One Pathway Is Not Enough: The Cabbage Stem Flea Beetle Psylliodes chrysocephala Uses Multiple Strategies to Overcome the Glucosinolate-Myrosinase Defense in Its Host Plants.

Franziska Beran,Michael Reichelt,Stefan Bartram,Sybille Lorenz,Anton Shekhov,Seung-Joon Ahn,Daniel G Vassão,Theresa Sporer,Franziska Betzin,Grit Kunert,Christian Paetz

doi:10.3389/fpls.2018.01754

Franziska Beran, Michael Reichelt + Show 9 more

Open Access

https://doi.org/10.3389/fpls.2018.01754

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Abstract

The cabbage stem flea beetle (Psylliodes chrysocephala) is a key pest of oilseed rape in Europe, and is specialized to feed on Brassicaceae plants armed with the glucosinolate-myrosinase defense system. Upon tissue damage, the β-thioglucosidase enzyme myrosinase hydrolyzes glucosinolates (GLS) to form toxic isothiocyanates (ITCs) which deter non-adapted herbivores. Here, we show that P. chrysocephala selectively sequester GLS from their host plants and store these throughout their life cycle. In addition, P. chrysocephala metabolize GLS to desulfo-GLS, which implies the evolution of GLS sulfatase activity in this specialist. To assess whether P. chrysocephala can largely prevent GLS hydrolysis in ingested plant tissue by sequestration and desulfation, we analyzed the metabolic fate of 4-methylsulfinylbutyl (4MSOB) GLS in adults. Surprisingly, intact and desulfo-GLS together accounted for the metabolic fate of only 26% of the total ingested GLS in P. chrysocephala, indicating that most ingested GLS are nevertheless activated by the plant myrosinase. The presence of 4MSOB-ITC and the corresponding nitrile in feces extracts confirmed the activation of ingested GLS, but the detected amounts of unmetabolized ITCs were low. P. chrysocephala partially detoxifies ITCs by conjugation with glutathione via the conserved mercapturic acid pathway. In addition to known products of the mercapturic acid pathway, we identified two previously unknown cyclic metabolites derived from the cysteine-conjugate of 4MSOB-ITC. In summary, the cabbage stem flea beetle avoids ITC formation by specialized strategies, but also relies on and extends the conserved mercapturic acid pathway to prevent toxicity of formed ITCs.

Highlights

Crucifer-feeding insects encounter a potent chemical defense that consists of two components, the glucosinolates (GLS) and the β-thioglucoside hydrolase myrosinase (Halkier and Gershenzon, 2006)
We focus on the cabbage stem flea beetle, Psylliodes chrysocephala, a notorious pest of oilseed rape in Northern Europe (Williams, 2010; Zimmer et al, 2014)
Volatile GLS hydrolysis products attract adults in the field (Bartlet et al, 1992). These findings demonstrate that P. chrysocephala is highly adapted to GLS-containing plants, but how this specialist overcomes the ITC-based defense in its host plants is unknown

Summary

Introduction

Crucifer-feeding insects encounter a potent chemical defense that consists of two components, the glucosinolates (GLS) and the β-thioglucoside hydrolase myrosinase (Halkier and Gershenzon, 2006). GLS are water-soluble β-thioglucoside-N-hydroxysulfates with a variable amino acid-derived side chain and occur almost exclusively in Brassicales plants (Fahey et al, 2001; Agerbirk and Olsen, 2012). While GLS are non-toxic, their hydrolysis by the myrosinase liberates noxious isothiocyanates (ITCs) with strong reactivity toward thiol- and amine-groups in peptides and proteins (Brown and Hampton, 2011). GLS and myrosinases are stored separately in the plant and ITCs are released rapidly in damaged tissue (Matile, 1980; Andréasson and Jørgensen, 2003). Dietary ITCs depleted cysteine and glutathione levels and activated the catabolism of insect proteins in Spodoptera littoralis larvae, thereby leading to reduced larval weight gain (Jeschke et al, 2015)

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