Research On Methylation Of MLH1 Promoter In Arabidopsis Thaliana Induced By Cadmium

In recent years, the soil heavy metal pollution is increasingly serious. It has effects on our sustainable agriculture and ecological environmental quality. Moreover, it can harm human health through the food chain. Thus, early diagnostic methods of soil heavy metal pollution is particularly important. The diagnostic methods of soil pollution were developed from the initial chemical methods. However, diagnosis using only chemical methods could not reveal whole spectrum of the characteristics of soil quality. Therefore, the molecular markers for indicator method are required to complement chemical ones. Using bisulfite DNA sequencing technique, the present 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+42 to-346 in MLH1 promoter region in Arabidopsis plantlets exposed to Cd of 0-5.0 mg·L-1 cadmium (Cd) for 21 days. The methylation percentage of CpG, CHH and CHG sites was 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 percentage 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 that 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 cytosines residues, and the effect was dose-dependant; Chang percentage of hypermethylation for CpG9 site was 20.0% while chang percentage of hypomethylation for CpG6, CHH44 and CHG4 sites was 12.0%、20.0% and 20.0%, respectively, in the Cd-expoesd seedlings 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 polymorphism of these hot loci is the most sensitive biomarkers for early diagnosis and risk assessment of genotoxic effects of Cd pollution in ecotoxicology.