From Agronomy to Molecular Genetics and Proteomics in an Effort to Improve Nitrogen Use Efficiency in Crops - Crop Improvement: New Approaches and Modern Techniques - page 344

root; it is first reduced to nitrite in the cytosol by nitrate reductase (NR). Nitrite
gets translocated to the chloroplast and reduced into ammonium by nitrite reductase
(NiR), NH 4 + the end product is finally integrated into the amino acids via the GS/
GOGAT pathway (Mokhele et al. 2012 )
NitrateReductase(EC1:6:6:1)
Nitrate reductase (NR) in higher plants is thought to be a homodimer. The estimated
subunit size of the monomers is between 100-120 kDa (Caboche and Rouze 1990 ).
The monomer size is about 100 kDa. In higher plants each monomer constitutes
three functional domains, each of which has three redox centers formed by FAD,
heme and a molybdenum Co-factor (MoCo). These are present in the steochiom-
etry of 1:1:1 (Caboche and Rouze 1990 ). These redox centres catalytically transfer
two electrons from NAD(P)H to NO 3 - . According to Lillo and Ruoff ( 1992 ), there
is a second site for NADH, which is occupied in the allosteric, regulating the NR
activity. Thus, electrons are able to move from redox centres in one subunit to the
redox centres in second subunit, hence enabling the total electron transfer from
NADPH to NO 3 - in heterodimers. NR is found in cytoplasm of shoot and root cells
identified using either cell-fractionation and biochemical techniques or immune cy-
tochemistry (Fedorova et al. 1994 ). In the cytoplasm of the leaf cells, the NADH
required for the functioning of NR is supplied by either of two shatter mechanisms,
one involving the phosphate translocator located in the chloroplast envelope and
the other involving the malate oxaloacetate translocator located in the envelopes
of mitochondria. In root cells where NR can utilize both NADH and NADPH as
reductants, either the glucose 6-phosphate dehydrogenase or 6-phospogluconate
dehydrogenase, present in the cytoplasm or plastid, can supply NADPH (Bowsher
et al. 1993 ).
NitriteReductase(EC1.7.7.1)
Nitrite reductase (NiR) catalyses the 6 electron transfer reaction from reduced fer-
redoxin to NO 2 - , leading to the synthesis of NH 4 + . It is localized inside chloroplasts
of the leaf, also in plastids of the root tissues (Sechley et al. 1992 ). In both roots
and leaves, the reduced ferredoxinacts as an electron donor. The NiR enzymes are
monomeric proteins of about 63 kDa containing siroheem and a 4Fe-4S centre as
prosthetic groups (Seigel and Wilkerson 1989 ). Wray 1993 confirmed that the NiR
apoprotein is synthesised as a precursor of transit peptide carrying an N-terminal
extension which helps in recognition of the protein in chloroplast/plastid. This NiR
apoprotein is encoded by nuclear DNA.