N6-adenine methyltransferase which modifies the AA dinucleotide at the plant nuclear 18S rRNA nucleotides A1785 and A1786 (PubMed:22829145). Required for generating appropriate patterns of gene expression during root development, including the cell-specific expression of transcriptional regulators involved in root hair and non-hair cells patterning (PubMed:22829145). adenosine(1785)/adenosine(1786) in 18S rRNA + 4 S-adenosyl-L-methionine = 4 H(+) + N(6)-dimethyladenosine(1785)/N(6)-dimethyladenosine(1786) in 18S rRNA + 4 S-adenosyl-L-homocysteine Expressed in rapidly dividing tissues, including root meristems and lateral root primordia, developing cotyledons and leaves, petals, anther, pollen grains and silique abscission zone. The inability to isolate a null mutant suggests that DIM1A is essential for viability. Belongs to the class I-like SAM-binding methyltransferase superfamily. rRNA adenine N(6)-methyltransferase family.