tryptophan biosynthetic pathway.

In plants, the tryptophan biosynthetic pathway provides a number of important secondary metabolites including the growth regulator indole-3-acetic acid (IAA) and indole glucosinolate defense compounds. Genes encoding tryptophan pathway enzymes are transcriptionally induced by a variety of stress signals, presumably to increase the production of both tryptophan and secondary metabolites during defense responses. To understand the mechanism of transcriptional induction, we isolated altered tryptophan regulation (atr) mutants in Arabidopsis thaliana with activated transcription of tryptophan genes. One atr complementation group consisted of mutations in the cytochrome P450 gene CYP83B1. Mutant plants had constitutively activated expression of the ATR1 Myb factor gene, which was identified as a positive regulator of tryptophan genes via the atr mutant screen. cyp83B1 mutants were previously characterized as having defects in IAA homeostasis due to perturbation of secondary tryptophan metabolism. Our findings indicate that the upregulation of tryptophan pathway genes might also contribute to the overaccumulation of IAA in mutant plants. Moreover, we show that cyp83B1 mutants have lesion-mimic phenotypes, suggesting that multiple stress pathways are activated by loss of CYP83B1 function.