Interconversion between
glutamate and
2-oxoglutarate, which can be catalysed by
glutamate dehydrogenase (GDH), is a key reaction in plant
carbon (C) and
nitrogen (N) metabolism. However, the physiological role of plant GDH has been a controversial issue for several decades. To elucidate the function of GDH, the expression of GDH in various tissues of Arabidopsis thaliana was studied. Results suggested that the expression of two Arabidopsis GDH genes was differently regulated depending on the organ/tissue types and cellular C availability. Moreover, Arabidopsis mutants defective in GDH genes were identified and characterized. The two isolated mutants, gdh1-2 and gdh2-1, were crossed to make a double knockout mutant, gdh1-2/gdh2-1, which contained negligible levels of
NAD(H)-dependent GDH activity. Phenotypic analysis on these mutants revealed an increased susceptibility of gdh1-2/gdh2-1 plants to C-deficient conditions. This conditional phenotype of the double knockout mutant supports the catabolic role of GDH and its role in fuelling the TCA cycle during C
starvation. The reduced rate of
glutamate catabolism in the gdh2-1 and gdh1-2/gdh2-1 plants was also evident by the growth retardation of these mutants when
glutamate was supplied as the alternative N source. Furthermore,
amino acid profiles during prolonged dark conditions were significantly different between WT and the gdh mutant plants. For instance,
glutamate levels increased in WT plants but decreased in gdh1-2/gdh2-1 plants, and aberrant accumulation of several
amino acids was detected in the gdh1-2/gdh2-1 plants. These results suggest that GDH plays a central role in
amino acid breakdown under C-deficient conditions.