Agriculture Reference
In-Depth Information
opmental stage. The GS enzyme exists as a cytosolic form (GS1) present
in a variety of organ and tissues such as roots, leaves, phloem cells and a
plastidic form (GS2) localized in the chloroplasts of photosynthetic tissues
and the plastids of roots and etiolated tissues. It has also been proposed
that GS2 is located in the mitochondria [137]. However, in numerous pre-
vious studies using immunocytolocalization techniques, the presence of
the enzyme in the mitochondria has never been reported [138]. The rela-
tive proportions of GS1 and GS2 vary within the organs of the same plant
and between plant species, each GS isoform playing a specifi c role in a
given metabolic process, such as photorespiratory ammonia assimilation,
nitrate reduction, N translocation and recycling [134,139]. The enzyme
GOGAT also exists as two forms that have specifi c roles during primary
N assimilation or N recycling. A ferredoxin-dependent iseoenzyme (Fd-
GOGAT) is mainly involved, in conjunction with GS2, in the reassimi-
lation of photorespiratory ammonia and a pyridine nucleotide-dependent
isoenzyme (NADH-GOGAT; EC 1.4.1.14) involved in the synthesis of
glutamate both in photosynthetic and non-photosynthetic organs or tissues
to sustain plant growth and development [134,136]. Moreover, by virtue
of their differential mode of expression regulated either at the transcrip-
tional and post transcriptional levels, both GS and GOGAT isoenzymes
have been shown to play a specifi c role at particular stages of the plant life
cycle and under particular environmental conditions related mainly to the
mode of N nutrition [134,135,139].
The reversible reaction catalyzed by the enzyme glutamate dehydro-
genase (GDH; EC 1.4.1.2) [134], which has theoretically the capacity to
incorporate ammonia into 2-oxoglutarate to form glutamate, was origi-
nally thought to be the main enzyme involved in inorganic N assimilation
in plants. Later on, a number of experiments using
15
N labeling techniques
and mutants defi cient in GS and GOGAT have demonstrated that over 95%
of the ammonia made available to the plant is assimilated via the GS/GO-
GAT pathway [134,140]. A number of
15
N labeling experiments followed
by GCMS or NMR-spectroscopy analysis have shown that GDH operates
in the direction of glutamate deamination to provide organic acids notably
when the cell is C-limited [141,142]. The fi nding that under certain physi-
ological conditions GDH is able to assimilate ammonia also needs to be
taken into consideration, although the rate of glutamate synthesis is prob-
