Mitogen-activated Protein Kinase Activation after Effector Recognition in Tomato

The major virulence strategy of the phytopathogenic bacterium Pseudomonas syringae pv. tomato DC3000 is to secrete effector proteins into tomato cells to target the immune machinery. AvrPto and AvrPtoB are two such effectors from P. syringae, which disable a range of kinases in tomato (Solanum lycopersicum). Both effectors target surface-localized receptor-kinases to avoid bacterial recognition. In turn, tomato has evolved an intracellular effector-recognition complex composed of the NB-LRR protein Prf and the Pto kinase. AvrPto is an inhibitor of Pto kinase activity but, paradoxically, this kinase activity is a prerequisite for defence activation by AvrPto. Using biochemical approaches, we show that disruption of the Pto P+1 loop stimulates phosphorylation in trans, which is possible because the Pto/Prf complex is oligomeric. Following P+1 loop disruption and transphosphorylation, the Pto/Prf complex dissociates, leading to downstream signalling through mitogen-activated protein kinases (MAPKs). Hence, the Pto/\Prf complex is a sophisticated molecular trap for effectors and provides an excellent model to study the mechanism of MAPK activation. In the current study, we sought to investigate the mechanism of MAPK activation after Pto/\Prf recognition of the AvrPto/AvrPtoB effectors.