Suppression Of Plant Innate Immune Responses By Cucumber Mosaic Virus Movement Protein

Plant cells can sense the invasion of pathogenic microbes through the recognition of pathogen-associated molecular patterns(PAMPs)via cell-surface-localized pattern recognition receptors(PRRs),and initiate a series of immune responses,namely PAMP-triggered immunity(PTI).For inatance,Arabidopsis immune receptor FLS2 and EFR can recognize bacterial flagellin(or the epitope flg22)and elongation factor Tu(or the epitope elf18),resepectively.LYK5 can recognize fungal cell wall component chitin.Upon ligand perception,PRRs initiate a set of downstream responses,such as the transient production of reactive oxygen species(ROS),activation of mitogen-activated protein kinase(MAPK)and calcium-dependent protein kinases(CDPK)cascades,and transcriptional reprogramming.Successful microbes can evade host recognition and dampen the host immune system by secreting effector proteins into the host cell.Plant cells have evolved a set of resistance(R)proteins to further confront the effectors.The recognition of effectors by R proteins lead to the activation of effector-trigged immunity(ETI),which is a more robust immune response.Cucumber Mosaic Virus(CMV)is one of the most important plant viral pathogens and is able to infect many plant species.CMV-SD is one of the CMV strains,which can infect the model plant A.thaliana and N.benthamiana.CMV contains three positive-sense RNA strands named RNA1,RNA2,and RNA3.Among which,RNA3 encodes movement protein(MP),and coat protein(CP).In this study,we expressed MP and CP in tobacco and Arabidopsis by Agrobacterium-mediated transient expression and transformation and analyzed their effects on plant immune responses.Here we discovered that the expression of the CMV MP in A.thaliana and N.benthamiana can specially suppress plant PTI immune responses.ROS production triggered by flg22,elf18,and chitin is impaired in MP-expressing Arabidopsis and tobacco.Transgenic Arabidopsis plants expressing the MP were compromised in flg22 and elf18-induced immune activation and were more susceptible to Pseudomonas syringae pv.tomato(Pst)DC3000 hrcC strain infection.We noticed that MP does not affect the stability of immune receptor complex and the activation of the upstream immune signaling.Further study revealed that flg22-induced defense gene expression was compromised in MP transgenic plants indicating MP may target the downstream signaling components.We attempted to identify the MP targets using protein immunoprecipitation and mass spectrometric analysis,and identified a list of potential targets for MP indicating MP may target to plant intracellular component to suppress immune responses.Taken together,our findings offer a new molecular mechanism for the CMV MP protein in suppression of host PAMP-triggered immune responses.