| The plant innate immune system consists of two main branches, PAMP-triggered immunity (PTI) and effector-triggered immunity (ETI). Perception of pathogen-associated molecular patterns (PAMPs) activates PTI. However, pathogenic bacteria deploy virulence factors that suppress PTI, including translocated type III effector (T3E) proteins. In addition to suppressing PTI, T3Es also activate the second branch of plant innate immunity, ETI, via activation of NB-LRR disease resistance proteins. ETI can be indirectly activated by PTI-suppressing T3Es. For instance, the Arabidopsis protein RIN4, which negatively regulates PTI, is targeted by the three sequence unrelated T3Es AvrRpm1, AvrB, and AvrRpt2 from P. syringae that suppress PTI. Additionally, RIN4 mediates recognition of these T3Es via the NB-LRR proteins RPM1 and RPS2. AvrRpm1 and AvrB phosphorylate RIN4, and AvrRpt2 cleaves it. RIN4 interacts in planta with, RPM1 and RPS2, and perturbation of RIN4 elicits their response to these T3Es. Structure-function analysis of RIN4 has revealed that (1) RIN4 has multiple domains capable of negatively regulating PTI; (2) one of these domains is critical for regulation of RPM1; (3) membrane targeting of RIN4 differentially affects its ability to regulate PTI and ETI; and (4) RIN4 differentially regulates RPM1 and RPS2. Collectively, our results indicate that RIN4 is multifunctional with separable activities involved in regulation of these two central branches of plant immunity. |
