Oryzalexin S biosynthesis: a cross-stitched disappearing pathway.

Unlabelled: Rice produces many diterpenoid phytoalexins and, reflecting the importance of these natural products in this important cereal crop plant, its genome contains three biosynthetic gene clusters () for such metabolism. The chromosome 4 () is largely associated with momilactone production, in part due to the presence of the initiating -copalyl diphosphate (CPP) synthase gene (). Oryzalexin S is also derived from -CPP. However, the relevant subsequently acting -stemarene synthase gene () is not located in the . Production of oryzalexin S further requires hydroxylation at carbons 2 and 19 (C2 and C19), presumably catalyzed by cytochrome P450 (CYP) monooxygenases. Here it is reported the closely related CYP99A2 and CYP99A3, whose genes are also found in the catalyze the necessary C19-hydroxylation, while the closely related CYP71Z21 and CYP71Z22, whose genes are found in the recently reported chromosome 7 (), catalyze subsequent hydroxylation at C2α. Thus, oryzalexin S biosynthesis utilizes two distinct , in a pathway cross-stitched together by . Notably, in contrast to the widely conserved , the is subspecies (ssp.) specific, being prevalent in ssp. japonica and only rarely found in the other major ssp. indica. Moreover, while the closely related -stemodene synthase was originally considered to be distinct from , it has now been reported to be a ssp. indica derived allele at the same genetic loci. Intriguingly, more detailed analysis indicates that is being replaced by (), suggesting introgression from ssp. indica to (sub)tropical japonica, with concurrent disappearance of oryzalexin S production.

Supplementary Information: The online version contains supplementary material available at 10.1007/s42994-022-00092-3.