Oxalate secretion by ectomycorrhizal Paxillus involutus is mineral-specific and controls calcium weathering from minerals

Trees and their associated rhizosphere organisms play a major role in mineral weathering driving calcium fluxes from the continents to the oceans that ultimately control long-term atmospheric CO2 and climate through the geochemical carbon cycle. Photosynthate allocation to tree roots and their mycorrhizal fungi is hypothesized to fuel the active secretion of protons and organic chelators that enhance calcium dissolution at fungal-mineral interfaces. This was tested using (14)CO2 supplied to shoots of Pinus sylvestris ectomycorrhizal with the widespread fungus Paxillus involutus in monoxenic microcosms, revealing preferential allocation by the fungus of plant photoassimilate to weather grains of limestone and silicates each with a combined calcium and magnesium content of over 10 wt.%. Hyphae had acidic surfaces and linear accumulation of weathered calcium with secreted oxalate, increasing significantly in sequence: quartz, granite < basalt, olivine, limestone < gabbro. These findings confirmed the role of mineral-specific oxalate exudation in ectomycorrhizal weathering to dissolve calcium bearing minerals, thus contributing to the geochemical carbon cycle.

Untangling above- and belowground mycorrhizal fungal networks in tropical orchids

Orchids typically depend on fungi for establishment from seeds, forming mycorrhizal associations with basidiomycete fungal partners in the polyphyletic group rhizoctonia from early stages of germination, sometimes with very high specificity. This has raised important questions about the roles of plant and fungal phylogenetics, and their habitat preferences, in controlling which fungi associate with which plants. In this issue of Molecular Ecology, Martos et al. (2012) report the largest network analysis to date for orchids and their mycorrhizal fungi, sampling a total of over 450 plants from nearly half the 150 tropical orchid species on Reunion Island, encompassing its main terrestrial and epiphytic orchid genera. The authors found a total of 95 operational taxonomic units of mycorrhizal fungi and investigated the architecture and nestedness of their bipartite networks with 73 orchid species. The most striking finding was a major ecological barrier between above- and belowground mycorrhizal fungal networks, despite both epiphytic and terrestrial orchids often associating with closely related taxa across all three major lineages of rhizoctonia fungi. The fungal partnerships of the epiphytes and terrestrial species involved a diversity of fungal taxa in a modular network architecture, with only about one in ten mycorrhizal fungi partnering orchids in both groups. In contrast, plant and fungal phylogenetics had weak or no effects on the network. This highlights the power of recently developed ecological network analyses to give new insights into controls on plant-fungal symbioses and raises exciting new hypotheses about the differences in properties and functioning of mycorrhiza in epiphytic and terrestrial orchids.

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 (14)CO(2) revealed plant-to-fungus allocation of photosynthates, with 17 times more (14)C 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.

Twelve testable hypotheses on the geobiology of weathering

Critical Zone (CZ) research investigates the chemical, physical, and biological processes that modulate the Earth's surface. Here, we advance 12 hypotheses that must be tested to improve our understanding of the CZ: (1) Solar-to-chemical conversion of energy by plants regulates flows of carbon, water, and nutrients through plant-microbe soil networks, thereby controlling the location and extent of biological weathering. (2) Biological stoichiometry drives changes in mineral stoichiometry and distribution through weathering. (3) On landscapes experiencing little erosion, biology drives weathering during initial succession, whereas weathering drives biology over the long term. (4) In eroding landscapes, weathering-front advance at depth is coupled to surface denudation via biotic processes. (5) Biology shapes the topography of the Critical Zone. (6) The impact of climate forcing on denudation rates in natural systems can be predicted from models incorporating biogeochemical reaction rates and geomorphological transport laws. (7) Rising global temperatures will increase carbon losses from the Critical Zone. (8) Rising atmospheric P(CO2) will increase rates and extents of mineral weathering in soils. (9) Riverine solute fluxes will respond to changes in climate primarily due to changes in water fluxes and secondarily through changes in biologically mediated weathering. (10) Land use change will impact Critical Zone processes and exports more than climate change. (11) In many severely altered settings, restoration of hydrological processes is possible in decades or less, whereas restoration of biodiversity and biogeochemical processes requires longer timescales. (12) Biogeochemical properties impart thresholds or tipping points beyond which rapid and irreversible losses of ecosystem health, function, and services can occur.

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.

Symbiotic germination and development of the myco-heterotroph Monotropa hypopitys in nature and its requirement for locally distributed Tricholoma spp

•  Germination and symbiotic development of the myco-heterotrophic plant Monotropa hypopitys were studied by sequential recovery of packets of seed buried in dune slacks in relation to distance from mature M. hypopitys and presence and absence of shoots of its autotrophic coassociate Salix repens. •  Fungal associates of M. hypopitys growing under S. repens in the dune slacks, and under S. caprea and Pinus sylvestris at two other locations in the UK, were identified by molecular analysis. •  While the earliest stage of germination could be found in the absence both of mature M. hypopitys, and S. repens, further development was dependent upon mycorrhizal colonisation, which was most common close to these plants. Molecular analysis showed that when growing with Salix, M. hypopitys associated with the Salix-specific ectomycorrhizal fungus Tricholoma cingulatum, whereas under Pinus it was colonised by the closely related, Pinaceae-specific, T. terreum. •  We establish the first definitive chronology of development of M. hypopitys and highlight its critical dependence upon, and specificity for, locally distributed Tricholoma species that link the myco-heterotroph to its autotrophic coassociates.

Novel in-growth core system enables functional studies of grassland mycorrhizal mycelial networks

•  A novel in-growth core system, enabling functional studies of natural communities of arbuscular mycorrhizal (AM) mycelia in soil is described and tested. •  The cores have windows covered with nylon mesh of 35 µm pore size that prevent in-growth of roots but permit penetration of AM hyphae. They were inserted into grassland turf and contained either sterilized sand and a 'bait' seedling of Trifolium repens or nonsterile natural soil without bait plants. The impacts of hyphal severance, achieved by periodic rotation of some of the cores, upon AM colonization of bait plants (experiment 1) and transfer of ³³ P from soil to plants outside the cores (experiment 2) were examined. •  Severance of AM hyphae reduced both AM colonization of bait plants and their shoot P concentrations. The shoot ³³ P concentrations of plants with mycelial access to ³³ PO₄ -labelled cores were 10-fold greater than those which had no mycelial access. •  It is concluded that this novel approach enables the functioning of mycorrhizal mycelial networks to be evaluated under conditions closely simulating those occurring in nature.

Hyperaccumulation of Zn by Thlaspi caerulescens can ameliorate Zn toxicity in the rhizosphere of cocropped Thlaspi arvense

The metal hyperaccumulating plant Thlaspi caerulescens is effective in depleting plant-available metals from the soil. We hypothesized that this reduction of toxic metals in the rhizosphere of T. caerulescens would increase the growth of less metal-tolerant plants with their roots permitted to intermingle and develop coincident rhizospheres. The extent of rhizosphere interaction between T. caerulescens and a coplanted nonaccumulator species, Thlaspi arvense, was controlled using barriers. Two media with elevated concentrations of water-extractable Zn were prepared by enriching one soil with zinc oxide (ZnO) or zinc sulfide (ZnS). The shoot mass of T. arvense was increased by 30% when its roots were permitted to intermingle with those of T. caerulescens in the ZnO treatment. The concomitant 2-3-fold reduction in shoot Zn concentration in T. arvense confirmed that its improved growth was associated with reduced uptake and phytotoxicity of Zn. Thlaspi arvense also showed increased growth and reduced metal uptake when cocropped with T. caerulescens in the ZnS treatment. We conclude that the strong Zn accumulation by T. caerulescens might enhance the establishment and development of surrounding less-tolerant species on soils that are naturally- or anthropogenically-enriched with metals.

Rates and quantities of carbon flux to ectomycorrhizal mycelium following 14C pulse labeling of Pinus sylvestris seedlings: effects of litter patches and interaction with a wood-decomposer fungus

We used a novel digital autoradiographic technique that enabled, for the first time, simultaneous visualization and quantification of spatial and temporal changes in carbon allocation patterns in ectomycorrhizal mycelia. Mycorrhizal plants of Pinus sylvestris L. were grown in microcosms containing non-sterile peat. The time course and spatial distribution of carbon allocation by P. sylvestris to mycelia of its mycorrhizal partners, Paxillus involutus (Batsch) Fr. and Suillus bovinus (L.): Kuntze, were quantified following 14C pulse labeling of the plants. Litter patches were used to investigate the effects of nutrient resource quality on carbon allocation. The wood-decomposer fungus Phanerochaete velutina (D.C.: Pers.) Parmasto was introduced to evaluate competitive and territorial interactions between its mycelial cords and the mycelial system of S. bovinus. Growth of ectomycorrhizal mycelium was stimulated in the litter patches. Nearly 60% of the C transferred from host plant to external mycorrhizal mycelium (> 2 mm from root surfaces) was allocated to mycelium in the patches, which comprised only 12% of the soil area available for mycelial colonization. Mycelia in the litter patch most recently colonized by mycorrhizal mycelium received the largest investment of carbon, amounting to 27 to 50% of the total 14C in external mycorrhizal mycelium. The amount of C transfer to external mycelium of S. bovinus following pulse labeling was reduced from a maximum of 167 nmol in systems with no saprotroph to a maximum of 61 nmol in systems interacting with P. velutina. The 14C content of S. bovinus mycelium reached a maximum 24-36 h after labeling in control microcosms, but allocation did not reach a peak until 56 h after labeling, when S. bovinus interacted with mycelium of P. velutina. The mycelium of S. bovinus contained 9% of the total 14C in the plants (including mycorrhizae) at the end of the experiment, but this was reduced to 4% in the presence of P. velutina. The results demonstrate the dynamic manner in which mycorrhizal mycelia deploy C when foraging for nutrients. The inhibitory effect of the wood-decomposer fungus P. velutina on C allocation to external mycorrhizal mycelium has important implications for nutrient cycling in forest ecosystems.

Symbiotic germination and development of myco-heterotrophic plants in nature: ontogeny of Corallorhiza trifida and characterization of its mycorrhizal fungi

The processes of symbiotic germination and seedling development were analysed in the myco-heterotrophic orchid Corallorhiza trifida, seeds of which were buried in 'packets' either adjacent to or at varying distances from adult plants in defined communities of ectomycorrhizal tree species. Germination occurred within eight months of burial under Betula-Alnus and within seven months under Salix repens. It was always associated with penetration of the suspensor by a clamp-forming mycorrhizal fungus. Four distinct developmental stages were defined and the rates of transition through these stages were plotted. There was no evidence of a relationship between extent of germination or rate of development and the presence of naturally distributed plants of C. trifida at the spatial scale of 1 m. The best germination and the most rapid rate of development of C. trifida seedlings occurred in a Salix repens community located at a considerable distance from any extant C. trifida population. Determination of internal transcribed spacer (ITS) RFLPs and of gene sequences of the fungi involved in symbiotic germination and growth of C. trifida, revealed them to belong exclusively to the Thelephora-Tomentella complex of the Thelephoraceae. These fungi are known also to be ectomycorrhizal associates of trees. It is hypothesized that the rate of growth of the C. trifida seedlings is determined by the ability of the fungal symbionts to transfer carbon from their ectomycorrhizal co-associates.

Symbiotic germination and development of myco-heterotrophic plants in nature: transfer of carbon from ectomycorrhizal Salix repens and Betula pendula to the orchid Corallorhiza trifida through shared hyphal connections

Seedlings of the myco-heterotrophic orchid Corallorhiza trifida which had been germinated in the field in mesh bags developed hyphal links and mycorrhizas with Betula pendula and Salix repens, but not with Pinus sylvestris, when transplanted into soil microcosms. The fungus connecting the myco-heterotroph to Betula and Salix formed endomycorrhiza in the orchid with typical pelotons, but formed ectomycorrhizas with the autotrophs. The orchid plants, when linked to Betula and Salix by fungal hyphae, gained 6-14% in weight over 25-28 wk. In microcosms supporting P. sylvestris, and in control microcosms which lacked autotrophs, the Corallorhiza plants lost 13% of their weight over the same period. In the course of the 28-wk experimental period new Corallorhiza seedlings, in addition to those added as part of the experiment, appeared in the microcosms containing Salix and Betula but not in the Pinus microcosms. Shoots of Betula and Salix plants grown in association with Corallorhiza were fed with ¹⁴ CO₂ , and the movement of the isotope was subsequently traced by a combination of digital autoradiography and tissue oxidation. Direct transfer of C from both autotrophs to the myco-heterotroph occurred in all cases where the associates had become connected by a shared fungal symbiont. Orchid seedlings lacking these hyphal connections, introduced to the microcosms as controls immediately before isotope feeding, failed to assimilate significant amounts of C. The results provide the first experimental confirmation that growth of Corallorhiza trifida can be sustained by supply of C received directly from an autotrophic partner through linked fungal mycelia.

Phosphodiesterase as mycorrhizal P sources: I. Phosphodiesterase production and the utilization of DNA as a phosphorus source by the ericoid mycorrhizal fungus Hymenoscyphus ericae

These results provide the first account of the breakdown and utilization of DNA by an ericoid mycorrhizal fungus, and of its phosphodiesterase activity in vitro. Hymenoscyphus ericae (Read) Korf & Kernan grew well on DNA as a sole source of phosphorus (P), achieving greater mycelium dry weight than on an equivalent concentration of P supplied as orthophosphate. Some characteristics of the production and activity of extracellular (culture filtrate) and cell-wall bound phosphodiesterase in H. ericae are reported. At least part of the phosphodiesterase activity is attributed to the exonuclease F nucleotide diesterase on the basis of its high affinity for a specific substrate of this enzyme, The pH optima for extra-cellular and cell-wall-bound diesterase are acidic (pH 4.0-5.5), with considerable activity maintained in the pH range typical of organic soils under ericaceous plants (pH 3.0-4.5). The production of phosphodiesterase was not substrate-induced, since highest specific activity for wall-bound enzyme was found in culture grown without organic or inorganic P. The results are discussed in relation to the ecology and biology of acid organic soils on which ericaceous plants are dominant.

Phosphodiesters as mycorrhizal P sources: II. Ericoid mycorrhiza and the utilization of nuclei as a phosphorus and nitrogen source by Vaccinium macrocarpon

Mycorrhizal plants of Vaccinium mocrocarpon Aiton used nuclei from salmon sperm as a sole source of phosphorus (P) and achieved similar yields, P content and P concentration to plants crown with orthophosphate. Mycorrhizal infection significantly increased the effectiveness of utilization of both inorganic and organic (nuclei) sources of P by Vaccinium but in the case of the organic source this involved providing access to P which was completely unavailable to the uninfected plants. The results provide further support for the view that ericoid mycorrhizas have a crucial role in direct recycling of nutrients from organic matter, independent of the mineralizing activities of saprotrophic micro-organisms.

The biology of mycorrhiza in the Ericaceae: XVII. The role of mycorrhizal infection in the regulation of iron uptake by ericaceous plants

The role of the ericoid mycorrhizal endophyte Hymenoscyphus ericae (Read) Korf & Kernan in the regulation of iron uptake by ericaceous plants is investigated. Growth of the fungus is not inhibited in solution cultures containing a range (0-144 [ig ml^-1 ) of iron concentration ([Fe]_ext ) typical of those obtained in extracts from heathland soil. Absorption of iron by the fungus occurs rapidly at low [Fe]_ext but more slowly as [Fe]_ext increases. Concentrations of iron in the mycelium ([Fe]_mye ), reach 6000μg g^-1 dry weight. Mycorrhizal (M) roots of Vaccinium macrocarpon Ait. and Calluna vulgaris L. (Hull) showed a very high affinity for iron at low [Fe]_ext , a feature not shown by non-mycorrhizal (NM) roots. The involvement of a hydroxamate siderophore in the absorption of Fe by M plants at low [Fe]_ext is suggested. Concentrations of iron in shoots ([Fe]_ext of NM plants of V. macrocarpon increase linearly with increasing [Fe]_ext while those of M plants fall in the mid-range of [Fe]_ext relative to initial and final values. Ratios of [Fe]_ext to [Fe]_ext are lower in M than in NM plants across the range of [Fe]_ext examined. The extent of involvement of mycorrhizal infection in excluding the metal from shoots as [Fe]_ext increases, is discussed, and the importance of the mechanisms of iron capture and storage in the root are assessed in terms of iron availability in natural heathlands.

Proteinase activity in mycorrhizal fungi: I. The effect of extracellular pH on the production and activity of proteinase by ericoid endophytes from soils of contrasted pH

The effect of pH on the production and specific activity of the extracellular proteinase enzymes of two ecologically distinct ericoid mycorrhizal fungi is described. The proteinase of Hymenoscyphus ericae (Read), Korf & Kernan, isolated from roots of Calluna vulgaris (L.) Hull growing in soil of pH 35, was compared with a similar enzyme from an endophyte of the calcicolous alpine shrub Rhodothamnus chamaecistus (L.) Reichenb. growing in soil of pH 6.5. The fungi were grown in liquid culture at pH values ranging from 3.0 to 8.0 with pure protein, bovine serum albumin, as sole source of N. Both fungi yielded an extracellular acid proteinase with pH optimum for activity between 20 and 30. The production and activity of these enzymes was strongly affected by pH of the culture medium. Maximum enzyme production during exponential growth occurred in both fungi at a culture pH of 4.0-5.0, whereas higher pH treatments severely inhibited enzyme production. The acid proteinase of H. ericae was tolerant of extreme acidity and retained near-optimal activity in solutions of pH 2.0. In contrast, the activity of the enzyme from the Rhodothamnus endophyte was almost completely inhibited at this pH. However, proteinase from the Rhodothamnus endophyte retained activity at much higher pH values than did the proteinase from H. ericae. Unlike H. ericae, the isolated endophyte of Rhodothamnus was able to grow and use protein as sole source of N at pH 7.0 and 8.0. The effects of pH on enzyme production and upon growth of the fungi are discussed in relation to the characteristics of the environments of their host plants.