Methyltransferase that adds a methyl group to the ribose of the last nucleotide of small RNAs (sRNAs). This protects the 3'-end of sRNAs from uridylation activity and subsequent degradation. Can methylate 3'-end of microRNAs (miRNAs), small interfering RNAs (siRNas) and trans-acting small interfering RNAs (ta-siRNAs). Involved in plant development through its role in small RNAs processing. Required for the specification of reproductive organ identities and the probable repression of A class genes. May control floral determinacy possibly by regulating the expression of the C class floral homeotic gene AGAMOUS (AG). S-adenosyl-L-methionine + small RNA 3'-end nucleotide = H(+) + S-adenosyl-L-homocysteine + small RNA 3'-end 2'-O-methylnucleotide Binds 1 Mg(2+) ion per subunit. Binds small RNA duplexes as monomer. Expressed in stems, leaves and inflorescences. Reduced organ size, altered rosette leaf shape and increased number of coflorescences. Urydilation of miRNA 3'-ends. Belongs to the methyltransferase superfamily. HEN1 family.