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Compartmental analysis by tracer efflux (CATE) is fundamental to examinations of membrane transport, allowing study of solute movement among subcellular compartments with high temporal, spatial, and chemical resolution. CATE can provide a wealth of information about fluxes and pool sizes in complex systems, but is a mathematically intensive procedure, and there is a need for software designed to fully, easily, and dynamically analyse results from CATE experiments. Here we present vaCATE (Visualized Automation of Compartmental Analysis by Tracer Efflux), a software package that meets these criteria. A robust suite of test cases using CATE datasets from experiments with intact rice ('Oryza sativa' L.) root systems reveals the high fidelity of vaCATE and the ease with which parameters can be extracted, using a three-compartment model and a curve-stripping procedure to distinguish them on the basis of variable exchange rates. vaCATE was developed using Python 2.7 and can be used in most situations where compartmental analysis is required. Funding Statement: This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ontario Graduate Scholarship Fund (OGS).

Although deemed a non-essential mineral nutrient, silicon (Si) is clearly beneficial to plant growth and development, particularly under stress conditions, including salinity and drought. Here, we review recent research on the physiological, biochemical, and molecular mechanisms underlying Si-induced alleviation of osmotic and ionic stresses associated with salinity and drought. We distinguish between changes observed in the apoplast (i.e. suberization, lignification, and silicification of the extracellular matrix; transpirational bypass flow of solutes and water), and those of the symplast (i.e. transmembrane transport of solutes and water; gene expression; oxidative stress; metabolism), and discuss these features in the context of Si biogeochemistry and bioavailability in agricultural soils, evaluating the prospect of using Si fertilization to increase crop yield and stress tolerance under salinity and drought conditions.

Rapid sodium cycling across the plasma membrane of root cells is widely thought to be associated with Na+ toxicity in plants. However, the efflux component of this cycling is not well understood. Efflux of Na+ from root cells is believed to be mediated by SOS1, although expression of this Na+/H+ antiporter has been localized to the vascular tissue and root meristem. Here, we used a chambered cuvette system in which the distal root of intact salinized barley and Arabidopsis thaliana plants (wild-type and sos1) were isolated from the bulk of the root by a silicone-acrylic barrier, so that we could compare patterns of 24Na+ efflux in these two regions of root. In barley, steady-state release of 24Na+ was about four times higher from the distal root than from the bulk roots. In the distal root, 24Na+ release was pronouncedly decreased by elevated pH (9.2), while the bulk-root release was not significantly affected. In A. thaliana, tracer efflux was about three times higher from the wild-type distal root than from the wild-type bulk root and also three to four times higher than both distal- and bulk-root fluxes of Atsos1 mutants. Elevated pH also greatly reduced the efflux from wild-type roots. These findings support a significant role of SOS1-mediated Na+ efflux in the distal root, but not in the bulk root.

Sudden elevations in external sodium chloride (NaCl) accelerate potassium (K(+)) efflux across the plasma membrane of plant root cells. It has been proposed that the extent of this acceleration can predict salt tolerance among contrasting cultivars. However, this proposal has not been considered in the context of plant nutritional history, nor has it been explored in rice (Oryza sativa L.), which stands among the world's most important and salt-sensitive crop species. Using efflux analysis with (42)K, coupled with growth and tissue K(+) analyses, we examined the short- and long-term effects of NaCl exposure to plant performance within a nutritional matrix that significantly altered tissue-K(+) set points in three rice cultivars that differ in salt tolerance: IR29 (sensitive), IR72 (moderate), and Pokkali (tolerant). We show that total short-term K(+) release from roots in response to NaCl stress is small (no more than 26% over 45 min) in rice. Despite strong varietal differences, the extent of efflux is shown to be a poor predictor of plant performance on long-term NaCl stress. In fact, no measure of K(+) status was found to correlate with plant performance among cultivars either in the presence or absence of NaCl stress. By contrast, shoot Na(+) accumulation showed the strongest correlation (a negative one) with biomass, under long-term salinity. Pharmacological evidence suggests that NaCl-induced K(+) efflux is a result of membrane disintegrity, possibly as result of osmotic shock, and not due to ion-channel mediation. Taken together, we conclude that, in rice, K(+) status (including efflux) is a poor predictor of salt tolerance and overall plant performance and, instead, shoot Na(+) accumulation is the key factor in performance decline on NaCl stress.

Rice provides staple food for 70% of world's human population and its increased global production has enabled crop scientist and farmers to starve off anticipated farming. The promise of modern technology in the area of nitrogen acquisition in rice is discussed.

This paper investigates the intergovernmental transfer systems in South Africa, more specifically, the underlying determinants and formula of the unconditional grants of the education component of the Provincial Equitable Share (PES). The PES formula is a weighted shares formula that depends on the demographic shares of the provinces (population) to determine the equitable share outcomes to be transferred to individual provinces. We find that the variables used have two main weaknesses: Firstly, the enrolment variable, which could be contaminated with school repeats and dropouts, is too broad to be an effective measure for representing the real beneficiaries of the transfers. Secondly, infrequent updates about the school-age cohort population variables render the desired PES grant transfers for education outdated as well as serially inconsistent. The paper also provides an analysis of the PES formula’s mathematical construct, followed by a few suggested variables to improve the relevance and accuracy of the formula of the revenue transfer. One such improvement is demonstrated by the inclusion of the Enrolment Survival Measure (ESR). More research and data are required to extrapolate nuanced results and to understand the socio-economic impacts of adopting these new variables in the PES formula going forward.

Main conclusion: A systematic analysis of NaCl-dependent, plasma-membrane depolarization (∆∆Ψ) in rice roots calls into question the current leading model of rapid membrane cycling of Na+ under salt stress.To investigate the character and mechanisms of Na+ influx into roots, Na+-dependent changes in plasma-membrane electrical potentials (∆∆Ψ) were measured in root cells of intact rice (Oryza sativa L., cv. Pokkali) seedlings. As external sodium concentrations ([Na+]ext) were increased in a step gradient from 0 to 100 mM, membrane potentials depolarized in a saturable manner, fitting a Michaelis-Menten model and contradicting the linear (non-saturating) models developed from radiotracer studies. Clear differences in saturation patterns were found between plants grown under low- and high-nutrient (LN and HN) conditions, with LN plants showing greater depolarization and higher affinity for Na+ (i.e., higher Vmax and lower Km) than HN plants. In addition, counterion effects on ∆∆Ψ were pronounced in LN plants (with ∆∆Ψ decreasing in the order: Cl− > SO42− > HPO42-), but not seen in HN plants. When effects of osmotic strength, Cl− influx, K+ efflux, and H+-ATPase activity on ∆∆Ψ were accounted for, resultant Km and Vmax values suggested that a single, dominant Na+-transport mechanism was operating under each nutritional condition, with Km values of 1.2 and 16 mM for LN and HN plants, respectively. Comparing saturating patterns of depolarization to linear patterns of 24Na+ radiotracer influx leads to the conclusion that electrophysiological and tracer methods do not report the same phenomena and that the current model of rapid transmembrane sodium cycling may require revision. [ABSTRACT FROM AUTHOR]

Most higher plants develop severe toxicity symptoms when grown on ammonium ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) as the sole nitrogen source. Recently, [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] toxicity has been implicated as a cause of forest decline and even species extinction. Although mechanisms underlying [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] toxicity have been extensively sought, the primary events conferring it at the cellular level are not understood. Using a high-precision positron tracing technique, we here present a cell-physiological characterization of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] acquisition in two major cereals, barley (Hordeum vulgare), known to be susceptible to toxicity, and rice (Oryza sativa), known for its exceptional tolerance to even high levels of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. We show that, at high external [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] concentration ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]), barley root cells experience a breakdown in the regulation of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] influx, leading to the accumulation of excessive amounts of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in the cytosol. Measurements of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] efflux, combined with a thermodynamic analysis of the transmembrane electrochemical potential for [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], reveal that, at elevated [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], barley cells engage a high-capacity [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]-efflux system that supports outward [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] fluxes against a sizable gradient. Ammonium efflux is shown to constitute as much as 80% of primary influx, resulting in a never-before-documented futile cycling of nitrogen across the plasma membrane of root cells. This futile cycling carries a high energetic cost (we record a 40% increase in root respiration) that is independent of N metabolism and is accompanied by a decline in growth. In rice, by contrast, a cellular defense strategy has evolved that is characterized by an energetically neutral, near-Nernstian, equilibration of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] at high [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Thus our study has characterized the primary events in [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] nutrition at the cellular level that may constitute the fundamental cause of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] toxicity in plants.

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]

Although the global nitrogen (N) cycle is largely driven by soil microbes, plant root exudates can profoundly modify soil microbial communities and influence their N transformations. A detailed understanding is now beginning to emerge regarding the control that root exudates exert over two major soil N processes – nitrification and N 2 fixation. We discuss recent breakthroughs in this area, including the identification of root exudates as nitrification inhibitors and as signaling compounds facilitating N-acquisition symbioses. We indicate gaps in current knowledge, including questions of how root exudates affect newly discovered microbial players and N-cycle components. A better understanding of these processes is urgent given the widespread inefficiencies in agricultural N use and their links to N pollution and climate change. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO 2 ) concentrations in the earth’s atmosphere are projected to rise from current levels near 400 ppm to over 700 ppm by the end of the 21st century. Projections over this time frame must take into account the increases in total net primary production (NPP) expected from terrestrial plants, which result from elevated CO 2 (eCO 2 ) and have the potential to mitigate the impact of anthropogenic CO 2 emissions. However, a growing body of evidence indicates that limitations in soil nutrients, particularly nitrogen (N), the soil nutrient most limiting to plant growth, may greatly constrain future carbon fixation. Here, we review recent studies about the relationships between soil N supply, plant N nutrition, and carbon fixation in higher plants under eCO 2 , highlighting key discoveries made in the field, particularly from free-air CO 2 enrichment (FACE) technology, and relate these findings to physiological and ecological mechanisms. [ABSTRACT FROM AUTHOR]

Although deemed a "non-essential" mineral nutrient, silicon (Si) is clearly beneficial to plant growth and development, particularly under stress conditions, including salinity and drought. Here, we review recent research on the physiological, biochemical, and molecular mechanisms underlying Si-induced alleviation of osmotic and ionic stresses associated with salinity and drought. We distinguish between changes observed in the apoplast (i.e., suberization, lignification, and silicification of the extracellular matrix; transpirational bypass flow of solutes and water), and those of the symplast (i.e., transmembrane transport of solutes and water; gene expression; oxidative stress; metabolism), and discuss these features in the context of Si biogeochemistry and bioavailability in agricultural soils, evaluating the prospect of using Si fertilization to increase crop yield and stress tolerance under salinity and drought conditions. [ABSTRACT FROM AUTHOR]

Rapid sodium cycling across the plasma membrane of root cells is widely thought to be associated with Na+ toxicity in plants. However, the efflux component of this cycling is not well understood. Efflux of Na+ from root cells is believed to be mediated by Salt Overly-Sensitive-1, although expression of this Na+/H+ antiporter has been localized to the vascular tissue and root meristem. Here, we used a chambered cuvette system in which the distal root of intact salinized barley and Arabidopsis thaliana plants (wild-type and sos1) were isolated from the bulk of the root by a silicone-acrylic barrier, so that we could compare patterns of 24Na+ efflux in these two regions of root. In barley, steady-state release of 24Na+ was about four times higher from the distal root than from the bulk roots. In the distal root, 24Na+ release was pronouncedly decreased by elevated pH (9.2), while the bulk-root release was not significantly affected. In A. thaliana, tracer efflux was about three times higher from the wild-type distal root than from the wild-type bulk root and also three to four times higher than both distal- and bulk-root fluxes of Atsos1 mutants. Elevated pH also greatly reduced the efflux from wild-type roots. These findings support a significant role of SOS1-mediated Na+ efflux in the distal root, but not in the bulk root. [ABSTRACT FROM AUTHOR]