Linking plants to rocks: ectomycorrhizal fungi mobilize nutrients from minerals

Plant nutrients, with the exception of nitrogen, are ultimately derived from weathering of primary minerals. Traditional theories about the role of ectomycorrhizal fungi in plant nutrition have emphasized quantitative effects on uptake and transport of dissolved nutrients. Qualitative effects of the symbiosis on the ability of plants to access organic nitrogen and phosphorus sources have also become increasingly apparent. Recent research suggests that ectomycorrhizal fungi mobilize other essential plant nutrients directly from minerals through excretion of organic acids. This enables ectomycorrhizal plants to utilize essential nutrients from insoluble mineral sources and affects nutrient cycling in forest systems.     

Iron ore weathering potentials of ectomycorrhizal plants

Plants in association with soil microorganisms play an important role in mineral weathering. Studies have shown that plants in symbiosis with ectomycorrhizal (ECM) fungi have the potential to increase the uptake of mineral-derived nutrients. However, it is usually difficult to study many of the different factors that influence ectomycorrhizal weathering in a single experiment. In the present study, we carried out a pot experiment where Pinus patula seedlings were grown with or without ECM fungi in the presence of iron ore minerals. The ECM fungi used included Pisolithus tinctorius, Paxillus involutus, Laccaria bicolor and Suillus tomentosus. After 24?weeks, harvesting of the plants was carried out. The concentration of organic acids released into the soil, as well as potassium and phosphorus released from the iron ore were measured. The results suggest that different roles of ectomycorrhizal fungi in mineral weathering such as nutrient absorption and transfer, improving the health of plants and ensuring nutrient circulation in the ecosystem, are species specific, and both mycorrhizal roots and non-mycorrhizal roots can participate in the weathering process of iron ore minerals.     

Biological weathering and the long-term carbon cycle: integrating mycorrhizal evolution and function into the current paradigm

The dramatic decline in atmospheric CO₂ evidenced by proxy data during the Devonian (416.0–359.2 Ma) and the gradual decline from the Cretaceous (145.5–65.5 Ma) onwards have been linked to the spread of deeply rooted trees and the rise of angiosperms, respectively. But this paradigm overlooks the coevolution of roots with the major groups of symbiotic fungal partners that have dominated terrestrial ecosystems throughout Earth history. The colonization of land by plants was coincident with the rise of arbuscular mycorrhizal fungi (AMF), while the Cenozoic (c. 65.5–0 Ma) witnessed the rise of ectomycorrhizal fungi (EMF) that associate with both gymnosperm and angiosperm tree roots. Here, we critically review evidence for the influence of AMF and EMF on mineral weathering processes. We show that the key weathering processes underpinning the current paradigm and ascribed to plants are actually driven by the combined activities of roots and mycorrhizal fungi. Fuelled by substantial amounts of recent photosynthate transported from shoots to roots, these fungi form extensive mycelial networks which extend into soil actively foraging for nutrients by altering minerals through the acidification of the immediate root environment. EMF aggressively weather minerals through the additional mechanism of releasing low molecular weight organic chelators. Rates of biotic weathering might therefore be more usefully conceptualized as being fundamentally controlled by the biomass, surface area of contact, and capacity of roots and their mycorrhizal fungal partners to interact physically and chemically with minerals. All of these activities are ultimately controlled by rates of carbon‐energy supply from photosynthetic organisms. The weathering functions in leading carbon cycle models require experiments and field studies of evolutionary grades of plants with appropriate mycorrhizal associations. Representation of the coevolution of roots and fungi in geochemical carbon cycle models is required to further our understanding of the role of the biota in Earth's CO₂ and climate history.     

Laccaria bicolor S238N improves Scots pine mineral nutrition by increasing root nutrient uptake from soil minerals but does not increase mineral weathering

The role of ectomycorrhizal fungi on mineral nutrient mobilization and uptake is crucial for tree nutrition and growth in temperate forest ecosystems. By using a “mineral weathering budget” approach, this study aims to quantify the effect of the symbiosis with the ectomycorrhizal model strain Laccaria bicolor S238N on mineral weathering and tree nutrition, carrying out a column experiment with a quartz/biotite substrate. Each column was planted with one Scots pine (Pinus sylvestris L.) non-mycorrhizal or mycorrhizal with L. bicolor, with exception of the abiotic control treatment. The columns were continuously supplied with a nutrient-poor solution. A mineral weathering budget was calculated for K and Mg. The pine shoot growth was significantly increased (73%) when plants were mycorrhizal with L. bicolor. Whatever their mycorrhizal status, pines increased mineral weathering by factors 1.5 to 2.1. No difference between non-mycorrhizal and mycorrhizal pine treatments was revealed, however, mycorrhizal pines assimilated significantly more K and Mg. This suggests that in our experimental conditions, L. bicolor S238N improved shoot growth and K and Mg assimilation in Scots pine mainly by increasing the uptake of dissolved nutrients, linked to a better exploration and exploitation of the soil by the mycorrhizal roots.     

Mycorrhizal types differ in ecophysiology and alter plant nutrition and soil processes

Mycorrhizal fungi benefit plants by improved mineral nutrition and protection against stress, yet information about fundamental differences among mycorrhizal types in fungi and trees and their relative importance in biogeochemical processes is only beginning to accumulate. We critically review and synthesize the ecophysiological differences in ectomycorrhizal, ericoid mycorrhizal and arbuscular mycorrhizal symbioses and the effect of these mycorrhizal types on soil processes from local to global scales. We demonstrate that guilds of mycorrhizal fungi display substantial differences in genome‐encoded capacity for mineral nutrition, particularly acquisition of nitrogen and phosphorus from organic material. Mycorrhizal associations alter the trade‐off between allocation to roots or mycelium, ecophysiological traits such as root exudation, weathering, enzyme production, plant protection, and community assembly as well as response to climate change. Mycorrhizal types exhibit differential effects on ecosystem carbon and nutrient cycling that affect global elemental fluxes and may mediate biome shifts in response to global change. We also note that most studies performed to date have not been properly replicated and collectively suffer from strong geographical sampling bias towards temperate biomes. We advocate that combining carefully replicated field experiments and controlled laboratory experiments with isotope labelling and ‐omics techniques offers great promise towards understanding differences in ecophysiology and ecosystem services among mycorrhizal types.     

Constraining the role of early land plants in Palaeozoic weathering and global cooling

How the colonization of terrestrial environments by early land plants over 400 Ma influenced rock weathering, the biogeochemical cycling of carbon and phosphorus, and climate in the Palaeozoic is uncertain. Here we show experimentally that mineral weathering by liverworts—an extant lineage of early land plants—partnering arbuscular mycorrhizal (AM) fungi, like those in 410 Ma-old early land plant fossils, amplified calcium weathering from basalt grains threefold to sevenfold, relative to plant-free controls. Phosphate weathering by mycorrhizal liverworts was amplified 9–13-fold over plant-free controls, compared with fivefold to sevenfold amplification by liverworts lacking fungal symbionts. Etching and trenching of phyllosilicate minerals increased with AM fungal network size and atmospheric CO₂ concentration. Integration of grain-scale weathering rates over the depths of liverwort rhizoids and mycelia (0.1 m), or tree roots and mycelia (0.75 m), indicate early land plants with shallow anchorage systems were probably at least 10-fold less effective at enhancing the total weathering flux than later-evolving trees. This work challenges the suggestion that early land plants significantly enhanced total weathering and land-to-ocean fluxes of calcium and phosphorus, which have been proposed as a trigger for transient dramatic atmospheric CO₂ sequestration and glaciations in the Ordovician.     

Atomic Force Microscopy of the fungi-mineral interface: applications in mineral dissolution, weathering and biogeochemistry

The interaction between mycorrhizal fungi and minerals is of fundamental importance in affecting the geochemical carbon cycle and CO(2) concentration in the atmosphere, alongside roles in soil creation and the release of nutrients. The symbiosis between the fungi and the plant, supported by photosynthesis in the host plant, has as one of its key features the interfacial zone where mineral and fungi come into contact. At this interface, the organism exudes a complex mixture of organic acids, chelating molecules, protons, and extracellular polysaccharide. In this review, examples will be given of recent Atomic Force Microscopy experiments to monitor the colonization of phyllosilicate minerals in sterile controlled microcosm environments containing only tree seedlings, mineral chips and mycorrhizal fungi. The surface activity of the colonizing fungal hyphae is extensive and complex. In complementary experiments involving exposure of minerals surfaces to single organic acids, it has been possible to monitor dissolution at the unit cell level and to extract activation energies for specific dissolution processes, for example 49kJmol(-1) for 100mM oxalic acid acting upon a biotite sample. The link between these simpler model experiments and the whole microcosm studies is illustrated partly by observations of fungal-colonized mineral surfaces from microcosms after careful removal of the organism and biolayer. These mineral surfaces give clear indications of basal plane modification and fungal weathering.     

丛枝菌根真菌参与下植物-土壤系统的养分交流及调控

近几年随着有机农业的发展,丛枝菌根的作用受到特别关注。丛枝菌根是由植物根系与丛枝菌根真菌(AMF)形成的一种共生体。在植物-AMF-土壤系统中,AMF为植物提供N、P等营养的同时从根系得到所需的C。概述了植物-AMF-土壤系统中C、N、P等营养物质的交流以及AMF与土壤微生物的互作关系。丛枝菌根的形成可显著提高植物对P的吸收,且在高P条件下多余的P可储存于AMF中。AMF对土壤N循环的影响相当复杂,可能参与调控N循环的多个过程,如硝化作用、反硝化作用和氨氧化作用等。在有机质丰富的土壤中AMF菌丝可快速扩增并吸收其中的N,主要供菌丝自身所需,只有一小部分传递给植物。AMF对土壤C库的影响尚存争议,可能存在时间尺度的差异。短期内可活化土壤C,而在长期尺度上可能有利于土壤C的储存。AMF能够通过改变土壤微生物群落结构而影响植物-土壤体系的物质交流。AMF与解磷菌、根瘤菌和放线菌的协同增效作用可促进土壤有机质的降解或增强其固氮能力;AMF对氨氧化菌的抑制作用可降低氨的氧化减少N2O的释放。AMF与外生共生真菌EMF共存时,表现出协同增效作用,但EMF的优先定殖会限制AMF的侵染。AMF不同类群之间则主要表现为竞争和拮抗关系。AMF与土壤微生物之间的互作关系受土壤无机环境的影响,在养分亏缺条件下微生物之间往往表现为竞争关系。因植物、AMF与土壤微生物之间存在复杂的互作关系,为此AMF并不总是表现出其对植物营养的促进作用。目前关于AMF的作用机理仍以假说为主,需要进一步的实验验证。在植物-AMF-土壤系统中N与C的交流和P与C的交流并未表现出一致性,对N、P循环相互关系的进一步探讨有助于深入理解植物-土壤体系中的养分循环。植物、AMF和土壤微生物的养分来源及其对养分的相对需求强度和吸收效率尚未可知,因此无法深入理解AMF在植物-土壤体系中养分交流和转化的作用。在方法上,传统的土壤学方法在养分动态研究中存在局限性,现代分子生物学手段和化学计量学的结合值得尝试。     

Recent 14C-labelled assimilate allocation to Scots pine seedling root and mycorrhizosphere compartments developed on reconstructed podzol humus,E- and B- mineral horizons

In northern boreal forests, podzolic soils prevail that comprise of a distinct upper organic humus/mor (O) horizon that is supported by underlying eluvial (E) and illuvial (B) mineral horizons. The dominant tree species, Scots pine (Pinus sylvestris L.), is known to be highly dependent on root symbiosis with ectomycorrhizal fungi that develop in constituent podzol horizons for growth in these nutrient limited soils. The aim of this microcosm-based study was a quantification of photosynthetically fixed ¹⁴C allocation, following standard pulse-feeding of 7-month-old Scots pine seedling shoots, to respective root and mycorrhizosphere compartments that developed in the reconstructed podzol (O, E and B) profile. Biomass of roots and mycorrhizas decreased with increasing soil depth but no soil origin, control forest vs. clear-cut area, related differences were observed. Similarly, no major soil origin- or podzol horizon-related differences in categorised ectomycorrhizal morphotypes and number of mycorrhizas, in relation to pooled root and mycorrhiza biomass, were detected. However, the total recovery of ¹⁴C-label was significantly higher in clear-cut soil microcosms compared to control counterparts. A significant finding was equivalent ¹⁴C-carbon allocation to roots and ectomycorrhizas in all three major, organic and mineral, podzol profile horizons studied. These carbon allocation data provide additional support for direct (or indirect) roles of roots and symbiotic mycorrhizal fungi in mineral weathering and biodegradation of organic ligands that are central for plant acquisition of growth limiting nutrients and the podzolization process in boreal forest ecosystems.     

Ecological aspects of mycorrhizal symbiosis: with special emphasis on the functional diversity of interactions involving the extraradical mycelium

Different symbiotic mycorrhizal associations between plantsand fungi occur, almost ubiquitously, in a wide range of terrestrialecosystems. Historically, these have mainly been consideredwithin the rather narrow perspective of their effects on theuptake of dissolved mineral nutrients by individual plants.More recent research has placed emphasis on a wider, multifunctionalperspective, including the effects of mycorrhizal symbiosison plant and microbial communities, and on ecosystem processes.This includes mobilization of N and P from organic polymers,release of nutrients from mineral particles or rock surfacesvia weathering, effects on carbon cycling, interactions withmyco-heterotrophic plants, mediation of plant responses to stressfactors such as drought, soil acidification, toxic metals, andplant pathogens, as well as a range of possible interactionswith groups of other soil micro-organisms. Mycorrhizal fungiconnect their plant hosts to the heterogeneously distributednutrients required for their growth, enabling the flow of energy-richcompounds required for nutrient mobilization whilst simultaneouslyproviding conduits for the translocation of mobilized productsback to their hosts. In addition to increasing the nutrientabsorptive surface area of their host plant root systems, theextraradical mycelium of mycorrhizal fungi provides a directpathway for translocation of photosynthetically derived carbonto microsites in the soil and a large surface area for interactionwith other micro-organisms. The detailed functioning and regulationof these mycorrhizosphere processes is still poorly understoodbut recent progress is reviewed and potential benefits of improvedunderstanding of mycorrhizosphere interactions are discussed. Key words: Arbuscular mycorrhiza, biotic interactions, carbon flow, ectomycorrhiza, ericoid mycorrhiza, mycelium, nutrient uptake, symbiosis, weathering Received 22 January 2008; Revised 7 February 2008 Accepted 7 February 2008     

Observations of canopy bromeliad roots compared with plants rooted in soils of a seasonal tropical forest,Chamela, Jalisco,Mexico

Roots of canopy bromeliads of a seasonal tropical forest were observed for mycorrhizal activity and compared with plants rooted in the soil during the later part of the growing season. No vesicular-arbuscular mycorrhizae or ectomycorrhizae were observed in the bromeliads. However, some interesting septate fungi were observed within the cortex of all samples where the roots were present in organic matter trapped in the canopy. All 15 soil-rooted plant species we observed were vesicular arbuscular mycorrhizal. While no known mycorrhizal types were apparently present in these canopy epiphytes, we cannot rule out the possible formation of symbioses between canopy epiphytes and other fungi in these habitats.     

Plants,mycorrhizal fungi and endobacteria: a dialog among cells and genomes

This review focuses on mycorrhizas, which are associations between fungi and the roots of 90% of terrestrial plants. These are the most common symbioses in the world; they involve about 6000 species of fungi distributed through all the fungal phyla and about 240000 species of plants, including forest and crop plants. Thanks to mycorrhizal symbiosis and nutrient exchanges, regulated by complex molecular signals, the plant improves its vegetative growth, while the fungus accomplishes its life cycle. Molecular and cellular analyses demonstrate that during colonization the cellular organization of the two eukaryotes is completely remodeled. For example, in cortical cells, structural modifications involve both the host and the microbiont. Recent studies revealed that in arbuscular mycorrhizas (AM), system complexity is increased by the presence of a third symbiont: a bacterium living inside the fungus. The presence of this resident genome makes the investigation of the molecular dialogues among the symbiotic partners even more complex. Molecular analysis showed that the bacterium has genes involved in the acquisition of mineral nutrients. The experimental data support the current view that mycorrhizal symbioses are often tripartite associations.     

丛枝菌根真菌与氮添加对不同根形态基因型水稻氮吸收的影响

植物主要依赖自身根系从土壤中获取矿质养分; 具有不同根形态的植物对于养分的吸收能力存在差异。丛枝菌根真菌(AMF)能与陆地植物根系形成共生关系, 帮助植物吸收矿质养分。但是, AMF对于植物根系养分吸收的促进效应是否会受根形态的影响还鲜有研究。该研究选取4种不同根形态基因型水稻(根毛缺陷突变体rhl1、侧根缺陷突变体iaa11、不定根缺失突变体arl1和野生型Kas)为研究对象, 设置2种施氮水平处理(低氮: 20 mg·kg^-1氨氮; 高氮: 100 mg·kg^-1氨氮), 利用稳定同位素¹⁵N示踪标记技术, 探究AMF和氮添加对不同根形态植物氮吸收的影响。研究结果发现, 相比低氮处理, 高氮处理下, rhl1、Kas、iaa11与arl1的茎叶¹⁵N浓度分别提高了60%、72%、128%与118%, 说明氮添加显著促进了水稻氮吸收, 且iaa11与arl1对氮添加的响应更强烈。在低氮水平下, AMF对rhl1、Kas、iaa11与arl1氮吸收的平均效应值分别为17%、31%、42%、51%, 表明AMF对于植物氮吸收的促进效应受根形态影响, iaa11与arl1对AMF的响应明显高于Kas与rhl1; 相较于低氮水平, 高氮水平下AMF对于不同根形态水稻氮吸收的促进效应都会显著降低, 表明氮添加削弱了AMF对植物氮吸收的促进效应。该研究阐明了4种不同根形态基因型水稻氮养分吸收存在显著差异, 其中氮吸收能力较弱的基因型水稻对AMF的响应更强, 该结果补充了植物与AMF在养分吸收上存在功能互补的控制实验证据。     

The Multifunctional Role of Ectomycorrhizal Associations in Forest Ecosystem Processes

Belowground biological interactions that occur among plant roots, microorganisms and animals are dynamic and substantially influence ecosystem processes. Among these interactions, the ectomycorrhizal (ECM) symbiosis is remarkable but unfortunately these associations have mainly been considered within the rather narrow perspective of their effects on the uptake of dissolved mineral nutrients by individual plants. More recent research has placed emphasis on a wider, multifunctional perspective, including the effects of ectomycorrhizal symbiosis on plant and microbial communities, and on ecosystem processes. This includes mobilization of N and P from organic polymers, release of nutrients from mineral particles or rock surfaces via weathering, effects on carbon cycling, interactions with mycoheterotrophic plants, mediation of plant responses to stress factors such as drought, soil acidification, toxic metals, and plant pathogens, rehabilitation and regeneration of degraded forest ecosystems, as well as a range of possible interactions with groups of other soil microorganisms. Ectomycorrhizas are almost invariably characterized by a Hartig net composed of highly branched hyphae which entirely surround the outer root cortical cells. The Hartig net is the place of massive bidirectional exchanges of nutrients between the host and the fungus. Through these branched hyphae ectomycorrhizal fungi connect their plant hosts to the heterogeneously distributed nutrients required for their growth, enabling the flow of energy-rich compounds required for nutrient mobilization whilst simultaneously providing conduits for the translocation of mobilized products back to their hosts. In addition to increasing the nutrient absorptive surface area of their host plant root systems, the extraradical mycelium of ectomycorrhizal fungi provides a direct pathway for translocation of photosynthetically derived carbon from their hosts to microsites in the soil and a large surface area for interaction with other soil micro-organisms. The detailed functioning and regulation of these mycorrhizosphere processes is still poorly understood and needs detailed molecular approach to study these mycorrhizosphere processes but recent progress in ectomycorrhizal associations is reviewed and potential benefits of improved understanding of mycorrhizosphere interactions are discussed.     

Role of Arbuscular Mycorrhizal Fungi in Uptake of Phosphorus and Nitrogen From Soil

Abstract

Colonization of plant roots by arbuscular mycorrhizal fungi can greatly increase the plant uptake of phosphorus and nitrogen. The most prominent contribution of arbuscular mycorrhizal fungi to plant growth is due to uptake of nutrients by extraradical mycorrhizal hyphae. Quantification of hyphal nutrient uptake has become possible by the use of soil boxes with separated growing zones for roots and hyphae. Many (but not all) tested fungal isolates increased phosphorus and nitrogen uptake of the plant by absorbing phosphate, ammonium, and nitrate from soil. However, compared with the nutrient demand of the plant for growth, the contribution of arbuscular mycorrhizal fungi to plant phosphorus uptake is usually much larger than the contribution to plant nitrogen uptake. The utilization of soil nutrients may depend more on efficient uptake of phosphate, nitrate, and ammonium from the soil solution even at low supply concentrations than on mobilization processes in the hyphosphere. In contrast to ectomycorrhizal fungi, nonsoluble nutrient sources in soil are used only to a limited extent by hyphae of arbuscular mycorrhizal fungi. Side effects of mycorrhizal colonization on, for example, plant health or root activity may also influence plant nutrient uptake.     

Trees,Mycorrhiza and Minerals –Field Relevance of in vitro Experiments

Ectomycorrhizal fungi are mutualistic symbionts of boreal forest trees and may mediate mineral weathering through their direct access to photosyntentically derived carbon. In soil, fungal mycelia (i) provide a large surface for nutrient uptake; (ii) induce intense colonization of nutrient rich substrates; (iii) cause local acidification and (iv) produce organic acids. Mechanisms of ectomyorrhizal fungi induced weathering in response to nutrient limiting growth conditions remains largely unresolved. This review summarizes how current knowledge on fungal weathering is affected by experimental setup and conditions, i.e., pure or symbiotic growth, nitrogen source, the mean of detecting weathering activity and species examined.     

Cost efficiency of nutrient acquisition and the advantage of mycorrhizal symbiosis for the host plant

Mycorrhizal symbiosis involves reciprocal transfer of carbon and nutrients between shoots on the one hand and roots colonized by symbiotic fungi on the other. Mycorrhizas may improve the mineral nutrient acquisition rates, but simultaneously increase the belowground demand for carbon. Mycorrhizal plants will have a selective advantage over non-mycorrhizal ones if they are more cost-efficient in terms of carbon cost per unit of acquired mineral nutrient. However, we demonstrate here in a simple model system that this is not a necessary condition. Mycorrhizas may evolve even when they are less cost-efficient, provided that photosynthesis and/or growth are strongly nutrient-limited. This result implies a unique hypothesis for the evolution of mycorrhizal associations which may be inherently cost-inefficient as compared to plant roots. Such symbioses may have evolved when the superior nutrient acquisition rate of fungi combines with the relatively high photosynthetic nutrient use efficiency of the host plant. Consequently, provided that mycorrhizas are really cost-inefficient, the selective advantage of mycorrhizal plants will disappear when an increase in the nutrient acquisition rate is not associated with a sufficiently high nutrient use efficiency of photosynthesis, as at high soil nutrient levels or due to a loss of leaf area, shading or low temperatures.     

A H+-ATPase That Energizes Nutrient Uptake during Mycorrhizal Symbioses in Rice and Medicago truncatula

Most plant species form symbioses with arbuscular mycorrhizal (AM) fungi, which facilitate the uptake of mineral nutrients such as phosphate from the soil. Several transporters, particularly proton-coupled phosphate transporters, have been identified on both the plant and fungal membranes and contribute to delivering phosphate from fungi to plants. The mechanism of nutrient exchange has been studied in plants during mycorrhizal colonization, but the source of the electrochemical proton gradient that drives nutrient exchange is not known. Here, we show that plasma membrane H⁺-ATPases that are specifically induced in arbuscule-containing cells are required for enhanced proton pumping activity in membrane vesicles from AM-colonized roots of rice (Oryza sativa) and Medicago truncatula. Mutation of the H⁺-ATPases reduced arbuscule size and impaired nutrient uptake by the host plant through the mycorrhizal symbiosis. Overexpression of the H⁺-ATPase Os-HA1 increased both phosphate uptake and the plasma membrane potential, suggesting that this H⁺-ATPase plays a key role in energizing the periarbuscular membrane, thereby facilitating nutrient exchange in arbusculated plant cells.     

Ectomycorrhizal weathering of the soil minerals muscovite and hornblende

Ectomycorrhizal fungi are hypothesized to enhance mineral weathering in forest soils. Several studies have shown an increased uptake of mineral-derived nutrients by trees when in symbiosis with ectomycorrhizal fungi. However, it is difficult to determine from these studies if the improved nutrient uptake is the result of increased weathering or better exploitation of the substrate by the ectomycorrhizal fungi. In a pot experiment, Pinus sylvestris (Scots pine) seedlings were grown with or without ectomycorrhizal fungi, and with or without the mineral muscovite as the only potassium (K) source or the mineral hornblende as the only magnesium (Mg) source. After 27 wk, all pools of non-mineral-bound K or Mg were determined. The ectomycorrhizal fungus Paxillus involutus increased weathering of muscovite but not hornblende. The other ectomycorrhizal fungi tested, Piloderma croceum and Suillus bovinus, did not increase weathering of either muscovite or hornblende compared with the nonmycorrhizal trees. The P. involutus-mediated mobilization of K from muscovite resulted in increased K content of root plus adhering hyphae, but not of shoots. In conclusion, ectomycorrhizal fungi may increase weathering of minerals in response to nutrient deficiencies, but this response is species specific.     

Nitrogen transport in the ectomycorrhiza association: the Hebeloma cylindrosporum-Pinus pinaster model

The function of the ectomycorrhizal mutualism depends on the ability of the fungal symbionts to take up nutrients (particularly nitrogen) available in inorganic and/or organic form in the soil and to translocate them (or their metabolites) to the symbiotic roots. A better understanding of the molecular mechanisms underlying nutrient exchanges between fungus and plant at the symbiotic interface is necessary to fully understand the function of the mycorrhizal symbioses. The present review reports the characterization of several genes putatively involved in nitrogen uptake and transfer in the Hebeloma cylindrosporum-Pinus pinaster ectomycorrhizal association. Study of this model system will further clarify the symbiotic nutrient exchange which plays a major role in plant nutrition as well as in resistance of plants against pathogens, heavy metals, drought stress, etc. Ultimately, ecological balance is maintained and/or improved with the help of symbiotic associations, and therefore, warrant further understanding.