Glutamate dehydrogenase is essential in the acclimation of Virgilia divaricata, a legume indigenous to the nutrient-poor Mediterranean-type ecosystems of the Cape Fynbos

Abstract

Glutamate dehydrogenase (NAD(H)- GDH, EC 1.4.1.2) is an important enzyme in nitrogen (N) metabolism. It serves as a link between C and N metabolism, in its role of assimilating ammonia into glutamine or deaminating glutamate into 2-oxoglutarate and ammonia. GDH may also have a key in the N assimilation of legumes growing in P-poor soils. Virgilia divaricata is such a legume, growing in the nutrient limited soils of the mediterranean-type Cape fynbos ecosystem. In order to understand the role of GDH in the nitrogen nutrition of V. divaricata, the aim of this study was to identify the GDH gene transcripts, their relative expressions and enzyme activity in P-stressed roots and nodules during N metabolism. During P deficiency there was a reduction in total plant biomass as well as total plant P concentration. The analysis of the GDH cDNA sequences in V. divaricata revealed the presence of GHD1 and GHD2 subunits, these corresponding to the GDH1, GDH-B and GDH3 genes of legumes and non-legume plants. The relative expression of GDH1 and GDH2 genes in the roots and nodules, indicates that two the subunits were differently regulated depending on the organ type, rather than P supply. Although both transcripts appeared to be ubiquitously expressed in the roots and nodules, the GDH2 transcript evidently predominated over those of GDH1. Furthermore, the higher expression of both GDH transcripts in the roots than nodules, suggests that roots are more reliant on on GDH in P-poor soils, than nodules. With regards to GHD activity, both aminating and deaminating GDH activities were differently affected by P deficiency in roots and nodules. This may function to assimilate N and regulate internal C and N in the roots and nodules. The variation in GDH1 and GDH2 transcript expression and GDH enzyme activities, indicate that the enzyme may be regulated by post-translational modification, instead of by gene expression during P deficiency in V. divaricata.