In microorganisms, one of the routes of glutamate synthesis is by combining
ammonium ion with α-ketoglutarate in a reaction catalyzed by glutamate dehydrogenase
(GDH) (Meers et al.
, 1970). Since the major route of glutamate synthesis
in plants occurs via the GS/GOGAT pathway, a parallel GDH-catalyzed route for
glutamate seems highly redundant. However, plants possess GDH enzymes,
whose metabolic functions have long been and still are highly controversial. The
metabolic status of plants largely depends on mineral nitrogen availability from
the soil (or from nitrogen fixing microorganisms) and carbon fixation from photosynthesis.
Since the availability of carbon and nitrogen depends on environmental
factors and may also be limiting, plants have evolved efficient ways to capture
nitrogen and carbon and to regulate the partition between sugars and nitrogenous
compounds to optimize plant growth and reproduction (Miflin and Habash, 2002;
Stitt et al.
, 2002). Since the GDH reaction is easily reversible leading to the release
of ammonium ion from glutamate, it could function in the conversion of glutamate
into organic acids under conditions of limiting carbon fixation. Indeed the
catabolic function of GDH in deaminating glutamate was demonstrated directly
[C] and 31
[P] nuclear magnetic resonance studies (Aubert et al.
, 2001). This
function has been indirectly implied by a number of physiological, biochemical,
and molecular studies that have been discussed before (Hirel and Lea, 2001;
Ireland and Lea, 1999; Lea and Ireland, 1999; Miflin and Habash, 2002).
In contrast to the well-documented catabolic functions of plant GDH, it is
possible that the enzyme may also operate in parallel to GOGAT in the aminating
direction of glutamate biosynthesis. Analyses of plants with reduced GOGAT
activity, either due to genetic mutation or due to expression of GOGAT antisense
constructs (Cordoba et al.
, 2003; Coschigano et al.
, 1998; Ferrario-Mery et al.
2002a,b; Lancien et al.
, 2002), suggested that GOGAT is the major enzyme responsible
for glutamate biosynthesis in plants. Hence, a possible anabolic (aminating)
activity of GDH, if it exists, contributes relatively little to overall glutamate
biosynthesis. Nevertheless, isolated mitochondria from potato plants can combine 15
[N]-labeled ammonium ion and α-ketoglutarate into 15
[N] glutamate (Aubert et al.
, 2001), suggesting that plant GDH can catalyze some glutamate synthesis
under specific metabolic conditions. A plausible limited anabolic activity of GDH
has indirectly been supported by other studies. Melo-Oliveira et al.
that seedlings of an Arabidopsis gdh1
null mutant grew slower than wild-type
seedlings, in particular with respect to root elongation, on media containing
high levels of inorganic nitrogen. Thus, the Arabidopsis
GDH1 appears to play a
nonredundant role in assimilating ammonium ion into glutamine under conditions
of excess inorganic nitrogen. Even so, the Arabidopsis
GDH1 is likely to
contribute minimally to nitrogen assimilation under regular growth conditions
when nitrogen fertilization is not in excess.
Another indirect support for some compensatory aminating function of GDH
was observed in transgenic tobacco plants in which Fd-GOGAT activity was
significantly reduced by an antisense approach (Ferrario-Mery et al.
, 2002a). Under conditions of reduced photorespiration (high CO2
), reduction of the
Fd-GOGAT activity affected neither the deaminating nor the aminating activity
of GDH. Yet, upon transport to air, there was a significant increase in the aminating,
but not the deaminating, activity of GDH in the transgenic lines, which was
also correlated with increased ammonium ion levels in these plants. These results
suggest that under conditions of reduced Fd-GOGAT activity and high rates of
photorespiration, GDH may compensate for the reduced GOGAT activity
(Ferrario-Mery et al.
Thus, the accumulating data suggest that in addition to the major catabolic
activity of GDH, the enzyme may also assist GOGAT in glutamate biosynthesis
under conditions of extensive photorespiration or excess nitrogen fertilization.
Nevertheless, such an aminating activity of the plant GDH would be minor
compared to that of GOGAT and may become important metabolically only
when GOGAT activity is compromised. Additional studies, using dynamic flux,
are needed to unequivocally demonstrate whether plant GDH enzymes function
in the anabolic direction of glutamate biosynthesis.
In other studies, microbial GDH genes were expressed in transgenic plants,
using the constitutive 35S promoter. Expression of an Escherichia coli GDH in
transgenic tobacco plants improved plant biomass production and also rendered
the plants more tolerant than wild-type plants to a glutamine synthetase inhibitor
(Ameziane et al.
, 2000). Similarly, expression of a Neurospora intermedia
transgenic tobacco plants improved plant growth under low nitrogen (Wang and
Tian, 2001). These results imply that the heterologous microbial GDH enzymes
contributed to nitrogen use efficiency of the transgenic plants by operating in
the aminating direction of glutamate synthesis. However, whether this function
is associated with specific biochemical characteristics of the microbial GDH
enzymes that are either present or not present in the plant counterparts remains
to be elucidated.