CHAPTER 14: Defense mechanisms mediated by chitin in rice-blast interactions

Chitin is widely distributed in fungal cell walls, and its fragments act as potent elicitors in several plant systems. Indeed, pretreatment of rice seedlings with the chitin elicitor results in increased resistance to rice blast. Our histochemical and reverse genetic analyses suggest that the rice blast fungus possesses a mechanism to avoid generation of the chitin elicitor in the first-invaded cell, but none of the genes encoding chitin-binding domains that were tested was responsible for the mechanism. The release of the chitin elicitor during the infection process is thus limited, and the chitin elicitor induces basal resistance, which is nonspecific but not strong enough to avoid infection completely. Therefore, we manipulated cellular responses upon sensing the chitin to evoke an HR-like response by modifying CEBiP, a receptor for the chitin elicitor, with the intracellular region of XA21. Rice plants constitutively expressing the chimeric receptor displayed enhanced resistance to rice blast. In future studies using molecular breeding for disease resistance, a combination approach in which both the chimeric receptor and chitinase are overexpressed simultaneously, or using another receptor-like protein kinase that induces more effective HR, would be helpful.