Investigation of cytochrome P450s in the biosynthesis of rice phytoalexins

Rice (Oryza sativa) has evolved a set of metabolites known as phytoalexins in response to biotic and abiotic stress factors. The rice phytoalexins almost invariably belong to the labdane-related diterpenoid (LRD) superfamily. The phytoalexins identified in rice thus far are momilactones A-E, phytocassanes A-F, oryzalides A and B, oryzalexins A-F, oryzalexin S, ent-10-oxodepressin and sakuranetin, all of which but the last two are LRDs. An important group of enzymes in diterpenoid biosynthesis are cytochromes P450 (CYPs) which catalyze various tailoring reactions on LRD olefins to produce bioactive diterpenoids. Intriguingly, two biosynthetic gene clusters (BGCs), composed of CYPs and upstream cyclases dedicated to LRD production, have been discovered in the rice genome. The chromosome two BGC (c2BGC) contains a set of closely related CYP76M sub-family members that function in LRD biosynthesis. The CYP76Ms from the c2BGC and two other closely-related sub-family members were functionally characterized. Recent data have also suggested an interdependence of these two rice BGCs in LRD biosynthesis. Here this interdependence is further elaborated upon, along with presentation of potential phytocassane and oryzalide biosynthetic networks.