Suppression and Activation of the Plant Immune System by Pseudomonas syringae Effectors AvrPto and AvrPtoB

Pseudomonas syringae uses a type III secretion system to deliver a large number of effector proteins into the plant cell where they suppress plant immune responses and promote pathogen virulence. Two of the best studied type III effectors are AvrPto and AvrPtoB, each of which has multiple structural domains with distinct activities in the plant cell. Certain tomato accessions express the Pto protein kinase that recognizes AvrPto and AvrPtoB, activating effector-triggered immunity. In plants lacking Pto, AvrPto and AvrPtoB suppress PAMP-triggered immunity by interfering with pattern recognition receptor complexes. The effectors exploit host protein modification processes such as phosphorylation, myristoylation, and ubiquitination to promote their virulence activity. AvrPto and AvrPtoB therefore are consummate manipulators of the host immune response and plants are engaged in an ongoing molecular ‘arms race’ to monitor and react to their presence.