Maintains chromatin CpG methylation that plays a role in genomic imprinting, regulation of embryogenesis and seed viability. Required for proper patterns of CG DNA methylation in dividing cells (By similarity). a 2'-deoxycytidine in DNA + S-adenosyl-L-methionine = a 5-methyl-2'-deoxycytidine in DNA + H(+) + S-adenosyl-L-homocysteine Expressed at low levels in vegetative and floral organs. Belongs to the class I-like SAM-binding methyltransferase superfamily. C5-methyltransferase family.