1. Nitrite reductase (NiRase) ¤ NO 2 - + 6 Fd red + 8 H + + 6 e -  NH 4 + + 6 Fd ox + 2H 2 O ¤ a single polypeptide containing iron-sulfur cluster and heme ¤ present in chloroplast or plastids of roots ¤ is encoded in the nucleus and synthesized in the cytoplasm with an N-terminal transit peptide that targets it to the plastids ¤ a small percentage (0.02-0.2%) of the NO 2 - reduced is released as N 2 O (06) Ferredoxin: a 2Fe-2S protein electron flow (NADPH for nongreen tissues) NO 3 -  NO 2 -  NH 4 +  Gln 12 ATP/N

2. Varied from species to species Both roots and shoots assimilate nitrate The xylem sap of plants grown with their roots exposed to nitrate solution

3. Ammonium assimilation ─ rapidly convert into amino acids nitrate assimilation, photorespiration (p. 169) [GS]  -ketoglutarate [GOGAT]

4. An alternative pathway

5. Glutamine synthetase (GS): NH 4 + + Glu + ATP → Gln + ADP + P i requires: Mg 2+, Mn 2+, Co 2+ two classes: cytosolic form: in germinating seeds or in the vascular bundles of roots and shoots, for intracellular transport. independent to light and carbohydrate levels plastidic form: dependent to light and carbohydrate levels root type: local consumption shoot chloroplast: reassimilate photorespiratory

6. Glutamate synthase (glutamine:2-oxoglutarate aminotransferase, GOGAT) electron supply NADH-GOGAT: Gln + α-KG + NADH + H + → 2 Glu + NAD + located in plastids of nonphotosynthetic tissues such as roots or vascular bundles of developing leaves ferredoxin-GOGAT: Gln + α-KG + Fd red → 2 Glu + Fdox in chloroplast: photorespiratory nitrogen metabolism or root plastids: incorporate glutamine Glutamate dehydrogenase (GDH): α-KG + NAD(P)H + NH 4 +  2 Glu + NAD(P)H + + H 2 O high Km for NH 4 +, the primary function is to deaminate Glu NADH-GDH: in mitochondria NADPH-GDH: in chloroplasts

7. Transamination: require pyridoxal phosphate (B 6 ) Asparate-malate shuttle Asparagine synthetase (AS) in the cytosol of roots and leaves and in nitrogen-fixing nodules, As rich NH 4 +, replace Gln

8. Asparagine (Asn, N) and Glutamine (Gln, Q) ─ link carbon and nitrogen metabolism ¤ Asn: the first amide to be identified in asparagus at 1806 ¤ functions: a protein precursor, nitrogen long-distance transport and long-term storage high N/C ratio: Asn (2N/4C), Gln (2N/5C), Glu (1N/5C), Asp (1N/4C) ¤ High levels of light and carbohydrates: plastid GS and Fd-GOGAT↑, AS↓  nitrogen assimilate into Glu and Gln ¤ Low levels of light and carbohydrates: plastid GS and Fd-GOGAT ↓, AS ↑  nitrogen assimilate into Asn