Possesses histone H3 methyltransferase activity in vitro (PubMed:24420930). Methylates 'Lys-4' of histone H3. H3 'Lys-4' methylation represents a specific tag for epigenetic transcriptional activation. Functions as a receptor for the lipid messenger phosphatidylinositol 5-phosphate (PI5P), which regulates negatively its transcriptional activation activity (By similarity). Involved in the regulation of flowering time and floral induction under long day (LD) conditions. Acts as an activator of flowering under LD conditions. May function through binding to EHD3, a repressor of GHD7 (PubMed:24420930). L-lysyl(4)-[histone H3] + S-adenosyl-L-methionine = H(+) + N(6)-methyl-L-lysyl(4)-[histone H3] + S-adenosyl-L-homocysteine Interacts with EHD3. Expressed in leaf blades and panicles. Late-flowering phenotype under long day (LD) conditions. Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily.