Advances in the Xoo-rice pathosystem interaction and its exploitation in disease management.

Bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo), is one of the devastating diseases of rice worldwide. The pathogen reported to cause 70% crop loss in some of the susceptible genotypes under disease favoring environments, viz., temperature ranging between 25 to 34°C and relative humidity more than 70%. In Xoo, about 245 genes govern the pathogenicity and host specificity. The hypersensitive response and pathogenicity (hrp) genes responsible for disease occurrence were clustered in the pathogenicity island of 31.3 Kb. The protein secreted through type three secretory system and type one secretory system mediates infection and establishment of the pathogen inside the host. However, elicitor molecules from Xoo triggered the resistant response in rice against the pathogen. An array of resistant genes (R genes) was known to be invoked by the host to combat the bacterial infection. To date, of the 45 Xa genes in rice, nine were cloned and characterized. The evolution of new races has made the task of developing resistant rice genotypes more challenging as it demands a comprehensive breeding strategy involving the best use of R genes from the existing gene pool. Thus, to combat the infection from the existing races and to slow down the emergence of new Xoo races, pyramiding two or more R genes was found to be effective against bacterial blight disease. In India, the successfully commercialized example includes the development of rice genotypes, viz., Improved Pusa Basmati- 1, Improved Samba Mahsuri, PR106, Type 3 Basmati, and Mahsuri with selected R genes, viz., xa5, Xa4, xa13 and Xa21 against bacterial blight resistance. This review primarily portray Xoo-rice interactions and provides opportunities for its effective management through sustainable technologies.