Cork is a water-impermeable, suberin-based material harboring lignin, (hemi)cellulose, and extractable small molecules (primarily triterpenoids). Extractables strongly influence the properties of suberin-based materials. Though these previous findings suggest a key role for triterpenoids in cork material quality, directly testing this idea is hindered in part because it is not known which genes control cork triterpenoid biosynthesis. Here, we used gas chromatography and mass spectrometry to determine that the majority (>85%) of non-polar extractables from cork were pentacyclic triterpenoids, primarily betulinic acid, friedelin, and hydroxy-friedelin. In other plants, triterpenoids are generated by oxidosqualene cyclases (OSCs). Accordingly, we mined Quercus suber EST libraries for OSC fragments to use in a RACE PCR-based approach and cloned three full-length OSC transcripts from cork (QsOSC1-3). Heterologous expression in Saccharomyces cerevisiae revealed that QsOSC1-3 respectively encoded enzymes with lupeol synthase, mixed α- and β-amyrin synthase, and mixed β-amyrin and friedelin synthase activities. These activities together account for the backbone structures of the major cork triterpenoids. Finally, we analyzed the sequences of QsOSC1-3 and other plant OSCs to identify residues associated with specific OSC activities, then combined this with analyses of Q. suber transcriptomic and genomic data to evaluate potential redundancies in cork triterpenoid biosynthesis.
Bibliographical noteFunding Information:
This work was supported in part by a Postdoctoral Research Fellowship in Biology (NSF PRFB IOS-1812037) to L.B., by a grant from the Cooperative Research Program for Agricultural Science & Technology Development [Project No. PJ00849903] Rural Development Administration, Republic of Korea to O.T.K., grants from the Natural Sciences and Engineering Research Council of Canada and the Canada Foundation for Innovation to R.J., and grants from Spanish Ministerio de Economía y Competitividad co-funded with ERDF (FEDER) Funds [AGL2015-67495-C2-1-R].