Immunosuppression induced by expression of a viral RNase enhances susceptibility of Plutella xylostella to microbial pesticides

摘要

Polydnaviruses are a group of insect DNA viruses and are characterized in their segmented genome that is located in the chromosome(s) of host wasps.A polydnavirus,Cotesia plutellae bracovirus (CpBV),encodes a viral ribonuclease (RNase) T2 in a specific segment #3 (CpBV-S3).This study tested its effect on gene expression associated with host immune responses in the diamondback moth,Plutella xylostella.Micro-injection of CpBV-S3 into nonparasitized larvae induced expression of its two encoded genes,CpBV-ORF301 (=CpBV-RNase T2) and CpBV-ORF302.In response to a bacterial challenge,four antimicrobial peptide genes (hemolin,gloverin,cecropin and lysozyme) and six phenoloxidase (PO)-associated genes (proPO-activating proteinase,PO,serine proteinase homolog and serpins 1-3) were up-regulated in their expressions.However,the transient expression of CpBV-S3 suppressed the expressions of cecropin,PO and serpin 1.Double-stranded RNA specific to the viral RNase T2 could specifically knockdown the viral gene expression and restored the three gene expressions suppressed in the larvae injected with CpBV-S3.The inhibitory activity of the viral RNase T2 on the target genes was further proven by the suppression of PO activation in response to bacterial challenge in the larvae injected with CpBV-S3.This immunosuppression by the expression of the viral RNase T2 resulted in significant increase of pathogen susceptibility ofP.xylostella against Bacillus thuringiensis or baculovirus infection.