Marker-assisted development and characterization of near-isogenic lines carrying the Rx4 gene for hypersensitive resistance to Xanthomonas euvesicatoria pv. perforans race T3 in tomato

Abstract

Bacterial spot caused by several species of Xanthomonas is a devastating disease resulting in severe fruit yield loss and quality decline in tomato production areas worldwide. Lack of resistant commercial varieties is among the most critical factors limiting the application of integrated management to control the disease. Although sources for resistance to all species and races have been discovered over years, breeding for resistance to the disease is slow because the resistance is quantitatively inherited and sometimes linked to poor horticultural traits. Development of near-isogenic lines (NILs) by introgressing the shortest DNA fragment of the resistance gene region from the donor into elite cultivars is a promising approach to eliminating the negative effect on horticultural traits. In the current study, a strategy of combining traditional backcross with molecular marker-assisted foreground and background selection was adopted to generate NILs carrying the Rx4 gene for resistance to Xanthomonas euvesicatoria pv. perforans race T3 strain. The donor parent is a wild species Solanum pimpinellifolium accession PI 128216, and the recurrent parent is an elite processing tomato breeding line OH88119. Disease evaluation was performed in F1, BC4F1, BC6F1, BC6F2, BC6F3, and BC6F4 generations by infiltration of race T3 strain Xv829 to leaves of plants. Meanwhile, fruit traits including fruit weight, fruit color parameters, soluble solid content, pH, and firmness were collected from replicated field trials in BC6F4 and BC6F5. Five NILs with 159–1901 kb DNA fragments in the Rx4 gene region were obtained. They all showed high level of hypersensitive resistance to race T3, and their fruit traits were not significantly different from the recurrent parent OH88119. However, fruit weight of one NIL Rx4–1812 carrying the largest DNA fragment of 1901 kb in the Rx4 gene region was 9.5–11.8% lower than OH8819 though the difference was not statistically significant. This might be due to the linkage drag of FW11.2, a QTL for fruit weight in the same region. Overall, the NILs created here can be used as sources for developing resistant varieties and deciphering the mechanism of resistance to bacterial spot in tomato.