Book excerpt: The use of natural variation is a powerful tool to study the evolution of plant resistance genes and pathogen effectors. In tomato, the Pto protein kinase interacts directly with two Pseudomonas syringae pv. tomato (Pst) effectors, AvrPto and AvrPtoB. This direct interaction is mediated by the binding of each effector through both a shared and a unique interface with Pto. The presence of two unique interfaces suggested that the recognition of these two effectors by Pto might have evolved independently. We conducted a screen of wild tomato accessions for their ability to mount effector-triggered immunity upon recognition of AvrPto or AvrPtoB to seek evidence of natural variation that would shed further light on how Pto-like kinases recognize and respond to two structurally different effectors. Our screen of wild relatives of tomatoes uncovered 22 accessions of Solanum chmielewskii (Schm) that recognize only AvrPtoB. Through further molecular characterization we found that a single histidine-to-aspartate substitution at position 193 in the activation domain of Schm Pto-2677 was sufficient to confer recognition of AvrPto in plant cells. The reciprocal substitution of aspartate-to-histidine-193 in Pto abolished AvrPto recognition, confirming the importance of this residue for signaling in response to AvrPto. Our results reveal that there are not only distinct binding interfaces involved in the Pto response to these effectors, but that there is also a difference in downstream signaling. Based on recent worldwide collections of Pst isolates it is known that race 1 strains have displaced race 0 strains and are now the most common strains found in the field. Race 0 strains express AvrPto or AvrPtoB and elicit resistance in Pto-expressing tomato lines, whereas race 1 strains lack these effectors and do not elicit resistance. We screened Pst isolates from infected field tomato plants across New York (NY) in 2015 and characterized them for their virulence and for the presence of specific effectors. We found that all isolates encode a functional AvrPto, which can be recognized by Pto. However, this recognition is 'masked' during later stages of infection, allowing development of mild bacterial speck symptoms in Pto-expressing tomatoes when vacuum infiltrated with high bacterial populations and under laboratory conditions. Our study demonstrates that introgression of Pto is still a viable strategy to manage bacterial speck of tomatoes in NY. ...