Plant growth regulation effects of triterpenoid saponins

Abstract

To investigate structure-activity relations between the sugar chain structures of triterpenoid saponins and their plant growth regulation effects, several monodesmosidic saponins with betulin as an aglycon were synthesized by chemical and enzymic reactions. Three triterpenoids (betulin, betulinic acid, oleanolic acid) and synthesized betulin glycosides were submitted to germination and growth regulation tests on alfalfa seeds. We concluded the following. Betulin had a slight growth inhibitory effect on alfalfa radicles. Betulin glycosides exhibited stronger effects than betulin, and betulin glycosides with two to four glucose residues as a sugar moiety had the greatest inhibitory activity. These characteristics of growth inhibitory effects were considerably different from those of phenolic compounds so far reported. Some betulin glycosides also showed a significant growth regulation effect on alfalfa hypocotyls. However, hypocotyl growth was less affected than radicle growth for all betulin glycosides. Among the triterpenoids, betulinic acid had stronger growth inhibitory effects on alfalfa radicles than betulin, suggesting the importance of the carboxyl group at the C-28 position for the inhibitory effects of lupane-type triterpenoids. On the other hand, no germination regulation effects on alfalfa seeds were observed for any of the betulin glycosides or triterpenoids examined.