BAT1, a bidirectional amino acid transporter in <Emphasis Type="Italic">Arabidopsis</Emphasis>

The Arabidopsis thaliana At2g01170 gene is annotated as a putative gamma amino butyric acid (GABA) permease based on its sequence similarity to a yeast GABA transporting gene (UGA4). A cDNA of At2g01170 was expressed in yeast and analyzed for amino acid transport activity. Both direct measurement of amino acid transport and yeast growth experiments demonstrated that the At2g01170 encoded-protein exhibits transport activity for alanine, arginine, glutamate and lysine, but not for GABA or proline. Significantly, unlike other amino acid transporters described in plants to date, At2g01170 displayed both export and import activity. Based on that observation, it was named bidirectional amino acid transporter 1 (BAT1). Sequence comparisons show BAT1 is not a member of any previously defined amino acid transporter family. It does share, however, several conserved protein domains found in a variety of prokaryotic and eukaryotic amino acid transporters, suggesting membership in an ancient family of transporters. BAT1 is a single copy gene in the Arabidopsis genome, and its mRNA is ubiquitously expressed in all organs. A transposon—GUS gene-trap insert in the BAT1 gene displays GUS localization in the vascular tissues (Dundar in Ann Appl Biol, 2009) suggesting BAT1 may function in amino acid export from the phloem into sink tissues.     

Contribution of amino compounds to dissolved organic nitrogen in forest soils

Dissolved organic nitrogen (DON) may play an important role in plantnutrition and nitrogen fluxes in forest ecosystems. In spite of the apparentimportance of DON, there is a paucity of information concerning its chemicalcomposition. However, it is exactly this chemical characterization that isrequired to understand the importance of DON in ecosystem processes. Theprimaryobjective of this study was to characterize the distribution of free aminoacidsand hydrolyzable peptides/proteins in the DON fraction of Oa horizon leachatesalong an extreme edaphic gradient in northern California. Insitu soil solutions were extracted by centrifugation from Oahorizonscollected beneath Pinus muricata (Bishop pine) andCupressus pygmaea (pygmy cypress) on slightlyacidic/fertile and highly acidic/infertile sites. DON accounted for 77 to99% of the total dissolved nitrogen in Oa horizon leachates. Nitrogen infree amino acids and alkyl amines ranged from 0.04–0.07 mgN/L on the low fertility site to 0.45–0.49 mg N/L onthe high fertility site, and accounted for 1.5 to 10.6% of the DON fraction.Serine, glutamic acid, leucine, ornithine, alanine, aspartic acid andmethylamine were generally the most abundant free amino compounds. Combinedamino acids released by acid hydrolysis accounted for 48 to 74% of theDON, suggesting that proteins and peptides were the main contributor to DON inOa horizon leachates. Together, nitrogen from free andcombined amino compounds accounted for 59 to 78% of the DON. Most of theDON was found in the hydrophobic fraction, which suggests the presence ofprotein/peptide-polyphenol complexes or amino compounds associated withhumic substances. Because free and combined amino acids can be an importantnitrogen source for some plants, soil DON may play an important role in plantnutrition and ecosystem function.     

Fluctuations in nitrate reductase activity,and nitrate and organic nitrogen concentrations of succulent plants under different nitrogen and water regimes

The CAM (Crassulacean acid metabolism) succulent species Kalanchoe daigremontiana, K. tubiflora and Crassula argentea, and the succulent C₃ species Peperomia obtusifolia, were cultivated in pure culture in open-air conditions under two different regimes of nitrogen and water supply. At specified intervals during the course of vegetative growth, biomass, nitrate reductase activity (NRA), nitrate concentration, and organic nitrogen concentration of whole plants were measured. After 100 days of cultivation the leaf conductance of Crassula and Peperomia was measured at intervals for the duration of a day. Behaviour of all four species was strongly influenced by the cultivation regime. This was apparent in terms of productivity and variable flucturations in NRA, nitrate concentration, and organic nitrogen concentration during the vegetative period. Increase in biomass was mostly connected with a decrease in all other investigated parameters, especially under conditions of water and/or nitrogen deficiency. The typical reaction of the CAM species Crassula to limited netrogen but adequate soil water was to reduce leaf conductance during light, whereas the C₃ plant Peperomia increased conductance in comparison with plants having a nitrogen suppy. The NRA of all plant species was reduced by both soil nitrate deficiency and drought. The succulent plant species, which are specially adapted to drought, neither took up nor used nitrate when water was limited. This was particularly the case for the CAM species, but less so for the C₃ Peperomia, which showed very high concentrations of nitrate and organic nitrogen, but low NRA and biomass gain. A formula was derived to express the nitrogen use efficiency (NUE) of the species, i.e. the ability of a plant to use nitrogen over a specific period of growth. NUE was shown to increase with age for the crassulacean species but to decrease for the C₃ Peperomia. Furthermore, NUE varied with the different nutrient levels in a species-specific manner, with high values for NUE not necessarily coupled to high productivity, and with NUE of the C₃ species generally higher than that of CAM species.     

Degradation of proteins by enzymes exuded by Allium porrum roots–A potentially important strategy for acquiring organic nitrogen by plants

Nitrogen is one of the crucial elements that regulate plant growth and development. It is well-established that plants can acquire nitrogen from soil in the form of low-molecular-mass compounds, namely nitrate and ammonium, but also as amino acids. Nevertheless, nitrogen in the soil occurs mainly as proteins or proteins complexed with other organic compounds. Proteins are believed not to be available to plants. However, there is increasing evidence to suggest that plants can actively participate in proteolysis by exudation of proteases by roots and can obtain nitrogen from digested proteins. To gain insight into the process of organic nitrogen acquisition from proteins by leek roots (Allium porrum L. cv. Bartek), casein, bovine serum albumin and oxidized B-chain of insulin were used; their degradation products, after exposure to plant culture medium, were studied using liquid chromatography–mass spectrometry (LC–MS). Casein was degraded to a great extent, but the level of degradation of bovine serum albumin and the B-chain of insulin was lower. Proteases exuded by roots cleaved proteins, releasing low-molecular-mass peptides that can be taken up by roots. Various peptide fragments produced by digestion of the oxidized B-chain of insulin suggested that endopeptidase, but also exopeptidase activity was present. After identification, proteases were similar to cysteine protease from Arabidopsis thaliana. In conclusion, proteases exuded by roots may have great potential in the plant nitrogen nutrition.     

Organic and inorganic nitrogen uptake in lichens

In order to learn more about nitrogen (N) acquisition in lichens, and to see whether different lichens differ in their affinity to various N sources, N uptake was measured in 14 various lichen associations (species). These species represented various morphologies (fruticose or foliose), contrasting microhabitat preferences (epiphytic or terricolous), and had green algal, cyanobacterial or both forms of photobionts. N was supplied under non-limiting conditions as an amino acid mixture, ammonium, or nitrate, using ¹⁵N to quantify uptake. Carbonyl cyanide m-chlorophenylhydrazone (CCCP) was used to separate active and passive uptake. Thallus N, amino acids, soluble polyol concentrations, and the biont-specific markers chlorophyll a and ergosterol were quantified, aiming to test if these metabolites or markers were correlated with N uptake capacity. Ammonium uptake was significantly greater and to a higher extent passive, relative to the other two N sources. Nitrate uptake differed among lichen photobiont groups, cyanobacterial lichens having a lower uptake rate. All lichens had the capacity to assimilate amino acids, in many species at rates equal to nitrate uptake or even higher, suggesting that organic N compounds could potentially have an important role in the N nutrition of these organisms. There were no clear correlations between N uptake rates and any of the measured metabolites or markers. The relative uptake rates of ammonium, nitrate and amino acids were not related to morphology or microhabitat.Abbreviations CCCP Carbonyl cyanide m-chlorophenylhydrazone - Chl Chlorophyll - N Nitrogen     

Partitioning of assimilates in fruiting tomato plants

Tomato is a potentially high-yield crop with a harvest index of about 65%. During fruiting, fruit growth accounts for 80 to 90% of the plant fresh weight gain and fruits are therefore the strongest sinks for assimilate.At initiation, an inflorescence is a weak sink in comparison with apical shoots. When assimilate supply is inadequate, the inflorescence has a reduced level of endogenous cytokinin and the degree of abortion is inversely related to the activity of sucrose hydrolase. Application of cytokinin plus gibberellins to the inflorescence increases its capacity to attract assimilate at the expense of apical shoots.At fruit set, cell division is activated and the ovary starts to accumulate reducing sugars and starch. Both the final cell number and the potential cell size are determined in the first two weeks and may be related to the levels of cytokinin and auxin.At the early stage of rapid growth a fruit accumulates imported assimilates, mainly in the forms of hexoses and starch. The rate of starch accumulation increases with the absolute fruit growth rate and affects the final soluble solids content of a fruit. The change in the fruit growth rate during fruit development does not coincide with the changes in the endogenous hormone levels of the fruit. A fruit competes for assimilate with others mainly in the same truss.     

Photoheterotrophy and light-dependent uptake of organic and organic nitrogenous compounds by Planktothrix rubescens under low irradiance

1. Planktothrix rubescens is the dominant photoautotrophic organism in Lake Zürich, a prealpine, deep, mesotrophic freshwater lake with an oxic hypolimnion. Over long periods of the year, P. rubescens accumulates at the metalimnion and growth occurs in situ at irradiance near the photosynthesis compensation point. Experiments were conducted to evaluate the contribution of photoheterotrophy, heterotrophy and light‐dependent uptake of nitrogenous organic compounds to the carbon and nitrogen budget of this cyanobacterium under conditions of restricted availability of light quanta. 2. We used both purified natural populations of P. rubescens from the depth of 9 m and an axenic culture grown under low irradiance at 11 μmol m^?2 s^?1 on a light : dark cycle (10 : 14 h) to determine the uptake rates of various amino acids, urea, glucose, fructose, acetate and inorganic carbon. The components were added to artificial lake water in low amounts that simulated the naturally occurring potential concentrations. 3. The uptake rates of acetate and amino acids (glycine, serine, glutamate and aspartate) were strongly enhanced at low irradiance as compared with the dark. However, no difference was observed in the uptake of arginine, which was taken up at high rates under both treatments. The uptake rates of glucose, fructose and urea were very low under all conditions. Similar results were obtained for both axenic P. rubescens and for purified natural populations of P. rubescens that were separated from bacterioplankton and other phytoplankton. 4. Metalimnetic P. rubescens that was stratified at low irradiance for weeks exhibited much higher uptake rates than filaments that were entrained in the deepening surface mixed layer and experienced higher irradiance. The added organic compounds contributed up to 62% to the total carbon uptake of metalimnetic P. rubescens. On the basis of a molar C : N ratio of 4.9, the nitrogen uptake as organic compounds satisfied up to 84% of the nitrogen demand. 5. The experiments indicate that photoheterotrophy and light‐dependent uptake of nitrogenous organic compounds may contribute significantly to the carbon and nitrogen budget of filaments at low irradiance typical for growth of P. rubescens in the metalimnion and at the bottom of the surface mixed layer.     

A simple model for nitrogen-limited plant growth and nitrogen allocation

BACKGROUND: and Aims In many studies of nitrogen-limited plant growth a linear relationship has been found between relative growth rate and plant nitrogen concentration, showing a negative intercept at a plant nitrogen concentration of zero. This relationship forms the basis of the nitrogen productivity theory. On the basis of empirical findings, several authors have suggested that there is also a distinctive relationship between allocation and plant nitrogen concentration. The primary aim of this paper is to develop a simple plant growth model that quantifies this relationship in mathematical terms. The model was focused on nitrogen allocation to avoid the complexity of differences in nitrogen concentrations in the different plant compartments. The secondary aim is to use the model for examining the processes that underlie the empirically based nitrogen productivity theory. METHODS: In the construction of the model we focused on the formation and degradation of biologically active nitrogen in enzymes involved in the photosynthetic process (photosynthetic nitrogen). It was assumed that, in nitrogen-limiting conditions, the formation of photosynthetic nitrogen is proportional to nitrogen uptake. Furthermore it was assumed that the degradation of photosynthetic nitrogen is governed by first-order kinetics. Model predictions of nitrogen allocation were compared with data from literature describing four studies of growth. Model predictions of whole plant growth were compared with the above-mentioned nitrogen productivity theory. KEY RESULTS: Allocation predictions agreed well with the investigated empirical data. The ratio of leaf nitrogen and plant nitrogen declines linearly with the inverse of plant nitrogen concentration. Nitrogen productivity is proportional to this ratio. Predictions for whole-plant growth were in accordance with the nitrogen productivity theory. CONCLUSIONS: The agreement between model predictions and empirical findings suggests that the derived equation for nitrogen allocation and its relationship to plant nitrogen concentration might be generally applicable. The negative intercept in the linear relationship between relative growth rate and plant nitrogen concentration is interpreted as being equal to the degradation constant of photosynthetic nitrogen.     

A role for shoot protein in shoot-root dry matter allocation in higher plants

BACKGROUND AND AIMS: It is stated in many recent publications that nitrate (NO3-) acts as a signal to regulate dry matter partitioning between the shoot and root of higher plants. Here we challenge this hypothesis and present evidence for the viewpoint that NO3- and other environmental effects on the shoot:root dry weight ratio (S:R) of higher plants are often related mechanistically to changes in shoot protein concentration. METHODS: The literature on environmental effects on S:R is reviewed, focusing on relationships between S:R, growth and leaf NO3- and protein concentrations. A series of experiments carried out to test the proposal that S:R is dependent on shoot protein concentration is highlighted and new data are presented for tobacco (Nicotiana tabacum). KEY RESULTS/EVIDENCE: Results from the literature and new data for tobacco show that S:R and leaf NO3- concentration are not significantly correlated over a range of environmental conditions. A mechanism involving the relative availability of C and N substrates for growth in shoots can explain how shoot protein concentration can influence shoot growth and hence root growth and S:R. Generally, results in the literature are compatible with the hypothesis that macronutrients, water, irradiance and CO2 affect S:R through changes in shoot protein concentration. In detailed studies on several species, including tobacco, a linear regression model incorporating leaf soluble protein concentration and plant dry weight could explain the greater proportion of the variation in S:R within and between treatments over a wide range of conditions. CONCLUSIONS: It is concluded that if NO3- can influence the S:R of higher plants, it does so only over a narrow range of conditions. Evidence is strong that environmental effects on S:R are often related mechanistically to their effects on shoot protein concentration.     

Seasonal uptake of 15N-nitrate and distribution of absorbed nitrogen in peach trees

The absorption and distribution of N was measured monthly throught a calendar year in 3-year old peach trees (Prunus persica (L) c.v. Maycrest) grafted on Nemaguard rootstock. Plants were grown on siliceous sand in 500-L pots and fertilized with a solution containing ¹⁵N enriched KNO₃. During flowering and fruit set (March) approximately 7% of N found in new growth came from the fertilizer and the remainder came from the N stored in the old organs. Maximum N absorption took place during the periods of fruit ripening and maximal vegetative growth (May to August). This nitrogen was relocated from leaves to woody tissues and stored as reserve-N before leaf fall. In the following growth season reserve-N was used for flower development and new shoot growth. The N absorbed during plant dormancy was quite low and remained in the stem bark and roots mainly as soluble-N.     

Alpine plants show species-level differences in the uptake of organic and inorganic nitrogen

As an estimate of species-level differences in the capacity to take up different forms of N, we measured plant uptake of ¹⁵N-NH₄ ⁺, ¹⁵N-NO₃ ^– and ¹⁵N, [1]-¹³C glycine within a set of herbaceous species collected from three alpine community types. Plants grown from cuttings in the greenhouse showed similar growth responses to the three forms of N but varied in the capacity to take up NH₄ ⁺, NO₃ ^– and glycine. Glycine uptake ranged from approximately 42% to greater than 100% of NH₄ ⁺ uptake; however, four out of nine species showed significantly greater uptake of either NH₄ ⁺ or NO₃ ^– than of glycine. Relative concentrations of exchangeable N at the sites of plant collection did not correspond with patterns of N uptake among species; instead, species from the same community varied widely in the capacity to take up NH₄ ⁺, NO₃ ^–, and glycine, suggesting the potential for differentiation among species in resource (N) use.     

Sectoriality and physiological organisation in herbaceous plants: an overview

The physiological organisation of plants is considered in relation to the carbon economy of plant parts. Although assimilate is partitioned according to the relative strength of sinks, in many species there is also a very close relationship between partitioning and shoot phyllotaxy, giving rise to sectorial patterns of allocation whereby only certain sinks are supported by any source leaf. Essentially these sinks are in the same orthostichy as the source leaf. This constraint of the vascular architecture on assimilate distribution to developing sinks such as leaves, flowers and fruits is not always absolute, as following the loss of their principal source leaves these sinks can in many cases be supplied with assimilate by other leaves via new inter-orthostichy pathways. The supply of assimilate to major sinks such as developing fruits becomes more and more localised with time so that a fruit in an axillary position becomes largely supported by its subtending leaf; the reproductive node—a metamer-can thus be regarded as a relatively autonomous unit of the plant (an IPU). Similary, once established after a developmental phase of assimilate import, tiller ramets and branches in unitary plants tend to become physiologically autonomous modules. However, the functional autonomy of tillers is reversed following defoliation or shading as they are then sustained by the import of assimilate, subject to its availability, from unaffected tillers. Consequently the plant becomes physiologically integrated by the flow of assimilate from one part to another. The mainly autonomous ramets of many stoloniferous and rhizomatous species display a similar pattern of physiological integration in response to source manipulation, but in some species the ramets appear to maintain their independent functioning as a normal feature of the carbon allocation within the clone. In other clonal species, as the clone develops and becomes more structurally complex, vascular constraints start to restrict the movement of resources, and the clone becomes composed of a number of semi-autonomous IPUs. In unitary plants branches appear to remain very physiologically isolated in terms of their carbon economy once they become established, irrespective of a range of source-sink manipulations.These different patterns of physiological integration and organisation are discussed in relation to different strategies of assimilate utilisation and conservation.     

Variation in nitrogen source utilisation by Pisolithus isolates maintained in axenic culture

The influence of various inorganic and organic nitrogen sources on biomass production by 17 isolates of Australian Pisolithus spp. was investigated before and after a 3-year maintenance period in axenic culture. While some isolates produced similar or higher amounts of biomass on NH₄ ⁺ or certain amino acids after the maintenance period, there was a general trend to reduced biomass production on these substrates. Biomass production by most isolates on bovine serum albumin increased significantly after maintenance. The data are discussed in relation to the use of axenic culture growth experiments for investigations of inter- and intraspecific physiological variation in ectomycorrhizal fungi. Accepted: 22 February 2001     

Carbon and nitrogen allocation in Lolium perenne in response to elevated atmospheric CO2 with emphasis on soil carbon dynamics

The effect of elevated CO2 on the carbon and nitrogen distribution within perennial ryegrass (L. perenne L.) and its influence on belowground processes were investigated. Plants were homogeneously 14C-labelled in two ESPAS growth chambers in a continuous 14C-CO2 atmosphere of 350 and 700 L L-1 CO2 and at two soil nitrogen regimes, in order to follow the carbon flow through all plant and soil compartments.After 79 days, elevated CO2 increased the total carbon uptake by 41 and 21% at low (LN) and high nitrogen (HN) fertilisation, respectively. Shoot growth remained unaffected, whereas CO2 enrichment stimulated root growth by 46% and the root/soil respiration by 111%, irrespective of the nitrogen concentration. The total 14C-soil content increased by 101 and 28% at LN and HN, respectively. The decomposition of the native soil organic matter was not affected either by CO2 or by the nitrogen treatment.Elevated CO2 did not change the total nitrogen uptake of the plant either at LN or at HN. Both at LN and HN elevated CO2 significantly increased the total amount of nitrogen taken up by the roots and decreased the absolute and relative amounts translocated to the shoots.The amount of soil nitrogen immobilised by micro-organisms and the size of the soil microbial biomass were not affected by elevated CO2, whereas both were significantly increased at the higher soil N content.Most striking was the 88% increase in net carbon input into the soil expressed as: 14C-roots plus total 14C-soil content minus the 12C-carbon released by decomposition of native soil organic matter. The net carbon input into the soil at ambient CO2 corresponded with 841 and 1662 kg ha-1 at LN and HN, respectively. Elevated CO2 increased these amounts with an extra carbon input of 950 and 1056 kg ha-1. Combined with a reduced decomposition rate of plant material grown at elevated CO2 this will probably lead to carbon storage in grassland soils resulting in a negative feed back on the increasing CO2 concentration of the atmosphere.     

Impact of culture media glucose levels on the intestinal uptake of organic cations

There are several data concerning transporters expression and/or regulation in cell lines maintained in different conditions, such as medium glucose concentration. This work aimed to evaluate the influence of two different extracellular glucose concentrations, commonly used in culture media, on the intestinal absorption of organic cations. Thus, the effect of 5.5 mM glucose and 25 mM glucose (HG) in culture media, was studied on [³H]-MPP⁺ (1-methyl-4-phenylpyridinium iodide) uptake in Caco-2 cells. Expression of human organic cation transporter type 1 (hOCT1) and human organic cation transporter type 3 (hOCT3) was investigated in cells cultured at both glucose concentrations. [³H]-MPP⁺ uptake, as well as its affinity for the transporter, were significantly decreased in HG cells. Moreover, hOCT3 mRNA levels were reduced in HG cells. Functional confirmation of this result was made using hOCT3 inhibitors. In conclusion, maintenance of Caco-2 cells (commonly used in several in vitro studies on membrane transport) in HG conditions affects organic cation transport at the intestinal level. Hence, results obtained in these conditions must be analysed with great care, since extracellular glucose levels may originate changes in organic cation nutrient and drug bioavailability.