Cytosine methylation alterations play key roles in gene regulation in plants interacting with biotrophic pathogens. For resistant and susceptible interactions of cucumber with the powdery mildew pathogen, Podosphaera xanthii, methylation of DNA cytosines were monitored by methylation-sensitive amplifed polymorphism (MSAP) analysis from immediately prior to 96 h post inoculation (hpi). In both resistant and susceptible interactions, methylation levels signifcantly changed over time, but a signifcant association was observed for decreasing number of loci with demethylated CCGG motif in the susceptible cultivar compared to increasing numbers in the resistant cultivar. Sequencing of six demethylated MSAP fragments from the resistant cultivar and one from susceptible cultivar showed that all contained promoter and cis-acting regulatory elements (CAREs). For the resistant cultivar, three of these were in possible regulatory regions located within or near tRNA coding genes, and those three plus two others contained CAREs associated with response to biotic stress and defense hormones SA, MeJA, ABA and ET. One possible reason for demethylation in the resistant genomic tRNA genes may be that demethylation can de-suppress gene expression resulting in increasing the availability of certain amino acids for defense proteins.
