A Rapid Seedling Resistance Assay Identifies Wild Tomato Lines That Are Resistant to Pseudomonas syringae pv. tomato Race 1

Bacterial speck caused by Pseudomonas syringae has historically been controlled by the Pto/Prf gene cluster. Emerging strains like P. syringae pv. tomato race 1 overcome resistance conferred by Pto/Prf, and can cause serious crop loss under appropriate environmental conditions. We developed a rapid assay to screen wild tomato seedlings for resistance to P. syringae pv. tomato race 1. We established the seedling resistance assay using the well-characterized P. syringae pv. tomato race 0 strain, DC3000, which is recognized in tomato cultivars carrying Pto/Prf (PtoR) and causes disease in isogenic lines lacking this cluster (PtoS). We optimized infectious conditions for P. syringae on tomato seedlings and demonstrated that tomato seedlings respond like adult tomato plants in critical measures of susceptibility and immunity, including the hypersensitive response, rapid ion leakage, restricted bacterial proliferation, and phenotypic resistance. After establishing infectious conditions for P. syringae pv. tomato race 1 on tomato seedlings, we screened 96 wild accessions and identified two accessions with strong P. syringae pv. tomato race 1 resistance, Solanum neorickii LA1329 and S. habrochaites LA1253, which are also resistant to bacterial infection as adult plants. This rapid high throughput seedling assay has many advantages, including reduced plant growth time and large sample sizes, and will allow for large-scale screening of resistance in tomato.