Defining essential processes in plant pathogenesis with Pseudomonas syringae pv. tomato DC3000 disarmed polymutants and a subset of key type III effectors.

Pseudomonas syringae pv. tomato DC3000 and its derivatives cause disease in tomato, Arabidopsis and Nicotiana benthamiana. The primary virulence factors include a repertoire of 29 effector proteins injected into plant cells by the type III secretion system and the phytotoxin coronatine. The complete repertoire of effector genes and key coronatine biosynthesis genes have been progressively deleted and minimally reassembled to reconstitute basic pathogenic ability in N. benthamiana, and in Arabidopsis plants that have mutations in target genes that mimic effector actions. This approach and molecular studies of effector activities and plant immune system targets have highlighted a small subset of effectors that contribute to essential processes in pathogenesis. Most notably, HopM1 and AvrE1 redundantly promote an aqueous apoplastic environment, and AvrPtoB and AvrPto redundantly block early immune responses, two conditions that are sufficient for substantial bacterial growth in planta. In addition, disarmed DC3000 polymutants have been used to identify the individual effectors responsible for specific activities of the complete repertoire and to more effectively study effector domains, effector interplay and effector actions on host targets. Such work has revealed that AvrPtoB suppresses cell death elicitation in N. benthamiana that is triggered by another effector in the DC3000 repertoire, highlighting an important aspect of effector interplay in native repertoires. Disarmed DC3000 polymutants support the natural delivery of test effectors and infection readouts that more accurately reveal effector functions in key pathogenesis processes, and enable the identification of effectors with similar activities from a broad range of other pathogens that also defeat plants with cytoplasmic effectors.