Identification of a Candidate Gene in Solanum habrochaites for Resistance to a Race 1 Strain of Pseudomonas syringae pv. tomato.

Bacterial speck disease caused by Pseudomonas syringae pv. tomato (Pst) is a persistent problem on tomato (Solanum lycopersicum L.). Resistance against race 0 Pst strains is conferred by the Pto protein, which recognizes either of two pathogen effectors: AvrPto or AvrPtoB. However, current tomato varieties do not have resistance to the increasingly common race 1 strains, which lack these effectors. We identified accessions of Solanum habrochaites S. Knapp & D. M. Spooner that are resistant to the race 1 strain T1. Genome sequence comparisons of T1 and two Pst strains that are virulent on these accessions suggested that known microbe-associated molecular patterns (MAMPs) or effectors are not involved in the resistance. We developed an F2 population from a cross between one T1-resistant accession, LA2109, and a susceptible tomato cultivar to investigate the genetic basis of this resistance. Linkage analysis using whole-genome sequence of 58 F2 plants identified quantitative trait loci (QTL), qRph1, in a 5.8-Mb region on chromosome 2, and qRph2, in a 52.4-Mb region on chromosome 8, which account for 24 and 26% of the phenotypic variability, respectively. High-resolution mapping of qRph1 confirmed it contributed to T1 resistance and delimited it to a 1060-kb region containing 139 genes, including three encoding receptor-like proteins (RLPs) and 17 encoding receptor-like protein kinases (RLKs). One RLK gene, Solyc02g072470, is a promising candidate for qRph1, as it is highly expressed in LA2109 and induced on treatment with MAMPs. qRph1 might be useful for enhancing resistance to race 1 strains and its future characterization could provide insights into the plant immune system.