Involved in melatonin biosynthesis (PubMed:29901843). Can function as acetylserotonin O-methyltransferase (PubMed:29901843). Catalyzes the transfer of a methyl group onto N-acetylserotonin, producing melatonin (N-acetyl-5-methoxytryptamine) (PubMed:29901843). Involved in the regulation of jasmonate- and brassinosteroid-mediated plant growth and defense responses (PubMed:26864209). N-acetylserotonin + S-adenosyl-L-methionine = H(+) + melatonin + S-adenosyl-L-homocysteine Aromatic compound metabolism; melatonin biosynthesis; melatonin from serotonin: step 1/2. Expressed in roots, leaf sheaths, flag leaves and panicles. Reduced plant growth and grain yield (PubMed:26864209, PubMed:29901843). Reduced content of chlorophyll in leaves (PubMed:26864209, PubMed:29901843). Reduced number and size of chloroplasts (PubMed:29901843). Altered structure of chloroplasts (PubMed:29901843). Reduced root length and lateral root number (PubMed:29901843). Formation of hypersensitive response (HR)-like lesions (PubMed:26864209). Early heading and premature leaf senescence (PubMed:29901843). Accumulation of the antimicrobial compounds momilactones and phytocassanes, and constitutive expression of pathogenesis-related genes (PubMed:26864209). Over-accumulation of jasmonates and elevated levels of brassinosteroid (PubMed:26864209). Increased leaf lamina bending (PubMed:26864209). Overexpression of MTS1 in transgenic lines enhances grain yield. Belongs to the methyltransferase superfamily. LCMT family.