In plants and animals, the incorporation of a methyl group in the carbon 5 of cytosine (5mC) generates a stable epigenetic mark that regulates gene expression. The 5mC levels can be modified by environmental conditions and thus affect responses to stress. We have recently demonstrated that the genome of Arabidopsis thaliana severely reduces the content of 5mC upon the attack of Pseudomonas syringae pv. tomato (Pst). Infected plants also display decondensation of centromeric chromatin. Pst-induced DNA hypomethylation occurs in the absence of DNA replication suggesting it may result from the action of helix-hairpin-helix DNA glycosylases, which mediate active DNA demethylation in Arabidopsis. Here we evaluate the involvement of a novel putative Arabidopsis DNA glycosylase, ACDI (Abnormal Chromatin Decondensation in Infection), in the activation of defenses against Pst. We constructed Arabidopsis T-DNA and siRNA mutant plants impaired in ACDI, as well as ADCI over-expressing lines. Abnormal features of chromatin condensation and susceptibility to Pst were found in these plants. Our results suggest that this putative glycosylase may participate in the phenomena of Pst-induced chromatin decondensation and the regulation of biotic defense responses.
