Manipulation of saponin biosynthesis by RNA interference-mediated silencing of β-amyrin synthase gene expression in soybean

Soybean seeds contain substantial amount of diverse triterpenoid saponins that influence the seed quality, although little is known about the physiologic functions of saponins in plants. We now describe the modification of saponin biosynthesis by RNA interference (RNAi)-mediated gene silencing targeted to beta-amyrin synthase, a key enzyme in the synthesis of a common aglycon of soybean saponins. We identified two putative beta-amyrin synthase genes in soybean that manifested distinct expression patterns with regard to developmental stage and tissue specificity. Given that one of these genes, GmBAS1, was expressed at a much higher level than the other (GmBAS2) in various tissues including the developing seeds, we constructed two RNAi vectors that encode self-complementary hairpin RNAs corresponding to the distinct regions of GmBAS1 under the control of a seed-specific promoter derived from the soybean gene for the alpha' subunit of the seed storage protein beta-conglycinin. These vectors were introduced independently into soybean. Six independent transgenic lines exhibited a stable reduction in seed saponin content, with the extent of saponin deficiency correlating with the beta-amyrin synthase mRNA depletion. Although some transgenic lines produced seeds almost devoid of saponins, no abnormality in their growth was apparent and the antioxidant activity of their seeds was similar to that of control seeds. These results suggest that saponins are not required for seed development and survival, and that soybean seeds may therefore be amenable to the modification of triterpenoid saponin content and composition through molecular biologic approaches.