Glutathione is involved in plant defense against the gall midge Mayetiola destructor

Glutathione, a tripeptide thiol of γ-glutamylcysteinylglycine, exists abundantly and widely in nearly all organisms.  Glutathione participates in various physiological processes involved in redox reactions by serving as an electron donor/acceptor.  In this study, we found that the abundance of total glutathione increased up to 60% in resistant plants within 72 hours following attack by the gall midge Mayetiola destructor, the so called Hessian fly.  The increase in total glutathione abundance, however, is coupled with an unbalanced activation of glutathione metabolic pathways.  The activity and gene expression of glutathione peroxidases, which convert reduced glutathione (GSH) to oxidized glutathione (GSSG), increased in infested resistant plants.  However, the enzymatic activity and gene expression of glutathione reductases, which converts GSSG back to GSH, did not change.  This unbalanced regulation of the glutathione oxidation/reduction cycle indicates the existence of an alternative pathway to regenerate GSH from GSSG to maintain a stable GSSG/GSH ratio.  Our data support the hypothesis that GSSG are transported from cytosol to apoplast to serve as an oxidant for class III peroxidases to generate reactive oxygen species for plant defense against Hessian fly larvae.  Our results shall provide a foundation for elucidating the molecular processes involved in glutathione leading to plant resistance to Hessian fly and potentially other pests as well.