Glycosylation-dependent Turnover of Triterpenoid Saponins Controls Insect Deterrence

Plants 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 pattern, we investigated how insect digestion transforms hederagenin-derived saponins and how these transformations influence their defensive activity.

Larvae of Plutella xylostella were fed Brassica napus leaf discs painted with structurally defined hederagenin-derived saponins. Saponin composition in painted 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.

Larvae selectively converted higher-glycosylated hederagenin-derived saponins into monoglucosides, resulting in a marked shift in saponin composition between ingested material and larval frass. Non-choice feeding assays showed that monodesmosidic saponins strongly deter 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.

Our findings demonstrate that glycosylation governs both saponin activity and metabolic fate, revealing insect-mediated turnover as an important component of plant chemical defence during plant-herbivore interactions.