Methylation of cytosine residues is a key process for regulation of chromatin structure and gene expression in several species. We have previously demonstrated that the Arabidopsis genome display a 70% reduction of 5-methyl cytosine (5-mC) content at 24 hours post-infection with Pseudomonas. The main targets of this modification are CpG and/or asymmetric cytosines located in Arabidopsis centromeric regions. Although transcriptional activity of pericentric coding sequences is not substantially altered, we here demonstrate that 5-mC loss causes main changes in the chromatin organization of intephasic nuclei from infected cells. DAPI staining combined with inmunolabelling of 5-mC with monoclonal antibodies showed that reduction of 5-mC residues at centromeres precedes decondensation of these heterochromatic regions. Both responses were only observed at the inoculation sites of infected tissues but not at distal sites of challenged leaves. Neither molecular markers of cell cycle progression nor BrdU incorporation was detected at bacterial-treated tissues, indicating that an active demethylation mechanism might operate. Whether this event is part of Pseudomonas pathogenicity strategies or it is involved in Arabidopsis defense responses remains to be elucidated.
