Rice Genetics IV - IRRI books - International Rice Research Institute

Suppression of ROS production and cell deathby the dominant-negative OsRac1We produced a dominant-negative form of OsRac1 by changing threonine at the 24thposition to asparagine, fused with the 35S promoter (Fig. 3), and introduced it into thesl mutant. The mutant dominant-negative form is able to suppress activities of all Racgenes within the same cell. In untransformed sl cell cultures, H 2 O 2 production wasinduced by calyculin A, a protein phosphatase inhibitor, and the induction was inhibitedby DPI. In contrast, carbonic anhydrase (CA)-induced H 2 O 2 production was inhibitedin the sl cells transformed with OsRac1-T24N, indicating that OsRac wasrequired for activation of H 2 O 2 production in the sl cells. To examine whether thedominant-negative OsRac1 suppresses lesion formation in sl plants, leaves ofuntransformed and transformed sl plants were inoculated with the rice blast fungus,Magnaporthe grisea. The results showed that lesion formation induced by the funguswas strongly inhibited in transformed plants. These results strongly suggest the roleof OsRac in induction of ROI production and cell death in rice.Role of OsRac1 for R gene–mediated blast resistance in riceTo test whether OsRac1 is involved in R gene–mediated disease resistance in rice, wechose the rice blast fungus M. grisea, since a series of R genes corresponding toseveral races has been identified (Notteghem et al 1994) and a gene-for-gene relationshiphas been established in this plant-pathogen interaction (Valent and Chumley 1994).More recently, Pi-b and Pi-ta genes, the R genes for blast resistance, were isolatedand shown to be members of the NBS-LRR class of plant R genes (Wang et al 1999,Jia et al 2000).Protein kinaseAVR R gene Rac?NADPH oxidaseO 2_ /H2 O 2Scavengers• Peroxidase• Catalase• MetallothioneinH2 OHeterotrimeric GproteinCell deathGene expression ofphytoalexinResistanceFig. 3. Model of the signaling pathways for R gene–mediated disease resistanceof rice.326 Shimamoto et al