Histone methyltransferase specifically required for trimethylation of 'Lys-4' of histone H3 (H3K4me3) and is crucial for both sporophyte and gametophyte development (PubMed:21037105, PubMed:20937886). Function as a diurnal 'writer' to counteract the nocturne 'eraser' demethylase activity of JMJ14 thus orchestrating the circadian rythm of histone modifications (e.g. H3K4me3) and modulating the rythmic expression of diurnal target genes; this mechanism relies also on the circadian clock oscillators CCA1 and LHY (PubMed:31429787). 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 Expressed in roots, leaves, stems and inflorescences. Circadian-regulation with peak levels occurring in the morning under diurnal but not constant light conditions in a CCA1- and LHY-dependent manner. Pleiotropic effects on plant growth and development, including dwarf size, aberrant root development and sterile flowers (PubMed:20937886, PubMed:21037105). Decreased levels of CCA1 and LHY circadian oscillators transcription factors as well as of other clock genes such as TOC1, PRR5, PRR7, PRR9, GI, ELF3, ELF4, and LUX associated with reduced H3K4me3 levels near their promoters (PubMed:31429787). Belongs to the class V-like SAM-binding methyltransferase superfamily. Histone-lysine methyltransferase family. TRX/MLL subfamily.