The regulation of nitrate and ammonium transport systems in plants

Inorganic nitrogen concentrations in soil solutions vary across several orders of magnitude among different soils and as a result of seasonal changes. In order to respond to this heterogeneity, plants have evolved mechanisms to regulate NO3- and NH4+ influx. In addition, efflux analysis using N-13 has revealed that there is a co-ordinated regulation of all component fluxes within the root, including biochemical fluxes. Physiological studies have demonstrated the presence of two high-affinity transporter systems (HATS) for NO3- and one HATS for NH4+ in roots of higher plants. By contrast, in Arabidopsis thaliana there exist seven members of the NRT2 family encoding putative HATS for NO3- and five members of the AMT1 family encoding putative HATS for NH4+. The induction of high-affinity NO3- transport and Nrt2.1 and Nrt2.2 expression occur in response to the provision of NO3-, while down-regulation of these genes appear to be due to the effects of glutamine. High-affinity NH4+ transport and AMT1.1 expression also appear to be subject to down-regulation by glutamine. In addition, there is evidence that accumulated NO, and NH4+ may act post-transcriptionally on transporter function. The present challenge is to resolve the functions of all of these genes. In Aspergillus nidulans and Chlamydomonas reinhardtii there are but two high-affinity NO3- transporters and these appear to have undergone kinetic differentiation that permits a greater efficiency of NO3- absorption over the wide range of concentration normally found in nature. Such kinetic differentiation may also have occurred among higher plant transporters. The characterization of transporter function in higher plants is currently being inferred from patterns of gene expression in roots and shoots, as well as through studies of heterologous expression systems and knockout mutants.