Architecture of the wood-wide web: Rhizopogon spp. genets link multiple Douglas-fir cohorts

Arbuscular mycorrhizal fungi equalize differences in plant fitness and facilitate plant species coexistence through niche differentiation

Mycorrhizal fungi are essential to the establishment of the vast majority of plant species but are often conceptualized with contradictory roles in plant community assembly. On the one hand, host-specific mycorrhizal fungi may allow a plant to be competitively dominant by enhancing growth. On the other hand, host-specific mycorrhizal fungi with different functional capabilities may increase nutrient niche partitioning, allowing plant species to coexist. Here, to resolve the balance of these two contradictory forces, we used a controlled greenhouse study to manipulate the presence of two main types of mycorrhizal fungus, ectomycorrhizal fungi and arbuscular mycorrhizal fungi, and used a range of conspecific and heterospecific competitor densities to investigate the role of mycorrhizal fungi in plant competition and coexistence. We find that the presence of arbuscular mycorrhizal fungi equalizes fitness differences between plants and stabilizes competition to create conditions for host species coexistence. Our results show how below-ground mutualisms can shift outcomes of plant competition and that a holistic view of plant communities that incorporates their mycorrhizal partners is important in predicting plant community dynamics.

Ectomycorrhizal fungi of Douglas-fir retain newly assimilated carbon derived from neighboring European beech

Ectomycorrhizal (ECM) fungi distribute tree-derived carbon (C) via belowground hyphal networks in forest ecosystems. Here, we asked the following: (1) Is C transferred belowground to a neighboring tree retained in fungal structures or transported within the recipient tree? (2) Is the overlap of ectomycorrhizal fungi in mycorrhizal networks related to the amount of belowground C transfer? We used potted sapling pairs of European beech (Fagus sylvatica) and North-American Douglas-fir (Pseudotsuga menziesii) for 13CO2 pulse-labeling. We compared 13C transfer from beech (donor) to either beech or Douglas-fir (recipient) and identified the ECM species. We measured the 13C enrichment in soil, plant tissues, and ECM fractions of fungal-containing parts and plant transport tissues. In recipients, only fungal-containing tissue of ectomycorrhizas was significantly enriched in 13C and not the plant tissue. Douglas-fir recipients shared on average one ECM species with donors and had a lower 13C enrichment than beech recipients, which shared on average three species with donors. Our results support that recently assimilated C transferred belowground is shared among fungi colonizing tree roots but not among trees. In mixed forests with beech and Douglas-fir, the links for C movement might be hampered due to low mycorrhizal overlap with consequences for soil C cycling.

Effects of fungicide treatments on mycorrhizal communities and carbon acquisition in the mixotrophic Pyrola japonica (Ericaceae)

Pyrola japonica, a member of the family Ericaceae, is a mixotroph that grows on forest floors and obtains carbon (C) from both its photosynthesis and its mycorrhizal fungi. Its mycorrhizal community is dominated by Russulaceae. However, the mechanism of its C acquisition and its flexibility are not well understood. Our aim was to assess the impact of disturbance of the mycorrhizal fungal communities on C acquisition by P. japonica. We repeatedly applied a fungicide (Benomyl) to soils around P. japonica plants in a broad-leaved forest of central Japan, in order to disturb fungal associates near roots. After fungicide treatment, P. japonica roots were collected and subjected to barcoding by next-generation sequencing, focusing on the ITS2 region. The rate of mycorrhizal formation and α-diversity did not significantly change upon fungicide treatments. Irrespective of the treatments, Russulaceae represented more than 80% of the taxa. Leaves and seeds of the plants were analysed for 13C stable isotope ratios that reflect fungal C gain. Leaf and seed δ13C values with the fungicide treatment were significantly lower than those with the other treatments. Thus the fungicide did not affect mycorrhizal communities in the roots, but disturbed mycorrhizal fungal pathways via extraradical hyphae, and resulted in a more photosynthetic behaviour of P. japonica for leaves and seeds.

Maintenance of host specialisation gradients in ectomycorrhizal symbionts

Many fungi that form ectomycorrhizas exhibit a degree of host specialisation, and individual trees are frequently colonised by communities of mycorrhizal fungi comprising species that fall on a gradient of specialisation along genetic, functional and taxonomic axes of variation. By contrast, arbuscular mycorrhizal fungi exhibit little specialisation. Here, we propose that host tree root morphology is a key factor that gives host plants fine-scale control over colonisation and therefore opportunities for driving specialisation and speciation of ectomycorrhizal fungi. A gradient in host specialisation is likely driven by four proximate mechanistic 'filters' comprising partner availability, signalling recognition, competition for colonisation, and symbiotic function (trade, rewards and sanctions), and the spatially restricted colonisation seen in heterorhizic roots enables these mechanisms, especially symbiotic function, to be more effective in driving the evolution of specialisation. We encourage manipulation experiments that integrate molecular genetics and isotope tracers to test these mechanisms, alongside mathematical simulations of eco-evolutionary dynamics in mycorrhizal symbioses.

The mycorrhizal symbiosis: research frontiers in genomics, ecology, and agricultural application

Mycorrhizal symbioses between plants and fungi are vital for the soil structure, nutrient cycling, plant diversity, and ecosystem sustainability. More than 250 000 plant species are associated with mycorrhizal fungi. Recent advances in genomics and related approaches have revolutionized our understanding of the biology and ecology of mycorrhizal associations. The genomes of 250+ mycorrhizal fungi have been released and hundreds of genes that play pivotal roles in regulating symbiosis development and metabolism have been characterized. rDNA metabarcoding and metatranscriptomics provide novel insights into the ecological cues driving mycorrhizal communities and functions expressed by these associations, linking genes to ecological traits such as nutrient acquisition and soil organic matter decomposition. Here, we review genomic studies that have revealed genes involved in nutrient uptake and symbiosis development, and discuss adaptations that are fundamental to the evolution of mycorrhizal lifestyles. We also evaluated the ecosystem services provided by mycorrhizal networks and discuss how mycorrhizal symbioses hold promise for sustainable agriculture and forestry by enhancing nutrient acquisition and stress tolerance. Overall, unraveling the intricate dynamics of mycorrhizal symbioses is paramount for promoting ecological sustainability and addressing current pressing environmental concerns. This review ends with major frontiers for further research.

Sustainable Development versus Extractivist Deforestation in Tropical, Subtropical, and Boreal Forest Ecosystems: Repercussions and Controversies about the Mother Tree and the Mycorrhizal Network Hypothesis

This research reviews the phenomenon of extractive deforestation as a possible trigger for cascade reactions that could affect part of the forest ecosystem and its biodiversity (surface, aerial, and underground) in tropical, subtropical, and boreal forests. The controversy and disparities in criteria generated in the international scientific community around the hypothesis of a possible link between "mother trees" and mycorrhizal networks in coopetition for nutrients, nitrogen, and carbon are analyzed. The objective is to promote awareness to generate more scientific knowledge about the eventual impacts of forest extraction. Public policies are emphasized as crucial mediators for balanced sustainable development. Currently, the effects of extractive deforestation on forest ecosystems are poorly understood, which requires caution and forest protection. Continued research to increase our knowledge in molecular biology is advocated to understand the adaptation of biological organisms to the new conditions of the ecosystem both in the face of extractive deforestation and reforestation. The environmental impacts of extractive deforestation, such as the loss of biodiversity, soil degradation, altered water cycles, and the contribution of climate change, remain largely unknown. Long-term and high-quality research is essential to ensure forest sustainability and the preservation of biodiversity for future generations.

Age-Related Conservation in Plant-Soil Feedback Accompanied by Ectomycorrhizal Domination in Temperate Forests in Northeast China

This study investigates the effects of forest aging on ectomycorrhizal (EcM) fungal community and foraging behavior and their interactions with plant-soil attributes. We explored EcM fungal communities and hyphal exploration types via rDNA sequencing and investigated their associations with plant-soil traits by comparing younger (~120 years) and older (~250 years) temperate forest stands in Northeast China. The results revealed increases in the EcM fungal richness and abundance with forest aging, paralleled by plant-soil feedback shifting from explorative to conservative nutrient use strategies. In the younger stands, Tomentella species were prevalent and showed positive correlations with nutrient availability in both the soil and leaves, alongside rapid increases in woody productivity. However, the older stands were marked by the dominance of the genera Inocybe, Hymenogaster, and Otidea which were significantly and positively correlated with soil nutrient contents and plant structural attributes such as the community-weighted mean height and standing biomass. Notably, the ratios of longer-to-shorter distance EcM fungal exploration types tended to decrease along with forest aging. Our findings underscore the integral role of EcM fungi in the aging processes of temperate forests, highlighting the EcM symbiont-mediated mechanisms adapting to nutrient scarcity and promoting sustainability in plant-soil consortia.

Fungal and bacterial communities and their associations in snow-free and snow covered (sub-)alpine Pinus cembra forest soils

BACKGROUND

In Europe, Pinus cembra forests cover subalpine and alpine areas and they are of high conservational and ecological relevance. These forests experience strong seasonality with alternating snow-free and snow covered periods. Although P. cembra is known for mycorrhization and mycorrhizae usually involve fungi, plants and bacteria, the community compositions of fungi and bacteria and their associations in (sub-)alpine P. cembra forests remain vastly understudied. Here, we studied the fungal and bacterial community compositions in three independent (sub-)alpine P. cembra forests and inferred their microbial associations using marker gene sequencing and network analysis. We asked about the effect of snow cover on microbial compositions and associations. In addition, we propose inferring microbial associations across a range of filtering criteria, based on which we infer well justified, concrete microbial associations with high potential for ecological relevance that are typical for P. cembra forests and depending on snow cover.

RESULTS

The overall fungal and bacterial community structure was comparable with regards to both forest locations and snow cover. However, occurrence, abundance, and diversity patterns of several microbial taxa typical for P. cembra forests differed among snow-free and snow covered soils, e.g. Russula, Tetracladium and Phenoliphera. Moreover, network properties and microbial associations were influenced by snow cover. Here, we present concrete microbial associations on genus and species level that were repeatedly found across microbial networks, thereby confirming their ecological relevance. Most importantly, ectomycorrhizal fungi, such as Basidioascus, Pseudotomentella and Rhizopogon, as well as saprobic Mortierella changed their bacterial association partners depending on snow cover.

CONCLUSION

This is the first study researching fungal-bacterial associations across several (sub-)alpine P. cembra forests. The poorly investigated influence of snow cover on soil fungi and bacteria, especially those mycorrhizing P. cembra roots, but also saprobic soil organisms, underlines the relevance of forest seasonality. Our findings highlight that the seasonal impact of snow cover has significant consequences for the ecology of the ecosystem, particularly in relation to mycorrhization and nutrient cycling. It is imperative to consider such effects for a comprehensive understanding of the functioning resilience and responsiveness of an ecosystem.

Mother trees, altruistic fungi, and the perils of plant personification

There are growing doubts about the true role of the common mycorrhizal networks (CMN or wood wide web) connecting the roots of trees in forests. We question the claims of a substantial carbon transfer from 'mother trees' to their offspring and nearby seedlings through the CMN. Recent reviews show that evidence for the 'mother tree concept' is inconclusive or absent. The origin of this concept seems to stem from a desire to humanize plant life but can lead to misunderstandings and false interpretations and may eventually harm rather than help the commendable cause of preserving forests. Two recent books serve as examples: The Hidden Life of Trees and Finding the Mother Tree.

Phenoptosis and the Various Types of Natural Selection

In the first description of evolution, the fundamental mechanism is the natural selection favoring the individuals best suited for survival and reproduction (selection at the individual level or classical Darwinian selection). However, this is a very reductive description of natural selection that does not consider or explain a long series of known phenomena, including those in which an individual sacrifices or jeopardizes his life on the basis of genetically determined mechanisms (i.e., phenoptosis). In fact, in addition to (i) selection at the individual level, it is essential to consider other types of natural selection such as those concerning: (ii) kin selection and some related forms of group selection; (iii) the interactions between the innumerable species that constitute a holobiont; (iv) the origin of the eukaryotic cell from prokaryotic organisms; (v) the origin of multicellular eukaryotic organisms from unicellular organisms; (vi) eusociality (e.g., in many species of ants, bees, termites); (vii) selection at the level of single genes, or groups of genes; (viii) the interactions between individuals (or more precisely their holobionts) of the innumerable species that make up an ecosystem. These forms of natural selection, which are all effects and not violations of the classical Darwinian selection, also show how concepts as life, species, individual, and phenoptosis are somewhat not entirely defined and somehow arbitrary. Furthermore, the idea of organisms selected on the basis of their survival and reproduction capabilities is intertwined with that of organisms also selected on the basis of their ability to cooperate and interact, even by losing their lives or their distinct identities.

Perspectives on Computation in Plants

Plants thrive in virtually all natural and human-adapted environments and are becoming popular models for developing robotics systems because of their strategies of morphological and behavioral adaptation. Such adaptation and high plasticity offer new approaches for designing, modeling, and controlling artificial systems acting in unstructured scenarios. At the same time, the development of artifacts based on their working principles reveals how plants promote innovative approaches for preservation and management plans and opens new applications for engineering-driven plant science. Environmentally mediated growth patterns (e.g., tropisms) are clear examples of adaptive behaviors displayed through morphological phenotyping. Plants also create networks with other plants through subterranean roots-fungi symbiosis and use these networks to exchange resources or warning signals. This article discusses the functional behaviors of plants and shows the close similarities with a perceptron-like model that could act as a behavior-based control model in plants. We begin by analyzing communication rules and growth behaviors of plants; we then show how we translated plant behaviors into algorithmic solutions for bioinspired robot controllers; and finally, we discuss how those solutions can be extended to embrace original approaches to networking and robotics control architectures.

Positive citation bias and overinterpreted results lead to misinformation on common mycorrhizal networks in forests

A common mycorrhizal network (CMN) is formed when mycorrhizal fungal hyphae connect the roots of multiple plants of the same or different species belowground. Recently, CMNs have captured the interest of broad audiences, especially with respect to forest function and management. We are concerned, however, that recent claims in the popular media about CMNs in forests are disconnected from evidence, and that bias towards citing positive effects of CMNs has developed in the scientific literature. We first evaluated the evidence supporting three common claims. The claims that CMNs are widespread in forests and that resources are transferred through CMNs to increase seedling performance are insufficiently supported because results from field studies vary too widely, have alternative explanations or are too limited to support generalizations. The claim that mature trees preferentially send resources and defence signals to offspring through CMNs has no peer-reviewed, published evidence. We next examined how the results from CMN research are cited and found that unsupported claims have doubled in the past 25 years; a bias towards citing positive effects may obscure our understanding of the structure and function of CMNs in forests. We conclude that knowledge on CMNs is presently too sparse and unsettled to inform forest management.

Ectomycorrhizal Networks in the Anthropocene: From Natural Ecosystems to Urban Planning

Trees acquire hydric and mineral soil resources through root mutualistic associations. In most boreal, temperate and Mediterranean forests, these functions are realized by a chimeric structure called ectomycorrhizae. Ectomycorrhizal (ECM) fungi are highly diversified and vary widely in their specificity toward plant hosts. Reciprocally, association patterns of ECM plants range from highly specialist to generalist. As a consequence, ECM symbiosis creates interaction networks, which also mediate plant-plant nutrient interactions among different individuals and drive plant community dynamics. Our knowledge of ECM networks essentially relies on a corpus acquired in temperate ecosystems, whereas the below-ground facets of both anthropogenic ECM forests and inter-tropical forests remain poorly investigated. Here, we successively (1) review the current knowledge of ECM networks, (2) examine the content of early literature produced in ECM cultivated forests, (3) analyze the recent progress that has been made in understanding the place of ECM networks in urban soils, and (4) provide directions for future research based on the identification of knowledge gaps. From the examined corpus of knowledge, we reach three main conclusions. First, the emergence of metabarcoding tools has propelled a resurgence of interest in applying network theory to ECM symbiosis. These methods revealed an unexpected interconnection between mutualistic plants with arbuscular mycorrhizal (AM) herbaceous plants, embedding ECM mycelia through root-endophytic interactions. This affinity of ECM fungi to bind VA and ECM plants, raises questions on the nature of the associated functions. Second, despite the central place of ECM trees in cultivated forests, little attention has been paid to these man-made landscapes and in-depth research on this topic is lacking. Third, we report a lag in applying the ECM network theory to urban soils, despite management initiatives striving to interconnect motile organisms through ecological corridors, and the highly challenging task of interconnecting fixed organisms in urban greenspaces is discussed. In particular, we observe a pauperized nature of resident ECM inoculum and a spatial conflict between belowground human pipelines and ECM networks. Finally, we identify the main directions of future research to make the needed link between the current picture of plant functioning and the understanding of belowground ECM networks.

Genetic mechanisms of aging in plants: What can we learn from them?

Plants hold all records in longevity. Their aging is a complex process. In the presented review, we analyzed published data on various aspects of plant aging with focus on any inferences that could shed a light on aging in animals and help to fight it in human. Plant aging can be caused by many factors, such as telomere depletion, genomic instability, loss of proteostasis, changes in intercellular interaction, desynchronosis, autophagy misregulation, epigenetic changes and others. Plants have developed a number of mechanisms to increase lifespan. Among these mechanisms are gene duplication ("genetic backup"), the active work of telomerases, abundance of meristematic cells, capacity of maintaining the meristems permanently active and continuous activity of phytohormones. Plant aging usually occurs throughout the whole perennial life, but could be also seasonal senescence. Study of causes for seasonal aging can also help to uncover the mechanisms of plant longevity. The influence of different factors such as microbiome communities, glycation, alternative oxidase activity, mitochondrial dysfunction on plant longevity was also reviewed. Adaptive mechanisms of long-lived plants are considered. Further comparative study of the mechanisms underlying longevity of plants is necessary. This will allow us to reach a potentially new level of understanding of the aging process of plants.

An Artificial Intelligence Agent That Synthesises Visual Abstractions of Natural Forms to Support the Design of Human-Made Habitat Structures

Biodiversity is in a state of global collapse. Among the main drivers of this crisis is habitat degradation that destroys living spaces for animals, birds, and other species. Design and provision of human-made replacements for natural habitat structures can alleviate this situation. Can emerging knowledge in ecology, design, and artificial intelligence (AI) help? Current strategies to resolve this issue include designing objects that reproduce known features of natural forms. For instance, conservation practitioners seek to mimic the function of rapidly disappearing large old trees by augmenting utility poles with perch structures. Other approaches to restoring degraded ecosystems employ computational tools to capture information about natural forms and use such data to monitor remediation activities. At present, human-made replacements of habitat structures cannot reproduce significant features of complex natural forms while supporting efficient construction at large scales. We propose an AI agent that can synthesise simplified but ecologically meaningful representations of 3D forms that we define as visual abstractions. Previous research used AI to synthesise visual abstractions of 2D images. However, current applications of such techniques neither extend to 3D data nor engage with biological conservation or ecocentric design. This article investigates the potential of AI to support the design of artificial habitat structures and expand the scope of computation in this domain from data analysis to design synthesis. Our case study considers possible replacements of natural trees. The application implements a novel AI agent that designs by placing three-dimensional cubes – or voxels – in the digital space. The AI agent autonomously assesses the quality of the resulting visual abstractions by comparing them with three-dimensional representations of natural trees. We evaluate the forms produced by the AI agent by measuring relative complexity and features that are meaningful for arboreal wildlife. In conclusion, our study demonstrates that AI can generate design suggestions that are aligned with the preferences of arboreal wildlife and can support the development of artificial habitat structures. The bio-informed approach presented in this article can be useful in many situations where incomplete knowledge about complex natural forms can constrain the design and performance of human-made artefacts.

Common Mycorrhizae Network: A Review of the Theories and Mechanisms Behind Underground Interactions

Most terrestrial plants establish symbiotic associations with mycorrhizal fungi for accessing essential plant nutrients. Mycorrhizal fungi have been frequently reported to interconnect plants via a common mycelial network (CMN), in which nutrients and signaling compounds can be exchanged between the connected plants. Several studies have been performed to demonstrate the potential effects of the CMN mediating resource transfer and its importance for plant fitness. Due to several contrasting results, different theories have been developed to predict benefits or disadvantages for host plants involved in the network and how it might affect plant communities. However, the importance of the mycelium connections for resources translocation compared to other indirect pathways, such as leakage of fungi hyphae and subsequent uptake by neighboring plant roots, is hard to distinguish and quantify. If resources can be translocated via mycelial connections in significant amounts that could affect plant fitness, it would represent an important tactic for plants co-existence and it could shape community composition and dynamics. Here, we report and critically discuss the most recent findings on studies aiming to evaluate and quantify resources translocation between plants sharing a CMN and predict the pattern that drives the movement of such resources into the CMN. We aim to point gaps and define open questions to guide upcoming studies in the area for a prospect better understanding of possible plant-to-plant interactions via CMN and its effect in shaping plants communities. We also propose new experiment set-ups and technologies that could be used to improve previous experiments. For example, the use of mutant lines plants with manipulation of genes involved in the symbiotic associations, coupled with labeling techniques to track resources translocation between connected plants, could provide a more accurate idea about resource allocation and plant physiological responses that are truly accountable to CMN.

Timing of Resource Addition Affects the Migration Behavior of Wood Decomposer Fungal Mycelia

Studies of fungal behavior are essential for a better understanding of fungal-driven ecological processes. Here, we evaluated the effects of timing of resource (bait) addition on the behavior of fungal mycelia when it remains in the inoculum and when it migrates from it towards a bait, using cord-forming basidiomycetes. Experiments allowed mycelium to grow from an inoculum wood across the surface of a soil microcosm, where it encountered a new wood bait 14 or 98 d after the start of growth. After the 42-d colonization of the bait, inoculum and bait were individually moved to a dish containing fresh soil to determine whether the mycelia were able to grow out. When the inoculum and bait of mycelia baited after 14 d were transferred to new soil, there was 100% regrowth from both inoculum and bait in Pholiota brunnescens and Phanerochaete velutina, indicating that no migration occurred. However, when mycelium was baited after 98 d, 3 and 4 out of 10 replicates of P. brunnescens and P. velutina, respectively, regrew only from bait and not from inoculum, indicating migration. These results suggest that prolonged periods without new resources alter the behavior of mycelium, probably due to the exhaustion of resources.

Shared friends counterbalance shared enemies in old forests

Mycorrhizal mutualisms are nearly ubiquitous across plant communities. Yet, it is still unknown whether facilitation among plants arises primarily from these mycorrhizal networks or from physical and ecological attributes of plants themselves. Here, we tested the relative contributions of mycorrhizae and plants to both positive and negative biotic interactions to determine whether plant-soil feedbacks with mycorrhizae neutralize competition and enemies within multitrophic forest community networks. We used Bayesian hierarchical generalized linear modeling to examine mycorrhizal-guild-specific and mortality-cause-specific woody plant survival compiled from a spatially and temporally explicit data set comprising 101,096 woody plants from three mixed-conifer forests across western North America. We found positive plant-soil feedbacks for large-diameter trees: species-rich woody plant communities indirectly promoted large tree survival when connected via mycorrhizal networks. Shared mycorrhizae primarily counterbalanced apparent competition mediated by tree enemies (e.g., bark beetles, soil pathogens) rather than diffuse competition between plants. We did not find the same survival benefits for small trees or shrubs. Our findings suggest that lower large-diameter tree mortality susceptibility in species-rich temperate forests resulted from greater access to shared mycorrhizal networks. The interrelated importance of aboveground and belowground biodiversity to large tree survival may be critical for counteracting increasing pathogen, bark beetle, and density threats.

Mixing tree species associated with arbuscular or ectotrophic mycorrhizae reveals dual mycorrhization and interactive effects on the fungal partners

Recent studies found that the majority of shrub and tree species are associated with both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. However, our knowledge on how different mycorrhizal types interact with each other is still limited. We asked whether the combination of hosts with a preferred association with either AM or EM fungi increases the host tree roots' mycorrhization rate and affects AM and EM fungal richness and community composition.We established a tree diversity experiment, where five tree species of each of the two mycorrhiza types were planted in monocultures, two-species and four-species mixtures. We applied morphological assessment to estimate mycorrhization rates and next-generation molecular sequencing to quantify mycobiont richness.Both the morphological and molecular assessment revealed dual-mycorrhizal colonization in 79% and 100% of the samples, respectively. OTU community composition strongly differed between AM and EM trees. While host tree species richness did not affect mycorrhization rates, we observed significant effects of mixing AM- and EM-associated hosts in AM mycorrhization rate. Glomeromycota richness was larger in monotypic AM tree combinations than in AM-EM mixtures, pointing to a dilution or suppression effect of AM by EM trees. We found a strong match between morphological quantification of AM mycorrhization rate and Glomeromycota richness. Synthesis. We provide evidence that the combination of hosts differing in their preferred mycorrhiza association affects the host's fungal community composition, thus revealing important biotic interactions among trees and their associated fungi.

Location, but not defensive genotype, determines ectomycorrhizal community composition in Scots pine (Pinus sylvestris L.) seedlings

For successful colonization of host roots, ectomycorrhizal (EM) fungi must overcome host defense systems, and defensive phenotypes have previously been shown to affect the community composition of EM fungi associated with hosts. Secondary metabolites, such as terpenes, form a core part of these defense systems, but it is not yet understood whether variation in these constitutive defenses can result in variation in the colonization of hosts by specific fungal species.We planted seedlings from twelve maternal families of Scots pine (Pinus sylvestris) of known terpene genotype reciprocally in the field in each of six sites. After 3 months, we characterized the mycorrhizal fungal community of each seedling using a combination of morphological categorization and molecular barcoding, and assessed the terpene chemodiversity for a subset of the seedlings. We examined whether parental genotype or terpene chemodiversity affected the diversity or composition of a seedling's mycorrhizal community.While we found that terpene chemodiversity was highly heritable, we found no evidence that parental defensive genotype or a seedling's terpene chemodiversity affected associations with EM fungi. Instead, we found that the location of seedlings, both within and among sites, was the only determinant of the diversity and makeup of EM communities.These results show that while EM community composition varies within Scotland at both large and small scales, variation in constitutive defensive compounds does not determine the EM communities of closely cohabiting pine seedlings. Patchy distributions of EM fungi at small scales may render any genetic variation in associations with different species unrealizable in field conditions. The case for selection on traits mediating associations with specific fungal species may thus be overstated, at least in seedlings.

Plant carbohydrate depletion impairs water relations and spreads via ectomycorrhizal networks

Under prolonged drought and reduced photosynthesis, plants consume stored nonstructural carbohydrates (NSCs). Stored NSC depletion may impair the regulation of plant water balance, but the underlying mechanisms are poorly understood, and whether such mechanisms are independent of plant water deficit is not known. If so, carbon costs of fungal symbionts could indirectly influence plant drought tolerance through stored NSC depletion. We connected well-watered Pinus ponderosa seedling pairs via ectomycorrhizal (EM) networks where one seedling was shaded (D) and the other kept illuminated (LD) and compared responses to seedling pairs in full light (L). We measured plant NSCs, osmotic and water potential, and transfer of ¹³ CO₂ through EM to explore mechanisms linking stored NSCs to plant water balance regulation and identify potential tradeoffs between plant water retention and EM fungi under carbon-limiting conditions. NSCs decreased from L to LD to D seedlings. Even without drought, NSC depletion impaired osmoregulation and turgor maintenance, both of which are critical for drought tolerance. Importantly, EM networks propagated NSC depletion and its negative effects on water retention from carbon stressed to nonstressed hosts. We demonstrate that NSC storage depletion influences turgor maintenance independently of plant water deficit and reveal carbon allocation tradeoffs between supporting fungal symbionts and retaining water.

Modeling mycorrhizal fungi dispersal by the mycophagous swamp wallaby (Wallabia bicolor)

Despite the importance of mammal-fungal interactions, tools to estimate the mammal-assisted dispersal distances of fungi are lacking. Many mammals actively consume fungal fruiting bodies, the spores of which remain viable after passage through their digestive tract. Many of these fungi form symbiotic relationships with trees and provide an array of other key ecosystem functions. We present a flexible, general model to predict the distance a mycophagous mammal would disperse fungal spores. We modeled the probability of spore dispersal by combining animal movement data from GPS telemetry with data on spore gut-retention time. We test this model using an exemplar generalist mycophagist, the swamp wallaby (Wallabia bicolor). We show that swamp wallabies disperse fungal spores hundreds of meters-and occasionally up to 1,265 m-from the point of consumption, distances that are ecologically significant for many mycorrhizal fungi. In addition to highlighting the ecological importance of swamp wallabies as dispersers of mycorrhizal fungi in eastern Australia, our simple modeling approach provides a novel and effective way of empirically describing spore dispersal by a mycophagous animal. This approach is applicable to the study of other animal-fungi interactions in other ecosystems.

Symbiosis and stress: how plant microbiomes affect host evolution

Existing paradigms for plant microevolution rarely acknowledge the potential impacts of diverse microbiomes on evolutionary processes. Many plant-associated microorganisms benefit the host via access to resources, protection from pathogens, or amelioration of abiotic stress. In doing so, they alter the plant's perception of the environment, potentially reducing the strength of selection acting on plant stress tolerance or defence traits or altering the traits that are the target of selection. We posit that the microbiome can affect plant microevolution via (1) manipulation of plant phenotypes in ways that increase plant fitness under stress and (2) direct microbial responses to the environment that benefit the plant. Both mechanisms might favour plant genotypes that attract or stimulate growth of the most responsive microbial populations or communities. We provide support for these scenarios using infectious disease and quantitative genetics models. Finally, we discuss how beneficial plant-microbiome associations can evolve if traditional mechanisms maintaining cooperation in pairwise symbioses, namely partner fidelity, partner choice and fitness alignment, also apply to the interactions between plants and diverse foliar and soil microbiomes. To understand the role of the plant microbiome in host evolution will require a broad ecological understanding of plant-microbe interactions across both space and time. This article is part of the theme issue 'The role of the microbiome in host evolution'.

Resilience of Rhizopogon-Douglas-fir mycorrhizal networks 25 years after selective logging

Rhizopogon vesiculosus and R. vinicolor are sister fungal species; they form ectomycorrhizas exclusively with Douglas-fir roots, and they are important in forming relatively large mycorrhizal networks, but they may be vulnerable to disturbance caused by logging practices. The main objective was to determine the resilience of mycorrhizal networks 25 years following removal of large hub trees. We predicted that the targeted removal of mature trees would reduce network connectedness compared with a non-harvested neighboring forest. Rhizopogon vesiculosus was nearly absent in the non-harvested plots, whereas both species were prominent in the harvested plots. Initially, network analysis was based only on networks formed by R. vinicolor because they were well represented in both treatments. These analyses showed that the R. vinicolor-Douglas-fir MN was more densely linked in the non-harvested plots than the harvested plots. When we accounted for differences in link and node density, there was still an edge difference and a greater vulnerability to fragmentation in harvested forests than in non-harvested forests. When both Rhizopogon sister species were included in the analysis, both treatments had similar connectivity and limited vulnerability to fragmentation. This suggests that when these forests transition from a regenerating to a non-regenerating state, the Rhizopogon network will lose R. vesiculosus but will maintain link density due to the colonization with R. vinicolor.

Rapid nitrogen loss from ectomycorrhizal pine germinants signaled by their fungal symbiont

Ectomycorrhizal fungi supply their plant partners with nitrogen but can also retain substantial amounts. The concentration of nitrogen in the soil and the amount of carbon supplied from the host seem to influence the proportion of N retained by the fungus. In an experiment designed to determine whether differential supply of nitrogen to two plants influenced nitrogen transfer from fungus to plant within a mycorrhizal network, we observed rapid, substantial loss of nitrogen from pine seedlings. The loss occurred when the mycorrhizal fungus experienced a sudden increase in nitrogen supply. We grew Pinus contorta seedlings in association with Suillus tomentosus in low-nitrogen microcosms where some nitrogen was accessible only by hyphae. After 70 days, foliage of some seedlings was treated with nitrogen. Three days later, hyphal nutrient media were replaced with water or a solution containing nitrogen. Foliar treatment did not affect nitrogen transfer by the fungus to shoots, but by day 75, seedling nitrogen contents had dropped by 60% in microcosms where nitrogen had been added to the hyphal compartments. Those seedlings retained only 55% of the nitrogen originally present in the seed. Loss of nitrogen did not occur if water was added or the hyphae were severed. Because of the severing effect, we concluded that S. tomentosus triggered the loss of seedling nitrogen. Nitrogen may have been lost through increased root exudation or transfer to the fungus. Access to nitrogen from nutrient-rich germinants would benefit rhizosphere microorganisms, including ectomycorrhizal fungi colonizing pine from spores after wildfire.

Share the wealth: Trees with greater ectomycorrhizal species overlap share more carbon

The mutualistic symbiosis between forest trees and ectomycorrhizal fungi (EMF) is among the most ubiquitous and successful interactions in terrestrial ecosystems. Specific species of EMF are known to colonize specific tree species, benefitting from their carbon source, and in turn, improving their access to soil water and nutrients. EMF also form extensive mycelial networks that can link multiple root-tips of different trees. Yet the number of tree species connected by such mycelial networks, and the traffic of material across them, are just now under study. Recently we reported substantial belowground carbon transfer between Picea, Pinus, Larix and Fagus trees in a mature forest. Here, we analyze the EMF community of these same individual trees and identify the most likely taxa responsible for the observed carbon transfer. Among the nearly 1,200 EMF root-tips examined, 50%-70% belong to operational taxonomic units (OTUs) that were associated with three or four tree host species, and 90% of all OTUs were associated with at least two tree species. Sporocarp 13 C signals indicated that carbon originating from labelled Picea trees was transferred among trees through EMF networks. Interestingly, phylogenetically more closely related tree species exhibited more similar EMF communities and exchanged more carbon. Our results show that belowground carbon transfer is well orchestrated by the evolution of EMFs and tree symbiosis.

The effects of ectomycorrhizal fungal networks on seedling establishment are contingent on species and severity of overstorey mortality

For tree seedlings in boreal forests, ectomycorrhizal (EM) fungal networks may promote, while root competition may impede establishment. Thus, disruption to EM fungal networks may decrease seedling establishment owing to the loss of positive interactions among neighbors. Widespread tree mortality can disrupt EM networks, but it is not clear whether seedling establishment will be limited by the loss of positive interactions or increased by the loss of negative interactions with surrounding roots. Depending upon the relative influence of these mechanisms, widespread tree mortality may have complicated consequences on seedling establishment, and in turn, the composition of future forests. To discern between these possible outcomes and the drivers of seedling establishment, we determined the relative importance of EM fungal networks, root presence, and the bulk soil on the establishment of lodgepole pine and white spruce seedlings along a gradient of beetle-induced tree mortality. We manipulated seedling contact with EM fungal networks and roots through the use of mesh-fabric cylinders installed in soils of lodgepole pine forests experiencing a range of overstorey tree mortality caused by mountain pine beetle. Lodgepole pine seedling survival was higher with access to EM fungal networks in undisturbed pine forests in comparison with that in beetle-killed stands. That is, overstorey tree mortality shifted fungal networks from being a benefit to a cost on seedling survival. In contrast, overstorey tree mortality did not change the relative strength of EM fungal networks, root presence and the bulk soil on survival and biomass of white spruce seedlings. Furthermore, the relative influence of EM fungal networks, root presence, and bulk soils on foliar N and P concentrations was highly contingent on seedling species and overstorey tree mortality. Our results highlight that following large-scale insect outbreak, soil-mediated processes can enable differential population growth of two common conifer species, which may result in species replacement in the future.

Spatial characteristics of volatile communication in lodgepole pine trees: Evidence of kin recognition and intra-species support

Plant interactions using volatile organic compounds, particularly in the context of kin recognition have received considerable attention in recent years, but several discrepancies and conflicting results have restricted our understanding. We propose that some of these discrepancies in literature are in part due to integral spatial characteristics of sites, and plant attributes. Chemotypic plasticity is commonly used to characterize kin, particularly in conifers. We studied constitutive and induced monoterpene chemotypes of non-attacked lodgepole pine trees within 30 m radii of pine trees attacked by mountain pine beetle. We tested the effects of volatile compounds emitted from the attacked trees on the non-attacked trees by challenge inoculations with a mountain pine beetle associated fungus. We found no relationship between constitutive monoterpene concentrations of the non-attacked trees and distance or direction from the attacked trees or site aspects. In contrast, the effects of volatile compounds were evident after inoculations, depending on distance from the attacked trees and site aspects. However, these interactions only emerged among chemotypically related trees. These results suggest that plants discriminate between chemical cues from kin and strangers, and the emitters likely aid only chemotypically related plants by emitting specific blends of volatiles that can only be deciphered by the receiving kin. These results further demonstrate the importance of incorporating spatial characteristics of sites and plant attributes in studies aimed at investigating intra-species interactions using volatile organic compounds.

Intraspecific Fine-Root Trait-Environment Relationships across Interior Douglas-Fir Forests of Western Canada

Variation in resource acquisition strategies enables plants to adapt to different environments and may partly determine their responses to climate change. However, little is known about how belowground plant traits vary across climate and soil gradients. Focusing on interior Douglas-fir (Pseudotsuga menziesii var. glauca) in western Canada, we tested whether fine-root traits relate to the environment at the intraspecific level. We quantified the variation in commonly measured functional root traits (morphological, chemical, and architectural traits) among the first three fine-root orders (i.e., absorptive fine roots) and across biogeographic gradients in climate and soil factors. Moderate but consistent trait-environment linkages occurred across populations of Douglas-fir, despite high levels of within-site variation. Shifts in morphological traits across regions were decoupled from those in chemical traits. Fine roots in colder/drier climates were characterized by a lower tissue density, higher specific area, larger diameter, and lower carbon-to-nitrogen ratio than those in warmer/wetter climates. Our results showed that Douglas-fir fine roots do not rely on adjustments in architectural traits to adapt rooting strategies in different environments. Intraspecific fine-root adjustments at the regional scale do not fit along a single axis of root economic strategy and are concordant with an increase in root acquisitive potential in colder/drier environments.

Host Phylogenetic Relatedness and Soil Nutrients Shape Ectomycorrhizal Community Composition in Native and Exotic Pine Plantations

Exotic non-native Pinus species have been widely planted or become naturalized in many parts of the world. Pines rely on ectomycorrhizal (ECM) fungi mutualisms to overcome barriers to establishment, yet the degree to which host specificity and edaphic preferences influence ECM community composition remains poorly understood. In this study, we used high-throughput sequencing coupled with soil analyses to investigate the effect of host plant identity, spatial distance and edaphic factors on ECM community composition in young (30-year-old) native (Pinus massoniana Lamb.) and exotic (Pinus elliottii Engelm.) pine plantations in China. The ECM fungal communities comprised 43 species with the majority belonging to the Thelephoraceae and Russulaceae. Most species were found associated with both host trees while certain native ECM taxa (Suillus) showed host specificity to the native P. massoniana. ECM fungi that are known to occur exclusively with Pinus (e.g., Rhizopogon) were uncommon. We found no significant effect of host identity on ECM communities, i.e., phylogenetically related pines shared similar ECM fungal communities. Instead, ECM fungal community composition was strongly influenced by site-specific abiotic factors and dispersal. These findings reinforce the idea that taxonomic relatedness might be a factor promoting ECM colonization in exotic pines but that shifts in ECM communities may also be context-dependent.

The continuing relevance of "older" mycorrhiza literature: insights from the work of John Laker Harley (1911-1990)

To new generations of scientists beginning their careers in research, we strongly recommend the practice of reading older literature. To illustrate the value of doing so, we highlight six insights of one of the most influential mycorrhiza researchers of the twentieth century, Jack Harley. These insights concerning mycotrophy, the new niche, the sheath, C cycling, N cycling, and mutualism were published prior to 1975 and so may have escaped the notice of many, but they laid the groundwork for some of the most important research of today.

Recent Insights on Biological and Ecological Aspects of Ectomycorrhizal Fungi and Their Interactions

The roots of most terrestrial plants are colonized by mycorrhizal fungi. They play a key role in terrestrial environments influencing soil structure and ecosystem functionality. Around them a peculiar region, the mycorrhizosphere, develops. This is a very dynamic environment where plants, soil and microorganisms interact. Interest in this fascinating environment has increased over the years. For a long period the knowledge of the microbial populations in the rhizosphere has been limited, because they have always been studied by traditional culture-based techniques. These methods, which only allow the study of cultured microorganisms, do not allow the characterization of most organisms existing in nature. The introduction in the last few years of methodologies that are independent of culture techniques has bypassed this limitation. This together with the development of high-throughput molecular tools has given new insights into the biology, evolution, and biodiversity of mycorrhizal associations, as well as, the molecular dialog between plants and fungi. The genomes of many mycorrhizal fungal species have been sequenced so far allowing to better understanding the lifestyle of these fungi, their sexual reproduction modalities and metabolic functions. The possibility to detect the mycelium and the mycorrhizae of heterothallic fungi has also allowed to follow the spatial and temporal distributional patterns of strains of different mating types. On the other hand, the availability of the genome sequencing from several mycorrhizal fungi with a different lifestyle, or belonging to different groups, allowed to verify the common feature of the mycorrhizal symbiosis as well as the differences on how different mycorrhizal species interact and dialog with the plant. Here, we will consider the aspects described before, mainly focusing on ectomycorrhizal fungi and their interactions with plants and other soil microorganisms.

Quantitative losses vs. qualitative stability of ectomycorrhizal community responses to 3 years of experimental summer drought in a beech-spruce forest

Forest ecosystems in central Europe are predicted to face an increasing frequency and severity of summer droughts because of global climate change. European beech and Norway spruce often coexist in these forests with mostly positive effects on their growth. However, their different below-ground responses to drought may lead to differences in ectomycorrhizal (ECM) fungal community composition and functions which we examined at the individual root and ecosystem levels. We installed retractable roofs over plots in Kranzberg Forest (11°39'42″E, 48°25'12″N; 490 m a.s.l.) to impose repeated summer drought conditions and assigned zones within each plot where trees neighboured the same or different species to study mixed species effects. We found that ECM fungal community composition changed and the numbers of vital mycorrhizae decreased for both tree species over 3 drought years (2014-2016), with the ECM fungal community diversity of beech exhibiting a faster and of spruce a stronger decline. Mixed stands had a positive effect on the ECM fungal community diversity of both tree species after the third drought year. Ectomycorrhizae with long rhizomorphs increased in both species under drought, indicating long-distance water transport. However, there was a progressive decline in the number of vital fine roots during the experiment, resulting in a strong reduction in enzyme activity per unit volume of soil. Hydrolytic enzyme activities of the surviving ectomycorrhizae were stable or stimulated upon drought, but there was a large decline in ECM fungal species with laccase activity, indicating a decreased potential to exploit nutrients bound to phenolic compounds. Thus, the ectomycorrhizae responded to repeated drought by maintaining or increasing their functionality at the individual root level, but were unable to compensate for quantitative losses at the ecosystem level. These findings demonstrate a strong below-ground impact of recurrent drought events in forests.

Comparative Genomics of the Ectomycorrhizal Sister Species Rhizopogon vinicolor and Rhizopogon vesiculosus (Basidiomycota: Boletales) Reveals a Divergence of the Mating Type B Locus

Divergence of breeding system plays an important role in fungal speciation. Ectomycorrhizal fungi, however, pose a challenge for the study of reproductive biology because most cannot be mated under laboratory conditions. To overcome this barrier, we sequenced the draft genomes of the ectomycorrhizal sister species Rhizopogon vinicolor Smith and Zeller and R. vesiculosus Smith and Zeller (Basidiomycota, Boletales)-the first genomes available for Basidiomycota truffles-and characterized gene content and organization surrounding their mating type loci. Both species possess a pair of homeodomain transcription factor homologs at the mating type A-locus as well as pheromone receptor and pheromone precursor homologs at the mating type B-locus. Comparison of Rhizopogon genomes with genomes from Boletales, Agaricales, and Polyporales revealed synteny of the A-locus region within Boletales, but several genomic rearrangements across orders. Our findings suggest correlation between gene content at the B-locus region and breeding system in Boletales with tetrapolar species possessing more diverse gene content than bipolar species. Rhizopogon vinicolor possesses a greater number of B-locus pheromone receptor and precursor genes than R. vesiculosus, as well as a pair of isoprenyl cysteine methyltransferase genes flanking the B-locus compared to a single copy in R. vesiculosus Examination of dikaryotic single nucleotide polymorphisms within genomes revealed greater heterozygosity in R. vinicolor, consistent with increased rates of outcrossing. Both species possess the components of a heterothallic breeding system with R. vinicolor possessing a B-locus region structure consistent with tetrapolar Boletales and R. vesiculosus possessing a B-locus region structure intermediate between bipolar and tetrapolar Boletales.

Transfer of 13 C between paired Douglas-fir seedlings reveals plant kinship effects and uptake of exudates by ectomycorrhizas

Processes governing the fixation, partitioning, and mineralization of carbon in soils are under increasing scrutiny as we develop a more comprehensive understanding of global carbon cycling. Here we examined fixation by Douglas-fir seedlings and transfer to associated ectomycorrhizal fungi, soil microbes, and full-sibling or nonsibling neighbouring seedlings. Stable isotope probing with 99% ¹³ C-CO₂ was applied to trace ¹³ C-labelled photosynthate throughout plants, fungi, and soil microbes in an experiment designed to assess the effect of relatedness on ¹³ C transfer between plant pairs. The fixation and transfer of the ¹³ C label to plant, fungal, and soil microbial tissue was examined in biomass and phospholipid fatty acids. After a 6 d chase period, c. 26.8% of the ¹³ C remaining in the system was translocated below ground. Enrichment was proportionally greatest in ectomycorrhizal biomass. The presence of mesh barriers (0.5 or 35 μm) between seedlings did not restrict ¹³ C transfer. Fungi were the primary recipients of ¹³ C-labelled photosynthate throughout the system, representing 60-70% of total ¹³ C-enriched phospholipids. Full-sibling pairs exhibited significantly greater ¹³ C transfer to recipient roots in two of four Douglas-fir families, representing three- and fourfold increases (+ c. 4 μg excess ¹³ C) compared with nonsibling pairs. The existence of a root/mycorrhizal exudation-hyphal uptake pathway was supported.

Photosynthesis and aboveground carbon allocation of two co-occurring poplar species in an urban brownfield

Phytoremediation, a technique used to reclaim heavy metal-contaminated soils, requires an understanding of plant physiological responses to heavy metals. However, the majority of studies documenting heavy metal impact on plant functioning have been performed in laboratory or greenhouse settings. We predicted that increased soil heavy metal concentrations reduce photosynthesis and biomass production in trees growing in metal contaminated soil in a naturally re-vegetated urban brownfield. Leaf gas exchange, leaf carbon and nitrogen concentration, and tree biomass were recorded and compared for Populus deltoides and Populus tremuloides growing in an urban brownfield. The CO₂ compensation point (CCP) differed significantly between soil metal concentrations and species, with P. deltoides displaying a greater CCP and P. tremuloides displaying a lower CCP as soil metal concentration increased, despite no changes in dark respiration for either species. In terms of biomass, only total branch weight (TBW) and leaf area (LA) differed significantly between soil metal concentrations, though the difference was largely attributable to variation in diameter at breast height (DBH). Furthermore, TBW and LA values for P. deltoides did not decrease with increasing soil metal concentration. Soil metal concentration, thus, had minimal effect on the relationship between tree age and DBH, and no effect on relationships of tree age and height or LA, respectively. Significant differences between soil metal concentrations and species were found for δ¹⁵N (isotopic nitrogen ratio) while leaf nitrogen content (% N) also differed significantly between species. Long-term water use efficiency derived from carbon isotope analysis (iWUE_isotope) differed significantly between trees grown on different soil metal concentrations and a significant species-metal concentration interaction was detected indicating that the two study species responded differentially to the soil metal concentrations. Specifically, P. tremuloides enhanced while P. deltoides reduced long-term iWUE_isotope as soil metal concentration increased, further emphasizing the importance of species and possible genotype selection for phytoremediation.