Functions outside the photorespiratory pathway in catalyzing 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. Ubiquitous. Mainly expressed in the shoot apical meristem and roots. Also detected in the leaf vasculature, especially in the protoxylem and adjacent cell layers. Does not display a lethal photorespiratory phenotype when grown at ambient carbon dioxide. May be due to a competing acceptor splice site. Belongs to the SHMT family.