Functional Analysis of Two Carboxylesterase Genes Involved in Beta-Cypermethrin and Phoxim Resistance in Plutella xylostella (L.)

Enhanced expression of carboxylesterase (CarE) genes is an important mechanism of insecticide resistance in pests. However, their roles in multi-insecticide resistance have rarely been reported. Herein, two CarE genes (PxαE6 and PxαE9) were identified; their relative expression levels in three multi-insecticide-resistant populations of P. xylostella (HN, GD-2017 and GD-2019) were 2.69- to 15.32-fold higher than those in the sensitive population, and they were considerably overexpressed at the larval stage and in the midgut of the 4th instar. PxαE6 and PxαE9 knockdown increased the susceptibility of GD-2019 larvae to phoxim or/and beta-cypermethrin. The recombinant PxαE6 and PxαE9 expressed in Escherichia coli exhibited high hydrolysis activity towards α-NA. GC–MS and LC–MS/MS assays revealed that PxαE9 could metabolize beta-cypermethrin and phoxim with efficiency determinations of 51.6% and 21.1%, respectively, while PxαE6 could metabolize phoxim with an efficiency of 12.0%. Homology modelling, molecular docking and molecular-dynamics simulation analyses demonstrated that beta-cypermethrin or/and phoxim could fit well into the active pocket and stably bind to PxαE6 or PxαE9. These results show that PxαE6 and PxαE9 overexpression were involved in resistance to beta-cypermethrin or/and phoxim in multi-insecticide-resistant P. xylostella populations, a finding which sheds light on the molecular mechanisms of multi-insecticide resistance in insect pests.