The saponin bomb: a nucleolar liquid-liquid phase-separated β-glucosidase hydrolyses triterpene saponins in Medicago truncatula

Abstract Plants often protect themselves from their own bioactive defense metabolites by storing them in less active forms. Consequently, plants also need systems allowing correct spatiotemporal reactivation of such metabolites, for instance under pathogen attack. Here we show that the model legume Medicago truncatula has evolved a two-component system composed of a β-glucosidase and triterpene saponins, which are physically separated from each other in intact cells. The β-glucosidase, which is stored in the nucleolus and subjected to liquid-liquid phase separation in intact cells, is released and united with its substrates only upon tissue damage, partly mediated by the surfactant action of the saponins themselves. Subsequently, enzymatic removal of carbohydrate groups from the saponins creates a pool of metabolites with an increased broad-spectrum antimicrobial activity. The evolution of this peculiar defense system benefited from both the intrinsic condensation abilities of the enzyme and the bioactivity properties of its substrates.