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Pathogen effectors target diverse subcellular organelles to manipulate the plant immune system. Although nucleolus has been emerged as a stress marker, and several effectors are localized in the nucleolus, the roles of nucleolar-targeted effectors remain elusive. In this study, we showed Phytophthora infestans infection of Nicotiana benthamiana results in nucleolar inflation during the transition from biotrophic to necrotrophic phase. Multiple P. infestans effectors were localized in the nucleolus: Pi23226 induced cell death in Nicotiana benthamiana and nucleolar inflation similar to that observed in the necrotrophic stage of infection, whereas its homolog Pi23015 and a deletion mutant (Pi23226ΔC) did not induce cell death or affect nucleolar size. RNA immunoprecipitation and iCLIP-seq analysis indicated that Pi23226 bound to the 3′-end of 25S rRNA precursors, resulting in the accumulation of unprocessed 27S pre-rRNAs. The nucleolar stress marker NAC082 was strongly upregulated under Pi23226-expressing conditions. Pi23226 subsequently inhibited global protein translation in host cells by interacting with ribosomes. Pi23226 enhanced P. infestans pathogenicity, indicating that Pi23226-induced ribosome malfunction and cell death was beneficial for pathogenesis in the host. Our results provide evidence for the molecular mechanism underlying RNA-binding effector activity in host ribosome biogenesis, and lead to new insights into the nucleolar action of effectors in pathogenesis.

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.

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