Imidacloprid is hydroxylated by Laodelphax striatellus CYP6AY3v2

Laodelphax striatellus (Fallén) is one of the most destructive pests of rice, and has developed high resistance to imidacloprid. Our previous work indicated a strong association between imidacloprid resistance and the overexpression of a cytochrome P450 gene CYP6AY3v2 in a L. striatellus imidacloprid resistant strain (Imid-R). In this study, a transgenic Drosophila melanogaster line that overexpressed the L. striatellus CYP6AY3v2 gene was established and was found to confer increased levels of imidacloprid resistance. Furthermore, CYP6AY3v2 was co-expressed with D. melanogaster cytochrome P450 reductase (CPR) in Spodoptera frugiperda 9 (SF9) cells. A carbon monoxide difference spectra analysis indicated that CYP6AY3v2 was expressed predominately in its cytochrome P450 (P450) form, which is indicative of a good-quality functional enzyme. The recombinant CYP6AY3v2 protein efficiently catalysed the model substrate P-nitroanisole to p-nitrophenol with a maximum velocity (V_max ) of 60.78 ± 3.93 optical density (mOD)/min/mg protein. In addition, imidacloprid itself was metabolized by the recombinant CYP6AY3v2/nicotinamide adenine dinucleotide 2'-phosphate reduced tetrasodium salt (NADPH) CPR microsomes in in vitro assays (catalytic constant (K_cat ) = 0.34 pmol/min/pmol P450, michaelis constant (K_m ) = 41.98 μM), and imidacloprid depletion and metabolite peak formation were with a time dependence. The data provided direct evidence that CYP6AY3v2 is capable of hydroxylation of imidacloprid and conferring metabolic resistance in L. striatellus.

Cytochrome P450 CYP4DE1 and CYP6CW3v2 contribute to ethiprole resistance in Laodelphax striatellus (Fallén)

Laodelphax striatellus Fallén (Hemiptera: Delphacidae), a destructive pest of rice, has developed high resistance to multiple insecticides, threatening the success of pest management programmes. The present study investigated ethiprole resistance mechanisms in a field population that is highly resistant to ethiprole. That population was used to establish a laboratory population that was subjected to further selection to produce a resistant strain. Target genes were cloned and compared between the resistant and the susceptible strains, the role of detoxification enzymes was examined, and the relative expression levels of 71 detoxification enzyme genes were tested using quantitative real time (RT)-PCR. The laboratory selection enhanced the resistance from 107-fold to 180-fold. The Rdl-type target site mutation seldom occurred in the resistant strain and is unlikely to represent the major mechanism underlying the observed resistance. Of the three important detoxification enzymes, only P450 monooxygenase was found to be associated with ethiprole resistance. Moreover, two genes, CYP4DE1 and CYP6CW3v2, were found to be overexpressed in the resistant strain. Furthermore, gene-silencing via a double-stranded RNA feeding test was carried out, and the results showed that the mRNA levels of CYP4DE1 and CYP6CW3v2 were reduced in the resistant strain, whereas ethiprole susceptibility was increased. These results suggest that CYP4DE1 and CYP6CW3v2 play an important role in ethiprole resistance in L. striatellus.