Induced Pib Expression and Resistance to Magnaporthe grisea are Compromised by Cytosine Demethylation at Critical Promoter Regions in Rice

Pib is a well‐characterized rice blast‐resistance gene belonging to the nucleotide binding site (NBS) and leucine‐rich repeat (LRR) superfamily. Expression of Pib was low under non‐challenged conditions, but strongly induced by the blast‐causing fungal pathogen Magnaporthe grisea, thereby conferring resistance to the pathogen. It is generally established that cytosine methylation of the promoter‐region often plays a repressive role in modulating expression of the gene in question. We report here that two critical regions of the Pib promoter were heavily CG cytosine‐methylated in both cultivars studied. Surprisingly, induced expression of Pib by M. grisea infection did not entail its promoter demethylation, and partial demethylation by 5‐azacytidine‐treatment actually reduced Pib expression relative to wild‐type plants. Accordingly, the blast disease‐resistance was compromised in the 5′‐azaC‐treated plants relative to wild‐type. In contrast, the disease susceptibility was not affected by the 5′‐azaC treatment in another two rice cultivars that did not contain the Pib gene, ruling out effects of other R genes and non‐specific genotoxic effects by the drug‐treatment as a cause for the compromised Pib‐conditioned blast‐resistance. Taken together, our results suggest that promoter DNA methylation plays a novel enhancing role in conditioning high‐level of induced expression of the Pib gene in times of M. grisea infection, and its conferred resistance to the pathogen.