Glutamine synthetase in higher plants: Regulation of gene and protein expression from the organ to the cell

Compared to other enzymatic systems, the regulation of GS isoenzyme expression shows a unique diversity. Considering that GS is one of the oldest existing and functioning genes found in all extant life forms, we can hypothesise that the evolution of metabolic pathways from primitive pre-procaryotes to lower and then higher plants might have gradually refined the function of GS to provide reduced nitrogen forms for the rest of the metabolism (Kumada et al. 1993). This refinement might explain the genetic and biological diversity encountered in the various modes of expression and regulation of higher plant GS isoenzymes both at the cellular and intracellular levels (Fig. 1). Although model plants are valuable sources of information helping to decipher fine regulatory control mechanisms (Lam et al. 1996), the study of this genetic diversity appears to be one of the most promising areas of research, necessary to better understand ammonia assimilation in plants and more generally improve nitrogen use efficiency.