Glutathion S-transfierases In The Omethoate Resistant Cotton Aphid, Aphis Gossypii (Glover)

Cotton aphid, Aphis gossypii Glover (Homoptera： Aphididae), emergesresistance nearly to all of the commercialized insecticide classes globally. In order toelucidate the resistant mechanism to omethoate in cotton aphid, two aphid strainshave been built in laboratory. The researches focus on the important enzymes whichare highly related to insecticide resistance in cotton aphid. Biochemical andmolecular methods were conducted, and the results revealed the adaptation toomethoate stress of cotton aphid.The susceptible strain exhibited a low LC50of omethoate as18.23mg/L. Andthe resistant strain had developed a high level resistance to omethoate by frequentlyselection. The LC50of omethoate as high as5657.90mg/L, and the resistant foldwas310.76when compared to the susceptible strain.The biochemical results of carboxylesterase（CarE）indicated that it was notinvolved in the omethoate resistance of cotton aphid. Compared with the susceptiblestrain, the susceptibility to omethoate of acetylcholinesterase (AChE) downregulatedof4.12-fold in the resistant strain. The special activity to acetylthiocholine iodidereduced from2.91±0.08to0.370.02mOD/mg pr. min. Correlatively, the Vmaxreduced from11.030.21to0.420.00nmol/mg pr. min, and the Kmreduced from1.970.11to0.610.01μM. So the downregulation of AChE susceptibility wasone of the important resistant mechanisms to omethoate in cotton aphid.1-ch1oro-2,4-dinitrobenzene (CDNB) was applied to determine the activity ofglutathione S-transferases (GST_s). The special activity was144.07±5.19nmol/mgpr. min in the resistant strain that slightly lower when compared to the susceptiblestrain which possessed higher activity of176.54±7.21nmol/mg pr. min. But theVmaxelevated from250.00±9.90to833.33±33.62nmol/mg pr. min and Kmelevatedfrom1.38±0.05to3.33±0.12mM. Thediethyl malate (DEM) was an antagonism with omethoate as the synergism ratio was0.11. The IC50to the resistant strain(2.32×10-4±0.14μM) was slightly higher (1.17-fold) than the susceptible strain(1.98×10-4±0.13μM). GST_s from the two strains exhibited differential catalyticmodels. In the susceptible strain, a biphasic kinetic mechanism was observed with abreakpoint depend on the GSH concentration, in the higher GSH concentrationcondition, GST_scatalyzed GSH conjugated with CDNB by a sequential mechanismand a ping-pong pathway predominated the conjugation in the lower GSHconcentration range. However, in the resistant strain, the single efficiently sequentialmechanism constantly catalyzed GSH conjugated with CDNB. A Delta GST_sDNAfragment was successfully amplified from the susceptible and resistant strainrespectively, which encoded the identical full GST_sprotein sequence (216aminoacid residues). The GST_sDNA copies was1.52-fold lower in the resistant strainwhile the mRNA transcription level was much lower (0.33-fold). It was assumedthat the GST_smay become insensitivity and was been compensated by efficientsingle ordered sequential mechanism in the high level omethoate resistance of cottonaphid.The researches provided evidences for the effect of AChE in omethoateresistance of cotton aphid, and detected the modification of GST_s under theomthoate stress. Which were important to elucidate the mechanism of omethoateresistance in A. gossypii.