The Role Of Serine Protease ACYPI4531 In Immune Responses Of The Pea Aphid,Acyrthosiphon Pisum

Aphids are one of the insects with piercing and sucking mouthparts.They have large numbers,strong reproduction ability,strong adaptability,and the ability to transmit plant viruses.Therefore,aphids have become world-class agricultural pests.The pea aphid(Acyrthosiphon pisum)belongs to the Aphididea.It is the first hemimetabolous insect and that genome has been published.Unlike other insects,the pea aphid has a unique immune system and its genome lacks of a large number of immune-related genes by genomic analyses.Nevertheless,the immune system of the pea aphid can effectively resist the invasion of pathogens.The clip domain serine protease plays an irreplaceable role in insect embryonic development,cell differentiation,digestion and metabolism,signal transduction and immune defense.In this study,RNA interference technique was used to silence the expression of serine protease ACYPI4531.We investigated the changes of expression of different bacteria and fungi infection,the aphids' survival,viability of bacteria and fungi,and phenoloxidase(PO)activity in the aphids after infection.The main experimental results are as follows:1.The bioinformatics analysis of serine protease ACYPI4531 and the prediction of its three-dimensional structure showed that the serine protease ACYPI4531 has typical structural characteristics of clip domain serine protease.2.After infection with Gram-positive bacterium Staphylococcus aureus,Gram-negative bacterium Pseudomonas aeruginosa and fungus Beauveria bassiana,the expression of serine protease ACYPI4531 in pea aphids was up-regulated at different time points.This indicates that serine protease ACYPI4531 participates in the immune responses of pea aphids.3.After silenced the ACYPI4531 and then infected with S.aureus,P.aeruginosa and B.bassiana,the survival rate of pea aphids were not significantly different from that of the control after the infection,but slightly lower than the control group.Secondly,the numbers of bacterial colonies and fungal spores in pea aphids were significantly higher than in the control group,which further indicated that serine protease ACYPI4531 participates in the immune response of pea aphids.4.The activity of PO in pea aphids was significantly affected by the interference of ACYPI4531 gene,and decreased after infection.Conclusion: Serine protease ACYPI4531 participates in the immune defense response to bacterial and fungal infections in the pea aphids.After silencing of ACYPI4531,PO activity was effectively reduced,suggesting that serine protease ACYPI4531 might be involved in the phenoloxidase cascade of pea aphids.