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Nitrogen (N) and potassium (K) are the two most abundantly acquired mineral elements by plants, and their acquisition pathways interact in complex ways. Here, we review pivotal interactions with respect to root acquisition, storage, translocation and metabolism, between the K+ ion and the two major N sources, ammonium (NH4+) and nitrate (NO3-). The intersections between N and K physiology are explored at a number of organizational levels, from molecular-genetic processes, to compartmentation, to whole plant physiology, and discussed in the context of both N-K cooperation and antagonism. Nutritional regulation and optimization of plant growth, yield, metabolism and water-use efficiency are also discussed. [ABSTRACT FROM AUTHOR]

K+ channels are among the best-characterized classes of membrane protein in plants. Nevertheless, in-planta demonstrations of traits emerging from molecular characterizations have often been insufficient or lacking altogether. Such linkages are, however, critical to our basic understanding of plant nutrition and to addressing 'real-world' issues that are faced in environmental and agricultural settings. Here, we cover some of the recent advances in K+ acquisition with particular focus on voltage-gated K+ channel functioning and regulation in roots, and highlight where linkages to in-planta behavior have been successfully made and, conversely, where such linkages are yet to be made. [ABSTRACT FROM AUTHOR]

Sodium (Na) toxicity is one of the most formidable challenges for crop production world-wide. Nevertheless, despite decades of intensive research, the pathways of Na+ entry into the roots of plants under high salinity are still not definitively known. Here, we review critically the current paradigms in this field. In particular, we explore the evidence supporting the role of nonselective cation channels, potassium transporters, and transporters from the HKT family in primary sodium influx into plant roots, and their possible roles elsewhere. We furthermore discuss the evidence for the roles of transporters from the NHX and SOS families in intracellular Na+ partitioning and removal from the cytosol of root cells. We also review the literature on the physiology of Na+ fluxes and cytosolic Na+ concentrations in roots and invite critical interpretation of seminal published data in these areas. The main focus of the review is Na+ transport in glycophytes, but reference is made to literature on halophytes where it is essential to the analysis. [ABSTRACT FROM AUTHOR]

Potassium (K+) flux into plant cells is a well-characterized ion transport phenomenon. By contrast, little is known about the mechanisms and regulation of K+ flux from the cell. Here, we present a radioisotopic analysis of K+ fluxes from roots of intact barley (Hordeum vulgare), in the context of recent discoveries in the molecular biology and electrophysiology of this process. Plants were labelled with 42K+, and kinetics of its release from roots were monitored at low (0.1 mM) or high (1.0 mM) external K concentration, [K+]ext, and with the application of channel modulators and nutrient shifts. At 0.1 (but not 1.0) mM [K+], where K+ efflux is thought to be mediated by K+- outward-rectifying channels, 42K+ efflux was inhibited by the channel blockers barium (Ba2+), caesium (Cs+), tetraethylammonium (TEA+), and lanthanum (La3+). Ammonium and nitrate (10 mM) stimulated and inhibited 42K+ efflux, respectively, while 10 mM [K+]ext or [Rb+]ext decreased it. No evidence for the involvement of ATP- binding cassettes, nonselective cation channels, or active K+-efflux pumps was found. Our study provides new evidence for the thermodynamic transition between high- and low-affinity transport, from the efflux perspective, identifying the operation of channels at low [K+], and the cessation of transmembrane efflux at high [K+]. [ABSTRACT FROM AUTHOR]

•Stimulation of potassium (K+) efflux by sodium (Na+) has been the subject of much recent attention, and its mechanism has been attributed to the activities of specific classes of ion channels. •The short-lived radiotracer 42K+ was used to test this attribution, via unidirectional K+-flux analysis at the root plasma membrane of intact barley ( Hordeum vulgare), in response to NaCl, KCl, NH4Cl and mannitol, and to channel inhibitors. •Unidirectional K+ efflux was strongly stimulated by NaCl, and K+ influx strongly suppressed. Both effects were ameliorated by elevated calcium (Ca2+). As well, K+ efflux was strongly stimulated by KCl, NH4Cl and mannitol , and NaCl also stimulated 13NH4+ efflux. The Na+-stimulated K+ efflux was insensitive to cesium (Cs+) and pH 4.2, weakly sensitive to the K+-channel blocker tetraethylammonium (TEA+) and quinine, and moderately sensitive to zinc (Zn2+) and lanthanum (La3+). •We conclude that the stimulated efflux is: specific neither to Na+ as effector nor K+ as target; composed of fluxes from both cytosol and vacuole; mediated neither by outwardly-rectifying K+ channels nor nonselective cation channels; attributable, alternatively, to membrane disintegration brought about by ionic and osmotic components; of limited long-term significance, unlike the suppression of K+ influx by Na+, which is a greater threat to K+ homeostasis under salt stress. [ABSTRACT FROM AUTHOR]

We present the first characterization of K+ optimization of N uptake and metabolism in an NH4+-tolerant species, tropical lowland rice (cv. IR-72). 13N radiotracing showed that increased K+ supply reduces futile NH4+ cycling at the plasma membrane, diminishing the excessive rates of both unidirectional influx and efflux. Pharmacological testing showed that low-affinity NH4+ influx may be mediated by both K+ and non-selective cation channels. Suppression of NH4+ influx by K+ occurred within minutes of increasing K+ supply. Increased K+ reduced free [NH4+] in roots and shoots by 50–75%. Plant biomass was maximized on 10 mm NH4+ and 5 mm K+, with growth 160% higher than 10 mm NO3--grown plants, and 220% higher than plants grown at 10 mm NH4+ and 0.1 mm K+. Unlike in NH4+-sensitive barley, growth optimization was not attributed to a reduced energy cost of futile NH4+ cycling at the plasma membrane. Activities of the key enzymes glutamine synthetase and phosphoenolpyruvate carboxylase (PEPC) were strongly stimulated by elevated K+, mirroring plant growth and protein content. Improved plant performance through optimization of K+ and NH4+ is likely to be of substantial agronomic significance in the world's foremost crop species. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented about plant transport mechanisms.

Potassium (K+) is the most abundant ion in the plant cell and is required for a wide array of functions, ranging from the maintenance of electrical potential gradients across cell membranes, to the generation of turgor, to the activation of numerous enzymes. The majority of these functions depend more or less directly upon the activities and regulation of membrane-bound K+ transport proteins, operating over a wide range of K+ concentrations. Here, we review the physiological aspects of potassium transport systems in the plasma membrane, re-examining fundamental problems in the field such as the distinctions between high- and low-affinity transport systems, the interactions between K+ and other ions such as NH4+ and Na+, the regulation of cellular K+ pools, the generation of electrical potentials and the problems involved in measurement of unidirectional K+ fluxes. We place these discussions in the context of recent discoveries in the molecular biology of K+ acquisition and produce an overview of gene families encoding K+ transporters. [ABSTRACT FROM AUTHOR]

It has long been believed that maintenance of low Na+ : K+ ratios in the cytosol of plant cells is critical to the plant’s ability to tolerate salinity stress. Direct measurements of such ratios, however, have been few. Here we apply the non-invasive technique of compartmental analysis, using the short-lived radiotracers 42K+ and 22Na+, in intact seedlings of barley ( Hordeum vulgare L.), to evaluate unidirectional plasma membrane fluxes and cytosolic concentrations of K+ and Na+ in root tissues, under eight nutritional conditions varying in levels of salinity and K+ supply. We show that Na+ : K+ ratios in the cytosol of root cells adjust significantly across the conditions tested, and that these ratios are poor predictors of the plant’s growth response to salinity. Our study further demonstrates that Na+ is subject to rapid and futile cycling at the plasma membrane at all levels of Na+ supply, independently of external K+, while K+ influx is reduced by Na+, from a similar baseline, and to a similar extent, at both low and high K+ supply. We compare our results to those of other groups, and conclude that the maintenance of the cytosolic Na+ : K+ ratio is not central to plant survival under NaCl stress. We offer alternative explanations for sodium sensitivity in relation to the primary acquisition mechanisms of Na+ and K+. [ABSTRACT FROM AUTHOR]

The classic biochemical pH-stat model of cytosolic pH regulation in plant cells presupposes a pH-dependent biosynthesis and degradation of organic acids, specifically malic acid, in the cytosol. This model has been used to explain the higher tissue accumulation of organic acids in nitrate (NO3–)-grown, relative to ammonium (NH4+)-grown, plants, the result of proposed cytosolic alkalinization by NO3– metabolism, and acidification by NH4+ metabolism. Here, a critical examination of the model shows that its key assumptions are fundamentally problematic, particularly in the context of the effects on cellular pH of nitrogen source differences. Specifically, the model fails to account for proton transport accompanying inorganic nitrogen transport, which, if considered, renders the H+ production of combined transport and assimilation (although not the accumulation) to be equal for NO3– and NH4+ as externally provided N sources. We show that the model's evidentiary basis in total-tissue mineral ion and organic acid analysis is not directly relevant to subcellular (cytosolic) pH homeostasis, while the analysis of the ionic components of the cytosol is relevant to this process. A literature analysis further shows that the assumed greater activity of the enzyme phosphoenolpyruvate (PEP) carboxylase under nitrate nutrition, which is a key characteristic of the biochemical pH-stat model as it applies to nitrogen source, is not borne out in numerous instances. We conclude that this model is not tenable in its current state, and propose an alternative model that reaffirms the anaplerotic role of PEP carboxylase within the context of N nutrition, in the production of carbon skeletons for amino acid synthesis. [ABSTRACT FROM AUTHOR]

Examines whether novel initiatives in rice genomics and physiology can contribute to global food security. Biological nitrogen fixation in rice; Primary nitrogen uptake; Metabolic assimilation of nitrogen into organic compounds; Points of intersection between photosynthesis and plant nitrogen use; Potential of functional genomics.

Summary • We report the first use of tracer 13 NH4 + ( 13 N-ammonium) efflux and retention data to analyse subcellular fluxes and compartmentation 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]

Summary • A comprehensive and pragmatic approach to the design of unidirectional ion transport experiments in plants is presented here, revising and simplifying classical models. • The kinetic constant for cytosolic ion exchange ( k c ) is critical to the understanding of the interrelated flux processes occurring simultaneously at the cellular level. This constant is most effectively estimated using the compartmental analysis by efflux method, which, by providing values for additional kinetic parameters (i.e. unidirectional influx and efflux) can be used to determine the extent of distortion inherent in assessments of influx at, or close to, the steady state. • Focusing on the kinetics of nitrogen exchange, with tracer efflux experiments in barley ( Hordeum vulgare ) using 13 NH4 + , conducted under perturbation conditions, it was demonstrated that a transitional state was rapidly established following a concentration shift, characterized by a restoration of the preperturbational (steady-state) k c value. It is concluded that a reasonably accurate estimate of unidirectional influx can be made when influx measurements are conducted subsequent to the establishment of this transitional state. • A mathematical treatment of unidirectional flux processes allows the exact determination of errors caused by ionic counterfluxes under steady-state conditions. [ABSTRACT FROM AUTHOR]

It has long been believed that maintenance of low Na+ : K+ ratios in the cytosol of plant cells is critical to the plant’s ability to tolerate salinity stress. Direct measurements of such ratios, however, have been few. Here we apply the non-invasive technique of compartmental analysis, using the short-lived radiotracers 42K+ and 22Na+, in intact seedlings of barley ( Hordeum vulgare L.), to evaluate unidirectional plasma membrane fluxes and cytosolic concentrations of K+ and Na+ in root tissues, under eight nutritional conditions varying in levels of salinity and K+ supply. We show that Na+ : K+ ratios in the cytosol of root cells adjust significantly across the conditions tested, and that these ratios are poor predictors of the plant’s growth response to salinity. Our study further demonstrates that Na+ is subject to rapid and futile cycling at the plasma membrane at all levels of Na+ supply, independently of external K+, while K+ influx is reduced by Na+, from a similar baseline, and to a similar extent, at both low and high K+ supply. We compare our results to those of other groups, and conclude that the maintenance of the cytosolic Na+ : K+ ratio is not central to plant survival under NaCl stress. We offer alternative explanations for sodium sensitivity in relation to the primary acquisition mechanisms of Na+ and K+. [ABSTRACT FROM AUTHOR]