Catalyzes the interconversion of serine and glycine with the conversion of tetrahydrofolate (THF) into 5,10-methylene-THF. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + glycine + H2O = (6S)-5,6,7,8-tetrahydrofolate + L-serine Inhibited by the antifolate drugs methotrexate and pemetrexed. One-carbon metabolism; tetrahydrofolate interconversion. Homotetramer (PubMed:31873125). Interacts with UBP16 (PubMed:23232097). Mostly expressed in flowers, less abundant in roots, inflorescence stems, and siliques, and barely detectable in leaves. Circadian-regulation. Belongs to the SHMT family.