Abstract
Antimicrobial diterpenoid compounds such as momilactones and phytocassanes are a major phytoalexin produced in rice upon pathogen attack. Recently, we reported that the momilactone biosynthetic genes are clustered in rice chromosome 4 and are coexpressed after the elicitation. In this report, we demonstrate that a novel gene cluster for phytocassanes biosynthesis is organized in rice chromosome 2, which consists of two diterpene cyclases genes for phytocassanes (OsCPS2 and OsKSL4) and six uncharacterized cytochrome P450 monooxygenases (P450) genes. Using genetic approaches, we first investigated functions of the CYP71Z7 gene, one of the P450 genes in the middle of the cluster. The knockdown of CYP71Z7 specifically suppressed the elicitor-inducible production of phytocassanes A, B, and D (C-2 oxygenated phytocassanes), whereas phytocassanes C, E, and the metabolic intermediate 1-deoxyphytocassane C (C-2 non-oxygenated phytocassanes) were detected to be more than 10 times accumulated to wild-type plant. These results suggest that the CYP71Z7 probably catalyzes C-2 hydroxylation of possible metabolic substrates phytocassane C and/or 1-deoxyphytocassane C to yield phytocassanes A, B, and D, and that phytocassanes biosynthetic genes are clustered in chromosome 2 like the momilacone biosynthetic gene cluster. We are now beginning to understand functions of remaining P450 genes (CYP76M5, M6, M7, M8) in the phytocassans biosynthetic gene cluster by generating their RNAi lines.
