Acclimation to temperature and temperature sensitivity of metabolism by ectomycorrhizal fungi

Ectomycorrhizal (ECM) fungi contribute significantly to ecosystem respiration, but little research has addressed the effect of temperature on ECM fungal respiration. Some plants have the ability to acclimate to temperature such that long‐term exposure to warmer conditions slows respiration at a given measurement temperature and long‐term exposure to cooler conditions increases respiration at a given measurement temperature. We examined acclimation to temperature and temperature sensitivity (Q₁₀) of respiration by ECM fungi by incubating them for a week at one of three temperatures and measuring respiration over a range of temperatures. Among the 12 ECM fungal isolates that were tested, Suillus intermedius, Cenococcum geophilum, and Lactarius cf. pubescens exhibited significant acclimation to temperature, exhibiting an average reduction in respiration of 20–45% when incubated at 23 °C compared with when incubated at 11 or 17 °C. The isolates differed significantly in their Q₁₀ values, which ranged from 1.67 to 2.56. We also found that half of the isolates significantly increased Q₁₀ with an increase in incubator temperature by an average of 15%. We conclude that substantial variation exists among ECM fungal isolates in their ability to acclimate to temperature and in their sensitivity to temperature. As soil temperatures increase, ECM fungi that acclimate may require less carbon from their host plants than fungi that do not acclimate. The ability of some ECM fungi to acclimate may partially ameliorate the anticipated positive feedback between soil respiration and temperature.     

Intraspecific physiological variation: implications for understanding functional diversity in ectomycorrhizal fungi

 Several thousand fungal species worlwide are thought to form ectomycorrhizas (ECM) with tree hosts and there is currently much interest in determining the functional significance of such diversity in natural and managed ecosystems. While only a few taxa have been investigated in detail, it is clear that ECM fungi display extensive intraspecific variation in a range of physiological and other life-history parameters. Thus, comparative investigations of single (or even a few) isolates of different species are unlikely to provide reliable information on functional capabilities. Extensive screening of taxonomically well-defined isolates is required. This must take into account spatial and temporal variation in gene expression in mycelia growing in axenic culture or in association with a host plant. Accepted: 29 June 1999     

Cryptic genetic structure and copy-number variation in the ubiquitous forest symbiotic fungus Cenococcum geophilum

Ectomycorrhizal (ECM) fungi associated with plants constitute one of the most successful symbiotic interactions in forest ecosystems. ECM support trophic exchanges with host plants and are important factors for the survival and stress resilience of trees. However, ECM clades often harbour morpho-species and cryptic lineages, with weak morphological differentiation. How this relates to intraspecific genome variability and ecological functioning is poorly known. Here, we analysed 16 European isolates of the ascomycete Cenococcum geophilum, an extremely ubiquitous forest symbiotic fungus with no known sexual or asexual spore-forming structures but with a massively enlarged genome. We carried out whole-genome sequencing to identify single-nucleotide polymorphisms. We found no geographic structure at the European scale but divergent lineages within sampling sites. Evidence for recombination was restricted to specific cryptic lineages. Lineage differentiation was supported by extensive copy-number variation. Finally, we confirmed heterothallism with a single MAT1 idiomorph per genome. Synteny analyses of the MAT1 locus revealed substantial rearrangements and a pseudogene of the opposite MAT1 idiomorph. Our study provides the first evidence for substantial genome-wide structural variation, lineage-specific recombination and low continent-wide genetic differentiation in C. geophilum. Our study provides a foundation for targeted analyses of intra-specific functional variation in this major symbiosis.     

Ectomycorrhizal fungi and their enzymes in soils: is there enough evidence for their role as facultative soil saprotrophs?

Although ectomycorrhizal (ECM) fungi are generally regarded as dependent upon the supply of carbon from their plant hosts, some recent papers have postulated a role for these fungi in the saprotrophic acquisition of carbon from soil. This theory was mainly based on the increase in enzymatic activity during periods of low photosynthate supply from tree hosts and emergence of the theory has led to a question about the overall influence of saprotrophy by ECM fungi on soil carbon turnover. However, I argue here that there is still not enough evidence to confirm this proposed function. My argument is based on inference from several lines of observation and concern over several aspects of the past studies. First, ECM fungi mainly inhabit deeper soil horizons, in which the availability of carbon compounds with positive energetic value is low. Second, the ability of ECM fungi to produce ligninolytic enzymes and cellulases is much weaker than that of saprotrophic basidiomycetes. This is most apparent in the low copy abundance of corresponding genes in the sequenced genomes of ECM species Laccaria bicolor and Amanita bisporigenes compared to the saprotrophic species Galerina marginata. I offer alternative hypotheses to explain the past observations of increased enzyme activity during starvation periods. These include, the induction of autolytic processes in ECM fungal mycelia or an attack on the host tissues to support escape from a dying root and to allow for a search for new hosts.     

Diversity and composition of ectomycorrhizal community on seedling roots: the role of host preference and soil origin

As the main source of inocula, ectomycorrhizal (ECM) fungal propagules are critical for root colonization and seedling survival in deforested areas. It is essential to know factors that may affect the diversity and composition of ECM fungal community on roots of seedlings planted in deforest areas during reforestation. We quantitatively evaluated the effect of host plant and soil origin on ECM fungal propagule community structure established on roots of Castanopsis fargesii, Lithocarpus harlandii, Pinus armandii, and Pinus massoniana growing in soils from local natural forests and from sites deforested by clear-cut logging in the 1950s and 1960s. ECM root tips were sampled in April, July, and October of 2006, and ECM fungal communities were determined using ECM root morphotyping, internal transcribed spacer (ITS)-RFLP, and ITS sequencing. A total of 36 ECM fungal species were observed in our study, and species richness varied with host species and soil origin. Decreased colonization rates were found in all host species except for L. harlandii, and reduced species richness was found in all host species except for P. armandii in soil from the deforested site, which implied the great changes in ECM fungal community composition. Our results showed that 33.3% variance in ECM fungal community composition could be explained by host plant species and 4.6% by soil origin. Results of indicator species analysis demonstrated that 14 out of 19 common ECM fungal species showed significant preference to host plant species, suggesting that the host preference of ECM fungi was one of the most important mechanisms in structuring ECM fungal community. Accordingly, the host plant species should be taken into account in the reforestation of deforested areas due to the strong and commonly existed host preference of ECM fungi.     

Mycorrhizal associations in woody plant species at the Mt. Usu volcano,Japan

We investigated the association between ectomycorrhizal (ECM) and arbuscular mycorrhizal (AM) fungi and pioneer woody plant species in areas devastated by the eruption of Mt. Usu, Japan, in 2000. We observed eight woody plant species at the research site, most of which were associated with ECM and/or AM fungi. In particular, dominant woody plant species Populus maximowiczii, Salix hultenii var. angustifolia and Salix sachalinensis were consistently associated with ECM fungi and erratically associated with AM fungi. We found one to six morphotypes in the roots of each ECM host and, on average, two in the roots of each seedling, indicating low ECM fungal diversity. ECM colonization ranged from 17 to 42% of root tips. Using morphotyping and molecular analyses, 15 ECM fungi were identified. ECM fungi differed greatly between hosts. However, Laccaria amethystea, Hebeloma mesophaeum, Thelephora terrestris and other Thelephoraceae had high relative colonization, constituting the majority of the ECM colonization in the roots of each plant species. These ECM fungi may be important for the establishment of pioneer woody plant species and further revegetation at Mt. Usu volcano.     

Diversity of ectomycorrhizal fungi in Britain: a test of the species–area relationship, and the role of host specificity

The host range of ectomycorrhizal (ECM) fungi in Britain was examined by compilation of a data matrix from published literature sources, based primarily on accounts of sporocarp associations with particular host genera. Information was gathered for 577 species of ECM fungi belonging to 51 genera, and 25 genera of host trees, representing the majority of ECM fungal species and host genera recorded in Britain.
Pronounced variation was recorded in the number of ECM fungal species associated with different host genera, with over 200 species recorded with Betula , Fagus , Pinus and Quercus . There was a positive linear relationship ( r ²=0·47, P =0·007) between the number of species of ECM fungi associated with different host genera and the total area occupied by each tree genus in Britain (both values log-transformed). There was also variation in the number of species of ECM fungi which were apparently specific to particular host genera, values ranging from zero (in 15 genera) to >40 in the case of Betula and Fagus . In total, 233 fungal species appeared to be specific to a single host genus (i.e. 40% of those surveyed). Comparison of the ECM mycota associated with different host genera by PCA accounted for 17% of the total variation, with genera belonging to the Fagaceae ( Quercus , Fagus and Castanea ) tending to cluster together, indicating a degree of overlap in their ECM associates. Exotic conifer species, which displayed a lower ECM diversity than would be expected from their distributional areas, were characterized by a high degree of overlap with the ECM associates of Pinus and Betula .
These results indicate that the abundance of different genera of host trees and variation in host specificity could provide a basis for understanding patterns of diversity in ECM fungi within Britain.     

Temporal variation of Bistorta vivipara‐associated ectomycorrhizal fungal communities in the High Arctic

Ectomycorrhizal (ECM) fungi are important for efficient nutrient uptake of several widespread arctic plant species. Knowledge of temporal variation of ECM fungi, and the relationship of these patterns to environmental variables, is essential to understand energy and nutrient cycling in Arctic ecosystems. We sampled roots of Bistorta vivipara ten times over two years; three times during the growing‐season (June, July and September) and twice during winter (November and April) of both years. We found 668 ECM OTUs belonging to 25 different ECM lineages, whereof 157 OTUs persisted throughout all sampling time‐points. Overall, ECM fungal richness peaked in winter and species belonging to Cortinarius, Serendipita and Sebacina were more frequent in winter than during summer. Structure of ECM fungal communities was primarily affected by spatial factors. However, after accounting for spatial effects, significant seasonal variation was evident revealing correspondence with seasonal changes in environmental conditions. We demonstrate that arctic ECM richness and community structure differ between summer (growing‐season) and winter, possibly due to reduced activity of the core community, and addition of fungi adapted for winter conditions forming a winter‐active fungal community. Significant month × year interactions were observed both for fungal richness and community composition, indicating unpredictable between‐year variation. Our study indicates that addressing seasonal changes requires replication over several years.     

Ectomycorrhizal fungi in Mexican Alnus forests support the host co-migration hypothesis and continental-scale patterns in phylogeography

To examine the geographic patterns in Alnus-associated ectomycorrhizal (ECM) fungal assemblages and determine how they may relate to host plant biogeography, we studied ECM assemblages associated with two Alnus species (Alnus acuminata and Alnus jorullensis) in montane Mexico and compared them with Alnus-associated ECM assemblages located elsewhere in the Americas. ECM root samples were collected from four sites in Mexico (two per host species), identified with ITS and LSU rRNA gene sequences, and assessed using both taxon- (richness, diversity, evenness indices) and sequence divergence-based (UniFrac clustering and significance) analyses. Only 23 ECM taxa were encountered. Clavulina, an ECM lineage never before reported with Alnus, contained the dominant taxon overall. ECM assemblage structure varied between hosts, but UniFrac significance tests indicated that both associated with similar ECM lineage diversity. There was a strikingly high sequence similarity among a diverse array of the ECM taxa in Mexico and those in Alnus forests in Argentina, the United States, and Europe. The Mexican and United States assemblages had greater overlap than those present in Argentina, supporting the host–ECM fungi co-migration hypothesis from a common north temperate origin. Our results indicate that Alnus-associated ECM assemblages have clear patterns in richness and composition across a wide range of geographic locations. Additional data from boreal western North America as well as the eastern United States and Canada will be particularly informative in further understanding the co-biogeographic patterns of Alnus and ECM fungi in the Americas.     

Comparative phylogenies and host specialization in the alder ectomycorrhizal fungi Alnicola, Alpova and Lactarius (Basidiomycota) in Europe

Background  

Mycorrhizal fungi form intimate associations with their host plants that constitute their carbon resource and habitat. Alnus spp. (Betulaceae) are known to host an exceptional species-poor and specialized ectomycorrhizal (ECM) fungal community compared to other tree species, but the host-specificity pattern and its significance in terms of fungal diversification and speciation remain poorly documented. The degree of parallel speciation, host switching, and patterns of biogeography were explored in the historical associations between alders and three ECM taxa of Basidiomycetes: Alnicola (Agaricales), Alpova (Boletales), and Lactarius (Russulales). The aim was to develop an evolutionary framework on host specificity and diversification of Basidiomycetes in this highly specialized plant-fungus symbiosis.     

Application of Ectomycorrhizal Fungi in Vegetative Propagation of Conifers

In forestry, vegetative propagation is important for the production of selected genotypes and shortening the selection cycles in genetic improvement programs. In vivo cutting production, in vitro organogenesis and somatic embryogenesis are applicable with conifers. However, with most coniferous species these methods are not yet suitable for commercial application. Large-scale production of clonal material using cuttings or organogenesis is hindered by rooting problems and difficulties in the maturation and conversion limit the use of somatic embryogenesis. Economically important conifers form symbiotic relationship mostly with ectomycorrhizal (ECM) fungi, which increase the fitness of the host tree. Several studies have shown the potential of using ECM fungi in conifer vegetative propagation. Inoculation with specific fungi can enhance root formation and/or subsequent root branching of in vivo cuttings and in vitro adventitious shoots. Germination of somatic embryos and subsequent root growth can also be improved by the use of ECM fungi. In addition, inoculation can increase the tree's ability to overcome the stress related to ex vitro transfer. A specific interaction between a fungal strain and tree clone occurs during root induction and germination of somatic embryos. Multiple rooting factors exist in this interaction that complicate the predictability of the response to inoculation. Fungal-specific factors that influence rooting responses to inoculation may include plant growth regulator production, modification of the rooting environment, and interactions with beneficial microbes. A combination of these factors may act synergistically to result in positive responses in tree genotypes that are compatible with the fungus.     

Mycorrhizal status of <Emphasis Type="Italic">Eucalyptus</Emphasis> plantations in south China and implications for management

The aim of this study is to assess the mycorrhizal status of Eucalyptus plantations in south China and to determine the need for inoculation. In four provinces in south China, 155 plantations were sampled for sporocarps of ectomycorrhizal (ECM) fungi, spores of arbuscular mycorrhizal (AM) fungi, and mycorrhizas over 2 years. This study revealed a low above-ground diversity of ECM fungi consisting of 15 taxa fruiting beneath Eucalyptus plantations. The most common ECM genera were Scleroderma and Pisolithus, but they were infrequent. A total of 21 AM fungi, mostly Glomus species, were recognized from spores collected from eucalypt plantations. Four Glomus species were frequently present in soils, but spore density and relative abundance of AM fungi were generally low. Eucalypt roots from all plantation sites were poorly colonized by either ECM fungi or AM fungi. A bioassay with E. urophylla as a bait host, using soils collected from 11 eucalypt plantations, confirmed low levels of inoculum of both ECM and AM fungi in field soil. This is the first integrated study on the mycorrhizal status of eucalypt plantations in China. Findings from this research can be used to encourage adoption of mycorrhizal technology by eucalypt nurseries in the region. The potential of using spores of compatible ECM fungi or collections for forest nurseries is discussed.     

Growth response of Afzelia africana Sm. seedlings to ectomycorrhizal inoculation in a nutrient-deficient soil

 The growth and mineral nutrition responses of seedlings of two provenances of Afzelia africana Sm. from Senegal and Burkina Faso, inoculated with four ectomycorrhizal (ECM) fungi (Scleroderma spp. and an unidentified isolate) from the same regions were assessed in a pot experiment in a savanna ECM-propagule-free soil deficient in NPK. There was little variation in the ability of the different fungal species to colonize roots of either provenance of A. africana or to produce external hyphal in soil. Root colonization by ECM fungi and their hyphal development were not related to mineral nutrition or ECM dependency. Differences in P, N, Mg and Ca concentrations in the leaves of inoculated and non-inoculated Afzelia seedlings were not always associated with production of biomass. Only leaf K concentration increased in both provenances after ECM inoculation. However, the Burkina Faso provenance responded better to inoculation with the two fungal isolates than the Senegal provenance in terms of biomass production. This was due to stimulation of root dry weight of the Burkina Faso provenance. Therefore, the hypothesis arises that non-nutritional rather than nutritional effects explain the contribution of ECM inoculation to the growth of A. africana seedlings. Accepted: 27 April 1999     

Revisiting the host effect on ectomycorrhizal fungal communities: implications from host–fungal associations in relict Pseudotsuga japonica forests

Host identity is among the most important factors in structuring ectomycorrhizal (ECM) fungal communities. Both host–fungal coevolution and host shifts can account for the observed host effect, but their relative significance in ECM fungal communities is not well understood. To investigate these two host-related mechanisms, we used relict forests of Pseudotsuga japonica, which is an endangered endemic species in Japan. As with other Asian Pseudotsuga species, P. japonica has been isolated from North American Pseudotsuga spp. since the Oligocene and has evolved independently as a warm-temperate species. We collected 100 soil samples from four major localities in which P. japonica was mixed with other conifers and broadleaf trees. ECM tips in the soil samples were subjected to molecular analyses to identify both ECM fungi and host species. While 136 ECM fungal species were identified in total, their communities were significantly different between host groups, confirming the existence of the host effect on ECM fungal communities. None of the 68 ECM fungal species found on P. japonica belonged to Pseudotsuga-specific lineages (e.g., Rhizopogon and Suillus subgroups) that are common in North America. Most of ECM fungi on P. japonica were shared with other host fungi or phylogenetically close to known ECM fungi on other hosts in Asia. These results suggest that after migrating, Pseudotsuga-specific fungal lineages may have become extinct in small isolated populations in Japan. Instead, most of the ECM fungal symbionts on P. japonica likely originated from host shifts in the region.     

Shifts in symbiotic associations in plants capable of forming multiple root symbioses across a long‐term soil chronosequence

Changes in soil nutrient availability during long‐term ecosystem development influence the relative abundances of plant species with different nutrient‐acquisition strategies. These changes in strategies are observed at the community level, but whether they also occur within individual species remains unknown. Plant species forming multiple root symbioses with arbuscular mycorrhizal (AM) fungi, ectomycorrhizal (ECM) fungi, and nitrogen‐(N) fixing microorganisms provide valuable model systems to examine edaphic controls on symbioses related to nutrient acquisition, while simultaneously controlling for plant host identity. We grew two co‐occurring species, Acacia rostellifera (N₂‐fixing and dual AM and ECM symbioses) and Melaleuca systena (AM and ECM dual symbioses), in three soils of contrasting ages (c. 0.1, 1, and 120 ka) collected along a long‐term dune chronosequence in southwestern Australia. The soils differ in the type and strength of nutrient limitation, with primary productivity being limited by N (0.1 ka), co‐limited by N and phosphorus (P) (1 ka), and by P (120 ka). We hypothesized that (i) within‐species root colonization shifts from AM to ECM with increasing soil age, and that (ii) nodulation declines with increasing soil age, reflecting the shift from N to P limitation along the chronosequence. In both species, we observed a shift from AM to ECM root colonization with increasing soil age. In addition, nodulation in A. rostellifera declined with increasing soil age, consistent with a shift from N to P limitation. Shifts from AM to ECM root colonization reflect strengthening P limitation and an increasing proportion of total soil P in organic forms in older soils. This might occur because ECM fungi can access organic P via extracellular phosphatases, while AM fungi do not use organic P. Our results show that plants can shift their resource allocation to different root symbionts depending on nutrient availability during ecosystem development.     

Variation in aluminum resistance among arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi mediate interactions between plants and soils, and are important where nutrient or metal concentrations limit plant growth. Variation in fungal response to edaphic conditions may influence the effectiveness of the plant-mycorrhizal association in some soil environments. Andropogon virginicus (broomsedge) colonizes disturbed sites in the eastern United States, including acidic mine soils where aluminum (Al) is phytotoxic, and Al resistance in broomsedge has been associated with colonization by the AM fungus Glomus clarum. In the present study, inter- and intra-specific variation to confer Al resistance to broomsedge was assessed among selected species of AM fungi. Broomsedge seeds were grown in sand culture inoculated with one of five isolates of three species of fungi (G. clarum, Acaulospora morrowiae, and Scutellospora heterogama). Plants were exposed to 0 or 400 µM Al in nutrient solution and harvested after 4 or 9 weeks of growth. Mean infection percentage, plant biomass, and plant tissue Al and phosphorus (P) concentrations were measured. G. clarum conferred the greatest Al resistance to broomsedge, with the lowest variability among isolates for colonization and growth inhibition by Al [tolerance indices (TI) between 22.4 and 92.7%]. Broomsedge plants colonized by A. morrowiae were consistently the most sensitive to Al, with little variation among isolates (TI between 1.6 and 12.1%). Al resistance by S. heterogama isolates was intermediate and wide-ranging (TI between 3.9 and 40.0%). Across all AM fungal isolates, resistance was associated with high rates of colonization and low tissue Al concentrations of broomsedge plants. The functional diversity in Al resistance displayed by these AM fungi reflect variation in acclimation mechanisms operating in the mycorrhizal symbiosis under environmental stress.     

Whose truffle is this? Distribution patterns of ectomycorrhizal fungal diversity in Tuber melanosporum brûlés developed in multi‐host Mediterranean plant communities

In the Mediterranean region, patches of vegetation recovering from disturbance and transiently dominated by shrubs produce one of the world's most prized fungi, the black truffle (Tuber melanosporum). In these successional plant communities, we have fragmentary knowledge of the distribution of T. melanosporum in space among ectomycorrhizal (ECM) host species and in time. Molecular identification of hosts (Restriction Fragment Length Polymorphism) and fungi (Internal Transcribed Spacer sequencing) and quantification of T. melanosporum mycelium (quantitative Polymerase Chain Reaction) were employed to evaluate the presence of T. melanosporum on four dominant ECM host species (Quercus ilex, Quercus coccifera, Arbutus unedo, Cistus albidus) and the extent to which their respective ECM communities shared fungal diversity, over the course of development of truffle grounds, from recent unproductive brûlés to senescent ones where production has stopped. We found that truffle grounds host rich communities in which multi‐host fungal species dominate in frequency. When considering both ECM tips and soil mycelia, we documented a dynamic and spatially heterogeneous pattern of T. melanosporum distribution in soils and a presence of ECM tips restricted to Q. ilex roots. This study advances our knowledge of the ecology of T. melanosporum, and provides insight into the extent of ECM fungal sharing among plant species that dominate Mediterranean landscapes.