Unique metabolism of different glucosinolates in larvae and adults of a leaf beetle specialised on Brassicaceae

Abstract

Brassicaceae plants contain glucosinolates, which are hydrolysed by myrosinases to toxic products such as isothiocyanates and nitriles, acting as defences. Herbivores have evolved various detoxification strategies, which are reviewed here. Larvae of Phaedon cochleariae (Coleoptera: Chrysomelidae) metabolise hydrolysis products of benzenic glucosinolates by conjugation with aspartic acid. In this study, we investigated whether P. cochleariae uses the same metabolic pathway for structurally different glucosinolates, whether the metabolism differs between adults and larvae and which hydrolysis products are formed as intermediates. Feeding experiments were performed with leaves of watercress (Nasturtium officinale, Brassicaceae) and pea (Pisum sativum, non-Brassicaceae), to which glucosinolates with structurally different side chains (benzenic, indole or aliphatic) or their hydrolysis products were applied. Samples were analysed by UHPLC-QTOF-MS/MS or TD–GC–MS. The same aspartic acid conjugates as previously identified in larvae were also detected as major metabolites of benzenic glucosinolates in adults. Indol-3-ylmethyl glucosinolate was mainly metabolised to N-(1H-indol-3-ylcarbonyl) glutamic acid in adults and larvae, while the metabolism of 2-propenyl glucosinolate remains unclear. The metabolism may thus proceed primarily via isothiocyanates rather than via nitriles, while the hydrolysis occurs independently of plant myrosinases. A detoxification by conjugation with these amino acids is not yet known from other Brassicaceae-feeders.

Development of a polyphagous leaf beetle on different host plant species and its detoxification of glucosinolates

“…When offering 4-hydroxybenzyl glucosinolate to G. tanaceti, we additionally detected 4-hydroxybenzaldehyde and 4-hydroxybenzoic acid, which were also present in the treated leaf samples. These metabolites were likewise found in larvae and adults of the oligophagous species Phaedon cochleariae (Coleoptera: Chrysomelidae) after feeding on 4-hydroxybenzyl glucosinolate-treated leaves (Table 1; Friedrichs et al, 2020Friedrichs et al, , 2022. Thus, the glucosinolate metabolism in G. tanaceti may be similar to that suggested for P. cochleariae (Figure 4; Friedrichs et al, 2020Friedrichs et al, , 2022.…”

“…These findings suggest that plant myrosinases are not necessarily involved in the detoxification mechanism. Gut microbiota may play a role in the formation of isothiocyanates, nitriles, and amines independent of plant myrosinases, as known for humans, other vertebrates, and insects (Mutlib et al, 2002;Angelino et al, 2015;Welte et al, 2016a, b;Liu et al, 2017;Liou et al, 2020;Sikorska-Zimny and Beneduce, 2021) and also suggested for P. cochleariae (Friedrichs et al, 2020(Friedrichs et al, , 2022.…”

“…These metabolites were likewise found in larvae and adults of the oligophagous species Phaedon cochleariae (Coleoptera: Chrysomelidae) after feeding on 4-hydroxybenzyl glucosinolate-treated leaves (Table 1; Friedrichs et al, 2020Friedrichs et al, , 2022. Thus, the glucosinolate metabolism in G. tanaceti may be similar to that suggested for P. cochleariae (Figure 4; Friedrichs et al, 2020Friedrichs et al, , 2022. In a first step, benzenic glucosinolates may be hydrolysed by plant myrosinases and/or gut microbiota of the insects (see below) to isothiocyanates and/or nitriles.…”

Metabolization and Sequestration of Plant Specialized Metabolites in Insect Herbivores: Current and Emerging Approaches

“…If artificial diets are not available for an insect system, leaf disks from accepted host plants can be painted or spiked with a PSM. For example, caterpillars of the monarch butterfly can be fed cardenolide compounds on discs of the cardenolide-free milkweed plant Asclepias tuberosa (Agrawal et al, 2021), while the mustard beetle Phaedon cochelariae can be fed glucosinolates on leaf discs of pea (Friedrichs et al, 2022). The PSMs are painted onto leaf discs dissolved in an organic solvent which evaporates, leaving only the PSM behind.…”

“…An untargeted metabolomics approach can therefore serve as an important first step to determine the metabolic fate of PSMs after insect consumption. For example, specialist herbivores of Brassicaceae plants have developed many unique strategies that allow them to cope with the toxic glucosinolate PSMs of their host plant, including conjugation of the compounds with glutathione or various amino acids, desulfation of the compound, or nitrile formation upon compound breakdown (Friedrichs et al, 2022). Novel pathways of glucosinolate metabolization with different products continue to be discovered (Jeanne Friedrichs et al, 2020), and untargeted metabolomics has been an essential tool in elucidating the fate of many PSMs (e.g., (Agrawal et al, 2021;Singh et al, 2022).…”