Distribution of Bacterial Blight Resistance Genes in the Main Cultivars and Application of Xa23 in Rice Breeding.

Bacterial blight (BB) is an important constraint on achieving a high and stable rice grain yield. An increasing number of BB resistance (R) genes have been identified and cloned to increase the available options for rice disease resistance breeding. However, it is necessary to understand the distribution ofRgenes in rice varieties for rational distribution and breeding. Here, we genotyped fiveRgenes,i.e. Xa4,Xa7,Xa21,Xa23, andXa27, in seventy main cultivars from Guangdong Province, South China using the corresponding specific markers. Our results showed that 61 varieties carriedXa4, only three varieties carriedXa27, andXa7,Xa21, orXa23was not detected in all tested varieties. Notably, only 33 varieties exhibited resistance to pathotype IVXoostrains. These results indicate thatXa4is no longer suitable for widespread use in rice breeding, althoughXa4is widely present in tested varieties. Remarkably, the strongly virulent BB strains of pathotype IX evolved quickly in southern China, andXa23was found to effectively confer resistance against the pathotype IX strains. Subsequently, we successfully bred two novel inbred rice varieties as also being restorer lines and two photoperiod- and thermo-sensitive genic male sterility (P/TGMS) lines using the broad-spectrum resistance geneXa23through marker-assisted selection (MAS) combined with phenotypic selection. All of the developed lines and derived hybrids exhibited enhanced resistance to BB with excellent yield performance. Our research may potentially facilitate both of the inbred and hybrid rice disease resistance breeding.