The Roles Of Two Clip-SPs In The Immune Responses Of The Pea Aphid, Acyrthosiphon Pisum

As a crucial agricultural pest, aphid, Acyrthosiphon pisum, is severely harmful to agricultural practice. A. pisum is a model of aphids. Studying pea aphid,s immune system will provide basis for biological control of aphids.We investigated the function of two clip domain serine proteases(clip-SPs) in the immune response of pea aphid in this study. Serine proteases play vital roles in immune system. When insects were infected by pathogens, recognization of pathogens by specific recognition recoptors triggers activation of the downstream serine proteases cascade, and eventual activation of innate immune pathways and responses, including: hemolymph coagulation, hemocytes adhesion, antimicrobial peptide synthesis and melanization to eliminate pathogens and parasites.Firstly, two clip-SPs, named Apvp and Splp respectively, were selected using semiquantitative PCR technology. Their expression were induced by bacterial and fungal infections. Their expressions did not change when infected by P. aeruginosa, while an obvious increase was observed when infected by S. aureus and B. bassiana.We used the RNA interference technology to knock down Apvp and Splp genes and subsequently infected pea aphids with the above-mentioned bacteria and fungus. The mortality of pea aphids was about 50% when only injected with dsApvp and dsSplp in a week. All the pea aphids died after P. aeruginosa infection in the dsApvp- and dsSplp-injected groups in two days; The mortality of the group injected with dsApvp was significantly higher than that of dsGFP-injected group, while there was no difference between the dsSplp-injected groups and dsGFP-injected group after S. aureus infection; In both of dsApvp- and dsSplp-injected groups, the survival of pea aphids was lower than that of dsGFP-injected group after B. bassiana infection. Those results indicated the mortality of pea aphid was affected after Apvp and Splp were knocked down.The proliferation of P. aeruginosa, S. aureus and GFP-B. bassiana in vivo of pea aphids after RNA interference was monitored using colony counting method. The numbers of CFU increased from 24 h to 48 h after P. aeruginosa and S. aureus infections and the dsApvp- and dsSplp-injected groups maintained at the same level after with P. aeruginosa infection in 48 h. The CFU in dsSplp-injected aphids was more higher than in dsApvp-injected aphids after with S. aureus infection in 48 h. There was no difference between dsApvp- and dsGFP-injected groups after with GFP-B. bassiana infection in 96 h, while the spores in dsSplp-injected aphids showed a higher load. Those results indicated that Apvp and Splp were capable to mediate pea aphid immune pathway to eliminate the invading pathogens in pea aphid.