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 345

GlutamineSynthetase(EC6.3.1.2)
Glutamine synthetase (GS) catalyzes the critical inclusion of inorganic ammonium
into glutamine. GS catalyzes the ATP-dependent condensation of NH 4 + with gluta-
mate to yield glutamine. The native GS protein weighs 350 kDa and is composed
of 8 almost identical subunits (Sechley et al. 1992 ; Nogueira et al. 2005 ). In leaves,
it is present in both chloroplasts (GS2) and cytoplasm (GS1) (Scarpeci et al. 2007 ).
The chloroplast form appears to play role in the assimilation of photorespiratory
NH 4 + (Freeman et al. 1990 ). The root enzyme too, is found in both cytoplasm and
plastids. In pea, the dominant form is plastidic, whereas in maize it is cystosolic
(Sakakibara et al. 1992 ). Studies carried out to encode the genes for GS has helped
to elucidate the function of each isoform are involved. Chloroplastic GS (GS2) is
believed to be having function in the re-assimilation of photorespiratory conditions
(Freeman et al. 1990 ). The gene for the cystosolic GS (GS3A in pea) is found to be
active in the phloem of the transgenic tobacco and alfalfa, indicating that it func-
tions primarily to produce glutamine for intercellular transfer (Fei et al. 2003 ). In
rice plants, cytosolic GS has been reported to be present in vascular bundle. In leaf
tissue, it is active in exporting nitrogen to 'sink' tissues (Gallais et al. 2006 ).
GlutamateSynthase(EC1.4.7.1and1.4.1.14)
Glutamate synthase (glutamine-2-oxoglutarate aminotransferase, GOGAT) is in-
volved in the reductive relocation of the GS to 2-oxoglutarate to create two mol-
ecules of glutamate. One of the glutamate molecules can then be cycled back as the
substrate for the GS reaction. This is GS-GOGAT cycle was defined by Lea and
Miflin ( 1974 ). Based on the nature of electron donor, two forms of GOGAT exists,
the ferredoxin-GOGAT and the NAD(P)H-GOGAT (Suzuki and Knaff 2005 ). In
rice leaves Fd-GOGAT is known to be present in mesophyll cells, consistent with a
job in photorespiratory nitrogen metabolism (Hayakawa et al. 1994 ). The NAD(P)
H-GOGAT occurs in vascular bundles of developing leaf blades, indicating a role
in the synthesis of glutamate from glutamine that is imported to the vascular bundle
from roots and senescing tissues (Tabuchi et al. 2007 ).
GlutamateDehydrogenase(EC1.4.1.2)
Glutamate dehydrogenase (GDH) is capable either of synthesizing or de-aminating
glutamate (Pahlich 1996 ). One isoform of the enzyme is localized in the mitochon-
dria. It uses NADH as the electron donor (Sechley et al. 1992 ). Another form that
has a specific requirement for NAD(P)H is present in the chloroplasts of photosyn-
thetic tissues. Levels of the NADH form of GDH increase with senescence or after