Natural Abundance of <Superscript>15</Superscript>N in Nitrogen-Limited Forests and Tundra Can Estimate Nitrogen Cycling Through Mycorrhizal Fungi: A Review

The hyphae of ectomycorrhizal and ericoid mycorrhizal fungi proliferate in nitrogen (N)-limited forests and tundra where the availability of inorganic N is low; under these conditions the most common fungal species are those capable of protein degradation that can supply their host plants with organic N. Although it is widely understood that these symbiotic fungi supply N to their host plants, the transfer is difficult to quantify in the field. A novel approach uses the natural ¹⁵N:¹⁴N ratios (expressed as δ¹⁵N values) in plants, soils, and mycorrhizal fungi to estimate the fraction of N in symbiotic trees and shrubs that enters through mycorrhizal fungi. This calculation is possible because mycorrhizal fungi discriminate against ¹⁵N when they create compounds for transfer to plants; host plants are depleted in ¹⁵N, whereas mycorrhizal fungi are enriched in ¹⁵N. The amount of carbon (C) supplied to these fungi can be stoichiometrically calculated from the fraction of plant N derived from the symbiosis, the N demand of the plants, the fungal C:N ratio, and the fraction of N retained in the fungi. Up to a third of C allocated belowground, or 20% of net primary production, is used to support ectomycorrhizal fungi. As anthropogenic N inputs increase, the C allocation to fungi decreases and plant δ¹⁵N increases. Careful analyses of δ¹⁵N patterns in systems dominated by ectomycorrhizal and ericoid mycorrhizal symbioses may reveal the ecosystem-scale effects of alterations in the plant–mycorrhizal symbioses caused by shifts in climate and N deposition. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

13C and 15N in microarthropods reveal little response of Douglas-fir ecosystems to climate change

Understanding ecosystem carbon (C) and nitrogen (N) cycling under global change requires experiments maintaining natural interactions among soil structure, soil communities, nutrient availability, and plant growth. In model Douglas-fir ecosystems maintained for five growing seasons, elevated temperature and carbon dioxide (CO₂) increased photosynthesis and increased C storage belowground but not aboveground. We hypothesized that interactions between N cycling and C fluxes through two main groups of microbes, mycorrhizal fungi (symbiotic with plants) and saprotrophic fungi (free-living), mediated ecosystem C storage. To quantify proportions of mycorrhizal and saprotrophic fungi, we measured stable isotopes in fungivorous microarthropods that efficiently censused the fungal community. Fungivorous microarthropods consumed on average 35% mycorrhizal fungi and 65% saprotrophic fungi. Elevated temperature decreased C flux through mycorrhizal fungi by 7%, whereas elevated CO₂ increased it by 4%. The dietary proportion of mycorrhizal fungi correlated across treatments with total plant biomass (n= 4, r²= 0.96, P= 0.021), but not with root biomass. This suggests that belowground allocation increased with increasing plant biomass, but that mycorrhizal fungi were stronger sinks for recent photosynthate than roots. Low N content of needles (0.8–1.1%) and A horizon soil (0.11%) coupled with high C : N ratios of A horizon soil (25–26) and litter (36–48) indicated severe N limitation. Elevated temperature treatments increased the saprotrophic decomposition of litter and lowered litter C : N ratios. Because of low N availability of this litter, its decomposition presumably increased N immobilization belowground, thereby restricting soil N availability for both mycorrhizal fungi and plant growth. Although increased photosynthesis with elevated CO₂ increased allocation of C to ectomycorrhizal fungi, it did not benefit plant N status. Most N for plants and soil storage was derived from litter decomposition. N sequestration by mycorrhizal fungi and limited N release during litter decomposition by saprotrophic fungi restricted N supply to plants, thereby constraining plant growth response to the different treatments.     

Mechanisms of nutrient transport across interfaces in arbuscular mycorrhizas

Bidirectional nutrient transfer between the plant and the fungus is a key feature of arbuscular mycorrhizal symbiosis. The major nutrients exchanged between the symbiotic partners are reduced carbon, assimilated through the plant photosynthesis and phosphate, taken up by the fungal hyphae exploring soil microhabitats. This nutrient exchange takes place across the symbiotic interfaces which are bordered by the plant and fungal plasma membranes. This review provides an overview of the current knowledge of the mechanisms underlying nutrient transport processes in the symbiosis, with special emphasis on recent developments in the molecular biology of the plant and fungal primary (H⁺-ATPases) and secondary transporters.     

Effects of inorganic nitrogen and phosphorus supply on nitrogen aquisition from alanine by birch seedlings in symbiosis with Paxillus involutus

Northern forests are exposed to relatively high ammonia inputs due to high atmospheric deposition and the common practise of forest fertilization. It is not known how increased soil NH₄ ⁺concentrations affect acquisition of symbiosis-mediated N from organic sources. We examined the effect of inorganic N and P availability on N acquisition from alanine by 43 weeks old birch (Betula pendula) seedlings in symbiosis with the ectomycorrhizal fungus Paxillus involutus. The seedlings were exposed for 9 weeks to nutrient additions equivalent to 43 kg N and 6.4 kg P ha^-1 (low N and P availability), 250 kg N and 38 kg P ha^-1(high N and P availability) or to 250 kg N and 6.4 kg P ha^-1 (high N and low P availability). Carbon and nitrogen allocation between the symbionts was assessed by exposing the foliage to ¹⁴CO₂ and the mycelium to ¹⁵N-alanine or ¹⁵NH₄ ⁺ simultaneously and measuring the distribution of the isotopic tracers after a three-day chase period. High inorganic N combined with low P availability did not have marked effect on symbiosis-mediated N uptake from alanine, whilst high N and P availability reduced alanine-derived ¹⁵N translocation by the fungus to the plant. Shoot ¹⁵N concentration and concentration of ¹⁴C in the extramatrical mycelium correlated significantly across treatments pointing to controlled reciprocity of transactions between the partners.     

Response of plants to ectomycorrhizae in N-limited conditions: which factors determine its variation?

In the present work, the following hypotheses were tested: (1) the negative effects of mycorrhization over host plant productivity in N-limited conditions are due to N retention by the fungal partner and not due to excessive C drainage; (2) If mycorrhization results in decreased N uptake, the host plant decreases its C investment in fungal growth. The effects of mycorrhization over a wide range of combinations between N availability, N concentration in plant tissues, and degree of mycorrhizal colonization were studied in Pinus pinaster L. mycorrhizal with Pisolithus tinctorius. Several plant productivity parameters, the seedlings’ N status, chl a fluorescence (JIP test), and mycorrhizal colonization were measured. N was always limiting. A gradient of mycorrhizal effects over the host plant’s growth and vitality was successfully obtained. The mycorrhizal effects on plant growth and N uptake were very strongly and positively correlated, and no evidence was found of a C limitation to growth, confirming hypothesis 1. Indications were found that the plants continued to provide C to the fungus although the N supplied by it was increasingly lower, denying hypothesis 2. A new index, the mycorrhizal N demand–supply balance, was found to efficiently explain, and to have a curvilinear relation with, the variation in response to mycorrhization. The mycorrhizal effect on host plant growth was not related to a negative effect on its photosynthetic performance and, therefore, reflected changes in resource allocation between host plant and mycorrhizal fungus, not in plant vitality.     

Ectomycorrhizal fungi associated with Pinus sylvestris seedlings respond differently to increased carbon and nitrogen availability: implications for ecosystem responses to global change

The ectomycorrhizal (ECM) symbiosis can cause both positive and negative feedback with trees under elevated CO₂. Positive feedback arises if the additional carbon (C) increases both nutrient uptake by the fungus and nutrient transfer to the plant, whereas negative feedback results from increased nutrient uptake and immobilization by the fungus and reduced transfer to the plant. Because species of ECM fungi differ in their C and nitrogen (N) demand, understanding fungal species‐specific responses to variation in C and N supply is essential to predict impacts of global change. We investigated fungal species‐specific responses of ECM Scots pine (Pinus sylvestris) seedlings under ambient and elevated CO₂ (350 or 700 μL L⁻¹ CO₂) and under low and high mineral N availability. Each seedling was associated with one of the following ECM species: Hebeloma cylindrosporum, Laccaria bicolor and Suillus bovinus. The experiment lasted 103 days. During the final 27 days, seedlings were labeled with ¹⁴CO₂ and ¹⁵N. Most plant and fungal parameters were significantly affected by fungal species, CO₂ level and N supply. Interactions between fungal species and CO₂ were also regularly significant. At low N availability, elevated CO₂ had the smallest impact on the photosynthetic performance of seedlings inoculated with H. cylindrosporum and the largest impact on seedlings with S. bovinus. At ambient CO₂, increasing N supply had the smallest impact on seedlings inoculated with S. bovinus and the largest on seedlings inoculated with H. cylindrosporum. At low N availability, extraradical hyphal length increased after doubling CO₂ level, but this was significant only for L. bicolor. At ambient CO₂, increasing N levels reduced hyphal length for both H. cylindrosporum and S. bovinus, but not for L. bicolor. We discuss the potential interplay of two major elements of global change, elevated CO₂ and increased N availability, and their effects on plant growth. We conclude that increased N supply potentially relieves mycorrhiza‐induced progressive N limitation under elevated CO₂.     

Differential effects of mineral and organic N sources,and of ectomycorrhizal infection by Hebeloma cylindrosporum,on growth and N utilization in Pinus pinaster

The effect of ectomycorrhizal association of Pinus pinaster with Hebeloma cylindrosporum was investigated in relation to the nitrogen source supplied as mineral (NH₄⁺ or NO₃^?) or organic N (L ‐glutamate) and at 5 mol m^?3. Plants were grown for 14 and 16 weeks with mineral and organic N, respectively, and samples were collected during the last 6 weeks of culture. Total fungal biomass was estimated using glucosamine amount and its viability was assessed using the glucosamine to ergosterol ratio. Non‐mycorrhizal plants grew better with NH₄⁺ than with NO₃^? and grew very slowly when supplied with L ‐glutamate. The presence of the fungus decreased the growth of the host plant with mineral N whereas it increased it with L ‐glutamate. Whatever the N source, most of the living fungal biomass was associated with the roots, whereas the main part of the total biomass was assayed outside the root. The form of mineral N did not significantly affect N accumulation rates over the 42 d in control plants. In mycorrhizal plants grown on either N source, the fungal tissues developing outside of the root were always the main N sink. The ectomycorrhizal association did not change ¹⁵NH₄⁺ uptake rate by roots, suggesting that the growth decrease of the host‐plant was related to the carbon cost for fungal growth and N assimilation rather than to a direct effect on NH₄⁺ acquisition. In contrast, in NO₃^?‐grown plants, in addition to draining carbon for NO₃^? reduction the fungus competed with the root for NO₃^? uptake. With NH₄⁺ or NO₃^? feeding, although mycorrhizal association improved N accumulation in shoots, we concluded that it was unlikely that the fungus had supplied the plant with N. In L ‐glutamate‐grown plants, the presence of the fungus increased the proportion of glutamine in the xylem sap and improved both N nutrition and the growth rate of the host plant.     

First evidences that the ectomycorrhizal fungus Paxillus involutus mobilizes nitrogen and carbon from saprotrophic fungus necromass

Fungal succession in rotting wood shows a surprising abundance of ectomycorrhizal (EM) fungi during the late decomposition stages. To better understand the links between EM fungi and saprotrophic fungi, we investigated the potential capacities of the EM fungus Paxillus involutus to mobilize nutrients from necromass of Postia placenta, a wood rot fungus, and to transfer these elements to its host tree. In this aim, we used pure cultures of P. involutus in the presence of labelled Postia necromass (¹⁵N/¹³C) as nutrient source, and a monoxenic mycorrhized pine experiment composed of labelled Postia necromass and P. involutus culture in interaction with pine seedlings. The isotopic labelling was measured in both experiments. In pure culture, P. involutus was able to mobilize N, but C as well, from the Postia necromass. In the symbiotic interaction experiment, we measured high ¹⁵N enrichments in all plant and fungal compartments. Interestingly, ¹³C remains mainly in the mycelium and mycorrhizas, demonstrating that the EM fungus transferred essentially N from the necromass to the tree. These observations reveal that fungal organic matter could represent a significant N source for EM fungi and trees, but also a C source for mycorrhizal fungi, including in symbiotic lifestyle.     

Plant-driven weathering of apatite - the role of an ectomycorrhizal fungus

Ectomycorrhizal (EcM) fungi are increasingly recognized as important agents of mineral weathering and soil development, with far‐reaching impacts on biogeochemical cycles. Because EcM fungi live in a symbiotic relationship with trees and in close contact with bacteria and archaea, it is difficult to distinguish between the weathering effects of the fungus, host tree and other micro‐organisms. Here, we quantified mineral weathering by the fungus Paxillus involutus, growing in symbiosis with Pinus sylvestris under sterile conditions. The mycorrhizal trees were grown in specially designed sterile microcosms in which the supply of soluble phosphorus (P) in the bulk media was varied and grains of the calcium phosphate mineral apatite mixed with quartz, or quartz alone, were provided in plastic wells that were only accessed by their fungal partner. Under P limitation, pulse labelling of plants with ¹⁴CO₂ revealed plant‐to‐fungus allocation of photosynthates, with 17 times more ¹⁴C transferred into the apatite wells compared with wells with only quartz. Fungal colonization increased the release of P from apatite by almost a factor of three, from 7.5 (±1.1) × 10^?10 mol m^?2 s^?1 to 2.2 (±0.52) × 10^?9 mol m^?2 s^?1. On increasing the P supply in the microcosms from no added P, through apatite alone, to both apatite and orthophosphate, the proportion of biomass in roots progressively increased at the expense of the fungus. These three observations, (i) proportionately more plant energy investment in the fungal partner under P limitation, (ii) preferential fungal transport of photosynthate‐derived carbon towards patches of apatite grains and (iii) fungal enhancement of weathering rate, reveal the tightly coupled plant–fungal interactions underpinning enhanced EcM weathering of apatite and its utilization as P source.     

Arbuscular mycorrhiza improve growth,nitrogen uptake,and nitrogen use efficiency in wheat grown under elevated CO<Subscript>2</Subscript>

Effects of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis on plant growth, carbon (C) and nitrogen (N) accumulation, and partitioning was investigated in Triticum aestivum L. plants grown under elevated CO₂ in a pot experiment. Wheat plants inoculated or not inoculated with the AM fungus were grown in two glasshouse cells with different CO₂ concentrations (400 and 700 ppm) for 10 weeks. A ¹⁵N isotope labeling technique was used to trace plant N uptake. Results showed that elevated CO₂ increased AM fungal colonization. Under CO₂ elevation, AM plants had higher C concentration and higher plant biomass than the non-AM plants. CO₂ elevation did not affect C and N partitioning in plant organs, while AM symbiosis increased C and N allocation into the roots. In addition, plant C and N accumulation, ¹⁵N recovery rate, and N use efficiency (NUE) were significantly higher in AM plants than in non-AM controls under CO₂ enrichment. It is concluded that AM symbiosis favors C and N partitioning in roots, increases C accumulation and N uptake, and leads to greater NUE in wheat plants grown at elevated CO₂.     

Defoliation increases carbon limitation in ectomycorrhizal symbiosis of <Emphasis Type="Italic">Betula pubescens</Emphasis>

Boreal forest trees are highly dependent on root-colonizing mycorrhizal fungi. Since the maintenance of mycorrhizal symbiosis implies a significant carbon cost for the host plant, the loss of photosynthetic leaf area due to herbivory is expected to reduce the host investment in mycorrhizae. We tested this hypothesis in a common garden experiment by exposing ectomycorrhizal white birch (Betula pubescens Ehrh.) seedlings to simulated insect defoliation of 50 or 100% intensity during either the previous or the current summer or repeatedly during both seasons before harvest. The shoot and root growth of the seedlings were distinctly reduced by both 100% defoliation and repeated 50% defoliation, and they were more strongly affected by previous-year than current-year defoliation. The root to shoot ratio significantly decreased after 100% defoliation, indicating reduced proportional allocation to the roots. Ergosterol concentration (i.e. fungal biomass) in the fine roots decreased by 100% defoliation conducted either in the year of harvest or in both years. No such decrease occurred following the 100% defoliation conducted in the previous year, indicating the importance of current photosynthates for fungal symbionts. The trend was similar in the colonization percentage of thick-mantled mycorrhizae in the roots, the most marked decline occurring in the repeatedly defoliated seedlings. The present results thus support the prediction that the plant investment in ectomycorrhizae may decline as a response to foliage loss. Moreover, the colonization percentage of thick-mantled mycorrhizae correlated positively with the ratio of leaf to heterotrophic plant biomass in the defoliated birch seedlings, but not in the control ones. This tends to indicate a stronger carbon limitation of ectomycorrhizal colonization in defoliated seedlings.     

Expression of genes involved in symbiotic carbon and nitrogen transport in Pinus taeda mycorrhizal roots exposed to CO2 enrichment and nitrogen fertilization

As atmospheric carbon dioxide (CO₂) concentrations rise, one important mechanism by which plants can gain greater access to necessary soil nutrients is through greater investment in their mycorrhizal symbionts. In this study, we tested the hypotheses that (1) plants increase C allocation to ectomycorrhizal fungi (EMF) under elevated CO₂ conditions, (2) N fertilization decreases C allocation to EMF, and (3) EMF activity at the site of symbiotic C and nutrient exchange is enhanced with CO₂ enrichment. To test these hypotheses, we examined expression levels of Pinus taeda genes encoding monosaccharide transport (MST) and ammonium transport (AMT) proteins thought to be involved in symbiotic C and N movement, respectively, from mycorrhizal root tips exposed to CO₂ and N fertilization. We also examined EMF ribosomal RNA expression (18S rRNA) to determine EMF activity. There was a trend toward lower relative MST expression with increased CO₂. AMT expression levels showed no significant differences between control and treatment plots. EMF 18S rRNA expression was increased in CO₂-enriched plots and there was a marginally significant positive interactive effect of CO₂ and N fertilization on expression (p = 0.09 and 0.10, respectively). These results are consistent with greater C allocation to EMF and greater EMF metabolic activity under elevated CO₂ conditions, although selective allocation of C to particular EMF species and greater fungal biomass on roots are plausible alternative hypotheses.     

Mycorrhizal mediation of plant N acquisition and residue decomposition: Impact of mineral N inputs

Mycorrhizas are ubiquitous plant–fungus mutualists in terrestrial ecosystems and play important roles in plant resource capture and nutrient cycling. Sporadic evidence suggests that anthropogenic nitrogen (N) input may impact the development and the functioning of arbuscular mycorrhizal (AM) fungi, potentially altering host plant growth and soil carbon (C) dynamics. In this study, we examined how mineral N inputs affected mycorrhizal mediation of plant N acquisition and residue decomposition in a microcosm system. Each microcosm unit was separated into HOST and TEST compartments by a replaceable mesh screen that either prevented or allowed AM fungal hyphae but not plant roots to grow into the TEST compartments. Wild oat (Avena fatua L.) was planted in the HOST compartments that had been inoculated with either a single species of AM fungus, Glomus etunicatum, or a mixture of AM fungi including G. etunicatum. Mycorrhizal contributions to plant N acquisition and residue decomposition were directly assessed by introducing a mineral ¹⁵N tracer and ¹³C‐rich residues of a C₄ plant to the TEST compartments. Results from ¹⁵N tracer measurements showed that AM fungal hyphae directly transported N from the TEST soil to the host plant. Compared with the control with no penetration of AM fungal hyphae, AM hyphal penetration led to a 125% increase in biomass ¹⁵N of host plants and a 20% reduction in extractable inorganic N in the TEST soil. Mineral N inputs to the HOST compartments (equivalent to 5.0 g N m^?2 yr^?1) increased oat biomass and total root length colonized by mycorrhizal fungi by 189% and 285%, respectively, as compared with the no‐N control. Mineral N inputs to the HOST plants also reduced extractable inorganic N and particulate residue C proportion by 58% and 12%, respectively, in the corresponding TEST soils as compared to the no‐N control, by stimulating AM fungal growth and activities. The species mixture of mycorrhizal fungi was more effective in facilitating N transport and residue decomposition than the single AM species. These findings indicate that low‐level mineral N inputs may significantly enhance nutrient cycling and plant resource capture in terrestrial ecosystems via stimulation of root growth, mycorrhizal functioning, and residue decomposition. The long‐term effects of these observed alterations on soil C dynamics remain to be investigated.     

Limited transfer of nitrogen between wood decomposing and ectomycorrhizal mycelia when studied in the field

Transfer of ¹⁵N between interacting mycelia of a wood-decomposing fungus (Hypholoma fasciculare) and an ectomycorrhizal fungus (Tomentellopsis submollis) was studied in a mature beech (Fagus sylvatica) forest. The amount of ¹⁵N transferred from the wood decomposer to the ectomycorrhizal fungus was compared to the amount of ¹⁵N released from the wood-decomposing mycelia into the soil solution as ¹⁵N-NH₄. The study was performed in peat-filled plastic containers placed in forest soil in the field. The wood-decomposing mycelium was growing from an inoculated wood piece and the ectomycorrhizal mycelium from an introduced root from a mature tree. The containers were harvested after 41 weeks when physical contact between the two foraging mycelia was established. At harvest, ¹⁵N content was analyzed in the peat (total N and ¹⁵NH₄ ⁺) and in the mycorrhizal roots. A limited amount of ¹⁵N was transferred to the ectomycorrhizal fungus and this transfer could be explained by ¹⁵NH₄ ⁺ released from the wood-decomposing fungus without involving any antagonistic interactions between the two mycelia. Using our approach, it was possible to study nutritional interactions between basidiomycete mycelia under field conditions and this and earlier studies suggest that the outcomes of such interactions are highly species-specific and depend on environmental conditions such as resource availability.     

Nitrogen Uptake by Trees and Mycorrhizal Fungi in a Successional Northern Temperate Forest: Insights from Multiple Isotopic Methods

Forest succession may cause changes in nitrogen (N) availability, vegetation and fungal community composition that affect N uptake by trees and their mycorrhizal symbionts. Understanding how these changes affect the functioning of the mycorrhizal symbiosis is of interest to ecosystem ecology because of the fundamental roles mycorrhizae play in providing nutrition to trees and structuring forest ecosystems. We investigated changes in tree and mycorrhizal fungal community composition, the availability and uptake of N by trees and mycorrhizal fungi in a forest undergoing a successional transition (age-related loss of early successional tree taxa). In this system, 82–96% of mycorrhizal hyphae were ectomycorrhizal (EM). As biomass production of arbuscular mycorrhizal (AM) trees increased, AM hyphae comprised a significantly greater proportion of total fungal hyphae, and the EM contribution to the N requirement of EM-associated tree taxa declined from greater than 75% to less than 60%. Increasing N availability was associated with lower EM hyphal foraging and ¹⁵N tracer uptake, yet the EM-associated later-successional species Quercus rubra was nonetheless a stronger competitor for ¹⁵N than AM-associated Acer rubrum, likely due to the more extensive nature of the persistent EM hyphal network. These results indicate that successional increases in N availability and co-dominance by AM-associated trees have increased the importance of AM fungi in the mycorrhizal community, while down-regulating EM N acquisition and transfer processes. This work advances understanding of linkages between tree and fungal community composition, and indicates that successional changes in N availability may affect competition between tree taxa with divergent resource acquisition strategies.     

Are carbon and nitrogen exchange between fungi and the orchid Goodyera repens affected by irradiance?

Background and Aims The green orchid Goodyera repens has been shown to transfer carbon to its mycorrhizal partner, and this flux may therefore be affected by light availability. This study aimed to test whether the C and N exchange between plant and fungus is dependent on light availability, and in addition addressed the question of whether flowering and/or fruiting individuals of G. repens compensate for changes in leaf chlorophyll concentration with changes in C and N flows from fungus to plant.Methods The natural abundances of stable isotopes of plant C and N were used to infer changes in fluxes between orchid and fungus across natural gradients of irradiance at five sites. Mycorrhizal fungi in the roots of G. repens were identified by molecular analyses. Chlorophyll concentrations in the leaves of the orchid and of reference plants were measured directly in the field.Key Results Leaf δ¹³C values of G. repens responded to changes in light availability in a similar manner to autotrophic reference plants, and different mycorrhizal fungal associations also did not affect the isotope abundance patterns of the orchid. Flowering/fruiting individuals had lower leaf total N and chlorophyll concentrations, which is most probably explained by N investments to form flowers, seeds and shoot.Conclusions The results indicate that mycorrhizal physiology is relatively fixed in G. repens, and changes in the amount and direction of C flow between plant and fungus were not observed to depend on light availability. The orchid may instead react to low-light sites through increased clonal growth. The orchid does not compensate for low leaf total N and chlorophyll concentrations by using a ¹³C- and ¹⁵N-enriched fungal source.     

Carbon allocation in ectomycorrhizal plants at limited and optimal N supply: an attempt at 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 (RAR_N), 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.     

Mycorrhiza and seedling establishment in a subarctic meadow: Effects of fertilization and defoliation

Abstract. Question: How does changing resource availability induced by fertilization and defoliation affect seedling establishment and mycorrhizal symbiosis in a subarctic meadow? Location: 610 m a.s.l., Kilpisjärvi (69°03’N, 20°50’E), Finland. Methods: A short‐term full‐factorial experiment was established, with fertilization and defoliation of natural established vegetation as treatments. Seeds of two perennial herbs Solidago virgaurea and Gnaphalium norvegicum were sown in natural vegetation and their germination and growth followed. At the final harvest we measured the response in terms of arbuscular mycorrhizal (AM) colonization, biomass and nitrogen concentration of the seedlings and the established vegetation. Results: Germination rate was negatively affected by defoliation in the unfertilized plots. The shoot biomass of S. virgaurea seedlings was reduced by the defoliation and fertilization treatments, but not affected by their interaction. In G. norvegicum, the germination rate and the seedling shoot biomass were negatively correlated with moss biomass in the plots. In the established plants the arbuscular colonization rate was low and defoliation and fertilization treatments either increased or did not affect the colonization by AM fungi. In the seedlings, the colonization rate by AM fungi was high, but it was not affected by treatments. Both seedlings and established plants were colonized by dark septate fungi. Conclusions: Reduction of plant biomass by herbivores can have different effects on seedling growth in areas of high and low soil nutrient availability. The weak response of AM colonization to defoliation and fertilization suggests that AM symbiosis is not affected by altering plant resource availability under the conditions employed in this study.     

Cloning of a Picea abies monosaccharide transporter gene and expression – analysis in plant tissues and ectomycorrhizas

Ectomycorrhizas are formed between certain soil fungi and fine roots of woody plants. An important feature of this symbiosis is the supply of photoassimilates to the fungus. Hexoses, formed from sucrose in the common apoplast at the root/fungus interface, can be taken up by both plant and fungal monosaccharide transporters. Recently we characterised a monosaccharide transporter from the ectomycorrhizal fungus Amanita muscaria. This transporter was up-regulated in mycorrhizas, thus increasing the hexose uptake capacity of the fungal partner in symbiosis. In order to characterise host (Picea abies) root monosaccharide transporters, degenerate oligonucleotide primers, designed to match conserved regions from known plant hexose transporters, were used to isolate a cDNA fragment of a transporter by PCR. This fragment was used to identify a presumably full length clone (PaMST1) in a P. abies/A. muscaria mycorrhizal cDNA library. The entire cDNA code for an open reading frame of 513 amino acids, revealing best homology to H⁺/monosaccharide transporters from Ara- bidopsis, Saccharum and Ricinus. PaMST1 was highly expressed in the hypocotyl and in roots of P. abies seedlings, but not in needles. Mycorrhiza formation led to a slight reduction of PaMST1 expression. The results are discussed with special reference to carbon allocation in ectomycorrhizas. Received: 9 October 1999 / Accepted: 22 December 1999