Mycorrhizal networks: Mechanisms, ecology and modelling

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.

Explosive networking: The role of adaptive host radiations and ecological opportunity in a species-rich host-parasite assembly

Many species-rich ecological communities emerge from adaptive radiation events. Yet the effects of adaptive radiation on community assembly remain poorly understood. Here, we explore the well-documented radiations of African cichlid fishes and their interactions with the flatworm gill parasites Cichlidogyrus spp., including 10,529 reported infections and 477 different host-parasite combinations collected through a survey of peer-reviewed literature. We assess how evolutionary, ecological, and morphological parameters determine host-parasite meta-communities affected by adaptive radiation events through network metrics, host repertoire measures, and network link prediction. The hosts' evolutionary history mostly determined host repertoires of the parasites. Ecological and evolutionary parameters predicted host-parasite interactions. Generally, ecological opportunity and fitting have shaped cichlid-Cichlidogyrus meta-communities suggesting an invasive potential for hosts used in aquaculture. Meta-communities affected by adaptive radiations are increasingly specialised with higher environmental stability. These trends should be verified across other systems to infer generalities in the evolution of species-rich host-parasite networks.

A quantitative synthesis of soil microbial effects on plant species coexistence

SignificanceUnderstanding the processes that maintain plant diversity is a key goal in ecology. Many previous studies have shown that soil microbes can generate stabilizing or destabilizing feedback loops that drive either plant species coexistence or monodominance. However, theory shows that microbial controls over plant coexistence also arise through microbially mediated competitive imbalances, which have been largely neglected. Using data from 50 studies, we found that soil microbes affect plant dynamics primarily by generating competitive fitness differences rather than stabilizing or destabilizing feedbacks. Consequently, in the absence of other competitive asymmetries among plants, soil microbes are predicted to drive species exclusion more than coexistence. These results underscore the need for measuring competitive fitness differences when evaluating microbial controls over plant coexistence.

Ectomycorrhizal fungal communities of Swiss stone pine (Pinus cembra) depend on climate and tree age in natural forests of the Alps

Background and Aims

Pinus cembra represent a typical and important tree species growing in European subalpine and alpine habitats. The ectomycorrhizal (ECM) fungal communities associated to this tree under natural conditions are largely unknown.

Methods

In this study, we investigated the ECM fungal abundance and composition at four high-altitude sites (two northern-exposed and two southern-exposed habitats) in South Tyrol (Italy), and included also two different age classes of P. cembra. The ECM partners were characterized morphologically, and identified by rDNA ITS sequence analysis.

Results

The degree of mycorrhization in adult P. cembra was typically 100% in these natural habitats, with a total species diversity of 20 ECM species. The four high-altitude sites were similar concerning their species richness and mycobiont diversity, but they differed significantly in ECM species composition. Young P. cembra had a mycorrhization degree of 100% and a total of 10 species were observed. All mycorrhizal partners of naturally regenerated young P. cembra were only detected in one specific location, with the exception of Cenococcum sp. and Amphinema sp. which were detected at two sites. Young trees harbour a distinct ectomycorrhizal fungal diversity, which is clearly lower than the diversity detected in adult P. cembra trees. The P. cembra bolete (Suillus plorans) is the most important symbiotic partner of P. cembra at Southern Tyrolean high-altitude sites and is known for its strict, species-specific host association.

Conclusions

The ectomycorrhizal fungal community composition strongly depends on geographic region and on the slope exposure (north or south) of the site.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11104-022-05497-z.

Differences between the effects of plant species and compartments on microbiome composition in two halophyte Suaeda species

Root-related or endophytic microbes in halophytes play an important role in adaptation to extreme saline environments. However, there have been few comparisons of microbial distribution patterns in different tissues associated with halophytes. Here, we analyzed the bacterial communities and distribution patterns of the rhizospheres and tissue endosphere in two Suaeda species (S. salsa and S. corniculata Bunge) using the 16S rRNA gene sequencing. The results showed that the bacterial abundance and diversity in the rhizosphere were significantly higher than that of endophytic, but lower than that of bulk soil. Microbial-diversity analysis showed that the dominant phyla of all samples were Proteobacteria, Actinobacteria, Bacteroidetes, Acidobacteria and Firmicutes, among which Proteobacteria were extremely abundant in all the tissue endosphere. Heatmap and Linear discriminant analysis Effect Size (LEfSe) results showed that there were notable differences in microbial community composition related to plant compartments. Different networks based on plant compartments exhibited distinct topological features. Additionally, the bulk soil and rhizosphere networks were more complex and showed higher centrality and connectedness than the three endosphere networks. These results strongly suggested that plant compartments, and not species, affect microbiome composition.

Ectomycorrhizal fungi mediate belowground carbon transfer between pines and oaks

Inter-kingdom belowground carbon (C) transfer is a significant, yet hidden, biological phenomenon, due to the complexity and highly dynamic nature of soil ecology. Among key biotic agents influencing C allocation belowground are ectomycorrhizal fungi (EMF). EMF symbiosis can extend beyond the single tree-fungus partnership to form common mycorrhizal networks (CMNs). Despite the high prevalence of CMNs in forests, little is known about the identity of the EMF transferring the C and how these in turn affect the dynamics of C transfer. Here, Pinus halepensis and Quercus calliprinos saplings growing in forest soil were labeled using a 13CO2 labeling system. Repeated samplings were applied during 36 days to trace how 13C was distributed along the tree-fungus-tree pathway. To identify the fungal species active in the transfer, mycorrhizal fine root tips were used for DNA-stable isotope probing (SIP) with 13CO2 followed by sequencing of labeled DNA. Assimilated 13CO2 reached tree roots within four days and was then transferred to various EMF species. C was transferred across all four tree species combinations. While Tomentella ellisii was the primary fungal mediator between pines and oaks, Terfezia pini, Pustularia spp., and Tuber oligospermum controlled C transfer among pines. We demonstrate at a high temporal, quantitative, and taxonomic resolution, that C from EMF host trees moved into EMF and that C was transferred further to neighboring trees of similar and distinct phylogenies.

Origin makes a difference: Alternative responses of an AM-dependent plant to mycorrhizal inoculum from invaded and native soils under abiotic stress

The presence of invasive alien plants (IAPs) alters the composition of soil arbuscular mycorrhizal (AM) fungal communities. Although fundamental for plant development, plant responses to AM from invaded soils have not been widely explored, especially under environmental stress. We compared plant growth, P accumulation, root colonization and the photosynthetic responses of the native AM-dependent Plantago lanceolata growing in contact with AM fungi from communities invaded by Acacia dealbata Link (AMinv) or non-invaded communities (AMnat) exposed to water and light restriction (shade). Under optimal growing conditions, plants in contact with AMnat produced higher leaf biomass and accumulated more P. However, plant responses to different AM inocula varied as the level of stress increased. Inoculation with AMinv promoted plant growth and root length under light restriction. When plants grew in contact with AMnat under drought, leaf P increased under severe water restriction, and leaf and root P increased under intermediate water irrigation. Growing in contact with the AMnat inoculum promoted root P content in both full light and light restriction. Colonization rates of P. lanceolata roots were comparable between treatments, and plants maintained photosynthetic activity within similar ranges, regardless of the level of stress applied. Our results suggest that origin of the inoculum (native soils versus invaded soils) did not affect the ability of AM species therein to establish effective mutualistic associations with P. lanceolata roots but did influence plant responses depending on the type and level of the abiotic stress.

Multimodal correlative imaging and modelling of phosphorus uptake from soil by hyphae of mycorrhizal fungi

Phosphorus (P) is essential for plant growth. Arbuscular mycorrhizal fungi (AMF) aid its uptake by acquiring P from sources distant from roots in return for carbon. Little is known about how AMF colonise soil pore-space, and models of AMF-enhanced P-uptake are poorly validated. We used synchrotron X-ray computed tomography to visualize mycorrhizas in soil and synchrotron X-ray fluorescence/X-ray absorption near edge structure (XRF/XANES) elemental mapping for P, sulphur (S) and aluminium (Al) in combination with modelling. We found that AMF inoculation had a suppressive effect on colonisation by other soil fungi and identified differences in structure and growth rate between hyphae of AMF and nonmycorrhizal fungi. Our results showed that AMF co-locate with areas of high P and low Al, and preferentially associate with organic-type P species over Al-rich inorganic P. We discovered that AMF avoid Al-rich areas as a source of P. Sulphur-rich regions were found to be correlated with higher hyphal density and an increased organic-associated P-pool, whilst oxidized S-species were found close to AMF hyphae. Increased S oxidation close to AMF suggested the observed changes were microbiome-related. Our experimentally-validated model led to an estimate of P-uptake by AMF hyphae that is an order of magnitude lower than rates previously estimated - a result with significant implications for the modelling of plant-soil-AMF interactions.

Arbuscular Mycorrhizal Fungi Are an Influential Factor in Improving the Phytoremediation of Arsenic, Cadmium, Lead, and Chromium

The increasing expansion of mines, factories, and agricultural lands has caused many changes and pollution in soils and water of several parts of the world. In recent years, metal(loid)s are one of the most dangerous environmental pollutants, which directly and indirectly enters the food cycle of humans and animals, resulting in irreparable damage to their health and even causing their death. One of the most important missions of ecologists and environmental scientists is to find suitable solutions to reduce metal(loid)s pollution and prevent their spread and penetration in soil and groundwater. In recent years, phytoremediation was considered a cheap and effective solution to reducing metal(loid)s pollution in soil and water. Additionally, the effect of soil microorganisms on increasing phytoremediation was given special attention; therefore, this study attempted to investigate the role of arbuscular mycorrhizal fungus in the phytoremediation system and in reducing contamination by some metal(loid)s in order to put a straightforward path in front of other researchers.

Karst rocky desertification diverged the soil residing and the active ectomycorrhizal fungal communities thereby fostering distinctive extramatrical mycelia

Ectomycorrhizal fungi (EMF) are mutualists that play crucial roles in liberation, nutrient acquisition, transfer of growth-limiting resources and provision of water to host plants in terrestrial ecosystems, particularly in stressed prone climates. In this study, a field-based experiment was performed in Yunnan, China to assess the effect of karst rocky desertification (KRD) and natural forests (non-KRD) sites on the richness and composition of EMF communities. Inert sand-filled mesh bags were employed to characterize the active EMF and quantify the production of extramatrical mycelium (EMM). Results indicated that, EMF exhibited a significant differentiation among KRD and non-KRD sites, richness and diversity were higher across KRD areas, whereas the evenness showed the opposite trend. Ascomycota and Zygomycota were greater across KRD sites, however, Basidiomycota showed no difference across both study sites. The relative abundance of Clavaria, Butyriboletus, Odontia, Phyloporus, Helvella, Russula and Tomentella were higher across the KRD sites, whereas, Clavulinopsis, Endogone, Amanita, Inocybe and Clavulina were higher across the non-KRD sites. It's worth noting that, saprophytic (SAP) fungal community was found to be more abundant in the soil than the mesh bags at both sites particularly at KRD sites, which likely provide more free space and less competition for the EMF to thrive well in the mesh bags. In similar pattern, ergosterol concentration in mesh bags was observed relatively higher at KRD sites than the non-KRD sites. The Entoloma, Amanita, and Sebacina were found to be substantially higher in mesh bags than soil across both sites. Delicatula, Helvella and Tomentella on the other hand, showed higher relative abundance in mesh bags than soil over KRD sites, however they did not differ across non-KRD sites. Taken together, the presented results highlight relationship between the EMF community and the complex KRD environment, which is very important for the restoration of disturbed karst landscapes.

Old growth forests and large old trees as critical organisms connecting ecosystems and human health. A review

Old forests containing ancient trees are essential ecosystems for life on earth. Mechanisms that happen both deep in the root systems and in the highest canopies ensure the viability of our planet. Old forests fix large quantities of atmospheric CO2, produce oxygen, create micro-climates and irreplaceable habitats, in sharp contrast to young forests and monoculture forests. The current intense logging activities induce rapid, adverse effects on our ecosystems and climate. Here we review large old trees with a focus on ecosystem preservation, climate issues, and therapeutic potential. We found that old forests continue to sequester carbon and fix nitrogen. Old trees control below-ground conditions that are essential for tree regeneration. Old forests create micro-climates that slow global warming and are irreplaceable habitats for many endangered species. Old trees produce phytochemicals with many biomedical properties. Old trees also host particular fungi with untapped medicinal potential, including the Agarikon, Fomitopsis officinalis, which is currently being tested against the coronavirus disease 2019 (COVID-19). Large old trees are an important part of our combined cultural heritage, providing people with aesthetic, symbolic, religious, and historical cues. Bringing their numerous environmental, oceanic, ecological, therapeutic, and socio-cultural benefits to the fore, and learning to appreciate old trees in a holistic manner could contribute to halting the worldwide decline of old-growth forests.

Soil spore bank communities of ectomycorrhizal fungi in Pseudotsuga japonica forests and neighboring plantations

Ectomycorrhizal (EcM) fungal spores play an important role in seedling establishment and forest regeneration, especially in areas where compatible host tree species are absent. However, compared to other Pinaceae trees with a wide distribution, limited information is available for the interaction between the endangered Pseudotsuga trees and EcM fungi, especially the spore bank. The aim of this study was to investigate EcM fungal spore bank communities in soil in remnant patches of Japanese Douglas-fir (Pseudotsuga japonica) forest. We conducted a bioassay of 178 soil samples collected from three P. japonica forests and their neighboring arbuscular mycorrhizal artificial plantations, using the more readily available North American Douglas-fir (Pseudotsuga menziesii) as bait seedlings. EcM fungal species were identified by a combination of morphotyping and DNA sequencing of the ITS region. We found that EcM fungal spore banks were present not only in P. japonica forests but also in neighboring plantations. Among the 13 EcM fungal species detected, Rhizopogon togasawarius had the second highest frequency and was found in all plots, regardless of forest type. Species richness estimators differed significantly among forest types. The community structure of EcM fungal spore banks differed significantly between study sites but not between forest types. These results indicate that EcM fungal spore banks are not restricted to EcM forests and extend to surrounding forest dominated by arbuscular mycorrhizal trees, likely owing to the durability of EcM fungal spores in soils.

Harnessing Synergistic Biostimulatory Processes: A Plausible Approach for Enhanced Crop Growth and Resilience in Organic Farming

Simple Summary

Demand for organically grown crops has risen globally due to its healthier and safer food products. From a sustainability perspective, organic farming offers an eco-friendly cultivation system that minimizes agrochemicals and producing food with little or no environmental footprint. However, organic agriculture’s biggest drawback is the generally lower and variable yield in contrast to conventional farming. Compatible with organic farming, the selective use of biostimulants can close the apparent yield gap between organic and conventional cultivation systems. A biostimulant is defined as natural microorganisms (bacteria, fungi) or biologically active substances that are able to improve plant growth and yield through several processes. Biostimulants are derived from a range of natural resources including organic materials (composts, seaweeds), manures (earthworms, fish, insects) and extracts derived from microbes, plant, insect or animal origin. The current trend is indicative that a mixture of biostimulants is generally delivering better growth, yield and quality rather than applying biostimulant individually. When used correctly, biostimulants are known to help plants cope with stressful situations like drought, salinity, extreme temperatures and even certain diseases. More research is needed to understand the different biostimulants, key components, and also to adjust the formulations to improve their reliability in the field.

Abstract

Demand for organically grown food crops is rising substantially annually owing to their contributions to human health. However, organic farm production is still generally lower compared to conventional farming. Nutrient availability, content consistency, uptake, assimilation, and crop responses to various stresses were reported as critical yield-limiting factors in many organic farming systems. In recent years, plant biostimulants (BSs) have gained much interest from researchers and growers, and with the objective of integrating these products to enhance nutrient use efficiency (NUE), crop performance, and delivering better stress resilience in organic-related farming. This review gave an overview of direct and indirect mechanisms of microbial and non-microbial BSs in enhancing plant nutrient uptake, physiological status, productivity, resilience to various stressors, and soil-microbe-plant interactions. BSs offer a promising, innovative and sustainable strategy to supplement and replace agrochemicals in the near future. With greater mechanistic clarity, designing purposeful combinations of microbial and non-microbial BSs that would interact synergistically and deliver desired outcomes in terms of acceptable yield and high-quality products sustainably will be pivotal. Understanding these mechanisms will improve the next generation of novel and well-characterized BSs, combining microbial and non-microbial BSs strategically with specific desired synergistic bio-stimulatory action, to deliver enhanced plant growth, yield, quality, and resilience consistently in organic-related cultivation.

Plant and seed germination responses to global change, with a focus on CO2: A review

Earth atmospheric CO2 concentration has risen by over 35% since 1750 and is presently increasing by about 2 parts per million (ppm) every year. Due to contributions from human activity, CO2 is projected to keep rising in the predictable future and to double sometime during this century if fossil fuels burning remains. As a result, air temperature is projected to rise from 2 to 5 °C by 2100. Following this rise in CO2, some ecosystems will face challenges in the next few decades as plants will live in warmer temperatures, higher evaporating demand and widespread changes in drought lengths and severity. To yield healthy crops and forests in changing climate surroundings, it is vital to define whether elevated CO2 disturbs seed germination and plant formation, but even more, the physiological traits conferring drought tolerance. Here, we review the current understanding on the role that CO2 plays on plant growth and seed germination, as well as its impact during the exposure of abiotic stresses like drought and salinity.

Environmental drivers of forest biodiversity in temperate mixed forests - A multi-taxon approach

Harmonization of timber production and forest conservation is a major challenge of modern silviculture. For the establishment of ecologically sustainable forest management, the management-related environmental drivers of multi-taxon biodiversity should be explored. Our study reveals those environmental variables related to tree species diversity and composition, stand structure, litter and soil conditions, microclimate, landscape, and land-use history that determine species richness and composition of 11 forest-dwelling organism groups. Herbs, woody regeneration, ground-floor and epiphytic bryophytes, epiphytic lichens, terricolous saprotrophic, ectomycorrhizal, and wood-inhabiting macrofungi, spiders, carabid beetles, and birds were sampled in West Hungarian mature mixed forests. The correlations among the diversities and compositions of different organism groups were also evaluated. Drivers of organism groups were principally related to stand structure, tree species diversity and composition, and microclimate, while litter, soil, landscape, and land-use historical variables were less influential. The complex roles of the shrub layer, deadwood, and the size of the trees in determining the diversity and composition of various taxa were revealed. Stands with more tree species sustained higher stand-level species richness of several taxa. Besides, stands with different dominant tree species harbored various species communities of organism groups. Therefore, landscape-scale diversity of dominant tree species may enhance the diversity of forest-dwelling communities at landscape level. The effects of the overstory layer on forest biodiversity manifested in many cases via microclimate conditions. Diversity of organism groups showed weaker relationship with the diversity of other taxa than with environmental variables. According to our results, the most influential drivers of forest biodiversity are under the direct control of the actual silvicultural management. Heterogeneous stand structure and tree species composition promote the different organism groups in various ways. Therefore, the long-term maintenance of the structural and compositional heterogeneity both at stand and landscape scale is an important aspect of ecologically sustainable forest management.

Root-associated fungal community reflects host spatial co-occurrence patterns in a subtropical forest

Plant roots harbor and interact with diverse fungal species. By changing these belowground fungal communities, focal plants can affect the performance of surrounding individuals and the outcome of coexistence. Although highly host related, the roles of these root-associated fungal communities per se in host plant spatial co-occurrence is largely unknown. Here, we evaluated the host dependency of root-associated communities for 39-plant species spatially mapped throughout a 50-ha subtropical forest plot with relevant environmental properties. In addition, we explored whether the differentiation in root fungal associations among plant species can reflect their observed co-occurrence patterns. We demonstrated a strong host-dependency by discriminating the differentiation of root-associated fungal communities regardless of background soil heterogeneity. Furthermore, Random Forest modeling indicated that these nonrandom root fungal associations significantly increased our ability to explain spatial co-occurrence patterns, and to a greater degree than the relative abundance, phylogenetic relatedness, and functional traits of the host plants. Our results further suggested that plants harbor more abundant shared, "generalist" pathogens are likely segregated, while hosting more abundant unique, "specialist" ectomycorrhizal fungi might be an important strategy for promoting spatial aggregation, particularly between early established trees and the heterospecific adults. Together, we provide a conceptual and testable approach to integrate this host-dependent root fungal "fingerprinting" into the plant diversity patterns. We highlight that this approach is complementary to the classic cultivation-based scheme and can deepen our understanding of the community-level effect from overall fungi and its contribution to the pairwise plant dynamics in local species-rich communities.

Heterospecific Neighbor Plants Impact Root Microbiome Diversity and Molecular Function of Root Fungi

Within the forest community, competition and facilitation between adjacent-growing conspecific and heterospecific plants are mediated by interactions involving common mycorrhizal networks. The ability of plants to alter their neighbor's microbiome is well documented, but the molecular biology of plant-fungal interactions during competition and facilitation has not been previously examined. We used a common soil-plant bioassay experiment to study molecular plant-microbial interactions among rhizosphere communities associated with Pinus taeda (native host) and Populus trichocarpa (non-native host). Gene expression of interacting fungal and bacterial rhizosphere communities was compared among three plant-pairs: Populus growing with Populus, Populus with Pinus, and Pinus with Pinus. Our results demonstrate that heterospecific plant partners affect the assembly of root microbiomes, including the changes in the structure of host specific community. Comparative metatranscriptomics reveals that several species of ectomycorrhizal fungi (EMF) and saprotrophic fungi exhibit different patterns of functional and regulatory gene expression with these two plant hosts. Heterospecific plants affect the transcriptional expression pattern of EMF host-specialists (e.g., Pinus-associated Suillus spp.) on both plant species, mainly including the genes involved in the transportation of amino acids, carbohydrates, and inorganic ions. Alteration of root microbiome by neighboring plants may help regulate basic plant physiological processes via modulation of molecular functions in the root microbiome.

Plant Communication

Communication occurs when a sender emits a cue perceived by a receiver that changes the receiver's behavior. Plants perceive information regarding light, water, other nutrients, touch, herbivores, pathogens, mycorrhizae, and nitrogen-fixing bacteria. Plants also emit cues perceived by other plants, beneficial microbes, herbivores, enemies of herbivores, pollinators, and seed dispersers. Individuals responding to light cues experienced increased fitness. Evidence for benefits of responding to cues involving herbivores and pathogens is more limited. The benefits of emitting cues are also less clear, particularly for plant–plant communication. Reliance on multiple or dosage-dependent cues can reduce inappropriate responses, and plants often remember past cues. Plants have multiple needs and prioritize conflicting cues such that the risk of abiotic stress is treated as greater than that of shading, which is in turn treated as greater than that of consumption. Plants can distinguish self from nonself and kin from strangers. They can identify the species of competitor or consumer and respond appropriately. Cues involving mutualists often contain highly specific information.

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.

Consciousness and cognition in plants

Unlike animal behavior, behavior in plants is traditionally assumed to be completely determined either genetically or environmentally. Under this assumption, plants are usually considered to be noncognitive organisms. This view nonetheless clashes with a growing body of empirical research that shows that many sophisticated cognitive capabilities traditionally assumed to be exclusive to animals are exhibited by plants too. Yet, if plants can be considered cognitive, even in a minimal sense, can they also be considered conscious? Some authors defend that the quest for plant consciousness is worth pursuing, under the premise that sentience can play a role in facilitating plant's sophisticated behavior. The goal of this article is not to provide a positive argument for plant cognition and consciousness, but to invite a constructive, empirically informed debate about it. After reviewing the empirical literature concerning plant cognition, we introduce the reader to the emerging field of plant neurobiology. Research on plant electrical and chemical signaling can help shed light into the biological bases for plant sentience. To conclude, we shall present a series of approaches to scientifically investigate plant consciousness. In sum, we invite the reader to consider the idea that if consciousness boils down to some form of biological adaptation, we should not exclude a priori the possibility that plants have evolved their own phenomenal experience of the world. This article is categorized under: Cognitive Biology > Evolutionary Roots of Cognition Philosophy > Consciousness Neuroscience > Cognition.

Retention of Matured Trees to Conserve Fungal Diversity and Edible Sporocarps from Short-Rotation Pinus radiata Plantations in Ethiopia

This study is conducted in the short-rotation plantations from the Afromontane Region of Ethiopia. Sporocarps were sampled weekly in a set of permanent plots (100 m²) in young, medium-aged, and mature Pinus radiata (Don) plantations. Fungal richness, diversity, and sporocarp yields were estimated. Composite soil samples were also collected from each plot to determine explanatory edaphic variables for taxa composition. We collected 92 fungal taxa, of which 8% were ectomycorrhizal (ECM). Taxa richness, the Shannon diversity index, and ECM species richness were higher in mature stands. Interestingly, 26% of taxa were classified as edible. Sporocarp yield showed increasing trends towards matured stands. OM and C/N ratio significantly affected fungal composition and sporocarp production. The deliberate retention of mature trees in a patch form rather than clear felling of the plantations could be useful to conserve and promote fungal diversity and production, including valuable taxa such as Morchella, Suillus, and Tylopilus in older stands. This approach has important implications for forest floor microhabitats, which are important for macrofungal occurrence and production. Thus, this strategy could improve the economic outputs of these plantations in the Afromontane Region, while the mature trees could serve as a bridge for providing fungal inocula to the new plantations.

Tree mycorrhizal type and tree diversity shape the forest soil microbiota

There is limited knowledge on how the association of trees with different mycorrhizal types shapes soil microbial communities in the context of changing tree diversity levels. We used arbuscular (AM) and ectomycorrhizal (EcM) tree species as con- and heterospecific tree species pairs (TSPs), which were established in plots of three tree diversity levels including monocultures, two-species mixtures and multi-tree species mixtures in a tree diversity experiment in subtropical China. We found that the tree mycorrhizal type had a significant effect on fungal but not bacterial alpha diversity. Furthermore, only EcM but not AM TSPs fungal alpha diversity increased with tree diversity, and the differences between AM and EcM TSPs disappeared in multi-species mixtures. Tree mycorrhizal type, tree diversity and their interaction had significant effects on fungal community composition. Neither fungi nor bacteria showed any significant compositional variation in TSPs located in multi-species mixtures. Accordingly, the most influential taxa driving the tree mycorrhizal differences at low tree diversity were not significant in multi-tree species mixtures. Collectively, our results indicate that tree mycorrhizal type is an important factor determining the diversity and community composition of soil microbes, and higher tree diversity levels promote convergence of the soil microbial communities. SIGNIFICANCE STATEMENT: More than 90% of terrestrial plants have symbiotic associations with mycorrhizal fungi which could influence the coexisting microbiota. Systematic understanding of the individual and interactive effects of tree mycorrhizal type and tree species diversity on the soil microbiota is crucial for the mechanistic comprehension of the role of microbes in forest soil ecological processes. Our tree species pair (TSP) concept coupled with random sampling within and across the plots, allowed us the unbiased assessment of tree mycorrhizal type and tree diversity effects on the tree-tree interaction zone soil microbiota. Unlike in monocultures and two-species mixtures, we identified species-rich and converging fungal and bacterial communities in multi-tree species mixtures. Consequently, we recommend planting species-rich mixtures of EcM and AM trees, for afforestation and reforestation regimes. Specifically, our findings highlight the significance of tree mycorrhizal type in studying 'tree diversity - microbial diversity - ecosystem function' relationships.

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.

Management After Windstorm Affects the Composition of Ectomycorrhizal Symbionts of Regenerating Trees but Not Their Mycorrhizal Networks

Due to ongoing climate change, forests are expected to face significant disturbances more frequently than in the past. Appropriate management is intended to facilitate forest regeneration. Because European temperate forests mostly consist of trees associated with ectomycorrhizal (ECM) fungi, understanding their role in these disturbances is important to develop strategies to minimize their consequences and effectively restore forests. Our aim was to determine how traditional (EXT) and nonintervention (NEX) management in originally Norway spruce (Picea abies) forests with an admixture of European larch (Larix decidua) affect ECM fungal communities and the potential to interconnect different tree species via ECM networks 15 years after a windstorm. Ten plots in NEX and 10 plots in EXT with the co-occurrences of Norway spruce, European larch, and silver birch (Betula pendula) were selected, and a total of 57 ECM taxa were identified using ITS sequencing from ECM root tips. In both treatments, five ECM species associated with all the studied tree species dominated, with a total abundance of approximately 50% in the examined root samples. Because there were no significant differences between treatments in the number of ECM species associated with different tree species combinations in individual plots, we concluded that the management type did not have a significant effect on networking. However, management significantly affected the compositions of ECM symbionts of Norway spruce and European larch but not those of silver birch. Although this result is explained by the occurrence of seedlings and ECM propagules that were present in the original forest, the consequences are difficult to assess without knowledge of the ecology of different ECM symbionts.

Diesel fuel differentially affects hyphal healing in Gigaspora sp. and Rhizophagus irregularis

Hydrocarbon pollution is an increasing problem affecting soil ecosystems. However, some microorganisms can cope with these pollutants and even facilitate plant establishment and thus phytoremediation. Within soil, arbuscular mycorrhizal fungi (AMF) have developed several strategies to survive and flourish under adverse conditions. Among these is the hyphal healing mechanism (HHM), a process allowing hyphae to re-establish integrity after physical injury. This mechanism differs among species and genera of AMF. However, whether and to what extent hydrocarbon pollution impacts the HHM is unknown. Here, the HHM was monitored in vitro on two AMF strains, Rhizophagus irregularis MUCL 41833 and Gigaspora sp. MUCL 52331, under increasing concentrations of diesel (1, 2, and 5% v:v). The addition of diesel slowed-down the HHM in both fungi. On Gigaspora sp., this effect was limited and most hyphae were able to heal after injury. Conversely, all steps of healing were severely impaired in R. irregularis. That fungus reconnected the injured hyphae at a much lower frequency than the Gigaspora sp., instead investing its energy to link neighboring hyphae or roots, or developing new branches from uninjured hyphae.

Functions of mineral-solubilizing microbes and a water retaining agent for the remediation of abandoned mine sites

There has been a rapid increase in abandoned mines across China, Consequently, external-soil spray seeding technologies have emerged as a common method for their remediation. However, slope soils are typically unstable and easily collapsed and the nutrients absorbed by plants are insufficient, which complicate ecological restoration. For this study, we added mineral-solubilizing microbes and a water retaining agent to an external-soil spray seeding substrate in Lespedeza bicolor pots. We investigated the soil nutrients, soil enzyme activities, root growth parameters, root tensile properties, and root-reinforced soil shear strengths. The results revealed that the addition of microbes enhanced soil nutrients, soil enzyme activities, and the content of lignin and hemicellulose, which promoted root growth. Further, the addition of a water retaining agent promoted Lespedeza bicolor root growth but decreased the root tensile strength and force. Shear stress under the microbe treatment was more robust than without it. Finally, root growth was correlated with soil nutrients and enzyme activities, whereas the root tensile force and strength were correlated with lignin and cellulose. Our results suggested that the addition of mineral-solubilizing microbes had the capacity to enhance the quality of soils to facilitate the growth of plants. These results provide a new and viable strategy for the ecological restoration of abandon mine sites.

Hyphal and mycelial consciousness: the concept of the fungal mind

Like other cells, fungal hyphae show exquisite sensitivity to their environment. This reactiveness is demonstrated at many levels, from changes in the form of the hypha resulting from alterations in patterns of exocytosis, to membrane excitation, and mechanisms of wound repair. Growing hyphae detect ridges on surfaces and respond to restrictions in their physical space. These are expressions of cellular consciousness. Fungal mycelia show decision-making and alter their developmental patterns in response to interactions with other organisms. Mycelia may even be capable of spatial recognition and learning coupled with a facility for short-term memory. Now is a fruitful time to recognize the study of fungal ethology as a distinctive discipline within mycology.

Compatible Mycorrhizal Types Contribute to a Better Design for Mixed Eucalyptus Plantations

Mixed-species forest plantation is a sound option to facilitate ecological restoration, plant diversity and ecosystem functions. Compatible species combinations are conducive to reconstruct plant communities that can persist at a low cost without further management and even develop into natural forest communities. However, our understanding of how the compatibility of mycorrhizal types mediates species coexistence is still limited, especially in a novel agroforestry system. Here, we assessed the effects of mycorrhizal association type on the survival and growth of native woody species in mixed-species Eucalyptus plantations. To uncover how mycorrhizal type regulates plant-soil feedbacks, we first conducted a pot experiments by treating distinct mycorrhizal plants with soil microbes from their own or other mycorrhizal types. We then compared the growth response of arbuscular mycorrhizal plants and ectomycorrhizal plants to different soil microbial compositions associated with Eucalyptus plants. We found that the type of mycorrhizal association had a significant impact on the survival and growth of native tree species in the Eucalyptus plantations. The strength and direction of the plant-soil feedbacks of focal tree species depended on mycorrhizal type. Non-mycorrhizal plants had consistent negative feedbacks with the highest survival in the Eucalyptus plantations, whereas nitrogen-fixing plants had consistent positive feedbacks and the lowest survival. Arbuscular mycorrhizal and ectomycorrhizal plants performed varied feedback responses to soil microbes from distinct mycorrhizal plant species. Non-mycorrhizal plants grew better with Eucalyptus soil microbes while nitrogen-fixing plants grew worse with their own conspecific soil microbes. Different soil microbial compositions of Eucalyptus consistently increased the aboveground growth of arbuscular mycorrhizal plants, but the non-mycorrhizal microbial composition of the Eucalyptus soil resulted in greater belowground growth of ectomycorrhizal plants. Overall, Eucalyptus plants induced an unfavorable soil community, impeding coexistence with other mycorrhizal plants. Our study provides consistent observational and experimental evidence that mycorrhizal-mediated plant-microbial feedback on species coexistence among woody species. These findings are with important implications to optimize the species combinations for better design of mixed forest plantations.

Revealing hidden drivers of macrofungal species richness by analyzing fungal guilds in temperate forests, West Hungary

We explored the most influential stand-scaled drivers of ectomycorrhizal, terricolous saprotrophic, and wood-inhabiting (main functional groups) macrofungal species richness in mixed forests by applying regression models. We tested 67 potential explanatory variables representing tree species composition, stand structure, soil and litter conditions, microclimate, landscape structure, and management history. Within the main functional groups, we formed and modeled guilds and used their drivers to more objectively interpret the drivers of the main functional groups. Terricolous saprotrophic fungi were supported by air humidity and litter mass. Ectomycorrhizal fungi were suppressed by high soil nitrogen content and high air temperature. Wood saprotrophs were enhanced by litter pH (deciduous habitats), deadwood cover, and beech proportion. Wood saprotrophic guilds were determined often by drivers with hidden effects on all wood saprotrophs: non-parasites: total deadwood cover; parasites: beech proportion; white rotters: litter pH; brown rotters: air temperature (negatively); endophytes: beech proportion; early ruderals: deciduous stands that were formerly meadows; combative invaders: deciduous tree taxa; heart rotters: coarse woody debris; late stage specialists: deciduous deadwood. Terricolous saprotrophic cord formers positively responded to litter mass. Studying the drivers of guilds simultaneously, beech was a keystone species to maintain fungal diversity in the region, and coniferous stands would be more diverse by introducing deciduous tree species. Guilds were determined by drivers different from each other underlining their different functional roles and segregated substrate preferences. Modeling guilds of fungal species with concordant response to the environment would be powerful to explore and understand the functioning of fungal communities.

Mycorrhizal type influences plant density dependence and species richness across 15 temperate forests

Recent studies suggest that the mycorrhizal type associated with tree species is an important trait influencing ecological processes such as response to environmental conditions and conspecific negative density dependence (CNDD). However, we lack a general understanding of how tree mycorrhizal type influences CNDD strength and the resulting patterns of species abundance and richness at larger spatial scales. We assessed 305 species across 15 large, stem-mapped, temperate forest dynamics plots in Northeastern China and North America to explore the relationships between tree mycorrhizal type and CNDD, species abundance, and species richness at a regional scale. Tree species associated with arbuscular mycorrhizal (AM) fungi showed a stronger CNDD and a more positive relationship with species abundance than did tree species associated with ectomycorrhizal (ECM) fungi. For each plot, both basal area and stem abundance of AM tree species was lower than that of ECM tree species, suggesting that AM tree species were rarer than ECM tree species. Finally, ECM tree dominance showed a negative effect on plant richness across plots. These results provide evidence that tree mycorrhizal type plays an important role in influencing CNDD and species richness, highlighting this trait as an important factor in structuring plant communities in temperate forests.

Role of environmental factors in shaping the soil microbiome

The soil microbiome comprises one of the most important and complex components of all terrestrial ecosystems as it harbors millions of microbes including bacteria, fungi, archaea, viruses, and protozoa. Together, these microbes and environmental factors contribute to shaping the soil microbiome, both spatially and temporally. Recent advances in genomic and metagenomic analyses have enabled a more comprehensive elucidation of the soil microbiome. However, most studies have described major modulators such as fungi and bacteria while overlooking other soil microbes. This review encompasses all known microbes that may exist in a particular soil microbiome by describing their occurrence, abundance, diversity, distribution, communication, and functions. Finally, we examined the role of several abiotic factors involved in the shaping of the soil microbiome.

Response of the arbuscular mycorrhizal fungi diversity and community in maize and soybean rhizosphere soil and roots to intercropping systems with different nitrogen application rates

Maize (Zea mays L.)/soybean (Glycine max L.) intercropping has been widely practiced in China, because of its effectiveness in improving crop yield and nitrogen utilization efficiency. However, the responses of indigenous arbuscular mycorrhizal fungal (AMF) diversity and communities in rhizosphere soil and roots to intercropping systems with different nitrogen application rates remain unclear. In this study, a field experiment was conducted with split-plot design, and AMF communities in crop rhizosphere soil and roots in monoculture and intercropping systems treated with different levels of nitrogen fertilization were investigated using Illumina MiSeq sequencing. Nitrogen fertilization significantly decreased the AMF alpha-diversity in maize rhizosphere soil, and no significant differences were observed between monocultured and intercropped maize. The Shannon index of soybean rhizosphere soil was significantly higher in intercropping treatments than in monoculture treatments for the corresponding nitrogen levels. The AMF diversity in the roots of maize showed different trends to those in the soil. The dominant genera in the present study were Glomus_f_Glomeraceae, Paraglomus, and Gigaspora, which occupied 55.52%, 9.18%, and 8.20% of the rhizosphere soil and 65.35%, 5.32%, and 17.16% of the roots, respectively. Our study showed that the abundance of the dominant genus, Glomus_f_Glomeraceae in maize soil and roots significantly increased in intercropping treatments compared with monoculture treatments, and it also increased with the increase in nitrogen application levels. In soybean soil and roots, the abundance of Glomus_f_Glomeraceae decreased with the increase in nitrogen application levels. The results of the redundancy and correlations analyses indicated that the changes in the AMF diversity and community in intercropping areas were significantly associated with alterations of the soil total nitrogen and alkali-hydrolysable nitrogen due to the interactions between maize and soybeans in intercropping systems with different nitrogen fertilizer application rates.

Introduced Populations of an Invasive Tree Have Higher Soluble Sugars but Lower Starch and Cellulose

Native and introduced plant populations vary in leaf physiology, biochemistry, and biotic interactions. These aboveground traits may help invasive plants in competition for resources with co-occurring native species. Root physiological traits may affect invasive plant performance because of the roles of roots in resource absorption. The aim of this study was to test this prediction, using invasive Chinese tallow tree (Triadica sebifera), as a model species. Here we examined carbohydrate (soluble sugar, sucrose, fructose, starch, and cellulose) concentrations and the mass of roots, stems, and leaves, along with root water potential and arbuscular mycorrhizal fungi (AMF) colonization of soil-cultured T. sebifera seedlings from 10 native (China) and 10 introduced (United States) populations in a common garden. Introduced populations had a significantly greater stem and leaf mass than native populations but their root masses did not differ, so they had lower R:S. Introduced populations had higher soluble sugar concentrations but lower starch and cellulose concentrations in their leaves, stems, and roots. Introduced populations had more negative root water potentials and higher AMF colonization. Together, our results indicate that invasive plants shift their carbohydrate allocation, leading to faster growth and a greater aboveground allocation strategy. Higher AMF colonization and more negative water potential in invasive plants likely facilitate more efficient water absorption by the roots. Thus, such physiological variation in root characteristics could play a role in plant invasion success.

Arbuscular mycorrhizal fungi reduce cadmium leaching from polluted soils under simulated heavy rainfall

Cadmium (Cd)-polluted soils were collected from wasteland, farmland, and slopeland surrounding a lead-zinc mine in Yunnan Province, Southwest China. Maize plants (the host) were inoculated with arbuscular mycorrhizal fungi (AMF) in a dual-compartment cultivation system that included mycorrhizal and hyphal compartments as part of an AMF inoculation treatment and root and soil compartments as part of a the non-inoculation treatment. The effects of AMF on maize biomass and Cd uptake, soil aggregate composition, and Cd concentration in the interflow within two soil layers (0-20 and 20-40 cm) as well as the Cd leaching from these three Cd-polluted soils under simulated heavy rainfall (40 and 80 mm/h) were investigated. The results demonstrated that AMF led to increased maize biomass and Cd uptake. There were greater contents of total glomalin-related soil protein (T-GRSP) and >2.0 mm aggregates and lower Cd concentrations in the interflow and lower dissolved Cd leaching in the mycorrhizal and hyphal compartments than in the soil compartment. A two-way analysis of variance revealed that AMF significantly increased the contents of T-GRSP and >2.0 mm aggregates and reduced both Cd concentrations in the interflow and dissolved Cd leaching. Moreover, AMF interacted extensively with the roots and affected soil aggregate composition and Cd concentrations in the interflow. Under 40 mm/h of rainfall, the contents of T-GRSP and >2.0 mm aggregates were significantly negatively correlated with both Cd concentrations in the interflow and dissolved Cd leaching. In addition, the Cd concentrations in the interflow were significantly positively correlated with the amount of dissolved Cd leaching. Therefore, both AMF-reduced Cd concentrations in the interflow and Cd leaching from Cd-polluted soils were closely related to increased T-GRSP contents and macroaggregate proportion in the soils.

Unique and common traits in mycorrhizal symbioses

Mycorrhizas are among the most important biological interkingdom interactions, as they involve ~340,000 land plants and ~50,000 taxa of soil fungi. In these mutually beneficial interactions, fungi receive photosynthesis-derived carbon and provide the host plant with mineral nutrients such as phosphorus and nitrogen in exchange. More than 150 years of research on mycorrhizas has raised awareness of their biology, biodiversity and ecological impact. In this Review, we focus on recent phylogenomic, molecular and cell biology studies to present the current state of knowledge of the origin of mycorrhizal fungi and the evolutionary history of their relationship with land plants. As mycorrhizas feature a variety of phenotypes, depending on partner taxonomy, physiology and cellular interactions, we explore similarities and differences between mycorrhizal types. During evolution, mycorrhizal fungi have refined their biotrophic capabilities to take advantage of their hosts as food sources and protective niches, while plants have developed multiple strategies to accommodate diverse fungal symbionts. Intimate associations with pervasive ecological success have originated at the crossroads between these two evolutionary pathways. Our understanding of the biological processes underlying these symbioses, where fungi act as biofertilizers and bioprotectors, provides the tools to design biotechnological applications addressing environmental and agricultural challenges.

Mycorrhizas transfer carbon in a mature mixed forest

Mycorrhizal fungi transfer nutrients to plants in exchange for photosynthates. Plants allocate up to 20% of their carbon to mycorrhizal structures, mycelium and fruit bodies of their fungal partners. Individuals of mycorrhizal fungi may encompass hundreds of square metres of soil and defragmented litter, linking multiple plant individuals of different species and size (Figure 1). Using a free-air ¹³ CO₂ enrichment (web-FACE) technique in a mature forest, interspecific transfer accounted for 40% of fine root carbon after 5 years of back and forth transfer between trees. In this issue of Molecular Ecology, Rog, Rosenstock, Körner, and Klein (2020) show that closely related trees shared relatively more mycorrhizal fungi than distantly related trees in the same experimental site, which correlated to increased carbon sharing.

Microbiome Variation Across Two Hemlock Species With Hemlock Woolly Adelgid Infestation

The hemlock woolly adelgid (Adelges tsugae, HWA), an invasive insect, is devastating native hemlock populations in eastern North America, and management outcomes have so far had limited success. While many plant microbiomes influence and even support plant immune responses to insect herbivory, relatively little is known about the hemlock microbiome and its interactions with pathogens or herbivores such as HWA. Using 16S rRNA and ITS gene amplicon sequencing, we characterized the needle, branch, root, and rhizosphere microbiome of two hemlock species, Tsuga canadensis and T. sieboldii, that displayed low and high levels of HWA populations. We found that both archaeal/bacterial and fungal needle communities, as well as the archaeal/bacterial branch and root communities, varied in composition in both hemlock species relative to HWA population levels. While host species and plant-associated habitats explained a greater proportion of the variance in the microbiome than did HWA population level, high HWA populations were associated with enrichment of 100 likely fungal pathogen sequence variants across the four plant-associated habitats (e.g., needle, branch, root, rhizosphere) compared to trees with lower HWA populations. This work contributes to a growing body of literature linking plant pathogens and pests with the changes in the associated plant microbiome and host health. Furthermore, this work demonstrates the need to further investigate plant microbiome effects across multiple plant tissues to understand their influences on host health.

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.