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N Use By Plants
Nitrate Assimilation
Ammonia Assimilation
Glu, Gln, Asn, Gly, Ser
Asp, Ala, GABA
Val, Leu, Ileu, Thr, Lys
Pro, Arg, Orn
Non-protein AAs
Sulfate Assimilation
Cys, Met, AdoMet, ACC
His, Phe, Tyr, Tryp
Secondary Products
Onium Compounds
HORT640 - Metabolic Plant Physiology

Ammonia Assimilation and Recycling

The photorespiratory nitrogen cycle

Expasy: | RUBISCO [EC] | P-Glycolate phosphatase [EC] | Glycolate oxidase [EC] | Glyoxylate aminotransferase (Ala [EC], Ser [EC]) | Glycine decarboxylase [EC and] | Serine hydroxymethyltransferase [EC] | Hydroxypyruvate reductase [EC] | Malate dehydrogenase [EC] | Glycerate kinase [EC] | Glutamine synthetase [EC] | Glutamate synthase (ferredoxin) [EC] | Nitrate reductase [EC and] | Ferredoxin-nitrite reductase [EC] | Catalase [EC] |

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.

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Last Update: 10/01/09