Histone methyltransferase involved in regulation of flowering time. Required for the expression of the SOC1/AGL20 gene. Required for histone H3 trimethylation on 'Lys-4' (H3K4me3) at the SOC1 locus. Prevents trimethylation on 'Lys-27' (H3K27me3) at the same locus. L-lysyl(4)-[histone H3] + 3 S-adenosyl-L-methionine = 3 H(+) + N(6),N(6),N(6)-trimethyl-L-lysyl(4)-[histone H3] + 3 S-adenosyl-L-homocysteine Associates with centromeric constitutive heterochromatin. Late flowering. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. SET2 subfamily.