Genetic Improvement of Rice for Bacterial Blight Resistance: Present Status and Future Prospects

The production and productivity of rice has been challenged due to biotic and abiotic factors. Bacterial blight (BB) disease, caused by Xanthomonas oryzae pv. oryzae, is one of the important biotic stress factors, which reduces rice production by 20%–50%. The deployment of host plant resistance is the most preferred strategy for management of BB disease, and breeding disease resistant varieties remains a very economical and effective option. However, it is difficult to develop rice varieties with durable broad-spectrum resistance against BB using conventional approaches alone. Modern biotechnological tools, particularly the deployment of molecular markers, have facilitated the cloning, characterization and introgression of BB resistance genes into elite varieties. At least 46 BB resistance genes have been identified and mapped from diverse sources till date. Among these, 11 genes have been cloned and characterized. Marker-assisted breeding remains the most efficient approach to improve BB resistance by introducing two or more resistance genes into target varieties. Among the identified genes, xa5, xa13 and Xa21 are being widely used in marker-assisted breeding and more than 70 rice varieties or hybrid rice parental lines have been improved for their BB resistance alone or in combination with genes/QTLs conferring tolerance to other stress. We review the developments related to identification and utilization of various resistance genes to develop BB resistant rice varieties through marker-assisted breeding.