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Plants and mycorrhizal fungi form mutualistic relationships that affect how resources flow between organisms and within ecosystems. Common mycorrhizal networks (CMNs) could facilitate preferential transfer of carbon and limiting nutrients, but this remains difficult to predict. Do CMNs favour fungal resource acquisition at the expense of plant resource demands (a fungi-centric view), or are they passive channels through which plants regulate resource fluxes (a plant-centric view)? We used stable isotope tracers (13CO2 and 15NH3), plant traits, and mycorrhizal DNA to quantify above- and below-ground carbon and nitrogen transfer between 18 plant species along a 520-km latitudinal gradient in the Pacific Northwest, USA. Plant functional type and tissue stoichiometry were the most important predictors of interspecific resource transfer. Of ‘donor’ plants, 98% were 13C-enriched, but we detected transfer in only 2% of ‘receiver’ plants. However, all donors were 15N-enriched and we detected transfer in 81% of receivers. Nitrogen was preferentially transferred to annuals (0.26 ± 0.50 mg N per g leaf mass) compared with perennials (0.13 ± 0.30 mg N per g leaf mass). This corresponded with tissue stoichiometry differences. Synthesis Our findings suggest that plants and fungi that are located closer together in space and with stronger demand for resources over time are more likely to receive larger amounts of those limiting resources. Read the free Plain Language Summary for this article on the Journal blog.

Societal Impact Statement Summary Common mycorrhizal networks (CMN) capture the imagination of researchers and the public alike and have played a large role in communicating about mycorrhiza in general. With many of the claims about the functional importance of CMN recently under intense scrutiny, it becomes important to assess the literature on this topic. We systematically map experimental approaches, uncovering large data gaps, with evidence from field studies and data on ecosystem processes lacking. Very few studies meet the strictest recommended criteria, limiting our understanding of CMN‐mediated effects on plants and soils. A renewed research effort needs to unravel the contribution of CMNs under global environmental change. Common mycorrhizal networks (CMN) are a research area within mycorrhizal ecology, studying the effects of mycorrhizal fungi linking plant roots. Experimentally studying the functioning of such networks is challenging because of the presence of many different types of network links that give rise to potentially confounding effects. Here, we collect, collate, and describe the research evidence for common mycorrhizal networks, specifically for arbuscular mycorrhiza (AM), using a systematic mapping approach. We find that not all studies tested networks formed exclusively by AM fungi, but that other filamentous fungi were present and that very few articles report on experiments fulfilling the current stringent definition (demanding hyphal continuity between plant roots). Furthermore, most research is limited to controlled environmental conditions with certain plant, fungal, and resource‐transfer variables measured, while microbial community responses or ecosystem processes are neglected. Given the broad interest in this topic, we see a need to strengthen the evidence base on “common mycorrhizal networks” in AM fungi, necessitating a renewed research effort focusing on a range of levels of mechanistic resolution (from simple to complex networks with and without hyphal continuity). Additionally, neglected experimental situations (e.g., field studies in general) and microbial community or ecosystem‐level responses should be included in future research. [ABSTRACT FROM AUTHOR]

Summary: 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. [ABSTRACT FROM AUTHOR]

One of the most prevalent symbioses on Earth is that formed between the majority of land plants and arbuscular mycorrhizal (AM) fungi. Through these intimate associations, AM fungi transfer soil nutrients to their plant hosts in exchange for photosynthetically fixed carbon resources. It has been hypothesised that this nutritional mutualism is evolutionarily stable because both partners are in control of the exchange of resources and can discriminate between partners according to whichever offers the highest returns. However, in nature, plant–AM symbioses are exposed to a wealth of additional biotic and abiotic interactions which can affect the regulation of carbon‐for‐nutrient exchange between symbionts. Moreover, the extraradical hyphae of AM fungi make up underground networks that may be interactive or physically connected, known as common mycorrhizal networks (CMNs). These can link neighbouring plants, potentially further influencing resource distribution across the network. How these layers of complexity interact to influence resource regulation and allocation between plants and AM fungi is not often considered by experimental designs. Here, we review resource allocation in AM symbioses, scaling up from evidence from reductionist experimental systems using axenic root organ cultures to complex systems incorporating multiple neighbouring plants dealing with other, co‐occurring symbionts. As experimental designs increase in scale and ecologically relevant complexity, the carbon‐for‐nutrient exchange between plants and their AM symbionts is increasingly subject to disruption associated with the wider ecological context, such as the intricacies of the plant‐fungal interactions in a CMN or the presence of co‐occurring organisms. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

Summary: The minute 'dust seeds' of some terrestrial orchids preferentially germinate and develop as mycoheterotrophic protocorms near conspecific adult plants. Here we test the hypothesis that mycorrhizal mycelial connections provide a direct pathway for transfer of recent photosynthate from conspecific green orchids to achlorophyllous protocorms.Mycelial networks of Ceratobasidium cornigerum connecting green Dactylorhiza fuchsii plants with developing achlorophyllous protocorms of the same species were established on oatmeal or water agar before the shoots of green plants were exposed to 14CO2. After incubation for 48 h, the pattern of distribution of fixed carbon was visualised in intact entire autotrophic/protocorm systems using digital autoradiography and quantified in protocorms by liquid scintillation counting.Both methods of analysis revealed accumulation of 14C above background levels in protocorms, confirming that autotrophic plants supply carbon to juveniles via common mycorrhizal networks. Despite some accumulation of plant‐fixed carbon in the fungal mycelium grown on oatmeal agar, a greater amount of carbon was transferred to protocorms growing on water agar, indicating that the polarity of transfer may be influenced by sink strength.We suggest this transfer pathway may contribute significantly to the pattern and processes determining localised orchid establishment in nature, and that 'parental nurture' via common mycelial networks may be involved in these processes. [ABSTRACT FROM AUTHOR]

Mycoheterotrophy, whereby plants acquire both carbon and nutrients from a fungal partner, is an evolutionarily puzzling phenomenon. According to biological market models, mycoheterotrophs have nothing to offer and thus should be shunned as trading partners by discriminating fungi. Nevertheless, mycoheterotrophy is common, particularly among orchids, and an estimated 10% of all plant species are facultatively mycoheterotrophic at early stages in their life cycle. Reconciling mycoheterotrophy with biological market models, I describe how mycoheterotrophs could use arbitrage trading to net a profit of carbon and nutrients, without acquiring either from the abiotic environment. The model requires that mycoheterotrophs simultaneously buy and sell both carbon and nutrients, exploiting variability in the trading ratios offered by mycorrhizal fungi. The model relies on several conditions, including the ability of the mycoheterotroph to form indirect hyphal associations with two or more neighbouring autotrophic mycorrhizal associations, the existence of variable carbon:nutrient exchange ratios among these associations and the ability of mycoheterotrophs to invert the net‐direction of resource trade. Evidence that these conditions occur in a state of nature varies from incontrovertible to plausible given available models. The arbitrage model provides evolutionary rationale for mycoheterotrophy from both the plant and fungal perspective. Accordingly, mycoheterotrophs match trading ratios offered by autotrophic plants and, thus, need not be antagonists. The model makes novel predictions that distinguish it from source‐sink models, most notably in the existence of resource exchange inversions at the plant‐mycorrhizal interface. Finally, the model emphasizes market inefficiencies as the foundation on which mycoheterotrophs construct an arbitrage niche. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

We have analyzed the claims made in two highly popular books, which promulgate the idea that trees possess a number of human characteristics for which there is no sound scientific evidence. A critical evaluation of the mother tree hypothesis also reveals that much of the data given in support of this concept is flawed and perhaps even non-existent. This concept is also incompatible with many well-known observations on the growth of forest trees. Moreover, there is no evidence from peer-reviewed published studies to support the claim that mature trees in forests communicate preferentially with offspring through a common mycelial network. It also remains unclear whether carbon compounds transmitted in the mycorrhizae actually enter the root tissue of the receiver tree. In any event, numerous studies indicate that the amounts of carbon transferred are physiologically insignificant. 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. A recent article highlights the growing doubts about the true role of the common mycorrhizal networks [CMN or wood wide web (WWW)] connecting the roots of trees in forests. It questions the claim 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. [ABSTRACT FROM AUTHOR]

Background and aims: Clonal integration between ramets under heterogeneous environment has crucial implications for the clonal plants, is widely distribute in the arid ecosystems because it helps to transfer water and nutrients from the habitats with high moisture or fertility to lower ones. How the clonal integration affects the plant productivity and nutrient uptake under heterogeneous environment still remains unclear. Methods: Leymus chinensis and the neighbouring Stipa grandis grew at two soil moisture environments (homogeneous water content: 8% in both the donor and recipient compartments; or heterogeneous water content: 16% in the donor and 8% in the recipient compartments) and two root connections (connected or severed L. chinensis). After 4 weeks of growth, the plants of donor L. chinensis were labelled with either 15NH4+ or 15NO3−. Results: The biomass of recipient L. chinensis and S. grandis and N uptake rate by S. grandis were larger under heterogeneous water water content with connected roots than that with severed roots. The NO3− uptake rate was 40 times faster than that of NH4+ by all the plants irrespective of soil moisture condition and root connection. Consequently, clonal integration increased N translocation from donor to recipient ramets and subsequent N utilization by neighbouring S. grandis. Unexpectedly, NH4+ uptake by recipient L. chinensis with severed roots was 1.5 times faster than that with connected roots under homogeneous environment, this largely ascribed to that the translocation of NH4+ from donor to recipient L. chinensis through common mycorrhizal networks (CMNs). Conclusions: Clonal integration increases plant biomass and N uptake in heterogeneous environment and weakens them in homogeneous environment. Plants prefer direct NO3− uptake, whereas arbuscular mycorrhizal fungi preferentially translocate NH4+ to recipient clonal ramets. These findings indicate that clonal plants rely on the clonal integration of clonal roots and CMNs to acquire N from soil. [ABSTRACT FROM AUTHOR]

The invasive plant species, Solidago canadensis, often shares a common mycorrhizal network (CMN) with native plant species in southeast China. We ask whether the transfer of carbon (C) and nitrogen (N) among the invasive S. canadensis, the native Kummerowia striata and their CMN could contribute to the invasiveness of S. canadensis.We conducted a microcosm experiment in which a CMN was established on S. canadensis and K. striata. We used 13CO2 pulse‐labelling to quantify the relative contribution of recently fixed C from the two host species to the CMN. We also calculated the relative N distribution to each host species from the CMN.We found that 89% of the recently fixed C in the CMN originated from S. canadensis, while 11% originated from K. striata. We also found that the CMN distributed 77% of the N it absorbed to S. canadensis and 23% to K. striata.Our results suggest that the unequal distribution of N from a CMN to the invasive and native plant species is related to the unequal contribution of C from the plant species. This mechanism could contribute to invasion of native communities by alien species when there is a stoichiometric exchange of limiting nutrients for C, and when alien species contribute more C to the CMN than natives. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

In many species-rich subtropical and tropical forests, the canopy is dominated by trees that form symbioses with ectomycorrhizal fungi, while understorey trees typically form symbioses with arbuscular mycorrhizal fungi. We propose a virtuous phosphorus acquisition hypothesis to explain this structure. This hypothesis is based on evidence that ectomycorrhizal fungi are better equipped than arbuscular mycorrhizal fungi to acquire growth-limiting phosphorus from organic forms in soil through the production of enzymes and exudates. Ectomycorrhizal saplings also gain more benefit from the formation of fungal networks with neighbouring adult plants, which also suppress infection by pathogens. Canopy dominance provides plants with the opportunity to capture more carbon from the atmosphere and fuel these below-ground nutrient-foraging activities. Subtropical and tropical forests in Asia often comprise canopy dominant trees that form symbioses with ectomycorrhizal fungi, and species-rich understorey trees that form symbioses with arbuscular mycorrhizal fungi. We propose a virtuous phosphorus acquisition hypothesis to explain this distinct structure. The hypothesis is based on (i) seedlings being rapidly colonised by ectomycorrhizal fungi from established mycelial networks that generates positive feedback and resistance to pathogens, (ii) ectomycorrhizal fungi having evolved a suite of morphological, physiological, and molecular traits to enable them to capture phosphorus from a diversity of chemical forms, including organic forms, and (iii) allocation of photosynthate carbon from adult host plants to provide the energy needed to undertake these processes. [ABSTRACT FROM AUTHOR]

Background: Arbuscular mycorrhizal (AM) symbiosis has been referred to as the mother of all plant root symbioses as it predated the evolution of plant roots. The AM research is a multidisciplinary field at the intersection of soil science, mycology, and botany. However, in recent decades the nature and properties of soils, in which the AM symbiosis develops and functions, have received less attention than desired. Scope: In this review we discuss a number of recent developments in AM research. We particularly cover the role of AM symbiosis in acquisition of phosphorus, nitrogen, heavy metals and metalloids, as well as water by plants from soil; mycorrhizal effects on plant nutritional stoichiometry and on the carbon cycle; the hyphosphere microbiome; so-called facultative mycorrhizal plants; explanations for lack of mycorrhizal benefit; common mycorrhizal networks; and arbuscular and ectomycorrhizal ecosystems. Conclusion: We reflect on what has previously been described as mycorrhizal 'dogmas'. We conclude that these are in fact generalisations on the AM symbiosis that are well supported by multiple studies, while admitting that there potentially is a geographical bias in mycorrhizal research that developed in temperate and boreal regions, and that research in other ecosystems might uncover a greater diversity of viable mycorrhizal and non-mycorrhizal strategies than currently acknowledged. We also note an increasing tendency to overinterpret data, which may lead to stagnation of some research fields due to lack of experiments designed to test the mechanistic basis of processes rather than cumulating descriptive studies and correlative evidences. [ABSTRACT FROM AUTHOR]

Abstract Background The root mycobiome plays a fundamental role in plant nutrition and protection against biotic and abiotic stresses. In temperate forests or meadows dominated by angiosperms, the numerous fungi involved in root symbioses are often shared between neighboring plants, thus forming complex plant-fungus interaction networks of weak specialization. Whether this weak specialization also holds in rich tropical communities with more phylogenetically diverse sets of plant lineages remains unknown. We collected roots of 30 plant species in semi-natural tropical communities including angiosperms, ferns, and lycophytes, in three different habitat types on La Réunion island: a recent lava flow, a wet thicket, and an ericoid shrubland. We identified root-inhabiting fungi by sequencing both the 18S rRNA and the ITS2 variable regions. We assessed the diversity of mycorrhizal fungal taxa according to plant species and lineages, as well as the structure and specialization of the resulting plant-fungus networks. Results The 18S and ITS2 datasets are highly complementary at revealing the root mycobiota. According to 18S, Glomeromycotina colonize all plant groups in all habitats forming the least specialized interactions, resulting in nested network structures, while Mucoromycotina (Endogonales) are more abundant in the wetland and show higher specialization and modularity compared to the former. According to ITS2, mycorrhizal fungi of Ericaceae and Orchidaceae, namely Helotiales, Sebacinales, and Cantharellales, also colonize the roots of most plant lineages, confirming that they are frequent endophytes. While Helotiales and Sebacinales present intermediate levels of specialization, Cantharellales are more specialized and more sporadic in their interactions with plants, resulting in highly modular networks. Conclusions This study of the root mycobiome in tropical environments reinforces the idea that mycorrhizal fungal taxa are locally shared between co-occurring plants, including phylogenetically distant plants (e.g. lycophytes and angiosperms), where they may form functional mycorrhizae or establish endophytic colonization. Yet, we demonstrate that, irrespectively of the environmental variations, the level of specialization significantly varies according to the fungal lineages, probably reflecting the different evolutionary origins of these plant-fungus symbioses. Frequent fungal sharing between plants questions the roles of the different fungi in community functioning and highlights the importance of considering networks of interactions rather than isolated hosts.

Arbuscular mycorrhizal fungi (AMF) are considered biofertilizers for sustainable agriculture due to their ability to facilitate plant uptake of important mineral elements, such as nitrogen (N). However, plant mycorrhiza-dependent N uptake and interplant transfer may be highly context-dependent, and whether it is affected by aboveground herbivory remains largely unknown. Here, we used 15N labeling and tracking to examine the effect of aboveground insect herbivory by Spodoptera frugiperda on mycorrhiza-dependent N uptake in maize (Zea mays L.). To minimize consumption differences and 15N loss due to insect chewing, insect herbivory was simulated by mechanical wounding and oral secretion of S. frugiperda larvae. Inoculation with Rhizophagus irregularis (Rir) significantly improved maize growth, and N/P uptake. The 15N labeling experiment showed that maize plants absorbed N from soils via the extraradical mycelium of mycorrhizal fungi and from neighboring plants transferred by common mycorrhizal networks (CMNs). Simulated aboveground leaf herbivory did not affect mycorrhiza-mediated N acquisition from soil. However, CMN-mediated N transfer from neighboring plants was blocked by leaf simulated herbivory. Our findings suggest that aboveground herbivory inhibits CMN-mediated N transfer between plants but does not affect N acquisition from soil solutions via extraradical mycorrhizal mycelium. [ABSTRACT FROM AUTHOR]

Invasive weeds of the Asteraceae family are widespread in the world. Arbuscular mycorrhiza (AM) is one of the main factors contributing to the successful distribution of these species that is most clearly manifested in the subfamily Asteroideae. The benefits of plant-AMF symbiosis are most significant under unfavorable biotic and abiotic conditions. The specificity of the relationship between arbuscular mycorrhizal fungi (AMF) communities and plants and is determined at the presymbiotic stage. The AMF colonization level is higher in invasive species than in native ones, but AMF communities associated with Asteraceae invasive species are less diverse. AMF communities of Asteraceae invaders often include fewer common species (e.g., species belonging to Diversisporales). Invaders also reduce native AMF species richness in new areas. Arbuscular mycorrhizal fungi can form mycorrhizal networks that allow the redistribution of nutrients in plant communities. The most significant influence of AMF associated with invasive Asteraceae plants is seen in the formation of soil and rhizosphere microbiota, including the suppression of beneficial soil bacteria and fungi. This review could be useful in the development of practical recommendations for the use of AMF-based fertilizers. [ABSTRACT FROM AUTHOR]

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The extraradical hyphae of arbuscular mycorrhizal fungi (AMF) of one plant root system forage for the soil nutrients and induce the root colonization of the nearby plants, which leads to the formation of common mycorrhizal networks (CMNs) that interconnect roots. Inoculation with AMF can increase the root length, surface area and volume of seedlings in nutrient-limited karstic soils. Mycorrhizal symbioses can secrete glomalin to help promoting soil aggregates for water and nutrients storage, through an extended hyphae to absorb water and nutrients from long distances. AMF can boost rhizosphere soil enzyme activities, and may help to drive carbon sequestration. AMF also improve plant growth by advancing soil quality through influencing its structure and texture. As a result, AMF and CMNs benefit plants through improving soil quality and enhancing morphological (e.g., hyphal length, tillering, number of stolons per individual), physiological (e.g., water use efficiency) and productive (e.g., fresh and dry shoot and root weights) traits.

Background: The root mycobiome plays a fundamental role in plant nutrition and protection against biotic and abiotic stresses. In temperate forests or meadows dominated by angiosperms, the numerous fungi involved in root symbioses are often shared between neighboring plants, thus forming complex plant-fungus interaction networks of weak specialization. Whether this weak specialization also holds in rich tropical communities with more phylogenetically diverse sets of plant lineages remains unknown. We collected roots of 30 plant species in semi-natural tropical communities including angiosperms, ferns, and lycophytes, in three different habitat types on La Réunion island: a recent lava flow, a wet thicket, and an ericoid shrubland. We identified root-inhabiting fungi by sequencing both the 18S rRNA and the ITS2 variable regions. We assessed the diversity of mycorrhizal fungal taxa according to plant species and lineages, as well as the structure and specialization of the resulting plant-fungus networks. Results: The 18S and ITS2 datasets are highly complementary at revealing the root mycobiota. According to 18S, Glomeromycotina colonize all plant groups in all habitats forming the least specialized interactions, resulting in nested network structures, while Mucoromycotina (Endogonales) are more abundant in the wetland and show higher specialization and modularity compared to the former. According to ITS2, mycorrhizal fungi of Ericaceae and Orchidaceae, namely Helotiales, Sebacinales, and Cantharellales, also colonize the roots of most plant lineages, confirming that they are frequent endophytes. While Helotiales and Sebacinales present intermediate levels of specialization, Cantharellales are more specialized and more sporadic in their interactions with plants, resulting in highly modular networks. Conclusions: This study of the root mycobiome in tropical environments reinforces the idea that mycorrhizal fungal taxa are locally shared between co-occurring plants, including phylogenetically distant plants (e.g. lycophytes and angiosperms), where they may form functional mycorrhizae or establish endophytic colonization. Yet, we demonstrate that, irrespectively of the environmental variations, the level of specialization significantly varies according to the fungal lineages, probably reflecting the different evolutionary origins of these plant-fungus symbioses. Frequent fungal sharing between plants questions the roles of the different fungi in community functioning and highlights the importance of considering networks of interactions rather than isolated hosts. [ABSTRACT FROM AUTHOR]

More than 80% terrestrial plants establish mutualistic symbiosis with soil-borne arbuscular mycorrhizal fungi (AMF). These fungi not only significantly improve plant nutrient acquisition and stress resistance, but also mitigate heavy metal phytotoxicity, Furthermore, the extraradical mycorrhizal mycelia can form common mycorrhizal networks (CMNs) that link roots of multiple plants in a community. Here we show that the networks mediate migration of heavy metal cadmium (Cd) from maize (Zea mays L.) to soybean (Glycine max (Linn.) Merr.) plants. CMNs between maize and soybean plants were established after inoculation of maize plants with AMF Funneliformis mosseae. Application of CdCl2 in maize plants led to 64.4% increase in the shoots and 48.2% increase in the roots in Cd content in CMNs-connected soybean plants compared to the control without Cd treatment in maize. Meanwhile, although the CMNs-connected soybean plants did not directly receive Cd supply, they upregulated transcriptional levels of Cd transport-related genes HATPase and RSTK 2.13- and 5.96-fold, respectively, induced activities of POD by 44.8% in the leaves, and increased MDA by 146.2% in the roots. Furthermore, Cd addition inhibited maize growth but mycorrhizal colonization improved plant performance in presence of Cd stress. This finding demonstrates that mycorrhizal networks mediate the transfer of Cd between plants of different species, suggesting a potential to use CMNs as a conduit to transfer toxic heavy metals from main food crops to heavy metal hyperaccumulators via intercropping.

Invasive weeds of the Asteraceae family are widespread in the world. Arbuscular mycorrhiza (AM) is one of the main factors contributing to the successful distribution of these species that is most clearly manifested in the subfamily Asteroideae. The benefits of plant-AMF symbiosis are most significant under unfavorable biotic and abiotic conditions. The specificity of the relationship between arbuscular mycorrhizal fungi (AMF) communities and plants and is determined at the presymbiotic stage. The AMF colonization level is higher in invasive species than in native ones, but AMF communities associated with Asteraceae invasive species are less diverse. AMF communities of Asteraceae invaders often include fewer common species (e.g., species belonging to Diversisporales). Invaders also reduce native AMF species richness in new areas. Arbuscular mycorrhizal fungi can form mycorrhizal networks that allow the redistribution of nutrients in plant communities. The most significant influence of AMF associated with invasive Asteraceae plants is seen in the formation of soil and rhizosphere microbiota, including the suppression of beneficial soil bacteria and fungi. This review could be useful in the development of practical recommendations for the use of AMF-based fertilizers.

Invasive species, including more than three dozen Asteraceae, such as Solidago canadensis, Leucanthemum vulgare, Senecio inaequidens etc, pose serious threat to ecosystem health. Arbuscular mycorrhizal symbiosis is a key factor for distribution of invasive species of some Asteraceae tribes, including Astereae, Anthemideae, Senecioneae, Gnaphalieae, Cardueae, and Cichorieae. The formation of invasion-friendly plant communities has occurred through increasing nutrient and water availability, hormonal regulation, production of bioactive compounds, and mycorrhiza-induced resistance of host plants. Native species are displaced through the influence on soil microbiota, mycorrhizal and nutrient status of neighboring plants, and several other parameters. Allelopathic influences and symbiotic interactions with bacteria and other fungi can inhibit these processes. Understanding the mycorrhizal status of invasive weeds, in our opinion, is a necessary condition for their successful control.

Cascading effects are ubiquitous in nature and can modify ecological processes. Most plants have mutualistic associations with mycorrhizal fungi, and can be connected to neighboring plants through common mycorrhizal networks (CMNs). However, little is known about how the distribution of nutrients by CMNs to the interconnected plants is affected by higher trophic levels, such as parasitic plants. We hypothesized that parasitism would indirectly drive CMNs to allocate more nutrients to the nonparasitized neighboring plants rather than to the parasitized host plants, and that this would result in a negative‐feedback effect on the growth of the parasitic plant. To test this, we conducted a container experiment, where each container housed two in‐growth cores that isolated the root system of a single Trifolium pratense seedling. The formation of CMNs was either prevented or permitted using nylon fabric with a mesh width of 0.5 or 25 μm, respectively. In each container, either both T. pratense plants were not parasitized or only one was parasitized by the holoparasite Cuscuta australis. To quantify the nutrient distribution by CMNs to the host and neighboring plants, we used 15N labeling. Growth and 15N concentrations of C. australis and T. pratense were measured, as well the arbuscular mycorrhizal fungi–colonization rates of T. pratense. We found that parasitism by C. australis reduced the biomass of T. pratense. In the absence of the parasite, CMNs increased the 15N concentration of both T. pratense plants, but did not affect their biomass. However, with the parasite, the difference between host and neighboring T. pratense plants in 15N concentration and biomass were amplified by CMNs. Furthermore, CMNs decreased the negative effect of C. australis on growth of the host T. pratense plants. Finally, although CMNs did not influence the 15N concentration of C. australis, they reduced its biomass. Our results indicate that when T. pratense was parasitized by C. australis, CMNs preferentially distributed more mineral nutrients to the nonparasitized neighboring T. pratense plant, and that this had a negative feedback on the growth of the parasite. [ABSTRACT FROM AUTHOR]

In order to explore the influence of arbuscular mycorrhizal (AM) fungi in the rhizosphere of poisonous plants on the neighboring pasture grasses in the Tibetan Plateau Alpine meadow ecosystem, rhizosphere soils were collected from eight different poisonous plants in degraded grasslands and one from pasture grass in non-degraded grasslands (CK). The collected soils were used as inocula to assess the influence of indigenous AM fungi on the growth of two typical pasture grass species, Elymus nutans and Poa pratensis, in a bioassay experiment. Five growth parameters and two AM parameters were determined. The mycorrhizal responsiveness and the importance value were calculated. Significant differences between the eight poisonous plants and CK were observed. Compared to CK, rhizosphere soil from the eight poisonous plants had lower AM fungal spore densities. The growth of E. nutans and P. pratensis seedlings was depressed with the inoculation from poisonous plants rhizosphere soil. This study demonstrated that the presence of poisonous plants with grassland degradation altered inherent AM fungal community abundance, and could exert inhibition effects on the growth of pasture grasses. It may attribute to discover the important role of rhizosphere soil of different poisonous plants to AM fungal community on the Alpine meadow.

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Summary: Arbuscular mycorrhiza (AM) symbiosis occurs between obligate biotrophic fungi of the phylum Glomeromycota and most land plants. The exchange of nutrients between host plants and AM fungi (AMF) is presumed to be the main benefit for the two symbiotic partners. In this review article, we outline the current concepts of nutrient exchanges within this symbiosis (mechanisms and regulation). First, we focus on phosphorus and nitrogen transfer from the fungal partner to the host plant, and on the reciprocal transfer of carbon compounds, with a highlight on a possible interplay between nitrogen and phosphorus nutrition during AM symbiosis. We further discuss potential mechanisms of regulation of these nutrient exchanges linked to membrane dynamics. The review finally addresses the common mycorrhizal networks formed AMF, which interconnect plants from similar and/or different species. Finally the best way to integrate this knowledge and the ensuing potential benefits of AM into sustainable agriculture is discussed. [ABSTRACT FROM AUTHOR]

Mycorrhizal symbiosis has important implications for the diversity and productivity of plant communities. However, our mechanistic understanding of its influence on the outcome of plant-plant interactions is still expanding. In this review we propose a framework that might be useful to efficiently approach the effects of mycorrhizal fungi (MF) on plant-plant interactions. We propose several scenarios that can theoretically result in different outcomes of plant-plant interactions based on the combination of two processes: the diversity of resources provided by MF taxa to their host (resource dissimilarity) and contrasting ways of distributing those resources (resource distribution). Then, we illustrate our arguments with different ecological contexts where certain combinations of these two processes are prone to occur. The proposed framework suggests testable hypotheses that can contribute to elucidate relevant processes underlying the effects of mycorrhizal symbiosis on plant-plant interactions. [ABSTRACT FROM AUTHOR]

The formation of a common mycorrhizal network (CMN) between roots of different plant species enables nutrient transfers from one plant to another and their coexistence. However, almost all studies on nutrient transfers between CMN-connected plants have separately, but not simultaneously, been demonstrated under the same experimentation. Both conspecific and heterospecific seedlings of Cinnamomum camphora, Bidens pilosa, and Broussonetia papyrifera native to a karst habitat in southwest China were concurrently grown in a growth microcosm that had seven hollowed compartments (six around one in the center) being covered by 35.0-μm and/or 0.45-μm nylon mesh. The Ci. camphora in the central compartment was supplied with or without Glomus etunicatum and 15N to track N transfers between CMN-connected conspecific and heterospecific seedlings. The results showed as follows: significant greater nitrogen accumulations, biomass productions, 15N content, % Ntransfer, and the Ntransfer amount between receiver plant species ranked as Br. papyrifera≈Bi. pilosa > Ci. camphora under both M+ and M−, and as under M+ than under M− for Ci. camphora but not for both Bi. Pilosa and Br. papyrifera; the CMN transferred more nitrogen (15N content, % Ntransfer, and Ntransfer amount) from the donor Ci. camphora to the heterospecific Br. papyrifera and Bi. pilosa, with a lower percentage of nitrogen derived from transfer (%NDFT). These findings suggest that the CMN may potentially regulate the nitrogen transfer from a donor plant to individual heterospecific receiver plants, where the ratio of nitrogen derived from transfer depends on the biomass strength of the individual plants. [ABSTRACT FROM AUTHOR]

In order to explore the influence of arbuscular mycorrhizal (AM) fungi in the rhizosphere of poisonous plants on the neighboring pasture grasses in the Tibetan Plateau Alpine meadow ecosystem, rhizosphere soils were collected from eight different poisonous plants in degraded grasslands and one from pasture grass in non-degraded grasslands (CK). The collected soils were used as inocula to assess the influence of indigenous AM fungi on the growth of two typical pasture grass species, Elymus nutans and Poa pratensis, in a bioassay experiment. Five growth parameters and two AM parameters were determined. The mycorrhizal responsiveness and the importance value were calculated. Significant differences between the eight poisonous plants and CK were observed. Compared to CK, rhizosphere soil from the eight poisonous plants had lower AM fungal spore densities. The growth of E. nutans and P. pratensis seedlings was depressed with the inoculation from poisonous plants rhizosphere soil. This study demonstrated that the presence of poisonous plants with grassland degradation altered inherent AM fungal community abundance, and could exert inhibition effects on the growth of pasture grasses. It may attribute to discover the important role of rhizosphere soil of different poisonous plants to AM fungal community on the Alpine meadow. [ABSTRACT FROM AUTHOR]

Invasive species often reduce ecosystem services and lead to a serious threat to native biodiversity. Roots of invasive plants are often linked to roots of native plants by common mycorrhizal networks (CMNs) of arbuscular mycorrhizal (AM) fungi, but whether and how CMNs mediate interactions between invasive and native plant species remains largely uninvestigated. We conducted two microcosm experiments, one in which we amended the soil with mineral N and another in which we amended the soil with mineral P. In each experiment, we grew a pair of test plants consisting of Kummerowia striata (native to our research site) and Solidago canadensis (an invasive species). CMNs were established between the plants, and these were either left intact or severed. Intact CMNs increased growth and nutrient acquisition by S. canadensis while they decreased nutrient acquisition by K. striata in comparison with severed CMNs.15N and P analyses indicated that compared to severed CMNs, intact CMNs preferentially transferred mineral nutrients to S. canadensis. CMNs produced by different species of AM fungi had slightly different effects on the interaction between these two plant species. These results highlight the role of CMNs in the understanding of interactions between the invasive species S. canadensis and its native neighbor. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi form extensive common mycorrhizal networks (CMNs) that may interconnect neighboring root systems of the same or different plant species, thereby potentially influencing the distribution of limiting mineral nutrients among plants. We examined how CMNs affected intra- and interspecific interactions within and between populations of Andropogon gerardii, a highly mycorrhiza dependent, dominant prairie grass and Elymus canadensis, a moderately dependent, subordinate prairie species. We grew A. gerardii and E. canadensis alone and intermixed in microcosms, with individual root systems isolated, but either interconnected by CMNs or with CMNs severed weekly. CMNs, which provided access to a large soil volume, improved survival of both A. gerardii and E. canadensis, but intensified intraspecific competition for A. gerardii. When mixed with E. canadensis, A. gerardii overyielded aboveground biomass in the presence of intact CMNs but not when CMNs were severed, suggesting that A. gerardii with intact CMNs most benefitted from weaker interspecific than intraspecific interactions across CMNs. CMNs improved manganese uptake by both species, with the largest plants receiving the most manganese. Enhanced growth in consequence of improved mineral nutrition led to large E. canadensis in intact CMNs experiencing water-stress, as indicated by C isotope abundance. Our findings suggest that in prairie plant communities, CMNs may influence mineral nutrient distribution, water relations, within-species size hierarchies, and between-species interactions. [ABSTRACT FROM AUTHOR]

Little is known about the adaptive capacity of arbuscular mycorrhizal (AM) fungi to novel hosts. Here we assessed the possibility of two heterospecific AM fungal isolates to adaptively change, in terms of host biomass response, as a function of host plant identity, over the course of a growing season. First, we produced pure inocula of Rhizophagus clarus and Rhizophagus intraradices, each starting from a single spore. Second, we 'trained' each isolate individually in a community with two plants, sudangrass ( Sorgum bicolour subsp . drummondii) and leek ( Aliium ampeloprasum var. porrum), using a dual-compartment system to allow the establishment of a common mycorrhizal network between the two hosts. Third, we conducted a greenhouse experiment to reciprocally test each 'trained' clone, obtained from each compartment, either with the same (home), or the other host (away) under two contrasting phosphorus levels. Overall, results did not support adaptive responses of the AM fungi to their hosts (i.e., greater host biomass under 'home' relative to 'away' conditions), but the opposite (i.e., greater host biomass under 'away' relative to 'home' conditions) was more frequently observed. These changes in AM fungal symbiotic functioning open the possibility for relatively rapid genetic change of arbuscular mycorrhizal fungi in response to new hosts, which represents one step forward from in vitro experiments. [ABSTRACT FROM AUTHOR]

Mycorrhizal fungi can interconnect individual plants to form common mycorrhizal networks (CMNs). Recent work has shown that these networks can transport signals produced by plants in response to herbivore and pathogen infestation to neighbouring plants before they are themselves attacked. The speed of transfer to uninfested plants is such that the mechanism is likely to have measurable benefits for plant protection. At present, the nature of the signals and the degree of plant and fungal control of their delivery are unknown. Interplant communication of signals via mycorrhizal fungi means we must rethink our understanding of multitrophic interactions in nature, and the findings have led to several hypotheses concerning the evolutionary outcomes of the process. We urgently need to better understand the mechanisms of signal transfer in natural plant communities, and attempt to harness the formation of CMNs as a tool for sustainable pest management in agriculture. [ABSTRACT FROM AUTHOR]

Background Plants, though sessile, employ various strategies to defend themselves against herbivorous insects and convey signals of an impending herbivore attack to other plant(s). Strategies include the production of volatiles that include terpenoids and the formation of symbiotic associations with fungi, such as arbuscular mycorrhiza (AM). This constitutes a two-pronged above-ground/below-ground attack-defence strategy against insect herbivores. Scope Terpenoids represent an important constituent of herbivore-induced plant volatiles that deter herbivores and/or attract their predators. Terpenoids serve as airborne signals that can induce defence responses in systemic undamaged parts of the plant and also prime defence responses in neighbouring plants. Colonization of roots by AM fungi is known to influence secondary metabolism in plants; this includes alteration of the concentration and composition of terpenoids, which can boost both direct and indirect plant defence against herbivorous insects. Enhanced nutrient uptake facilitated by AM, changes in plant morphology and physiology and increased transcription levels of certain genes involved in the terpenoid biosynthesis pathway result in alterations in plant terpenoid profiles. The common mycorrhizal networks of external hyphae have added a dimension to the two-pronged plant defence strategy. These act as conduits to transfer defence signals and terpenoids. Conclusion Improved understanding of the roles of terpenoids in plant and AM defences against herbivory and of interplant signalling in natural communities has significant implications for sustainable management of pests in agricultural ecosystems. [ABSTRACT FROM AUTHOR]

Plants are often exposed to shade over different time scales and this may substantially affect not only their own growth, but also development and functioning of the energetically dependent organisms. Among those, the root symbionts such as arbuscular mycorrhizal (AM) fungi and rhizobia represent particularly important cases – on the one hand, they consume a significant share of plant carbon (C) budget and, on the other, they generate a number of important nutritional feedbacks on their plant hosts, often resulting in a net positive effect on their host growth and/or fitness. Here we discuss our previous results comparing mycorrhizal performance under different intensities and durations of shade (Konvalinková et al., 2015) in a broader context of previously published literature. Additionally, we review publicly available knowledge on the root colonization and mycorrhizal growth responses in AM plants under light deprivation. Experimental evidence shows that sudden and intensive decrease of light availability to a mycorrhizal plant triggers rapid deactivation of phosphorus transfer from the AM fungus to the plant already within a few days, implying active and rapid response of the AM fungus to the energetic status of its plant host. When AM plants are exposed to intensive shading on longer time scales (weeks to months), positive mycorrhizal growth responses (MGR) are often decreasing and may eventually become negative. This is most likely due to the high C cost of the symbiosis relative to the C availability, and failure of plants to fully compensate for the fungal C demand under low light. Root colonization by AM fungi often declines under low light intensities, although the active role of plants in regulating the extent of root colonization has not yet been unequivocally demonstrated. Quantitative information on the rates and dynamics of C transfer from the plant to the fungus is mostly missing, as is the knowledge on the involved molecular mechanisms. Therefore, these subjects deserve particular attention in the future.

Plants are often exposed to shade over different time scales and this may substantially affect not only their own growth, but also development and functioning of the energetically dependent organisms. Among those, the root symbionts such as arbuscular mycorrhizal (AM) fungi and rhizobia represent particularly important cases--on the one hand, they consume a significant share of plant carbon (C) budget and, on the other, they generate a number of important nutritional feedbacks on their plant hosts, often resulting in a net positive effect on their host growth and/or fitness. Here we discuss our previous results comparing mycorrhizal performance under different intensities and durations of shade (Konvalinková et al., 2015) in a broader context of previously published literature. Additionally, we review publicly available knowledge on the root colonization and mycorrhizal growth responses in AM plants under light deprivation. Experimental evidence shows that sudden and intensive decrease of light availability to a mycorrhizal plant triggers rapid deactivation of phosphorus transfer from the AM fungus to the plant already within a few days, implying active and rapid response of the AM fungus to the energetic status of its plant host. When AM plants are exposed to intensive shading on longer time scales (weeks to months), positive mycorrhizal growth responses (MGR) are often decreasing and may eventually become negative. This is most likely due to the high C cost of the symbiosis relative to the C availability, and failure of plants to fully compensate for the fungal C demand under low light. Root colonization by AM fungi often declines under low light intensities, although the active role of plants in regulating the extent of root colonization has not yet been unequivocally demonstrated. Quantitative information on the rates and dynamics of C transfer from the plant to the fungus is mostly missing, as is the knowledge on the involved molecular mechanisms. Therefore, these subjects deserve particular attention in the future. [ABSTRACT FROM AUTHOR]

Arbuscular mycorrhizal fungi (AMF) are considered biofertilizers for sustainable agriculture due to their ability to facilitate plant uptake of important mineral elements, such as nitrogen (N). However, plant mycorrhiza-dependent N uptake and interplant transfer may be highly context-dependent, and whether it is affected by aboveground herbivory remains largely unknown. Here, we used 15 N labeling and tracking to examine the effect of aboveground insect herbivory by Spodoptera frugiperda on mycorrhiza-dependent N uptake in maize ( Zea mays L.). To minimize consumption differences and 15 N loss due to insect chewing, insect herbivory was simulated by mechanical wounding and oral secretion of S. frugiperda larvae. Inoculation with Rhizophagus irregularis ( Rir ) significantly improved maize growth, and N/P uptake. The 15 N labeling experiment showed that maize plants absorbed N from soils via the extraradical mycelium of mycorrhizal fungi and from neighboring plants transferred by common mycorrhizal networks (CMNs). Simulated aboveground leaf herbivory did not affect mycorrhiza-mediated N acquisition from soil. However, CMN-mediated N transfer from neighboring plants was blocked by leaf simulated herbivory. Our findings suggest that aboveground herbivory inhibits CMN-mediated N transfer between plants but does not affect N acquisition from soil solutions via extraradical mycorrhizal mycelium., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 He, Lin, Zhang, Tong, Ding, Yao, Liu, Zeng, Chen and Song.)

Common mycorrhizal networks ( CMNs) influence competition between plants, but reports regarding their precise effect are conflicting. We studied CMN effects on phosphorus (P) uptake and growth of seedlings as influenced by various disruptions of network components., Tomato ( Solanum lycopersicon) seedlings grew into established networks of Rhizophagus irregularis and cucumber ( Cucumis sativus) in two experiments. One experiment studied seedling uptake of 32P in the network in response to cutting of cucumber shoots; the other analysed seedling uptake of P and nitrogen (N) in the presence of intact or severed arbuscular mycorrhizal fungus networks and at two soil P concentrations., Pre-established and intact networks suppressed growth of tomato seedlings. Cutting of cucumber shoots mitigated P deficiency symptoms of seedlings, which obtained access to P in the extraradical mycelium and thereby showed improved growth. Solitary seedlings growing in a network patch that had been severed from the CMN also grew much better than seedlings of the corresponding CMN., Interspecific and size-asymmetric competition between plants may be amplified rather than relaxed by CMNs that transfer P to large plants providing most carbon and render small plants P deficient. It is likely that grazing or senescence of the large plants will alleviate the network-induced suppression of seedling growth. [ABSTRACT FROM AUTHOR]

Abstract: Arbuscular mycorrhizal fungi (AMF) may colonize several plant species simultaneously, thus receiving their carbon from different plants. In previous work, we have used microcosms with flax (a C3 plant) and sorghum (a C4 plant), connected to a common mycorrhizal network, in order to track the carbon source of AMF, making use of the distinct 13C/12C isotope compositions of C3 and C4 plants (Walder et al., 2012). Here we compare three methods for analysing the stable carbon isotope composition of AMF. Bulk carbon isotope analysis of washed extraradical mycelium is possible, but has the drawback of potential contamination from non-mycorrhizal sources. Bulk carbon isotope analysis of isolated AMF spores yields more reliable results but is rather tedious. We explain, in some detail, a more refined analysis based on the extraction of lipids from soil, followed by analysis of an AMF biomarker, the fatty acid C16:1ω5. [Copyright &y& Elsevier]

This study aimed to elucidate the relationship between ericaceous understorey shrubs and the diversity and abundance of ectomycorrhizal fungi (EcMF) associated with the invasive Pinus strobus and native Pinus sylvestris. Seedlings of both pines were grown in mesocosms and subjected to three treatments simulating different forest microhabitats: (a) grown in isolation and grown with (b) Vaccinium myrtillus or (c) Vaccinium vitis-idaea. Ericaceous plants did not act as a species pool of pine mycobionts and inhibited the ability of the potentially shared species Meliniomyces bicolor to form ectomycorrhizae. Similarly, Ericaceae significantly reduced the formation of Thelephora terrestris ectomycorrhizae in P. sylvestris. EcMF species composition in the mesocosms was strongly affected by both the host species and the presence of an ericaceous neighbour. When grown in isolation, P. strobus root tips were predominantly colonised by Wilcoxina mikolae, whereas those of P. sylvestris were more commonly colonised by Suillus and Rhizopogon spp. Interestingly, these differences were less evident ( Suillus + Rhizopogon spp.) or absent ( W. mikolae) when the pines were grown with Ericaceae. P. strobus exclusively associated with Rhizopogon salebrosus s.l., suggesting the presence of host specificity at the intrageneric level. Ericaceous plants had a positive effect on colonisation of P. strobus root tips by R. salebrosus s.l. This study demonstrates that the interaction of selective factors such as host species and presence of ericaceous plants may affect the realised niche of the ectomycorrhizal fungi. [ABSTRACT FROM AUTHOR]

The majority of herbaceous plants are connected by arbuscular mycorrhizal (AM) fungi in complex networks, but how this affects carbon (C) and phosphorus (P) allocation among symbionts is poorly understood. We utilized a monoxenic AM system where hyphae from donor roots colonized two younger receiver roots of varying C status. AM fungal C allocation from donor to receiver compartments was followed by measuring the 13C contents in fungal- and plant-specific lipids, and P movement from a hyphal compartment was traced using 33P. Four times more 13C was translocated from donor to C-limited receiver roots, but C remained in fungal tissue. Root C status did not influence the overall AM colonization, but arbuscule density was twice as high in non-C-limited roots, and they received 10 times more 33P. The number of hyphal connections between compartments did not influence C and P allocation. Interestingly, there were more fungal storage lipids, but fewer structural lipids inside C-limited roots. Our results indicate that AM colonization may poorly reflect host quality as C can be supplied from neighboring roots. A mycocentric view of the symbiosis is proposed where C-delivering hosts are resource islands for the exchange of P for C, and C-limited hosts are storage units. [ABSTRACT FROM AUTHOR]

• Rhizanthella gardneri is a rare and fully subterranean orchid that is presumably obligately mycoheterotrophic. R. gardneri is thought to be linked via a common mycorrhizal fungus to co-occurring autotrophic shrubs, but there is no experimental evidence to support this supposition. •We used compartmentalized microcosms to investigate the R. gardneri tripartite relationship. 13CO2 was applied to foliage of Melaleuca scalena plants and [13C-15N]glycine was fed to the common mycorrhizal fungus, and both sources traced to R. gardneri plants. •In our microcosm trial, up to 5% of carbon (C) fed as 13CO2 to the autotrophic shrub was transferred to R. gardneri. R. gardneri also readily acquired soil C and nitrogen (N), where up to 6.2% of C and 22.5% of N fed as labelled glycine to soil was transferred via the fungus to R. gardneri after 240 h. •Our study confirms that R. gardneri is mycoheterotrophic and acquires nutrients via mycorrhizal fungus connections from an ectomycorrhizal autotrophic shrub and directly from the soil via the same fungus. This connection with a specific fungus is key to explaining why R. gardneri occurs exclusively under certain Melaleuca species at a very limited number of sites in Western Australia. [ABSTRACT FROM AUTHOR]