Rice defense mechanisms against Cochliobolus miyabeanus and Magnaporthe grisea are distinct

Responses of rice to Magnaporthe grisea and Cochliobolus miyabeanus were compared. In Tetep, a rice cultivar resistant to both fungi, pathogen inoculation rapidly triggered the hypersensitive reaction (HR), resulting in microscopic cell death. In rice cv. Nakdong, susceptible to both pathogens, M. grisea did not cause HR, whereas C. miyabeanus caused rapid cell death similar to that associated with HR, which appeared similar to that observed in cv. Tetep, yet failed to block fungal ramification. Treatment with conidial germination fluid (CGF) from C. miyabeanus induced rapid cell death in both cultivars, suggesting the presence of phytotoxins in CGF. Pretreatment of cv. Nakdong with CGF significantly increased resistance to M. grisea, while the same treatment was ineffective against C. miyabeanus. Similarly, in cv. Nakdong, benzothiadiazole (BTH) significantly increased resistance to M. grisea, but was ineffective against C. miyabeanus. Methyl jasmonate (MeJA) treatment appeared to be ineffective against either fungus. Increased resistance of cv. Nakdong to M. grisea by BTH or CCF treatment was correlated with more rapid induction of three monitored PR genes. Application of MeJA resulted in the expression of JAmyb in cv. Nakdong being induced faster than in untreated plants in response to M. grisea infection. In contrast, the expression pattern of the PR and JAmyb genes in response to C. miyabeanus was nearly identical between cvs. Nakdong and Tetep, and neither BTH nor MeJA treatment significantly modified their expression patterns in response to C. miyabeanus infection. Our results suggest that rice employs distinct mechanisms for its defense against M. grisea versus C. miyabeanus.