Glutathione S-transferases (GSTs; EC are a family

S-transferases (GSTs; EC
are a family of multifunctional phase II enzymes that play a crucial role in
the detoxification of many exogenous and endogenous xenobiotics compounds and
have been widely found in almost all living organisms (prokaryotic and
eukaryotic) (Booth et al., 1961; Tu and Akgül, 2005; Li et
al., 2007). The
enhanced metabolic capability of detoxification enzymes, such as
carboxylesterase (CarE), cytochrome P450 monooxygenases (P450) and GSTs are
important for insecticide resistance (Rufingier et al., 1999; Puinean et al., 2010; Cui et al., 2015). The major function of GSTs is conjugation of
electrophilic compounds with the thiol group of reduced glutathione (GSH), thus
making 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 into
six major classes: delta, epsilon, omega, sigma, theta, and zeta; there are
also several unclassified genes (Ranson et al., 2001). Different classes of GSTs can be distinguished
based 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., 2005). The first GST of the omega class (GSTO) was
identified through a bioinformatics analysis of expressed sequence tags in
humans (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 essential
physiological roles that differ from other GST classes (Meng et al., 2014). In GSTOs, a novel cysteine residue (Cys) is present
in the active site, where GSTs from other classes have canonical serine and
tyrosine residues (Caccuri et al., 2002). Insect GSTs display different substrate
specificities, catalytic activities and have unique N-terminal and C-terminal
extensions that are not observed in the other GST classes (Board, 2011). As GSTs can play role in detoxification of
various insecticides, a change in the GST activity is one mechanism of metabolic
resistance to insecticides (Ranson and Hemingway, 2005; Li et al., 2007).The omega class of GSTs is one of the largest
GST subfamilies, with multiple functions identified in various species.

            Aphids are common phloem-feeding
pests 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 aestivum
L.) (Kieckhefer and Gellner, 1992; Blackman and Eastop, 2000) and is also an important vector for the barley yellow dwarf
virus, which infects and damages wheat crops (Watson and Mulligan, 1960). Insecticides are stress factors that can affect many physical
and biochemical process in insects. Insect populations have increased over time
due to acquisition of insecticide resistance (Bass et al., 2014).

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Here, we report the identification and classification of an
omega class GST (RpGSTO1) from R. padi. The recombinant protein, RpGSTO1,
was expressed in Escherichia coli cells. The biochemical properties of
the purified recombinant GST protein were characterized. The transcriptional
patterns 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 defense
were also investigated. Our results provide valuable insight into the function
of RpGSTO1 in the stress response to insecticides.