Influence of different nutrient regimes on the regulation of carbon metabolism in Norway spruce [Picea abies (L.) Karst.] seedlings

Interactive effects of ozone exposure and nitrogen addition on tree root traits and biomass allocation pattern: An experimental case study and a literature meta-analysis

Ground-level ozone (O₃) pollution often co-occurs with anthropogenic nitrogen (N) deposition. Many studies have explored how O₃ and soil N affect aboveground structure and function of trees, but it remains unclear how belowground processes change over a spectrum of N addition and O₃ concentrations levels. Here, we explored the interactive impact of O₃ (five levels) and soil N (four levels) on fine and coarse root biomass and biomass allocation pattern in poplar clone 107 (Populus euramericana cv. '74/76'). We then evaluated the modifying effects of N on the responses of tree root biomass to O₃ via a synthesis of published literature. Elevated O₃ inhibited while N addition stimulated root biomass, with more pronounced effects on fine roots than on coarse root. The root:shoot (R:S) ratio was markedly decreased by N addition but remained unaffected by O₃. No interactive effects between O₃ and N were observed on root biomass and R:S ratio. The slope of log-log linear relationship between shoot and root biomass (i.e. scaling exponent) was increased by N, but not significantly affected by O₃. The analysis of published literature further revealed that the O₃-induced reduction in tree root biomass was not modified by soil N. The results suggest that higher N addition levels enhance faster allocation of shoot biomass while shoot biomass scales isometrically with root biomass across multiple O₃ levels. N addition does not markedly alter the sensitivity of root biomass of trees to O₃. These findings highlight that the biomass allocation exhibits a differential response to environmentally realistic levels of O₃ and N, and provide an important perspective for understanding and predicting net primary productivity and carbon dynamics in O₃-polluted and N-enriched environments.

Carbon allocation in ectomycorrhizal plants at limited optimal N supply: an attempt aat unraveling conflicting theories

With regard to mycorrhiza, conflicting theories try to explain how the balance between fungal demand for carbohydrates and the plant’s needs for nutrients varies, resulting in conflicting predictions. In order to evaluate current concepts, we investigated some metabolic parameters, which are indicative for plant carbon allocation in response to mycorrhization at limited and optimal N supply. Pinus pinaster seedlings were inoculated with living or dead (control) cultures of Pisolithus tinctorius, supplied with ammonium at 4 (limiting) or 7% d−1 (non-limiting) N relative addition rate (RARN), and followed development for 29 days. Mycorrhizal colonization of roots was quantified by the determination of ergosterol. A series of enzymes (sucrose and trehalose metabolism, anaplerosis) and metabolites (soluble carbohydrate, including trehalose; fructose 2,6 bisphosphate, free amino acids) relevant in the C/N exchange between symbionts, and in the carbon allocation and sink strength within the plant were assayed for 2-day-intervals for up to 14 days, and at 5-day-intervals for the rest of the experiment. The first 10 days reflected the establishment of mycorrhizal interaction, and the carbon allocation to the root was higher in M plants independent of N supply. Following this period, carbon allocation became N-related, higher at low, and lower at high N supply. The belowground C investment of M plants was dependent on N availability, but not on N gain. Finally, increased belowground C allocation was accompanied by a shift from plant to fungal metabolism.

Nitrogen mediates above-ground effects of ozone but not below-ground effects in a rhizomatous sedge

Ozone and atmospheric nitrogen are co-occurring pollutants with adverse effects on natural grassland vegetation. Plants of the rhizomatous sedge Carex arenaria were exposed to four ozone regimes representing increasing background concentrations (background-peak): 10-30, 35-55, 60-80 and 85-105 ppb ozone at two nitrogen levels: 12 and 100 kg N ha(-1) yr(-1). Ozone increased the number and proportion of senesced leaves, but not overall leaf number. There was a clear nitrogen x ozone interaction with high nitrogen reducing proportional senescence in each treatment and increasing the ozone dose (AOT40) at which enhanced senescence occurred. Ozone reduced total biomass due to significant effects on root biomass. There were no interactive effects on shoot:root ratio. Rhizome tissue N content was increased by both nitrogen and ozone. Results suggest that nitrogen mediates above-ground impacts of ozone but not impacts on below-ground resource translocation. This may lead to complex interactive effects between the two pollutants on natural vegetation.

The influence of external nitrogen on carbon allocation to Glomus intraradices in monoxenic arbuscular mycorrhiza

The influence of external nitrogen (N) on carbon (C) allocation and processes related to phosphorus (P) metabolism were studied in monoxenic arbuscular mycorrhiza (AM) cultures of Daucus carota. Fungal hyphae of Glomus intraradices proliferated from colonized roots growing on solid medium into C-free liquid minimal medium with two different N and P levels. Furthermore, we exposed the colonized roots to high or low N availability and then studied the mycelial development. Roots were provided with (13)C-glucose in order to follow the C allocation. The mycelium was analysed for phosphatase activity and transcription levels of two nutrient regulated genes. High N availability to the monoxenic AM root reduced the C allocation to the AM fungus while N availability to the mycelium was important for the upregulation of the fungal inorganic phosphorus (Pi)-transporter GiPT. We found that N availability can regulate nutritional processes in arbuscular mycorrhiza. We conclude that negative impacts of N on AM abundance are caused by reduced C allocation from the plant. Upregulation of the fungal Pi-transporter GiPT indicated that increased N availability might induce P limitation in the mycelium.

Source-sink balance and carbon allocation below ground in plants exposed to ozone

The role of tropospheric ozone in altering plant growth and development has been the subject of thousands of publications over the last several decades. Still, there is limited understanding regarding the possible effects of ozone on soil processes. In this review, the effects of ozone are discussed using the flow of carbon from the atmosphere, through the plant to soils, and back to the atmosphere as a framework. A conceptual model based on carbohydrate signaling is used to illustrate physiological changes in response to ozone, and to discuss possible feedbacks that may occur. Despite past emphasis on above-ground effects, ozone has the potential to alter below-ground processes and hence ecosystem characteristics in ways that are not currently being considered. Contents Summary 213 I. Introduction 213 II. Source-sink model: carbohydrate signaling 214 III. Effect of ozone on above-ground sources and sinks 216 IV. Decreased allocation below ground 218 V. Carbon flux to soils 220 VI. Soil food web 223 VII. Summary, conclusions and future research 223 Acknowledgements 223 References 223.

Phosphoenolpyruvate carboxylase in mistletoe leaves: Regulation of gene expression, protein content, and covalent modification

Seasonal changes in the activity of phosphoenolpyruvate carboxylase (PEPCase, EC 4.1.1.31), a key enzyme in the interaction of carbohydrate and nitrogen metabolism, were studied in leaves of the C3 semiparasitic mistletoe, Viscum album, growing on different host trees. Maximum extractable PEPCase activities were higher in leaves of mistletoes growing on Betula pendula and Alnus glutinosa hosts compared with those on the conifers, Abies alba and Larix decidua. Independent of host, maximum extractable PEPCase activities were high in spring and autumn while low in summer. Samples with higher PEPCase activities showed higher amounts of PEPCase protein and higher PEPCase mRNA levels. A curvilinear correlation between leaf total nitrogen content and the maximum extractable PEPCase activity as well as PEPCase mRNA level suggested that nitrogen might affect the activity of PEPCase of mistletoe by up-regulating gene expression. In addition to extractable activity, seasonal changes of the PEPCase activation state, the ratio of activities resulting from limited:non-limited assays, were found, which was correlated to the variation of malate content in leaves of mistletoe. ATP-dependent activation of PEPCase was characterized by an increase in I0.5(L-malate), indicating that PEPCase of leaves of mistletoes is probably regulated via phosphorylation.

Recent advances in exploring physiology and biodiversity of ectomycorrhizas highlight the functioning of these symbioses in ecosystems

Ectomycorrhizas, the dominating mycorrhizal symbiosis in boreal, temperate and some tropical forests, are formed by 5000-6000 species of the asco- and basidiomycetes. This high diversity of fungal partners allows optimal foraging and mobilisation of various nitrogen and phosphorus forms from organic soil layers. In this review, two approaches to study the functioning of this multitude of symbiotic associations are presented. On selected culture models, physiological and molecular investigations have shown that the supply of hexoses has a key function in controlling the plant-fungus interaction via partner-specific regulation of gene expression. Environmental factors which affect fungal carbon supply, such as increased nitrogen availability, also affect mycorrhiza formation. Based on such laboratory results, the adaptative capability of ectomycorrhizas to changing field conditions is discussed. The second approach consists of analysing the distribution of mycorrhizas in ecosystem compartments and to relate distribution patterns to variations of ecological factors. Recent advances in identification of fungal partners in ectomycorrhizas by analysing the internal transcribed spacer of ribosomal DNA are presented, which can help to resolve sampling problems in field studies. The limits of the laboratory and the field approaches are discussed. Despite some problems, this combined approach is the most promising. Direct investigation of gene expression, which has been introduced for soil bacteria, will be difficult in the case of mycorrhizal fungi which constitute organisms with functionally varying structures.

Effect of fertilization on ozone-induced changes in the metabolism of birch (Betula pendula) leaves

Cloned cuttings of Betula pendula Roth were grown in field fumigation chambers at Birmensdorf throughout one growing season in filtered air with either < 3 (control) or 90/40 nl l^-1 O₃ (day/night; ozone generated from pure oxygen). Each ozone regime was split into high and low soil nutrient regimes by watering plants with either a 0.05 % or a 0.005% solution of a fertilizer which contained macronutrients and micronutrients. Fertilization had a strong effect on plant growth, enzyme activities and the expression of ozone-induced effects at the biochemical level. The activities of PEPC and Rubisco were enhanced about threefold in the plants with high fertilization (HF). Significant effects of ozone were in most cases found only in the older leaves of the plants with low fertilization (LF), There, sucrose, glucose and fructose levels were enhanced. In both fertilization treatments, the number of starch granules along the minor veins was increased. These ozone effects point to a decreased or inhibited phloem loading. The increased PEPC activity and the enhanced malate levels in the ozone-exposed plants might be the result of a redirection of carbon flow from sucrose synthesis and translocation towards anapleurotic processes, which can feed detoxification and repair of ozone injury as indicated by enhanced respiration. These findings agree well with the observed effects of ozone in lowering the root: shoot biomass ratio. Although there was a marked reduction in the O₃ /LF plants, O₃ /HF plants showed no significant response. Inositol was decreased under ozone exposure in both fertilizer treatments, contrasting with the pattern for carbohydrates. These results demonstrate the role of fertilization as an important modifier of ozone-induced effects at the plant biochemical level. Well fertilized plants appear to cope better with the impact of ozone on metabolism.

Cyclic AMP, a possible regulator of glycolysis in the ectomycorrhizal fungus Amanita muscaria

The amounts of cyclic AMP (cAMP), fructose-2,6-bisphosphate (F26BP), trehalose and glycogen were determined in cell suspension cultures of the ectomycorrhiza-forming fungus Amanita muscaria (L. ex Fr.) Hooker. For the assay of cAMF a protocol was developed that enabled the detection of as little as 50 fmol of this secondary messenger by an enzyme-linked immuno assay (EIA). Values varied from < 1 and up to 5 pmol cAMP mg¹ d. wt according to the age of the fungal culture. Typically, a transient increase in cAMP occurred after c. 4 d of culture of the fungus on glucose-containing medium. This increase (up to 100%) was followed by the start of the logarithmic growth phase, and by a more persistent increase in F26BP. In parallel, glucose in the medium started to decrease, whilst the amounts of fungal carbohydrates, especially the disaccharide trehalose, increased, From these data we assume that a high initial rate of glucose uptake caused an increase in the fungal pools of storage carbohydrates and, via activation of an adenylate cyclase, of cAMP. According to data reported for yeast cells this should enhance the formation of F26BP by phosphorylation of relevant enzymes. In animal and yeast cells an increase in the concentration of F26BP stimulates glycolysis by activation of the ATP-dependent phosphofructokinase (PFK). A. muscaria also possesses an F26BP activated PFK and, under conditions of symbiosis, host-derived carbohydrates are supplied mainly in the form of glucose. The implications of these findings to the regulation of carbohydrate metabolism of symbiotic plant root/fungus structures (ectomycorrhiza) are discussed.

Molecular characterization of a phosphoenolpyruvate carboxylase in the gymnosperm Picea abies (Norway spruce)

Phosphoenolpyruvate carboxylase (PEPC) genes and cDNA sequences have so far been isolated from a broad range of angiosperm but not from gymnosperm species. We constructed a cDNA library from seedlings of Norway spruce (Picea abies) and identified cDNAs coding for PEPC. A full-length PEPC cDNA was sequenced. It consists of 3522 nucleotides and has an open reading frame (ORF) that encodes a polypeptide (963 amino acids) with a molecular mass of 109551. The deduced amino acid sequence revealed a higher similarity to the C3-form PEPC of angiosperm species (86-88%) than to the CAM and C4 forms (76-84%). The putative motif (Lys/Arg-X-X-Ser) for serine kinase, which is conserved in all angiosperm PEPCs analysed so far, is also present in this gymnosperm sequence. Southern blot analysis of spruce genomic DNA under low-stringency conditions using the PEPC cDNA as a hybridization probe showed a complex hybridization pattern, indicating the presence of additional PEPC-related sequences in the genome of the spruce. In contrast, the probe hybridized to only a few bands under high-stringency conditions. Whereas this PEPC gene is highly expressed in roots of seedlings, a low-level expression can be detected in cotyledons and adult needles. A molecular phyiogeny of plant PEPC including the spruce PEPC sequence revealed that the spruce PEPC sequence is clustered with monocot and dicot C3- form PEPCs including the only dicot C4 form characterized so far.

Changes in carbon partitioning or allocation due to ectomycorrhiza formation: biochemical evidence

Based on the in vitro synthesis of ectomycorrhizas between spruce seedlings and different fungal partners we assessed biochemical parameters connected to the regulation of both formation and utilization of sucrose. Mycorrhization of 4-month-old seedlings resulted in an increase of the activity of sucrose phosphate synthase, a key enzyme in the cytosolic formation of sucrose in source leaves. In parallel, the concentration of fructose 2, 6-bisphosphate (F26BP), a potent inhibitor of fructose 1, 6-bisphosphatase (FBPase), was decreased in needles. Both changes indicate an increased capacity for sucrose formation in mycorrhizal seedlings. To examine the fate of sucrose in the root, we compared transport properties of protoplasts from mycorrhiza-forming fungi (Amanita muscaria, Cenococcum geophilum) with apoplastic sucrose metabolism. Protoplasts from A. muscaria preferentially took up glucose (KM, 1.25 mM; Vmax, 18 pmol/(106 protoplasts∙min)), while sucrose was not transported. This is in accordance with the restriction of invertase activity (sucrose hydrolysis) to the host tissue. A distinction of different zones of symbiotic interaction (analysis of 0.5-mm-wide sections from the tip towards the base of single Picea abies – A. muscaria mycorrhizas) showed a decrease of sucrose in the zones exhibiting the highest fungal proportion (ergosterol), which was paralleled by a concomitant increase in trehalose. In addition, these zones of symbiotic interaction were characterized by high levels of F26BP (up to 1.8 pmol/mg dry weight), which exceeded those resulting from a mere addition of the levels contained in the single partners. An assay of potentially F26BP stimulated enzymes (phosphofructokinase (PFK), PPi-dependent fructose-6-phosphate phosphotransferase (PFP)) revealed that in fungal extracts PFK was stimulated by F26BP while in host tissue it was PFP. This partner-specific difference in stimulation by F26BP could play a pivotal role in creating the strong sink activity in mycorrhizal roots. Possible implications on the partner-specific regulation of glycolysis are discussed. Key words: carbon allocation, ectomycorrhizal symbiosis, fructose 2, 6-bisphosphate, hexose transport, regulation of glycolysis, sucrose metabolism.