Viral and bacterial plant pathogens suppress antiviral defense against flaviviruses in their insect vectors

A positive, single-stranded RNA virus member within the Flavivirus genus was identified and characterized infecting Myzus persicae. This new insect-specific virus (ISV), Myzus persicae flavivirus (MpFV), is 23,236 nucleotides in length and encodes a large polyprotein from a single open reading frame. Analysis of conserved domains showed that helicases, NS3-proteases, Fts-J methyltransferase, and an RNA-dependent RNA polymerase are present in the coded polyprotein. Aphid-infecting ISVs have been reported to interact with plant viruses within the vector, modulating its titer and manipulating aphid behavior and morphology. Small RNA (sRNA) profile analysis of the M. persicae sRNA profile demonstrated that the circulative plant virus, potato leafroll virus (PLRV), modified the aphid antiviral immunity against MpFV. Abundant sRNA reads matching MpFV were detected when aphids were fed on healthy plants, sucrose diet, and potato virus Y-infected plants. In contrast, no MpFV reads were detected in aphids that had acquired PLRV from infected plants or artificial diet sachets containing purified virions. While the titer of M. persicae densovirus (MpDNV) was previously reported to be regulated by expression of the PLRV silencing suppressor protein P0, P0 had no effect on MpFV titer in the aphid. MpFV was transmitted 100% vertically to the offspring, and exhibited tissue tropisms for the body rather than the head. By artificial diet assays, other aphid species, including Aphis gossypii (cotton aphid), Schizaphis graminum (greenbug aphid), Rhopalosiphum padi (bird cherry-oat aphid), and R. maidis (corn leaf aphid), acquired the MpFV. These findings further support the idea that PLRV suppresses aphid immunity against ISVs, suggest the existence of at least two distinct pathways for PLRV-induced aphid immune system modulation. To test whether other circulative plant pathogens suppress insect anti-viral immunity against insect-specific flaviviruses, we quantified the small RNA response of Diaphorina citri, vector of "Candidatus Liberibacter asiaticus" (CLas) associated with citrus greening disease and showed that CLas also suppresses D. citri anti-viral immunity against D. citri-like flavivirus (DcLFV). These data reveal an evolutionary conserved, unexpected role for diverse circulative plant pathogens in modulating anti-viral immunity in hemipteran vectors.