Postdoctoral research associate Cornell University Ithaca, New York
Body of Abstract:
Rationale:
Stilbenes like resveratrol are widely recognized for their health-promoting benefits which span from anti-cancer to anti-neurodegeneration, and have thus been successfully commercialized. Recently, O-methylated stilbenes such as pterostilbene and isorhapontigenin have emerged as promising candidates for their extraordinary pharmacological properties. Nonetheless, O-methylated stilbenes are sparsely distributed in crops and plant biomass. Their exact derivatization steps and enzymology involved are also poorly understood and thus significantly hinders efforts to fortify crops with O-methylated stilbenes via synthetic bioengineering.
Objectives & methods:
A combinatorial approach involving molecular biology, genetic engineering, protein structural analyses, and phylogenetic analyses were employed to elucidate how stilbenes are regioselectively produced in two closely related Saccharinae grasses, sorghum (Sorghum bicolor) and wild sugarcane (Saccharum spontaneumi).
Results:
We have recently identified two closely related crops, sorghum and wild sugarcane, as novel sources of O-methylated stilbenes but intriguingly, their methylation patterns are highly regioselective. We elucidated that sorghum had recruited a stilbene O-methyltransferase (SbSOMT) from canonical caffeic acid O-methyltransferases (COMTs) for pterostilbene (A-ring di-O-methylated) biosynthesis from resveratrol, only after the divergence of Sorghum genus from Saccharum genus. In contrast, a canonical SsCOMT is likely involved in isorhapontigenin (B-ring O-methylated) biosynthesis from piceatannol in wild sugarcane. Protein structural analyses pinpointed two key hydrophobic residues, Ile144 and Phe377, in SbSOMT that are vital for regioselectively orientating resveratrol for stilbene A-ring O-methylation, hence allowing pterostilbene accumulation in sorghum. By contrast, their equivalent residues in SsCOMT, Asn130 and Asn323, are hydrophilic which favor an opposite substrate orientation, thus greatly favoring 3สน-O-methylation of stilbene B-ring for isorhapontigenin production in wild sugarcane.
Conclusions:
We unveiled Saccharinae grasses as promising sources of O-methylated stilbenes, and provides novel insights into the regioselectivity of SOMT activities essential for tailoring the production of specific O-methylated stilbenes via synthetic biology.