Glutamate Dehydrogenase: An Enzyme withControversial Functions in Plants

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 by ¹³[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., 2000, 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. (1996) found 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., 2002a).

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 GDH in 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.