GlutathioneS-transferases (GSTs; EC 2.5.

1.18)are a family of multifunctional phase II enzymes that play a crucial role inthe detoxification of many exogenous and endogenous xenobiotics compounds andhave been widely found in almost all living organisms (prokaryotic andeukaryotic) (Booth et al., 1961; Tu and Akgül, 2005; Li etal., 2007). Theenhanced metabolic capability of detoxification enzymes, such ascarboxylesterase (CarE), cytochrome P450 monooxygenases (P450) and GSTs areimportant for insecticide resistance (Rufingier et al., 1999; Puinean et al.

, 2010; Cui et al., 2015). The major function of GSTs is conjugation ofelectrophilic compounds with the thiol group of reduced glutathione (GSH), thusmaking them less toxic, more soluble and easier to excrete from the cell (Enayati et al., 2005; Ketterman et al., 2011).

Cytosolic insect GSTs can be classified intosix major classes: delta, epsilon, omega, sigma, theta, and zeta; there arealso several unclassified genes (Ranson et al., 2001). Different classes of GSTs can be distinguishedbased on their primary amino acid sequences; identity is approximately 50%within a class and less than 30% among different classes (Sheehan et al., 2001; Mannervik et al.

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, 2005). The first GST of the omega class (GSTO) wasidentified through a bioinformatics analysis of expressed sequence tags inhumans (Board et al., 2000). GSTOs have since been found in plants, yeast,bacteria and insects (Dixon et al., 2002; Garcerá et al., 2006;Walters et al., 2009; Xun et al., 2010).

GSTOs have unique structures and play essentialphysiological roles that differ from other GST classes (Meng et al., 2014). In GSTOs, a novel cysteine residue (Cys) is presentin the active site, where GSTs from other classes have canonical serine andtyrosine residues (Caccuri et al., 2002). Insect GSTs display different substratespecificities, catalytic activities and have unique N-terminal and C-terminalextensions that are not observed in the other GST classes (Board, 2011). As GSTs can play role in detoxification ofvarious insecticides, a change in the GST activity is one mechanism of metabolicresistance to insecticides (Ranson and Hemingway, 2005; Li et al., 2007).The omega class of GSTs is one of the largestGST subfamilies, with multiple functions identified in various species.

            Aphids are common phloem-feedingpests found worldwide, and they damage plants by removing nutrients (Rabbinge et al., 1981). The bird cherry-oat aphid, Rhopalosiphum padi (L.)(Hemiptera: Aphididae), is a serious wheat pest in China (Wang et al., 2006). It can significantly reduce grain yields (Triticum aestivumL.) (Kieckhefer and Gellner, 1992; Blackman and Eastop, 2000) and is also an important vector for the barley yellow dwarfvirus, which infects and damages wheat crops (Watson and Mulligan, 1960).

Insecticides are stress factors that can affect many physicaland biochemical process in insects. Insect populations have increased over timedue to acquisition of insecticide resistance (Bass et al., 2014). Here, we report the identification and classification of anomega class GST (RpGSTO1) from R. padi. The recombinant protein, RpGSTO1,was expressed in Escherichia coli cells.

The biochemical properties ofthe purified recombinant GST protein were characterized. The transcriptionalpatterns of RpGSTO1 following exposure to various concentrations of ?-cypermethrin,isoprocarb, malathion and sulfoxaflor were analyzed.The potential roles of the RpGSTO1 fusion protein in antioxidant defensewere also investigated. Our results provide valuable insight into the functionof RpGSTO1 in the stress response to insecticides.