Differential expression of three oxidosqualene cyclase mRNAs in Glycyrrhiza glabra

The cultured cells and intact plants of Glycyrrhiza glabra (Fabaceae) produce betulinic acid and oleananetype triterpene saponins (soyasaponins and glycyrrhizin). To elucidate the regulation of triterpenoid biosynthesis in G. glabra, the cDNA of lupeol synthase, an oxidosqualene cyclase (OSC) responsible for betulinic acid biosynthesis, was cloned, and expression patterns of lupeol synthase and two additional OSCs, beta-amyrin synthase and cycloartenol synthase, were compared. The mRNA expression levels of lupeol synthase and beta-amyrin synthase were consistent with the accumulation of betulinic acid and oleanane-type triterpene saponins, respectively. The transcript of lupeol synthase was highly expressed in the cultured cells and root nodules. The transcript of beta-amyrin synthase, an OSC responsible for oleanane-type triterpene biosynthesis, was highly expressed in the cultured cells, root nodules and germinating seeds, where soyasaponin accumulates, and in the thickened roots where glycyrrhizin accumulates. In the cultured cells, the addition of methyl jasmonate up-regulated beta-amyrin synthase mRNA and soyasaponin biosynthesis, but down-regulated lupeol synthase mRNA. Furthermore, the addition of gibberellin A(3) down-regulated beta-amyrin synthase mRNA but not lupeol synthase mRNA in the cultured cells. The mRNA levels of cycloartenol synthase, an additional OSC responsible for sterol biosynthesis, in the intact plant and cultured cells were relatively constant in these experiments.