Abstract
Western redcedar (WRC; Thuja plicata) produces high amounts of oxygenated thujone monoterpenoids associated with resistance against herbivore feeding, particularly ungulate browsing. Thujones and other monoterpenoids accumulate in glandular structures in the foliage of WRC. Thujones are produced from (+)-sabinene by sabinol and sabinone. Using metabolite analysis, enzyme assays with WRC tissue extracts, cloning, and functional characterization of cytochrome P450 monooxygenases, we established that trans-sabin-3-ol but not cis-sabin-3-ol is the intermediate in thujone biosynthesis in WRC. Based on transcriptome analysis, full-length complementary DNA cloning, and characterization of expressed P450 proteins, we identified CYP750B1 and CYP76AA25 as the enzymes that catalyze the hydroxylation of (+)-sabinene to trans-sabin-3-ol. Gene-specific transcript analysis in contrasting WRC genotypes producing high and low amounts of monoterpenoids, including a glandless low-terpenoid clone, as well as assays for substrate specificity supported a biological role of CYP750B1 in α- and β-thujone biosynthesis. This P450 belongs to the apparently gymnosperm-specific CYP750 family and is, to our knowledge, the first member of this family to be functionally characterized. In contrast, CYP76AA25 has a broader substrate spectrum, also converting the sesquiterpene farnesene and the herbicide isoproturon, and its transcript profiles are not well correlated with thujone accumulation.