Because of the accumulation of heavy metals, Hydrilla verticillata (L.f.) Royle, 
a rooted submerged perennial aquatic herb, is being developed as a potential 
tool to clean the aquatic ecosystem polluted by heavy metals. However, its 
physiological responses for heavy metal remain to be elucidated. Here, through 
employing proteomics approach, we found that excess Cu significantly induced the 
expressions of four DNA methylation related proteins in H. verticillata, which 
were the homologues of two domains rearranged methyltransferases (DRM), a 
methyltransferases chromomethylase (CMT) and a histone H3 lysine-9 specific 
SUVH6-like (SUVH6). Consistently, a dramatic change in DNA methylation patterns 
was detected in excess Cu-exposed H. verticillata. Surprisingly, administration 
of the NADPH oxidase inhibitors, diphenylene iodonium (DPI) and imidazole (IMZ) 
that block production of reactive oxygen species (ROS) could trigger the 
remethylation of genomic sites that were demethylated by excess Cu, indicating 
that Cu-induced ROS might be another way to affect DNA methylation. Further 
analysis suggested this changed DNA methylation may be owing to the ROS-induced 
DNA damage. Taken together, our findings demonstrate that two different ways to 
influence DNA methylation in excess Cu-treated H. verticillata.