Functions in the photorespiratory pathway in catalyzing the interconversion of serine and glycine. Involved in controlling cell damage caused by abiotic stress, such as high light and salt and the hypersensitive defense response of plants. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + glycine + H2O = (6S)-5,6,7,8-tetrahydrofolate + L-serine One-carbon metabolism; tetrahydrofolate interconversion. Homotetramer (By similarity). Interacts with GLU1 (PubMed:19223513). Interacts with UBP16 (PubMed:23232097). Ubiquitous. Mostly expressed in leaves, less abundant in stems, flowers and siliques, and barely detectable in roots. Circadian-regulation. Induction by light. Ubiquitinated. Displays a lethal photorespiratory phenotype when grown at ambient carbon dioxide. Recessive mutant shmt1-1 (shm1-3) shows enhanced susceptibility to salt stress and to biotrophic and necrotrophic pathogens. Belongs to the SHMT family.