bioRxiv2014
Perception of pathogen associated molecular patterns (PAMPs) by cell surface localized pattern recognition receptors (PPRs), activates plant basal defense responses in a process known as PAMP/PRR-triggered immunity (PTI). In turn, pathogens deploy effector proteins that interfere with different steps in PTI signaling. However, our knowledge of PTI suppression by filamentous plant pathogens, i.e. fungi and oomycetes, remains fragmentary. Previous work revealed that BAK1/SERK3, a regulatory receptor of several PRRs, contributes to basal immunity against the Irish potato famine pathogen Phytophthora infestans. Moreover BAK1/SERK3 is required for the cell death induced by P. infestans elicitin INF1, a protein with characteristics of PAMPs. The P. infestans host-translocated RXLR-WY effector AVR3a is known to supress INF1-mediated defense by binding the E3 ligase CMPG1. In contrast, AVR3aKl-Y147del, a deletion mutant of the C-terminal tyrosine of AVR3a, fails to bind CMPG1 and suppress INF1 cell death. Here we studied the extent to which AVR3a and its variants perturb additional BAK1/SERK3 dependent PTI responses using the plant PRR FLAGELLIN SENSING 2 (FLS2). We found that all tested variants of AVR3a, including AVR3aKl-Y147del, suppress early defense responses triggered by the bacterial flagellin-derived peptide flg22 and reduce internalization of activated FLS2 from the plasma membrane without disturbing its nonactivated localization. Consistent with this effect of AVR3a on FLS2 endocytosis, we discovered that AVR3a associates with the Dynamin-Related Protein DRP2, a plant GTPase implicated in receptor-mediated endocytosis. Interestingly, DRP2 is required for ligand-induced FLS2 internalization but does not affect internalization of the growth receptor BRASSINOSTEROID INSENSITIVE 1 (BRI1). Furthermore, overexpression of DRP2 suppressed accumulation of reactive oxygen species triggered by PAMP treatment. We conclude that AVR3a associates with a key cellular trafficking and membrane-remodeling complex involved in immune receptor-mediated endocytosis and signaling. AVR3a is a multifunctional effector that can suppress BAK1/SERK3 mediated immunity through at least two different pathways.\n\nAUTHOR SUMMARYPlants have a basal layer of immunity to mount defense responses against invading pathogens; in turn, pathogens deploy effector proteins to subvert plant immunity and manipulate host processes to enable parasitic infection. The Irish potato famine pathogen Phytophthora infestans has a large set of effectors that target multiple host cellular sites. The best-characterized P. infestans effector AVR3a supports enhanced infection and suppresses the cell death induced by the P. infestans protein INF1-elicitin. Previous work demonstrated that the plant immunity co-receptor BAK1/SERK3 contributes to basal immunity to P. infestans and that the RXLR-WY effector of P. infestans suppresses BAK1/SERK3-mediated immunity by binding the E3 ligase protein CMPG1. Here we show that AVR3a suppresses additional defense responses mediated by BAK1/SERK3 independently of CMPG1. AVR3a reduces the endocytosis of the plant receptor FLS2, which recognizes the flagellin-derived peptide flg22 in a BAK1/SERK3 dependent manner. Furthermore, we demonstrate that AVR3a associates with the Dynamin-Related Protein DRP2, a plant GTPase involved in receptor-mediated endocytosis that is required for FLS2 internalization. Our work revealed that AVR3a is a multifunctional effector that perturbs cellular trafficking initiated at the cell periphery by at least two mechanisms, and that this effector associates with a key cellular trafficking and membrane-remodeling complex involved in immune receptor-mediated endocytosis and signaling.