2,5-Dihydroxybenzaldehyde in lettuce latex inhibits insect detoxification enzymes in Spodoptera litura larvae

Abstract Plant latex is a sticky emulsion exuded from laticifer once the plant is damaged. Latex is an essential component of plant defense against herbivores. Lettuce (Lactuca sativa L.) in the compositae family has relatively fewer insect herbivores compared with other leaf vegetables. Our bioassay showed that the larvae of a generalist lepidopteran pest Spodoptera litura (Fabricius) avoided feeding on living lettuce plants. However, the larvae rapidly damaged the excised leaves unable to produce latex. The methanol extract of lettuce latex exhibited antifeedant activity. Six compounds were isolated from lettuce latex. They were identified as 2,5-dihydroxybenzaldehyde (1), 3β-hydroxy-4,15-dehydrograndolide (2), annuolide D (3), lactucin (4), lactucopicrin (5), and hanphyllin (6). Bioassays showed that compounds 1 (2,5-dihydroxybenzaldehyde) and 6 (hanphyllin, a sesquiterpene lactone) inhibited larval growth of S. litura at the concentration of 100 µg/g. RNA-seq analyses showed that larval exposure to compound 1 down-regulated the genes associated with heterobiotic metabolism including drug metabolism-cytochrome P450, metabolism of xenobiotics by cytochrome P450, retinol metabolism, glutathione metabolism, drug metabolism-other enzymes (mainly uridine diphosphate glucuronyltransferase, UGTs). RT-qPCR further confirmed that 33 genes in the family of carboxylesterase (CarE), P450s and UGTs were down-regulated by compound 1. The activities of CarE, P450s and UGTs in the larvae fed on diets containing compound 1 were significantly lower than those fed on control diets. These findings suggest that secondary metabolites including 2,5-dihydroxybenzaldehyde in the latex plays a key role in protection of lettuce from insect herbivory.