Catalyzes methyl transfer from S-methylmethionine (SMM) to adenosyl-L-homocysteine (AdoMet). SMM degradation (by HMT-1, HMT-2, HMT-3 and HMT-4) and biosynthesis (by MMT1) constitute the SMM cycle in plants, which is probably required to achieve short term control of AdoMet level 