Cytoplasmic and nuclear Sw-5b NLR act both independently and synergistically to confer full host defense against tospovirus infection.

Abstract

Plant intracellular nucleotide-binding leucine-rich repeat (NLR) receptors play critical roles in mediating host immunity to pathogen attack. We use tomato Sw-5b::tospovirus as a model system to study the specific role of the compartmentalized plant NLR in dictating host defenses against the virus at different infection steps. We demonstrated here that tomato NLR Sw-5b distributes to the cytoplasm and nucleus, respectively, to play different roles in inducing host resistances against tomato spotted wilt orthotospovirus (TSWV) infection. The cytoplasmic-enriched Sw-5b induces a strong cell death response to inhibit TSWV replication. This host response is, however, insufficient to block viral intercellular and long-distance movement. The nuclear-enriched Sw-5b triggers a host defense that weakly inhibits viral replication but strongly impedes virus intercellular and systemic movement. Furthermore, the cytoplasmic and nuclear Sw-5b act synergistically to dictate a full host defense of TSWV infection. We further demonstrated that the extended N-terminal Solanaceae domain (SD) of Sw-5b plays critical roles in cytoplasm/nucleus partitioning. Sw-5b NLR controls its cytoplasm localization. Strikingly, the SD but not coil-coil domain is crucial for Sw-5b receptor to import into the nucleus to trigger the immunity. The SD was found to interact with importins. Silencing both importin α and β expression disrupted Sw-5b nucleus import and host immunity against TSWV systemic infection. Collectively, our findings suggest that Sw-5b bifurcates disease resistances by cytoplasm/nucleus partitioning to block different infection steps of TSWV. The findings also identified a new regulatory role of extra domain of a plant NLR in mediating host innate immunity.