Novel glucosinolate metabolism in larvae of the leaf beetle Phaedon cochleariae. (September 2020) - Friedrichs, Jeanne
Abstract: Plants of the Brassicales are defended by a binary system, in which glucosinolates are degraded by myrosinases, forming toxic breakdown products such as isothiocyanates and nitriles. Various detoxification pathways and avoidance strategies have been found that allow different herbivorous insect taxa to deal with the glucosinolate-myrosinase system of their host plants. Here, we investigated how larvae of the leaf beetle species Phaedon cochleariae (Coleoptera: Chrysomelidae), a feeding specialist on Brassicaceae, cope with this binary defence. We performed feeding experiments using leaves of watercress ( Nasturtium officinale, containing 2-phenylethyl glucosinolate as major glucosinolate and myrosinases) and pea ( Pisum sativum, lacking glucosinolates and myrosinases), to which benzenic glucosinolates (benzyl- or 4-hydroxybenzyl glucosinolate) were applied. Performing comparative metabolomics using UHPLC-QTOF-MS/MS, N -(phenylacetyl) aspartic acid, N -(benzoyl) aspartic acid and N -(4-hydroxybenzoyl) aspartic acid were identified as major metabolites of 2-phenylethyl-, benzyl- and 4-hydroxybenzyl glucosinolate, respectively, in larvae and faeces. This suggests that larvae of P. cochleariae metabolise isothiocyanates or nitriles to aspartic acid conjugates of aromatic acids derived from the ingested benzenic glucosinolates. Myrosinase measurements revealed activity only in second-instar larvae that were fed with watercress, but not in freshly moulted and starved second-instar larvae fed with pea leaves. Our results indicate that the predicted pathway can occur independently of the presence of plant myrosinases, because the same major glucosinolate-breakdown metabolites were found in the larvae feeding on treated watercress and pea leaves. A conjugation of glucosinolate-derived compounds with aspartic acid is a novel metabolic pathway that has not been described for other herbivores. Graphical abstract: Image 1 Highlights: Brassicales are defended by glucosinolates and myrosinases, which form toxic hydrolysis products when coming into contact. As a feeding specialist on Brassicaceae plants, the leaf beetle Phaedon cochleariae must be able to cope with this defence. Larvae of P. cochleariae metabolise glucosinolate breakdown products with aromatic acids to aspartic acid conjugates. This pathway can occur independently of the presence of plant myrosinases. Aspartic acid conjugation is a novel way of glucosinolate metabolism, so far not known from other Brassicaceae herbivores.