The previous chapter has described the roles that glycine decarboxylase (GDC) and serine hydroxymethyltransferase (SHMT) play in converting photorespiratory-derived glycine to serine, primarily to recover some of the carbon from the glycolate produced by the oxidative reactions of RUBISCO in C3 plants. As well as being needed for protein synthesis, glycine and serine are precursors of a variety of molecules essential to the growth and development of plant tissues. Glycine is required for the synthesis of, amongst other things, glutathione, porphyrins such as leghemoglobin in nitrogen-fixing root nodules, and purines which are needed for nucleic acid synthesis and, in some nodules, for ureide production. Serine plays a similar role as a precursor of biomolecules, including phospholipids, tryptophan and cysteine, and, under some stress conditions is involved in the synthesis of glycinebetaine (see chapter by Gorham, this volume, and Rhodes & Hanson, 1993). The interconversion of glycine and serine, with the concomitant production of methylene tetrahydrofolate, is also important to the plant as a source of one carbon units (see Cossins, 1980). In C3 leaves, photorespiration provides substantial amounts of glycine and serine which can be used for such syntheses. In non-photosynthetic tissue such as roots and developing and germinating seeds, there is a need for glycine and serine just as in the leaves, but there is no photorespiration to produce high levels of glycine and serine. Alternative, nonphotorespiratory routes for the production of glycine and serine exist in such tissues, as well as in the leaves of C3 plants.