Activation of Multidimensional Defenses in Camptotheca acuminata Seedlings Against Spodoptera frugiperda Larvae

Camptotheca acuminata, the primary botanical source of camptothecin (CPT), employs this monoterpenoid indole alkaloid as a key chemical defense against herbivores in addition to its established clinical pharmaceutical importance. Given that Spodoptera frugiperda infestations pose a severe threat to C. acuminata seedlings, we examined integrated, multi-layered defense mechanisms that combine physical barriers with chemical toxins to bolster plant resistance. Physiological analyses revealed that herbivory induces antioxidant enzymes such as superoxide dismutase (SOD) and catalase (CAT), alongside broader metabolic reprogramming. These responses are orchestrated by differential activation of jasmonic acid (JA) and salicylic acid (SA) signaling pathways, which together drive complex defense mobilization, including a marked increase in trichome density. Concurrently, insect herbivory activates the MYB-bHLH-WD40 (MBW) transcriptional complex to promote trichome development while upregulating core CPT biosynthetic genes. In particular, two cytochrome P450 genes, Ca32236 and CaCYP81BQ18, mediate the accumulation of 10-hydroxycamptothecin (10-HCPT), a derivative that is sparingly soluble in water, which enables alkaloid transport and sequestration to specialized storage sites, including trichomes. Collectively, these stress-responsive strategies confer potent insecticidal activity against S. frugiperda and provide valuable insights for improving protection in C. acuminata seedling plantations.