It is now recognized that in C₃ plants GS₁ is localized in the cytoplasm, but is present mainly in the phloem companion cells, and GS₂ is located in the chloroplasts.

Refixation of photorespiratory ammonium is mediated primarily by GS₂. Despite this efficient system for reassimilating ammonium, plants with normal activities of GS can lose ammonia to the atmosphere when grown on ammonium as sole N source. Ammonia emission is greatly elevated when GS is inhibited with methionine sulfoximine. Ammonia emission rates are also significantly increased in barley mutants with reduced GS activity (47 to 66% of wildtype) (Mattsson et al, 1997).

The photorespiratory N cycle requires an important metabolic interplay between mitochondria and peroxisome. Thus, the reduction of serine in the peroxisomes to glycerate (via hydroxypyruvate) requires reducing equivalents (NADH) provided by the mitochondria via the malate:oxaloacetate (OAA) shuttle. In this shuttle, OAA is reduced in the mitochondrial matrix by NADH generated during glycine oxidation. The malate generated in the mitochondria is then transported to the peroxisome where malate is use to regenerate OAA and provide the NADH required to sustain serine reduction to glycerate (Raghavendra et al, 1998).

Givan CV, Joy KW, Kleczkowski LA 1988 A decade of photorespiratory nitrogen cycling. Trends Biochem. Sci. 13: 433-437.

Lea PJ, Blackwell RD, Joy KW 1992 Ammonia assimilation in higher plants. In (K Mengel, DJ Pilbeam eds) "Nitrogen Metabolism of Plants", Clarendon Press, Oxford, pp 153-186.

Mattsson M, Hausler RE, Leegood RC, Lea PJ, Schjoerring JK 1997 Leaf-atmosphere NH₃ exchange in barley mutants with reduced activities of glutamine synthetase. Plant Physiol. 114: 1307-1312.

Raghavendra AS, Reumann S, Heldt HW 1998 Participation of mitochondrial metabolism in photorespiration. Reconstituted system of peroxisomes and mitochondria from spinach leaves. Plant Physiol. 116: 1333-1337.