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Glycine max symbiotic ammonium transporter 1 was first documented as a putative ammonium (NH4+) channel localized to the symbiosome membrane of soybean root nodules. We show that Glycine max symbiotic ammonium transporter 1 is actually a membrane- localized basic helix-loop-helix (bHLH) DNA-binding transcription factor now renamed Glycine max bHLH membrane 1 (GmbHLHm1). In yeast, GmbHLHm1 enters the nucleus and transcriptionally activates a unique plasma membrane NH4+ channel Saccharomyces cerevisiae ammonium facilitator 1. Ammonium facilitator 1 homologs are present in soybean and other plant species, where they often share chromosomal microsynteny with bHLHm1 loci. GmbHLHm1 is important to the soybean rhizobium symbiosis because loss of activity results in a reduction of nodule fitness and growth. Transcriptional changes in nodules highlight downstream signaling pathways involving circadian clock regulation, nutrient transport, hormone signaling, and cell wall modification. Collectively, these results show that GmbHLHm1 influences nodule development and activity and is linked to a novel mechanism for NH4+ transport common to both yeast and plants. [ABSTRACT FROM AUTHOR]

Summary • We report the first use of tracer 13 NH4 + N-ammonium) efflux and retention data to analyse subcellular fluxes and compartmentation of NH 13 in the leaves of a higher plant (wheat, Triticum aestivum ). • Leaf segments, 1–2 mm, were obtained from 8-d-old seedlings. The viability of the segments, and stability of NH 4 + in the leaves of a higher plant (wheat, Triticum aestivum ). • Leaf segments, 1–2 mm, were obtained from 8-d-old seedlings. The viability of the segments, and stability of NH 4 + acquisition over time, were confirmed using oxygen-exchange and NH 4 + -depletion measurements. Fluxes of NH 4 + and compartment sizes were estimated using tracer efflux kinetics and retention data. • Influx and efflux across the plasma membrane, half-lives of exchange and cytosolic pool sizes were broadly similar to those in root systems. As the external concentration of NH 4 + ([NH 4 + ] o ) increased from 10 µm to 10 mm, both influx and efflux greatly increased, with a sixfold increase in the ratio of efflux to influx. Half-lives were similar among treatments, except at [NH 4 + ] o = 10 mm, where they declined. Concentrations of NH 4 + in the cytosol ([NH 4 + ] c ) increased from 2.6 to 400 mm. • Although [NH 4 + ] c became large as [NH 4 + ] o increased, the ratio of [NH 4 + ] c to [NH 4 + ] o decreased more than sixfold. The apparently futile cycling of NH 4 + at high [NH 4 + ] o suggested by the large fluxes of NH 4 + in both directions across the membrane indicate that leaf cells respond to potentially toxic NH 4 + concentrations in a manner similar to root cells. [ABSTRACT FROM AUTHOR]

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 and influx. In addition, efflux analysis using 13N 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 and one HATS for in roots of higher plants. By contrast, in Arabidopsis thaliana there exist seven members of the NRT2 family encoding putative HATS for and five members of the AMT1 family encoding putative HATS for . The induction of high‐affinity transport and Nrt2.1 and Nrt2.2 expression occur in response to the provision of , while down‐regulation of these genes appear to be due to the effects of glutamine. High‐affinity transport and AMT1.1 expression also appear to be subject to down‐regulation by glutamine. In addition, there is evidence that accumulated and 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 transporters and these appear to have undergone kinetic differentiation that permits a greater efficiency of 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. [ABSTRACT FROM AUTHOR]

13NO3– and 13NH4+ compartmental analyses were carried out in seedling roots of trembling aspen (Populus tremuloides Michx.), lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and interior Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) at 0·1 and 1·5 mol m–3 external NO3– or NH4+ concentrations ([NO3–]o or [NH4+]o, respectively). At the lower [NO3–]o, the capacities and efficiencies of acquisition and accumulation of NO3–, based upon NO3– fluxes and cytoplasmic NO3– concentrations ([NO3–]c), were in the order aspen >> Douglas-fir > pine. At 1·5 mol m–3 [NO3–]o, the NO3– influx increased 18-fold in pine, four-fold in Douglas-fir and approximately 1·4-fold in aspen; in fact, at 1·5 mol m–3 [NO3–]o, the NO3– influx in pine was higher than in aspen. However, at high [NO3–]o, efflux also increased in the two conifers to a much greater extent than in aspen. In aspen, at both [NO3–]o, approximately 30% of the 13N absorbed was translocated to the shoot during 57 min of 13N loading and elution, compared with less than 10% in the conifers. At 0·1 mol m–3 [NH4+]o, influx and net flux were in the order: aspen > pine > Douglas-fir but the differences were much less than in NO3–... [ABSTRACT FROM AUTHOR]