Molecular Biology of Triterpenoid Biosynthesis in Glycyrrhiza Plants

Licorice (roots and stolons of Glycyrrhiza plants, Leguminosae) is a well-known natural sweetener, and its sweet constituent is an oleanane-type triterpene saponin, glycyrrhizin. Cultured cells of G. glabra do not produce glycyrrhizin, but produce two structurally different triterpenoid constituents, betulinic acid and soyasaponins. Both glycyrrhizin and soyasaponins are oleanane-type triterpenes, and glycyrrhizin is localized exclusively in the woody parts of thickened roots and stolons, whereas soyasaponins are localized mainly in seeds and rootlets of G. glabra. On the other hand, betulinic acid, a lupane-type triterpene, is localized in the cork layer of thickened roots and stolons. To elucidate the regulation of the triterpenoid biosyntheses in G. glabra, cDNAs of three oxidosqualene cylaces, cycloartenol synthase, -amyrin synthase and lupeol synthase, which are situated at the branching point of triterpenoid biosynthesis, were cloned and characterized. mRNA levels of the three oxidosuqualene cyclase were differentally regulated in cultured cells and intact plants of G. glabra. Exogenously applied methyl jasmonate (MeJA) stimulated soyasaponin biosynthesis in cultured cells, and mRNA levels of squalene synthase and -amyrin synthase were up-regulated by MeJA. In contrast, mRNA levels of squalene synthase and -amyrin synthase were down-regulated by yeast extract, which induced flavonoid biosynthesis in leguminous plants. These results suggested that the transcription of other enzymes involved in the late steps of saponin biosynthesis might be up-regulated by MeJA in the cultured licorice cells. _____________________________________________________________________________________________________________