Glycosylation-dependent Turnover of Triterpenoid Saponins Controls Insect Deterrence

Background and AimsPlants deploy triterpenoid saponins as chemical defences against herbivores, yet it remains unclear whether insect digestion detoxifies these compounds or generates equally or more active metabolites. Because saponin bioactivity depends strongly on glycosylation patterns, we examined the fate and defensive activity of hederagenin-derived saponins during herbivory. MethodsLarvae of Plutella xylostella were fed leaf discs containing structurally defined hederagenin-derived saponins. Saponin composition in treated leaves and larval frass was analysed by LC- qTOF-ESI-MS/MS. Feeding assays were used to compare the antifeedant activity of mono- and bidesmosidic forms. Key ResultsLarvae selectively metabolized complex hederagenin-derived saponins into simpler forms, with cellobiosides converted into monoglucosides during digestion, resulting in a marked shift in saponin composition between ingested material and frass. Feeding assays showed that monodesmosidic saponins strongly deterrer feeding, whereas bidesmosidic saponins were largely inactive. The loss of activity in bidesmosidic saponins was not explained by differential metabolism, indicating that glycosylation patterns directly determine biological function. ConclusionsInsect herbivores selectively modify saponin structures through deglycosylation, thereby altering their defensive properties. Our findings demonstrate that glycosylation governs both saponin activity and metabolic fate, highlighting insect-driven turnover as a critical component of plant chemical defence during plant-herbivore interactions. Issue SectionOriginal article