RIN4 fragments released by AvrRpt2 promote NDR1-dependent activation of RPS2.

Plant nucleotide-binding, leucine-rich-repeat (NLR) immune receptors recognize pathogen effectors and activate immunity. The NLR Resistance to Pseudomonas syringae2 (RPS2) recognizes Avirulence protein interacting with RPS2 (AvrRpt2), a Pseudomonas effector that promotes virulence by proteolytically cleaving a membrane-tethered host protein, RPM1-interacting protein 4 (RIN4). RIN4 cleavage by AvrRpt2 also activates RPS2. A model in which RPS2 is activated by elimination of RIN4 is consistent with the ectopic activity of RPS2 in plants lacking RIN4 but does not explain the link between AvrRpt2's virulence activity and RPS2 activation. We found that non-membrane-tethered RIN4 derivatives are potent cytosolic activators of RPS2. Activation of RPS2 by these RIN4 derivatives, like AvrRpt2-induced activation,and unlike ectopic activation in the absence of RIN4, requires the defense signaling protein NON-RACE-SPECIFIC DISEASE RESISTANCE 1 (NDR1). Cleavage products of RIN4 produced by AvrRpt2 play contrasting roles in the activation of RPS2, with the membrane-tethered C-terminal fragment suppressing RPS2 and the non-membrane-tethered internal fragment, dependent on compatibility with the C-terminal fragment, overcoming its suppression of RPS2.