Competition for nitrogen sources between European beech (Fagus sylvatica) and sycamore maple (Acer pseudoplatanus) seedlings

To investigate the short‐term consequences of direct competition between beech and sycamore maple on root N uptake and N composition, mycorrhizal seedlings of both tree species were incubated for 4 days (i.e. beech only, sycamore maple only or both together) in an artificial nutrient solution with low N availability. On the fourth day, N uptake experiments were conducted to study the effects of competition on inorganic and organic N uptake. For this purpose, multiple N sources were applied with a single label. Furthermore, fine roots were sampled and analysed for total amino acids, soluble protein, total nitrogen, nitrate and ammonium content. Our results clearly show that both tree species were able to use inorganic and organic N sources. Uptake of inorganic and organic N by beech roots was negatively affected in the presence of the competing tree species. In contrast, the presence of beech stimulated inorganic N uptake by sycamore maple roots. Both the negative effect of sycamore maple on N uptake of beech and the positive effect of beech on N uptake of sycamore maple led to an increase in root soluble protein in beech, despite an overall decrease in total N concentration. Thus, beech compensated for the negative effects of the tree competitor on N uptake by incorporating less N into structural N components, but otherwise exhibited the same strategy as the competitor, namely, enhancing soluble protein levels in roots when grown under competition. It is speculated that enhanced enzyme activities of so far unknown nature are required in beech as a defence response to inter‐specific competition.     

Plant sulfite oxidase as novel producer of H2O2: combination of enzyme catalysis with a subsequent non-enzymatic reaction step

Sulfite oxidase (EC 1.8.3.1) from the plant Arabidopsis thaliana is the smallest eukaryotic molybdenum enzyme consisting of a molybdenum cofactor-binding domain but lacking the heme domain that is known from vertebrate sulfite oxidase. While vertebrate sulfite oxidase is a mitochondrial enzyme with cytochrome c as the physiological electron acceptor, plant sulfite oxidase is localized in peroxisomes and does not react with cytochrome c. Here we describe results that identified oxygen as the terminal electron acceptor for plant sulfite oxidase and hydrogen peroxide as the product of this reaction in addition to sulfate. The latter finding might explain the peroxisomal localization of plant sulfite oxidase. 18O labeling experiments and the use of catalase provided evidence that plant sulfite oxidase combines its catalytic reaction with a subsequent non-enzymatic step where its reaction product hydrogen peroxide oxidizes another molecule of sulfite. In vitro, for each catalytic cycle plant SO will bring about the oxidation of two molecules of sulfite by one molecule of oxygen. In the plant, sulfite oxidase could be responsible for removing sulfite as a toxic metabolite, which might represent a means to protect the cell against excess of sulfite derived from SO2 gas in the atmosphere (acid rain) or during the decomposition of sulfur-containing amino acids. Finally we present a model for the metabolic interaction between sulfite and catalase in the peroxisome.     

Dynamics of amino acid redistribution in the carnivorous Venus flytrap (Dionaea muscipula) after digestion of 13C/15N‐labelled prey

• Amino acids represent an important component in the diet of the Venus flytrap (Dionaea muscipula), and supply plants with much needed nitrogen resources upon capture of insect prey. Little is known about the significance of prey‐derived carbon backbones of amino acids for the success of Dionaea's carnivorous life‐style.
• The present study aimed at characterizing the metabolic fate of ¹⁵N and ¹³C in amino acids acquired from double‐labeled insect powder. We tracked changes in plant amino acid pools and their δ¹³C‐ and δ¹⁵N‐signatures over a period of five weeks after feeding, as affected by contrasting feeding intensity and tissue type (i.e., fed and non‐fed traps and attached petioles of Dionaea).
• Isotope signatures (i.e., δ¹³C and δ¹⁵N) of plant amino acid pools were strongly correlated, explaining 60% of observed variation. Residual variation was related to contrasting effects of tissue type, feeding intensity and elapsed time since feeding. Synthesis of nitrogen‐rich transport compounds (i.e., amides) during peak time of prey digestion increased ¹⁵N‐ relative to ¹³C‐ abundances in amino acid pools. After completion of prey digestion, ¹³C in amino acid pools was progressively exchanged for newly fixed ¹²C. The latter process was most evident for non‐fed traps and attached petioles of plants that had received ample insect powder.
• We argue that prey‐derived amino acids contribute to respiratory energy gain and loss of ¹³CO₂ during conversion into transport compounds (i.e., 2 days after feeding), and that amino‐nitrogen helps boost photosynthetic carbon gain later on (i.e., 5 weeks after feeding).

     

Oxidation and reduction of sulfite contribute to susceptibility and detoxification of SO2 in Populus × canescens leaves

Key Message

The critical level for SO ₂ susceptibility of Populus × canescens is approximately 1.2 μL L ^?1 SO ₂ . Both sulfite oxidation and sulfite reduction and assimilation contribute to SO ₂ detoxification.

Abstract

In the present study, uptake, susceptibility and metabolism of SO₂ were analyzed in the deciduous tree species poplar (Populus × canescens). A particular focus was on the significance of sulfite oxidase (SO) for sulfite detoxification, as SO has been characterized as a safety valve for SO₂ detoxification in herbaceous plants. For this purpose, poplar plants were exposed to different levels of SO₂ (0.65, 0.8, 1.0, 1.2 μL L^?1) and were characterized by visible injuries and at the physiological level. Gas exchange parameters (stomatal conductance for water vapor, CO₂ assimilation, SO₂ uptake) of the shoots were compared with metabolite levels (sulfate, thiols) and enzyme activities [SO, adenosine 5′-phosphosulfate reductase (APR)] in expanding leaves (80–90 % expanded). The critical dosage of SO₂ that confers injury to the leaves was 1.2 μL L^?1 SO₂. The observed increase in sulfur containing compounds (sulfate and thiols) in the expanding leaves strongly correlated with total SO₂ uptake of the plant shoot, whereas SO₂ uptake rate was strongly correlated with stomatal conductance for water vapor. Furthermore, exposure to high concentration of SO₂ revealed channeling of sulfite through assimilatory sulfate reduction that contributes in addition to SO-mediated sulfite oxidation to sulfite detoxification in expanding leaves of this woody plant species.     

A Single-Pore Residue Renders the Arabidopsis Root Anion Channel SLAH2 Highly Nitrate Selective

In contrast to animal cells, plants use nitrate as a major source of nitrogen. Following the uptake of nitrate, this major macronutrient is fed into the vasculature for long-distance transport. The Arabidopsis thaliana shoot expresses the anion channel SLOW ANION CHANNEL1 (SLAC1) and its homolog SLAC1 HOMOLOGOUS3 (SLAH3), which prefer nitrate as substrate but cannot exclude chloride ions. By contrast, we identified SLAH2 as a nitrate-specific channel that is impermeable for chloride. To understand the molecular basis for nitrate selection in the SLAH2 channel, SLAC1 and SLAH2 were modeled to the structure of HiTehA, a distantly related bacterial member. Structure-guided site-directed mutations converted SLAC1 into a SLAH2-like nitrate-specific anion channel and vice versa. Our findings indicate that two pore-occluding phenylalanines constrict the pore. The selectivity filter of SLAC/SLAH anion channels is determined by the polarity of pore-lining residues located on alpha helix 3. Changing the polar character of a single amino acid side chain (Ser-228) to a nonpolar residue turned the nitrate-selective SLAH2 into a chloride/nitrate-permeable anion channel. Thus, the molecular basis of the anion specificity of SLAC/SLAH anion channels seems to be determined by the presence and constellation of polar side chains that act in concert with the two pore-occluding phenylalanines.     

Diurnal pattern of acetaldehyde emission by flooded poplar trees

The emission of the tropospheric trace gas acetaldehyde was determined in leaves of 4-month-old poplar trees ( Populus tremula × P. alba ) grown under controlled environmental conditions in a greenhouse. Using a dynamic cuvette system together with a high sensitivity laser-based photoacoustic detection unit, rates of acetaldehyde emission were measured with the high time resolution of about 15 min. Submergence of the roots resulted in the emission of acetaldehyde by the leaves. The emission increased linearly before reaching more or less steady-state values (ca 350 nmol m⁻² min⁻¹; ca 470 ng g⁻¹ dry weight min⁻¹) after approximately 6 h. Prolonged flooding of poplar trees resulted in a clear diurnal rhythm of acetaldehyde emission. The emission rates decreased when the light was switched off in the evening and peaked in the morning after the light was turned on again. This pattern significantly correlated with diurnal rhythms of stomatal conductance, photosynthesis, transpiration and with the concentrations of ethanol, the assumed precursor of acetaldehyde, in the xylem sap of flooded poplar trees. It may be concluded that under conditions of diminished stomatal conductance, acetaldehyde emission declines because its diffusive flux is reduced. Alternatively, reduced transpiration may decrease ethanol transport from the roots to the shoots and appreciable amounts of the acetaldehyde precursor ethanol are lacking in the leaves. The present results support the view that acetaldehyde emitted by the leaves of plants is derived from ethanol produced by alcoholic fermentation in submerged roots and transported to the leaves with the transpiration stream.     

Phloem transport of sulfur in Ricinus

Mature leaves of Ricinus communis fed with ³⁵SO ₄ ^2- in the light export labeled sulfate and reduced sulfur compounds by phloem transport. Only 1–2% of the absorbed radiosulfur is exported to the stem within 2–3 h, roughly 12% of ³⁵S recovered was in reduced form. The composition of phloem translocate moving down the stem toward the root was determined from phloem exudate: 20–40% of the ³⁵S moved in the form of organic sulfur compounds, however, the bulk of sulfur was transported as inorganic sulfate. The most important organic sulfur compound translocated was glutathione, carrying about 70% of the label present in the organic fraction. In addition, methionine and cysteine were involved in phloem sulfur transport and accounted for roughly 10%. Primarily, the reduced forms of both, glutathione and cysteine are prsent in the siever tubes.Abbreviations CySH cysteine - GSH glutathione - GSSG glutathione disulfide - NEM N-ethylmaleimide - CyS-SCy cystine