Arabidopsis PAD3, a gene required for camalexin biosynthesis, encodes a putative cytochrome P450 monooxygenase

Phytoalexins are low molecular weight antimicrobial compounds that are synt hesized in response to pathogen attack. The phytoalexin camalexin, an indol e derivative, is produced by Arabidopsis in response to infection with the bacterial pathogen Pseudomonas syringae. The phytoalexin deficient 3 (pad3) mutation, which causes a defect in camalexin production, has no effect on resistance to P. syringae but compromises resistance to the fungal pathogen Alternaria brassicicola. We have now isolated PAD3 by map-based cloning. T he predicted PAD3 protein appears to be a cytochrome P450 monooxygenase, si milar to those from maize that catalyze synthesis of the indole-derived sec ondary metabolite 2,4-dihydroxy-1,4-benzoxazin-3-one. The expression of PAD 3 is tightly correlated with camalexin synthesis and is regulated by PAD4 a nd PAD1. On the basis of these findings, we conclude that PAD3 almost certa inly encodes an enzyme required for camalexin biosynthesis. Moreover, these results strongly support the idea that camalexin does not play a major rol e in plant resistance to P. syringae infection, although it is involved in resistance to a fungal pathogen.