The bacterial type III-secreted protein AvrRps4 is a bipartite effector

Bacterial effector proteins secreted into host plant cells manipulate those cells to the benefit of the pathogen, but effector-triggered immunity (ETI) occurs when effectors are recognized by host resistance proteins. The RPS4/RRS1 pair recognizes the Pseudomonas syringae pv. pisi effector AvrRps4. AvrRps4 is processed in planta into AvrRps4(N) (133 amino acids), homologous to the N-termini of other effectors including the native P. syringae pv. tomato strain DC3000 effector HopK1, and AvrRps4(C) (88 amino acids). Previous data suggested that AvrRps4(C) alone is necessary and sufficient for resistance when overexpressed in heterologous systems. We show that delivering AvrRps4(C) from DC3000, but not from a DC3000 hopK1(-) strain, triggers resistance in the Arabidopsis accession Col-0. Delivering AvrRps4(C) in tandem with AvrRps4(N), or as a chimera with HopK1(N), fully complements AvrRps4-triggered immunity. AvrRps4(N) in the absence of AvrRps4(C) enhances virulence in Col-0. In addition, AvrRps4(N) triggers a hypersensitive response in lettuce that is attenuated by coexpression of AvrRps4(C), further supporting the role of AvrRps4(N) as a bona fide effector domain. Based on these results we propose that evolutionarily, fusion of AvrRps4(C) to AvrRps4(N) may have counteracted recognition of AvrRps4(N), and that the plant RPS4/RRS1 resistance gene pair was selected as a countermeasure. We conclude that AvrRps4 represents an unusual chimeric effector, with recognition in Arabidopsis by RPS4/RRS1 requiring the presence of both processed effector moieties.