Rice Genetics IV - IRRI books - International Rice Research Institute

Japonica rice variety Kinmaze, which was used to produce transgenic plants, carriesthe Pi-a resistance gene that is incompatible with race 31 of the blast fungus. Weinfected the leaf blade of transgenic rice plants expressing the constitutive activeOsRac1 with race 7 of the blast fungus by the punch infection method. Because race7 is compatible with var. Kinmaze, infected leaves of untransformed control plantsdeveloped typical disease symptoms. In contrast, transgenic plants were highly resistantor showed a reduced level of symptoms. These results indicate that rice plantstransformed with the constitutive active OsRac1 acquire resistance to a compatiblerace of the rice blast fungus and exhibit hypersensitive resistance (HR)-like responses.We next asked whether HR caused by an avirulent race of the rice blast fungus issuppressed by expression of the dominant-negative OsRac1. The leaf blades oftransgenic plants expressing the dominant-negative OsRac1 were infected with anavirulent race, 31, of the blast fungus. Typical HR accompanied by cell death wasobserved in untransformed control plants. In contrast, in transgenic lines examined,HR was clearly suppressed in the leaf blade and suppression was observed in repeatedexperiments, indicating that the dominant-negative OsRac1 suppresses R gene-specificresistance against rice blast fungus. Taken together, our results strongly suggestthat OsRac1 is an essential component of a resistance-signaling pathway for the blastfungus acting downstream of R genes.Activation of phytoalexin production in transgenic riceexpressing the constitutive active OsRac1We examined the amounts of momilactone A, a major phytoalexin of rice (Cartwrightet al 1977), in the leaves of transgenic rice. In all seven transgenic lines examined, thelevels of momilactone A were highly elevated and the increases were 19–180-foldhigher than the levels of the untransformed control plants. Transgenic lines expressingthe dominant-negative OsRac1 did not show increased levels of momilactone A.These results indicate that constitutive active OsRac1 activates phytoalexin synthesisto confer resistance to the blast fungus.Alteration of defense-related gene expression in transgenic riceexpressing the constitutive active OsRac1To further elucidate the basis for the blast resistance observed in transgenic rice expressingthe constitutive active OsRac1, we next examined the expression of twogenes that are closely associated with disease resistance in rice. Expression of D9 washighly activated in all lines examined. D9 was isolated from lesion-mimic mutants ofrice (Takahashi et al 1999) resistant to rice blast and found to be up-regulated in thesemutants. Its deduced amino acid sequence is highly homologous with that of terpenoidsynthase and may be involved in momilactone A synthesis (unpublished results).One of the rice peroxidase genes, POX22.3, which was originally isolated as a geneactivated during the resistance reactions of rice against Xanthomonas oryzae pv. oryzae(Chittoor et al 1997), was shown to be strongly down-regulated in the transgenic riceexpressing the constitutive active OsRac1 that acquired resistance to blast disease.This observation suggests that OsRac1 may also function to down-regulate H 2 O 2 scav-Molecular signaling in disease resistance of rice 327