Arabidopsis AtPRMT10 is a plant-specific type Ⅰ protein arginine methyltransferase that can asymmetrically dimethylate arginine 3 of histone H4 with auto-methylation activity.Mutations of AtPRMT10 derepress FLOWERING LOCUS C (FLC) expression resulting in a late-flowering phenotype.Here,to further investigate the biochemical characteristics of AtPRMT10,we analyzed a series of mutated forms of the AtPRMT10 protein.We demonstrate that the conserved “VLD” residues and “double-E loop” are essential for enzymatic activity of AtPRMT10.In addition,we show that Arg54 and Cys259 of AtPRMT10,two residues unreported in animals,are also important for its enzymatic activity.We find that Arg13 of AtPRMT10 is the auto-methylation site.However,substitution of Arg13 to Lys13 does not affect its enzymatic activity.In vivo complementation assays reveal that plants expressing AtPRMT10 with VLD-AAA,E143Q or E152Q mutations retain high levels of FLC expression and fail to rescue the late-flowering phenotype of atprmt10 plants.Taken together,we conclude that the methyltransferase activity of AtPRMT10 is essential for repressing FLC expression and promoting flowering in Arabidopsis.