Marker-assisted introgression of Xa39 improve rice resistance to five Xanthomonas oryzae pv. oryzae strains

Bacterial blight (BB), which is caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the most important rice diseases in Asian countries. Developing resistant rice hybrids and cultivars is the most effective and economical strategy for controlling bacterial disease. In this study, a novel dominant gene Xa39, which confers broad-spectrum resistance for BB, was introgressed into two rice restorer lines H6X6 and H6X11 by a marker-assisted introgression breeding approach, combined with artificial inoculation with five Xoo and rigorous phenotypic selections. Finally, the two restorer lines H6X6-Xa39 and H6X11-Xa39 carrying a broader spectrum of BB resistance were developed. H6X6-Xa39, H6X11-Xa39, and their hybrid combinations with three sterile lines exhibited the same level of resistance as that of resistant donor FF329. The newly developed BB-resistant restorers and their derived hybrids with sterile lines were mostly identical to their respective original versions for agronomic traits, suggesting considerable value of the Xa39 gene in breeding BB resistance for genetic improvement of hybrid rice. Our results provide important reference information for the utilization of the Xa39 gene in rice breeding program.