Regulation of amino acid uptake in conifers by exogenous and endogenous nitrogen

Although an accumulating amount of research clearly indicates that plants are capable of taking up exogenous amino acids, the actual importance of such organic N sources for plant N nutrition is under debate. In this study, we show that amino acid uptake by Scots pine (Pinus sylvestris L.) is significantly decreased by elevated internal NH(4)(+) levels, while it increases following exposure to exogenous amino acids. Furthermore, amino acid uptake is larger in N-deficient plants than in plants grown with a large access of N. The regulatory pattern of amino acid uptake shows important similarities to the regulation of NO(3)(-) and NH(4)(+) transport as well as to the regulation of yeast amino acid transporters. In addition, our data suggest that uptake may be regulated by factors not originating from N metabolism. The up-regulation of uptake in response to N deficiency suggests that amino acid uptake may be a significant contributor to the N economy of P. sylvestris.     

Dependence of amino acid composition upon nitrogen availability in birch (Betula pendula)

Small birch plants ( Betula pendula Roth .) were grown at different rates of exponentially increasing nitrogen supply. This resulted in plants with different relative growth rates and different internal nitrogen concentrations. Within a nitrogen treatment, both of these variables remained constant with time.
Free amino acids were measured in leaves and roots of the seedlings at two different harvests. At greater nitrogen supply, higher concentrations of total amino acid nitrogen were found in roots and leaves. The ratio of amino acid nitrogen to total nitrogen was low albeit greater at higher nitrogen supply. Higher concentrations of amino acid nitrogen were mainly due to high concentrations of citrulline, glutamine, γ-aminobuitric acid and arginine.
Greater leaf concentrations of amino acid nitrogen at higher nitrogen supply may be related lo increased concentrations in the xylem sap and/or may be indicative of small excesses of nitrogen with respect to current nitrogen usage in protein synthesis.     

Mineral nutrient imbalances and arginine concentrations in needles of Picea abies (L.) Karst. from two areas with different levels of airborne deposition

Summary This study evaluated the utility of free arginine concentrations as a possible alternative to mineral nutrient concentrations as an indicator of mineral nutrient imbalances in Norway spruce [Picea abies (L.) Karst.]. The concentrations of mineral nutrients and arginine were measured in the needles of spruce trees from two areas in Sweden, one with high (15–30 kg ha^–1 year^–1) airborne N deposition, and one with lower (1–4 kg ha^–1 year^–1) deposition. The spruce needles from the area with high deposition in southern Sweden had elevated concentrations of free arginine, especially on peat sites. No increase in concentrations was found in the low deposition area in northern Sweden. The arginine concentrations on different sampling occasions were consistent for each site and for individual trees. Trees on peat sites in the south seemed to suffer from P deficiency in relation to N availability. A tendency for K deficiency in needles from peat sites was also found. Needles from trees on mor plots showed acceptable levels of these nutrient elements. Sites in the northern area showed low N concentrations, but the ratios between the different mineral elements analyzed in this study and N were within ranges normally found. A low P/N ratio correlated to high free arginine concentration. The threshold for elevated arginine concentrations is crossed when P/N ratios drop below 0.07–0.08. A tendency for increased arginine levels when ratios between N and the other mineral elements are low was also found, although it was not as strong as that for the P/N ratio. The results are discussed in relation to mineral nutrient imbalances in spruce stands caused by airborne deposition.     

Purification and partial characterization of two genetic variants of placental alkaline phosphatase

The two most common variants of placental alkaline phosphatase, the F and S variants, were purified to homogeneity and characterized. Their molecular weights were determined by equilibrium ultracentrifugation and sodium dodecylsulfate polyacrylamide gel electrophoresis, which gave almost identical values for the two variants, 118,000 (F) and 119,000 (S). The amino acid compositions of the F and S variants presented here are found to be very similar. Differences between the two variants were found in specific activity (160 U/mg for F and 250 U/mg for S), isoelectric point (IP=4.5 for F and 4.7 for S), sedimentation coefficient (6.5×10^?13 sec for F and 6.4×10^?13 sec for S). Thus the structural differences observed for these enzyme variants seem to affect both the active site and the protein conformation.     

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     

Studies on the Crayfish Plague Fungus Aphanomyces astaci

A mycelial suspension of the crayfish plague fungus, Aphanomyces astaci, was able to produce large numbers of zoospores, when transferred to redistilled water, at 20°C, even after storage for months at 2°C. Spore production was greater in redistilled water than in tap water and heavier under shake conditions than under stationary ones. In buffered redistilled water sporulation occurred between pH 5 and 8 and the optimal range was about pH 5 to 7. Of the tested aliphatic alcohols, aldehydes, and carboxylic acids, the long analogues were more toxic to spore formation than the shorter ones. Ethylenediamine-tetraacetic acid (EDTA) prevented sporulation probably by removing some essential metal (s) with an affinity for EDTA near that of calcium. Calcium protected against the toxic effect of lithium, sodium, and potassium. Magnesium, only tested against lithium, had no such protecting effect. Cu²⁺, Ni²⁺, Zn²⁺, Co²⁺, K⁺ Mn²⁺, NH₄⁺, Li⁺, Na⁺, Ca²⁺, Mg²⁺ was the approximate order among tested cations in their ability to stop the swimming stage of the zoospores, the first mentioned being the most effective ones. Nitrate and acetate were more active in the same respect than sulphate, chloride, phosphate, or bicarbonate. The optimal pH range for swimming seemed to be pH 6–7.5, and the maximal range 4.5–9.0. The zoospores showed no chemotactic response to tested substances. The germination ability was as high in horse blood as in crayfish blood. A spore suspension stored for 2 months at 2°C still contained viable spores.     

Transporters in Arabidopsis roots mediating uptake of amino acids at naturally occurring concentrations

Recent studies of Arabidopsis have identified several transporters as being important for amino acid uptake. We used Arabidopsis plants with altered expression of lysine histidine transporter 1 (LHT1), amino acid permease 1 (AAP1) and amino acid permease 5 (AAP5) with the aim of disentangling the roles of each transporter in the uptake of different amino acids at naturally occurring concentrations (2-50 μM). LHT1 mutants displayed reduced uptake rates of L-Gln, L-Ala, L-Glu and L-Asp but not of L-Arg or L-Lys, while AAP5 mutants were affected in the uptake of L-Arg and L-Lys only. Double mutants (lht1aap5) exhibited reduced uptake of all tested amino acids. In the concentration range tested, AAP1 mutants did not display altered uptake rates for any of the studied amino acids. Expression analysis of amino acid transporter genes with important root functions revealed no major differences in the individual mutants other than for genes targeted for mutation. We conclude that LHT1 and AAP5, but not AAP1, are crucial for amino acid uptake at concentrations typically found in soils. LHT1 and AAP5 displayed complementary affinity spectra, and no redundancy with respect to gene expression was found between the two transporters, suggesting these two transporters have separate roles in amino acid uptake.     

Plant root distributions and nitrogen uptake predicted by a hypothesis of optimal root foraging

CO(2)-enrichment experiments consistently show that rooting depth increases when trees are grown at elevated CO(2) (eCO(2)), leading in some experiments to increased capture of available soil nitrogen (N) from deeper soil. However, the link between N uptake and root distributions remains poorly represented in forest ecosystem and global land-surface models. Here, this link is modeled and analyzed using a new optimization hypothesis (MaxNup) for root foraging in relation to the spatial variability of soil N, according to which a given total root mass is distributed vertically in order to maximize annual N uptake. MaxNup leads to analytical predictions for the optimal vertical profile of root biomass, maximum rooting depth, and N-uptake fraction (i.e., the proportion of plant-available soil N taken up annually by roots). We use these predictions to gain new insight into the behavior of the N-uptake fraction in trees growing at the Oak Ridge National Laboratory free-air CO(2)-enrichment experiment. We also compare MaxNup with empirical equations previously fitted to root-distribution data from all the world's plant biomes, and find that the empirical equations underestimate the capacity of root systems to take up N.     

Informing climate models with rapid chamber measurements of forest carbon uptake

Models predicting ecosystem carbon dioxide (CO₂) exchange under future climate change rely on relatively few real‐world tests of their assumptions and outputs. Here, we demonstrate a rapid and cost‐effective method to estimate CO₂ exchange from intact vegetation patches under varying atmospheric CO₂ concentrations_. We find that net ecosystem CO₂ uptake (NEE) in a boreal forest rose linearly by 4.7 ± 0.2% of the current ambient rate for every 10 ppm CO₂ increase, with no detectable influence of foliar biomass, season, or nitrogen (N) fertilization. The lack of any clear short‐term NEE response to fertilization in such an N‐limited system is inconsistent with the instantaneous downregulation of photosynthesis formalized in many global models. Incorporating an alternative mechanism with considerable empirical support – diversion of excess carbon to storage compounds – into an existing earth system model brings the model output into closer agreement with our field measurements. A global simulation incorporating this modified model reduces a long‐standing mismatch between the modeled and observed seasonal amplitude of atmospheric CO₂. Wider application of this chamber approach would provide critical data needed to further improve modeled projections of biosphere–atmosphere CO₂ exchange in a changing climate.     

Chiral Assay of Atenolol Present in Microdialysis and Plasma Samples of Rats Using Chiral CBH as Stationary Phase

Two different enantioselective chiral chromatographic methods were developed and validated to investigate the disposition of the β₁-receptor antagonist atenolol in blood and in brain extracellular fluid of rats (tissue dialysates). System A for the plasma samples was a one-column chromatographic system with a Chiral CBH column with an aqueous buffer as mobile phase into which cellobiose was added for selective regulation of the retention of the internal standard, (S)-metoprolol. The plasma samples were analysed after a simple extraction procedure. The limit of quantitation was 0.2 μg/ml for the atenolol enantiomers. The repeatability of the medium concentration quality control plasma sample (6.0 μg rac-atenolol/ml) was 11–18% for the enantiomers. The dynamic linear range of the plasma samples was 0.5–20 μg/ml. For system B, since atenolol is an extremely hydrophilic drug, the tissue dialysate sample required a much more sensitive system as compared to the plasma samples. A coupled column system was used for peak compression of the enantiomers in the eluate after the separation on the Chiral CBH column, hence increasing the detection sensitivity. The limit of quantification was 0.045 μg/ml for the atenolol enantiomers in artificial CSF. The repeatability of the medium concentration quality control samples (0.1 and 4.0 μg rac-atenolol/ml in artificial CSF and Hepes Ringer, respectively) was 2.8–9.3% for the two enantiomers. The dynamic linear range of the brain samples was 0.05–1.0 and 0.5–20 μg/ml in artificial CSF and Hepes Ringer, respectively. Chirality 9:329–334, 1997. © 1997 Wiley-Liss, Inc.