Abstract: Using bisulfite DNA sequencing technique, this study investigated the methylation patterns of 71 cytosine residues including 16 CpG sites, 6 CHG sites (H as C, A or T) and 49 CHH sites within the region 391 bp between nucleotides -346 to +42 in MLH1 promoter region in Arabidopsis plantlets exposed to 0-5.0 mg·L^-1 cadmium (Cd) for 21 days. The methylation percentages of CpG, CHH and CHG sites were 44.8%, 40.5% and 52.0%, respectively, in the control. With the increasing Cd level, the hyper and hypomethylated cytosine residues, and the methylation percentages of those residues for CpG, CHG and CHH sites were increased in the Cdtreated seedlings, and were higher at CpG and CHH sites than in the controls except for CHG sites. Among 71 cytosines in MLH1 promoter regions, methylation polymorphism of CpG6, CpG9, CHH44 and CHG4 sites was much more sensitive to Cd stress than other cytosine residues, and the effect was dosedependent. The percentage of hypermethylation change for CpG9 site was 20.0%, while the percentages of hypomethylation change for CpG6, CHH44 and CHG4 sites were 12.0%, 20.0% and 20.0%, respectively, in the Cdexpoesd seedlings at Cd level of 0.25 mg·L^-1. The presence of significant methylation polymorphism of cytocine residues for the above four hot spots in MLH1 promoter of Cdtreated seedlings, prior to the onset of leaf number, fresh mass and chlorophyll content of shoots, suggests that methylation polymorphisms of these hot loci are the most sensitive biomarkers for early diagnosis and risk assessment of genotoxic effects of Cd pollution.

Key words: soil nutrient, land-use type, topographic factor, geostatistics, small watershed.