Characterization of an inducible P450 hydroxylase involved in the rice diterpene phytoalexin biosynthetic pathway.

Ent-isopimara-8(14),15-dien-3 beta-ol (1) was isolated from uv-irradiated rice (Oryza sativa L.) leaves. Since 1 was converted to the rice diterpene phytoalexins and oryzalexins D (ent-isopimara-8(14),15-diene-3 beta,7 alpha-diol) and E (ent-isopimara-8(14),15-diene-3 beta,9 alpha-diol) in uv-irradiated rice leaf microsome fraction in the presence of oxygen and NADPH, it was concluded that 1 was the biosynthetic precursor of these oryzalexins. This enzyme reaction was inhibited by cytochrome P450 inhibitors such as piperonyl butoxide, SKF-525A, paclobutorazole, metyrapone, cytochrome c, and carbon monoxide. The lack of inhibitory activity of 1 for spore germination of Pyricularia oryzae indicates that the final cytochrome P450-dependent hydroxylation steps in converting 1 to oryzalexins D and E are essential for the production of rice phytoalexins with antifungal activities.