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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 of  R  genes in rice varieties for rational distribution and breeding. Here, we genotyped five  R  genes,  i.e. Xa4 ,  Xa7 ,  Xa21 ,  Xa23 , and  Xa27 , in seventy main cultivars from Guangdong Province, South China using the corresponding specific markers. Our results showed that 61 varieties carried  Xa4 , only three varieties carried  Xa27 , and  Xa7 ,  Xa21 , or  Xa23  was not detected in all tested varieties. Notably, only 33 varieties exhibited resistance to pathotype IV  Xoo  strains. These results indicate that  Xa4  is no longer suitable for widespread use in rice breeding, although  Xa4  is widely present in tested varieties. Remarkably, the strongly virulent BB strains of pathotype IX evolved quickly in southern China, and  Xa23  was 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 gene  Xa23  through 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.

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