The Negative Regulative Roles of BdPGRPs in the Imd Signaling Pathway of Bactrocera dorsalis

Peptidoglycan recognition proteins (PGRPs) are key regulators in insects’ immune response, functioning as sensors to detect invading pathogens and as scavengers of peptidoglycan (PGN) to reduce immune overreaction. However, the exact function of PGRPs in Bactrocera dorsalis is still unclear. In this study, we identified and functionally characterized the genes BdPGRP-LB, BdPGRP-SB(1) and BdPGRP-SC(2) in B. dorsalis. The results showed that BdPGRP-LB, BdPGRP-SB(1) and BdPGRP-SC(2) all have an amidase-2 domain, which has been shown to have N-Acetylmuramoyl-l-Alanine amidase activity. The transcriptional levels of BdPGRP-LB and BdPGRP-SC(2) were both high in adult stages and midgut tissues; BdPGRP-SB(1) was found most abundantly expressed in the 2nd instar larvae stage and adult fat body. The expression of BdPGRP-LB and BdPGRP-SB(1) and AMPs were significantly up-regulated after injury infected with Escherichia coli at different time points; however, the expression of BdPGRP-SC(2) was reduced at 9 h, 24 h and 48 h following inoculation with E. coli. By injection of dsRNA, BdPGRP-LB, BdPGRP-SB(1) and BdPGRP-SC(2) were knocked down by RNA-interference. Silencing of BdPGRP-LB, BdPGRP-SB(1) and BdPGRP-SC(2) separately in flies resulted in over-activation of the Imd signaling pathway after bacterial challenge. The survival rate of the ds-PGRPs group was significantly reduced compared with the ds-egfp group after bacterial infection. Taken together, our results demonstrated that three catalytic PGRPs family genes, BdPGRP-LB, BdPGRP-SB(1) and BdPGRP-SC(2), are important negative regulators of the Imd pathway in B. dorsalis.