Abstract
Sesame (Sesamum indicum L.) plant contains large amounts of acteoside, a typical phenylethanoid glycoside (PhG) that exhibits various pharmacological activities. Although there is increasing interest in the biosynthesis of PhGs for improving their production, the pathway remains to be clarified. In this study, we established sesame-cultured cells and performed transcriptome analysis of methyl jasmonate (MeJA)-treated cultured cells to identify enzyme genes responsible for glucosylation and acylation in acteoside biosynthesis. Among the genes annotated as UDP-sugar dependent glycosyltransferase (UGT) and acyltransferase (AT), 34 genes and one gene, respectively, were up-regulated by MeJA in accordance with acteoside accumulation. Based on phylogenetic analysis, five UGT genes (SiUGT1-5) and one AT gene (SiAT1) were selected as candidate genes involved in acteoside biosynthesis. Additionally, two AT genes (SiAT2-3) were selected based on sequence identity. Enzyme assays using recombinant SiUGT proteins revealed that SiUGT1, named UGT85AF10, had the highest glucosyltransferase activity among the five candidates against hydroxytyrosol to produce hydroxytyrosol 1-O-glucoside. SiUGT1 also exhibited glucosyltransferase activity against tyrosol to produce salidroside (tyrosol 1-O-glucoside). SiUGT2, named UGT85AF11, had similar activity against hydroxytyrosol and tyrosol. Enzyme assay using the recombinant SiATs indicated that SiAT1 and SiAT2 had activity transferring the caffeoyl group to hydroxytyrosol 1-O-glucoside and salidroside (tyrosol 1-O-glucoside) but not to decaffeoylacteoside. The caffeoyl group was attached mainly at the 4-positon of glucose of hydroxytyrosol 1-O-glucoside, followed by attachment at the 6-position and the 3-position of glucose. Based on our results, we propose an acteoside biosynthetic pathway induced by MeJA treatment in sesame.
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