Transient, rapid increase of cytosolic Ca2+ upon pathogen infection is essential for plant pathogen associated molecular pattern (PAMP) triggered immunity (PTI). Several cyclic nucleotide gated channels (CNGCs) have been implicated; however, their regulatory mechanisms remain elusive. Here, the Phytophthora infestans RXLR effector AVRblb2 family targeted NbCNGC18, 19, 20 at the plasma membrane, inhibiting Ca2+ influx and PTI. AVRblb2 required calmodulin (CaM) and calmodulin like (CML) proteins as co factors to interact with N. benthamiana CNGCs (NbCNGCs), forming the AVRblb2, CaM/CML, NbCNGCs complex. After recognizing PAMPs, NbCNGC18 formed active heteromeric channels with other CNGCs, potentially providing selectivity for diverse signals to fine tune cytosolic Ca2+ levels and responses. AVRblb2 suppressed the Ca2+ influx and oxidative burst induced by NbCNGC18 heteromeric complexes. Silencing CNGC18, CNGC20, and CNGC25 compromised the effect of AVRblb2 on P. infestans virulence, confirming that AVRblb2 contributed to virulence by targeting CNGCs. Our findings delineated the regulatory mechanism and role of effector targeted Ca2+ channels in plant innate immunity.
Pathogen effectors can suppress various plant immune responses, suggesting their multiple targets in the host. To understand the mechanisms underlying plasma membrane-associated and effector-mediated immunity, we performed Phytophthora capsici CRISIS screening. In Nicotiana benthamiana, cell death induced by the RxLR effector CRISIS2 is inhibited by irreversible plasma membrane H+-ATPase (PMA) activator fusicoccin. Biochemical and gene silencing analyses revealed that CRISIS2 physically and functionally associates with PMAs and induces cell death independent of immune receptors. CRISIS2 induces apoplastic alkalization by suppressing PMA activity by association with the C-terminal regulatory domain of PMA. In planta expression of CRISIS2 significantly enhanced virulence of P. capsici, whereas host-induced gene silencing of CRISIS2 compromised disease symptom and biomass of P. capsici. Furthermore, co-immunoprecipitation assays revealed CRISIS2 associates with BAK1, the co-receptor of PRRs. CRISIS2 interferes with the FLS2-BAK1 complex induced by flagellin perception and impairs downstream signaling from PRR complex. Proteomics and gene silencing assays identified putative PRRs negatively regulate the virulence of P. capsici in N. benthamiana as interactors of CRISIS2 and BAK1. Our study identified a novel RxLR effector playing multiple roles in suppression of plant defense and induction of cell death to support the pathogen hemibiotrophic life cycle in the host plant.
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