DNA methylation is a post-replicative modification that modulates gene expression and genome stability in many eukaryotes including plants. Methylated DNA is generally associated with reduced gene expression and suppressed homologous recombination. The most abundant methylated residue in eukaryotic genomes is 5-methylcytosine (5-mC) which, in plants, is mainly found in repetitive sequences and is located at symmetrical (CpG, CpNpG) and asymmetric (CH) sites. Once established, the patterns of DNA methylation can be perpetuated over replication by the activity of DNA methyltransferases. Alternatively, methylation patterns can be altered by processes involving de novo DNA methylation or demethylation.
We investigated whether the A. thaliana genome modifies its DNA methylation status at early steps of interaction with Pseudomonas syringae pv. tomato DC3000 (Pst). Surprisingly, the plant genomic DNA displays a massive reduction over its basal 5-mC level 24 hr after pathogen inoculation. This response takes place in the absence of plant DNA replication and precedes plant cell death. Arabidopsis genomic fragments which are targets of Pst-induced altered methylation were identified by AFLP assays monitoring the presence of 5-mC at anonymous CpG, CpNpG and CH sites. Those involving peri/centromeric repeats were further characterized to determine that not every 5-mC of these sequences becomes hypomethylated under infection. Centromeric hypomethylation is associated with cytological alterations of heterochromatin which could even lead to loosening of chromocenters. Together these results indicate that an active demethylation process affecting specific cytosine methylation sites is induced in plant tissues treated with Pst. The involvement of such modification in defense or pathogenicity responses deserves to be determined.
