Histone demethylase that demethylates 'Lys-27' (H3K27me) of histone H3, thus acting as a positive regulator of gene expression (PubMed:33107825). Demethylates tri-methylated (H3K27me3) and di-methylated (H3K27me2) H3K27me (PubMed:33107825). Inactive on H3K27me1, H3K4me3, H3K9me2 and H3K36me3 (PubMed:33107825). Acts as a repressor of the photoperiodic flowering pathway and of FT (PubMed:33107825). May also be active on H3K4me. Binds around the transcription start site of the FT locus. Required for epigenetic reprogramming by resetting the expression of the floral repressor FLC locus, thus aluviating cold-mediated FLC epigenetically silencing occurring during vernalization and preventing inapropriate epigenetic states inheritence (PubMed:25219852, PubMed:33107825). Together with REF6, required for H3K27me3 resetting (especially in constitutive heterochromatin within the pericentromeric regions) and transgenerational inheritance of histone marks, thus acting in safeguarding genome and epigenome integrity during sexual reproduction. 2-oxoglutarate + N(6),N(6),N(6)-trimethyl-L-lysyl(27)-[histone H3] + O2 = CO2 + formaldehyde + N(6),N(6)-dimethyl-L-lysyl(27)-[histone H3] + succinate 2-oxoglutarate + N(6),N(6)-dimethyl-L-lysyl(27)-[histone H3] + O2 = CO2 + formaldehyde + N(6)-methyl-L-lysyl(27)-[histone H3] + succinate Interacts with BZR2 (via N-terminus). Expressed at low levels in seedlings, cotyledons and leaves (PubMed:25219852). Detected in inflorescences, stems, roots and siliques but not in shoot apical meristems or root tips. Accumulates in flowers and embryos (PubMed:25219852). Early flowering a reduced number of rosette leaves at bolting stage, but normal development of all other organs (PubMed:33107825, PubMed:25219852). Hyper-methylated genes with accumulation of H3K27me3 histone marks (PubMed:33107825). Reduced FLC expression (PubMed:25219852). Partially redundant with JMJ14. Brassinosteroid-insensitive phenotype. Plants lacking both REF6 and ELF6 have several growth defects, such as increased number of petals, reduced silique length, embryos with patterning defects, and pleiotropic defects in leaf morphology, such as serrations and downward curling; these defects are caused by epimutations arising in offspring lineage due to a lack of H3K27me3 resetting during sexual reproduction (PubMed:33107825). Belongs to the JHDM3 histone demethylase family. Truncated N-terminus.