Mapping and characterization of the Rx3 gene for resistance to Xanthomonas euvesicatoria pv. euvesicatoria race T1 in tomato.

KEY MESSAGE: Rx3 encodes a typical CC-NBS-LRR resistance protein and confers the resistance to Xanthomonas euvesicatoria pv. euvesicatoria race T1 causing bacterial spot in tomato. Bacterial spot caused by at least four species of Xanthomonas is an epidemic disease severely affecting tomato production worldwide. The use of resistant cultivars is an economical and effective approach to control the disease. An unimproved tomato breeding line Hawaii 7988 has been considered as the most reliable source for resistance to X. euvesicatoria pv. euvesicatoria race T1, and the Rx3 locus located at a 4.53-Mb region on chromosome 5 (SL4.0) is the major locus for resistance to race T1 in this line. In the current study, the Rx3 locus was firstly located to a 1.05-Mb region based on comparisons of marker polymorphisms between the susceptible line Ohio 88119 and resistant lines Hawaii 7998, Ohio 9834 and FG02-7530. Using recombinant inbred lines (F5:6, F6:7, and F7:8) derived from a cross between Ohio 88119 and Ohio 9834, the Rx3 locus was finally mapped to a 64.3-kb interval between markers MG-Rx3-4 and MG-Rx3-A6. The Solyc05g053980 gene, designated as Rx3, encoding a coiled-coil nucleotide-binding leucine-rich repeat protein was considered as the candidate for the Rx3 locus. Expression of the gene could be induced by the infection of race T1 strain. Knockout of the Solyc05g053980 gene through CRISPR/Cas9 editing system in the resistant line FG02-7530 decreased resistance to race T1 strain. These results provide a close step for understanding the resistance mechanism to race T1 in Hawaii 7998 and guide tomato breeders accordingly to improve bacterial spot disease resistance in tomato.