D3386: The fate of a Solanum steroidal alkaloid toxin in the cabbage looper (Trichoplusia ni): Qualitative and quantitative dimensions of detoxification

Plants produce complex chemical defenses against herbivores, resulting in the emergence of detoxification strategies in phytophagous insects. While enzymatic detoxification and target site mutagenesis are well-documented, the quantitative contribution of excretion remains less studied. We focus on the Trichoplusia ni, a generalist herbivore, to elucidate the detoxification of a steroidal alkaloid, solanidine, produced in the host plant Solanum tuberosum. Through larval feeding experiments and chemical analysis of metabolites using high-resolution mass spectrometry, we identify solanidine 3-O-b-glucopyranoside and solanidine 3-phosphate as major detoxification products of solanidine. Glycosylation and phosphorylation reactions, not previously observed in cabbage looper, were found to be prominent detoxification pathways of solanidine. Modified solanidine derivatives exhibit reduced lipophilicity, preventing passive transport as predicted by physicochemical analyses, and only solanidine was detected in body tissue. In addition, a T. ni mutant strain, with knocked out midgut cadherin conferring resistance to Bt toxins, was also investigated to examine how Bt resistance impacts steroidal alkaloid detoxification. T. ni larvae from the cadherin knockout strain showed lower solanidine uptake compared to the wild-type strain but similar excretion kinetics. Our study expands the detoxification enzyme repertoire in cabbage loopers, emphasizing the complexity of detoxification mechanisms in generalist herbivores.