Histone methyltransferase (PubMed:18375658). Dimethylates 'Lys-4' of histone H3 (H3K4me2) (PubMed:18375658, PubMed:24102415). H3 'Lys-4' methylation represents a specific tag for epigenetic transcriptional activation (PubMed:18375658). Methylates only a limited fraction of nucleosomes of target genes (e.g. NAP and XTH33) (PubMed:18375658). Involved in epigenetic regulation of the floral repressor FLC and FT to prevent the transition from vegetative to reproductive development (PubMed:18375656, PubMed:24102415). N(6)-methyl-L-lysyl-[histone] + S-adenosyl-L-methionine = H(+) + N(6),N(6)-dimethyl-L-lysyl-[histone] + S-adenosyl-L-homocysteine Expressed in roots, leaves and flowers and, to a lower extent, in young seedlings. Accumulates in adult plants, especially in rosette leaves and roots (but not in the vasculature and in tips) (PubMed:18375658). Weakly expressed in inflorescence nodes and at the base of the flowers (PubMed:18375658). In flowers, present in pollen and, at low levels, at the tips of the stigma (PubMed:18375658). In seedlings, observed in the vasculature and in the shoot apical meristems (PubMed:18375656). Activated via O-glycosylation. No obvious phenotype except a slightly delayed abscission of sepals and petals after fertilization (PubMed:18375658). Reduced dimethylated 'Lys-4' histone H3 (H3K4me2) but normal trimethylated 'Lys-4' histone H3 (H3K4me3) at FLC, FT, NAP and XTH33 nucleosomes (PubMed:18375658, PubMed:24102415). Accelerated transition from vegetative to reproductive development (early flowering), especially in medium-day conditions (12 hours light / 12 hours dark), due to FLC and FT epigenetic misregulation (PubMed:18375656, PubMed:24102415). Altered transcription levels of target genes (PubMed:18375658). Decreased XTH33 but normal WRKY70 transcript levels in atx2 plants (PubMed:18375658). Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily. The predicted gene has been split into 2 genes: At1g05830 and At1g05835.