Hiding in a crowd—does diversity facilitate persistence of a low-quality fungal partner in the mycorrhizal symbiosis?

Effect the accumulation of bioactive constituents of a medicinal plant (Salvia Miltiorrhiza Bge.) by arbuscular mycorrhizal fungi community

BACKGROUND

Arbuscular mycorrhizal fungi (AMF) form symbiotic relationships with various terrestrial plants and have attracted considerable interest as biofertilizers for improving the quality and yield of medicinal plants. Despite the widespread distribution of AMFs in Salvia miltiorrhiza Bunge's roots, research on the impact of multiple AMFs on biomass and active ingredient accumulations has not been conducted. In this study, the effects of five native AMFs (Glomus formosanum, Septoglomus constrictum, Rhizophagus manihotis, Acaulospora laevis, and Ambispora gerdemannii) and twenty-six communities on the root biomass and active ingredient concentrations of S. miltiorrhiza were assessed using the total factor design method.

RESULTS

Thirty-one treatment groups formed symbiotic relationships with S. miltiorrhiza based on the pot culture results, and the colonization rate ranged from 54.83% to 89.97%. AMF communities had higher colonization rates and total phenolic acid concentration than single AMF, and communities also appeared to have higher root fresh weight, dry weight, and total phenolic acid concentration than single inoculations. As AMF richness increased, there was a rising trend in root biomass and total tanshinone accumulations (ATTS), while total phenolic acid accumulations (ATP) showed a decreasing trend. This suggests that plant productivity was influenced by the AMF richness, with higher inoculation benefits observed when the communities contained three or four AMFs. Additionally, the affinities of AMF members were also connected to plant productivity. The inoculation effect of closely related AMFs within the same family, such as G. formosanum, S. constrictum, and R. manihotis, consistently yielded lower than that of mono-inoculation when any combinations were applied. The co-inoculation of S. miltiorrhiza with nearby or distant AMFs from two families, such as G. formosanum, R. manihotis, and Ac. laevis or Am. gerdemannii resulted in an increase of ATP and ATTS by more than 50%. AMF communities appear to be more beneficial to the yield of bioactive constituents than the single AMF, but overall community inoculation effects are related to the composition of AMFs and the relationship between members.

CONCLUSION

This study reveals that the AMF community has great potential to improve the productivity and the accumulation of bioactive constituents in S. miltiorrhiza, indicating that it is an effective way to achieve sustainable agricultural development through using the AMF community.

Plant productivity response to inter- and intra-symbiont diversity: Mechanisms, manifestations and meta-analyses

Symbiont diversity can have large effects on plant growth but the mechanisms generating this relationship remain opaque. We identify three potential mechanisms underlying symbiont diversity-plant productivity relationships: provisioning with complementary resources, differential impact of symbionts of varying quality and interference between symbionts. We connect these mechanisms to descriptive representations of plant responses to symbiont diversity, develop analytical tests differentiating these patterns and test them using meta-analysis. We find generally positive symbiont diversity-plant productivity relationships, with relationship strength varying with symbiont type. Inoculation with symbionts from different guilds (e.g. mycorrhizal fungi and rhizobia) yields strongly positive relationships, consistent with complementary benefits from functionally distinct symbionts. In contrast, inoculation with symbionts from the same guild yields weak relationships, with co-inoculation not consistently generating greater growth than the best individual symbiont, consistent with sampling effects. The statistical approaches we outline, along with our conceptual framework, can be used to further explore plant productivity and community responses to symbiont diversity, and we identify critical needs for additional research to explore context dependency in these relationships.

Root diameter, host specificity and arbuscular mycorrhizal fungal community composition among native and exotic plant species

Plant root systems rely on a functionally diverse range of arbuscular mycorrhizal fungi to, among other benefits, extend their nutrient foraging. Extended nutrient foraging is likely of greatest importance to coarse-rooted plants, yet few studies have examined the link between root traits and arbuscular mycorrhizal fungal community composition. Here, we examine the relationship between root diameter and the composition of arbuscular mycorrhizal fungal communities in a range of native and exotic plant species. We characterized the arbuscular mycorrhizal fungal communities of 30 co-occurring native and exotic montane grassland/shrubland plant species in New Zealand. We found that plant root diameter and native/exotic status both strongly correlated with arbuscular mycorrhizal fungal community composition. Coarse-rooted plants had a lower diversity of mycorrhizal fungi compared with fine-rooted plants and associated less with generalist fungal partners. Exotic plants had a lower diversity of fungi and fewer associations with nondominant families of arbuscular mycorrhizal fungi compared with native plants. These observational patterns suggest that plants may differentially associate with fungal partners based on their root traits, with coarse-rooted plants being more specific in their associations. Furthermore, exotic plants may associate with dominant arbuscular mycorrhizal fungal taxa as a strategy in invasion.

Co-occurring epiphytic orchids have specialized mycorrhizal fungal niches that are also linked to ontogeny

Mycorrhizal symbiosis has been related to the coexistence and community assembly of coexisting orchids in few studies despite their obligate dependence on mycorrhizal partners to establish and survive. In hyper-diverse environments like tropical rain forests, coexistence of epiphytic orchids may be facilitated through mycorrhizal fungal specialization (i.e., sets of unique and dominant mycorrhizal fungi associated with a particular host species). However, information on the role of orchid mycorrhizal fungi (OMF) in niche differentiation and coexistence of epiphytic orchids is still scarce. In this study, we sought to identify the variation in fungal preferences of four co-occurring epiphytic orchids in a tropical rainforest in Costa Rica by addressing the identity and composition of their endophytic fungal and OMF communities across species and life stages. We show that the endophytic fungal communities are formed mainly of previously recognized OMF taxa, and that the four coexisting orchid species have both a set of shared mycorrhizal fungi and a group of fungi unique to an orchid species. We also found that adult plants keep the OMF of the juvenile stage while adding new mycobionts over time. This study provides evidence for the utilization of specific OMF that may be involved in niche segregation, and for an aggregation mechanism where adult orchids keep initial fungal mycobionts of the juvenile stage while adding others.

Physiological and transcriptomic response of Medicago truncatula to colonization by high- or low-benefit arbuscular mycorrhizal fungi

Arbuscular mycorrhizal (AM) fungi form a root endosymbiosis with many agronomically important crop species. They enhance the ability of their host to obtain nutrients from the soil and increase the tolerance to biotic and abiotic stressors. However, AM fungal species can differ in the benefits they provide to their host plants. Here, we examined the putative molecular mechanisms involved in the regulation of the physiological response of Medicago truncatula to colonization by Rhizophagus irregularis or Glomus aggregatum, which have previously been characterized as high- and low-benefit AM fungal species, respectively. Colonization with R. irregularis led to greater growth and nutrient uptake than colonization with G. aggregatum. These benefits were linked to an elevated expression in the roots of strigolactone biosynthesis genes (NSP1, NSP2, CCD7, and MAX1a), mycorrhiza-induced phosphate (PT8), ammonium (AMT2;3), and nitrate (NPF4.12) transporters and the putative ammonium transporter NIP1;5. R. irregularis also stimulated the expression of photosynthesis-related genes in the shoot and the upregulation of the sugar transporters SWEET1.2, SWEET3.3, and SWEET 12 and the lipid biosynthesis gene RAM2 in the roots. In contrast, G. aggregatum induced the expression of biotic stress defense response genes in the shoots, and several genes associated with abiotic stress in the roots. This suggests that either the host perceives colonization by G. aggregatum as pathogen attack or that G. aggregatum can prime host defense responses. Our findings highlight molecular mechanisms that host plants may use to regulate their association with high- and low-benefit arbuscular mycorrhizal symbionts.

Mycorrhizal fungi control phosphorus value in trade symbiosis with host roots when exposed to abrupt 'crashes' and 'booms' of resource availability

Biological market theory provides a conceptual framework to analyse trade strategies in symbiotic partnerships. A key prediction of biological market theory is that individuals can influence resource value - meaning the amount a partner is willing to pay for it - by mediating where and when it is traded. The arbuscular mycorrhizal symbiosis, characterised by roots and fungi trading phosphorus and carbon, shows many features of a biological market. However, it is unknown if or how fungi can control phosphorus value when exposed to abrupt changes in their trade environment. We mimicked an economic 'crash', manually severing part of the fungal network (Rhizophagus irregularis) to restrict resource access, and an economic 'boom' through phosphorus additions. We quantified trading strategies over a 3-wk period using a recently developed technique that allowed us to tag rock phosphate with fluorescing quantum dots of three different colours. We found that the fungus: compensated for resource loss in the 'crash' treatment by transferring phosphorus from alternative pools closer to the host root (Daucus carota); and stored the surplus nutrients in the 'boom' treatment until root demand increased. By mediating from where, when and how much phosphorus was transferred to the host, the fungus successfully controlled resource value.

Arbuscular mycorrhizal fungus identity modulates growth effects of endophyte-infected grasses on neighboring plants

Endophytes of grasses have been reported to affect the colonization by arbuscular mycorrhizal fungi (AMF) not only of their dual host plant but also of neighboring non-endophyte-infected plants. However, studies investigating the impact of AMF identity on the effects of endophyte-infected grasses on neighboring plants are rare. In this study, we investigated the influence of Leymus chinensis litter type (NL, no litter; E-, endophyte-free litter; E-E+, half E+ and half E- litter; E+, endophyte-infected litter) on Stipa krylovii growth with different AMF species (Claroideoglomus etunicatum, CE; Funneliformis mosseae, FM; Claroideoglomus claroideum, CC; Rhizophagus intraradices, RI). The results showed that the root biomass of S. krylovii tended to decrease with the increase of E+ litter in the mycorrhiza-free treatment. With AMF inoculation, the effects of E+ litter on the AMF colonization rate and root biomass of S. krylovii varied with AMF species. Structural equation modeling (SEM) showed E+ litter could modulate the growth of S. krylovii indirectly via changes in AMF colonization rate, but this effect was related to AMF species.

Effects of single and multiple species inocula of arbuscular mycorrhizal fungi on the salinity tolerance of a Bangladeshi rice (Oryza sativa L.) cultivar

Soil salinization due to sea level rise and groundwater irrigation has become an important agronomic problem in many parts of the world. Symbiosis between crop species and arbuscular mycorrhizal fungi (AMF) may alleviate salt stress-induced detrimental effects on crop growth and yield, for example, through helping the host plant to selectively absorb potassium while avoiding uptake of excessive sodium. Here, we performed a greenhouse experiment to evaluate growth, grain yield, and salt tolerance of a Bangladeshi rice cultivar under three levels of salt stress (0, 75, and 120 mM) after inoculation with three different AMF species from three different genera (Funnelliformis mosseae (BEG12), Acaulospora laevis (BEG13), and Gigaspora margarita (BEG34)), singly and in combination. We found that under salt stress, AMF inoculation enhanced total chlorophyll concentration, shoot K⁺/Na⁺ ratio, and lowered shoot Na⁺/root Na⁺ ratio, accompanied by increased root biomass, spikelet fertility, and grain yield compared with the non-inoculated control plants. Specifically, we found that the combination of BEG13 and BEG34 increased rice yield by 125 and 143% as compared with the non-inoculated controls, at the 75 and 120mM salt levels, respectively. In general, the low AMF diversity treatments (one species or a combination of two AMF species) were found to be the most effective in mediating salt stress tolerance for the majority of the measured crop performance variables. Overall, our results indicate that specific AMF species can promote the salt tolerance and productivity of rice, likely by increasing photosynthetic efficiency and restricting Na⁺ uptake and transport from root to shoot in AMF-inoculated plants.

Inoculum handling alters the strength and direction of plant-microbe interactions

The pooling of soil samples in plant-microbe interaction studies is commonly employed, but the impact of sample handling has rarely been explored experimentally. Concerns have been raised that sample pooling may reduce biological variation leading to inflated type I errors or may alter the magnitude of microbial effects observed, invalidating the results achieved. To assess the impact of inocula pooling on plant-microbe interactions, we examined the reciprocal influence of unpooled and pooled soil microbial inocula on growth of Solidago altissima and Schizachyrium scoparium, with and without inoculum sterilization. Soil pooling had no effect on the variance among replicates in either plant species. However, pooling dramatically altered the magnitude and direction of microbial impacts on plant performance. Pooling of Solidago altissima soil increased the antagonistic effects on growth of both target species. In contrast, pooling of Schizachyrium scoparium soil shifted impacts on Solidago altissima from effectively neutral to slightly positive. Pooling in this system altered both the strength and direction of plant-microbe interactions relative to unpooled soils. Therefore soil mixing should be avoided when the research goal is to determine naturally occurring interaction strengths, even within a single habitat.

Does Inoculation with Arbuscular Mycorrhizal Fungi Reduce Trunk Disease in Grapevine Rootstocks?

Ilyonectria is a weak pathogen known for causing black foot disease in young vines, infecting roots and vascular tissues at the basal end of the rootstock and restricting the movement of water and nutrients. This negatively impacts vine establishment during transplant into the vineyard. Arbuscular mycorrhizal (AM) fungi are symbiotic fungi that associate with most plants and have been shown to mitigate the infection and effect of pathogens. This greenhouse study was designed to determine if the mycorrhizal fungi could mitigate Ilyonectria infection and whether this was dependent on inoculation timing. ‘Riparia gloire’ grapevine rootstocks (Vitis riparia) were infected with Ilyonectria either after AM fungi, at the same time as AM fungi, or to roots that were not inoculated by AM fungi. We measured the abundance using specific markers for both the pathogen and AM fungi. Colonization by AM fungi did not suppress Ilyonectria, but instead increased the abundance of Ilyonectria. Further, mycorrhizal rootstocks did not have enhanced growth effects on physiological parameters when compared to non-mycorrhizal rootstocks. These findings stand in contrast to the general perception that AM fungi provide protection against root pathogens.

Drought modulates interactions between arbuscular mycorrhizal fungal diversity and barley genotype diversity

Droughts associated with climate change alter ecosystem functions, especially in systems characterized by low biodiversity, such as agricultural fields. Management strategies aimed at buffering climate change effects include the enhancement of intraspecific crop diversity as well as the diversity of beneficial interactions with soil biota, such as arbuscular mycorrhizal fungi (AMF). However, little is known about reciprocal relations of crop and AMF diversity under drought conditions. To explore the interactive effects of plant genotype richness and AMF richness on plant yield under ambient and drought conditions, we established fully crossed diversity gradients in experimental microcosms. We expected highest crop yield and drought tolerance at both high barley and AMF diversity. While barley richness and AMF richness altered the performance of both barley and AMF, they did not mitigate detrimental drought effects on the plant and AMF. Root biomass increased with mycorrhiza colonization rate at high AMF richness and low barley richness. AMF performance increased under higher richness of both barley and AMF. Our findings indicate that antagonistic interactions between barley and AMF may occur under drought conditions, particularly so at higher AMF richness. These results suggest that unexpected alterations of plant-soil biotic interactions could occur under climate change.

When and where plant-soil feedback may promote plant coexistence: a meta-analysis

Plant-soil feedback (PSF) theory provides a powerful framework for understanding plant dynamics by integrating growth assays into predictions of whether soil communities stabilise plant-plant interactions. However, we lack a comprehensive view of the likelihood of feedback-driven coexistence, partly because of a failure to analyse pairwise PSF, the metric directly linked to plant species coexistence. Here, we determine the relative importance of plant evolutionary history, traits, and environmental factors for coexistence through PSF using a meta-analysis of 1038 pairwise PSF measures. Consistent with eco-evolutionary predictions, feedback is more likely to mediate coexistence for pairs of plant species (1) associating with similar guilds of mycorrhizal fungi, (2) of increasing phylogenetic distance, and (3) interacting with native microbes. We also found evidence for a primary role of pathogens in feedback-mediated coexistence. By combining results over several independent studies, our results confirm that PSF may play a key role in plant species coexistence, species invasion, and the phylogenetic diversification of plant communities.

Tracking plant preference for higher-quality mycorrhizal symbionts under varying CO2 conditions over multiple generations

The symbiosis between plants and root-colonizing arbuscular mycorrhizal (AM) fungi is one of the most ecologically important examples of interspecific cooperation in the world. AM fungi provide benefits to plants; in return plants allocate carbon resources to fungi, preferentially allocating more resources to higher-quality fungi. However, preferential allocations from plants to symbionts may vary with environmental context, particularly when resource availability affects the relative value of symbiotic services. We ask how differences in atmospheric CO 2-levels influence root colonization dynamics between AMF species that differ in their quality as symbiotic partners. We find that with increasing CO 2-conditions and over multiple plant generations, the more beneficial fungal species is able to achieve a relatively higher abundance. This suggests that increasing atmospheric carbon supply enables plants to more effectively allocate carbon to higher-quality mutualists, and over time helps reduce lower-quality AM abundance. Our results illustrate how environmental context may affect the extent to which organisms structure interactions with their mutualistic partners and have potential implications for mutualism stability and persistence under global change.

Are there keystone mycorrhizal fungi associated to tropical epiphytic orchids?

In epiphytic orchids, distinctive groups of fungi are involved in the symbiotic association. However, little is known about the factors that determine the mycorrhizal community structure. Here, we analyzed the orchid mycorrhizal fungi communities associated with three sympatric Cymbidieae epiphytic tropical orchids (Cyrtochilum flexuosum, Cyrtochilum myanthum, and Maxillaria calantha) at two sites located within the mountain rainforest of southern Ecuador. To characterize these communities at each orchid population, the ITS2 region was analyzed by Illumina MiSeq technology. Fifty-five mycorrhizal fungi operational taxonomic units (OTUs) putatively attributed to members of Serendipitaceae, Ceratobasidiaceae and Tulasnellaceae were identified. Significant differences in mycorrhizal communities were detected between the three sympatric orchid species as well as among sites/populations. Interestingly, some mycorrhizal OTUs overlapped among orchid populations. Our results suggested that populations of studied epiphytic orchids have site-adjusted mycorrhizal communities structured around keystone fungal species. Interaction with multiple mycorrhizal fungi could favor orchid site occurrence and co-existence among several orchid species.

Arbuscular mycorrhizal fungal community composition affected by original elevation rather than translocation along an altitudinal gradient on the Qinghai-Tibet Plateau

Elucidating arbuscular mycorrhizal (AM) fungal responses to elevation changes is critical to improve understanding of microbial function in ecosystems under global asymmetrical climate change scenarios. Here we examined AM fungal community in a two-year reciprocal translocation of vegetation-intact soil blocks along an altitudinal gradient (3,200 m to 3,800 m) in an alpine meadow on the Qinghai-Tibet Plateau. AM fungal spore density was significantly higher at lower elevation than at higher elevation regardless of translocation, except that this parameter was significantly increased by upward translocation from original 3,200 m to 3,400 m and 3,600 m. Seventy-three operational taxonomic units (OTUs) of AM fungi were recovered using 454-pyrosequencing of 18S rDNA sequences at a 97% sequence similarity. Original elevation, downward translocation and upward translocation did not significantly affect AM fungal OTU richness. However, with increasing altitude the OTU richness of Acaulosporaceae and Ambisporaceae increased, but the OTU richness of Gigasporaceae and Glomeraceae decreased generally. The AM fungal community composition was significantly structured by original elevation but not by downward translocation and upward translocation. Our findings highlight that compared with the short-term reciprocal translocation, original elevation is a stronger determinant in shaping AM fungal community in the Qinghai-Tibet alpine meadow.

Prospects for arbuscular mycorrhizal fungi (AMF) to assist in phytoremediation of soil hydrocarbon contaminants

Arbuscular mycorrhizal fungi (AMF) form mutualistic associations with the roots of 80-90% of vascular plant species and may constitute up to 50% of the total soil microbial biomass. AMF have been considered to be a tool to enhance phytoremediation, as their mycelium create a widespread underground network that acts as a bridge between plant roots, soil and rhizosphere microorganisms. Abundant extramatrical hyphae extend the rhizosphere thus creating the hyphosphere, which significantly increases the area of a plant's access to nutrients and contaminants. The paper presents and evaluates the role and significance of AMF in phytoremediation of hydrocarbon contaminated sites. We focused on (1) an impact of hydrocarbons on arbuscular mycorrhizal symbiosis, (2) a potential of AMF to enhance phytoremediation, (3) determinants that influence effectiveness of hydrocarbon removal from contaminated soils. This knowledge may be useful for selection of proper plant and fungal symbionts and crucial to optimize environmental conditions for effective AMF-mediated phytoremediation. It has been concluded that three-component phytoremediation systems based on synergistic interactions between plant roots, AMF and hydrocarbon-degrading microorganisms demonstrated high effectiveness in dissipation of organic pollutants in soil.

Conditions Promoting Mycorrhizal Parasitism Are of Minor Importance for Competitive Interactions in Two Differentially Mycotrophic Species

Interactions of plants with arbuscular mycorrhizal fungi (AMF) may range along a broad continuum from strong mutualism to parasitism, with mycorrhizal benefits received by the plant being determined by climatic and edaphic conditions affecting the balance between carbon costs vs. nutritional benefits. Thus, environmental conditions promoting either parasitism or mutualism can influence the mycorrhizal growth dependency (MGD) of a plant and in consequence may play an important role in plant-plant interactions. In a multifactorial field experiment we aimed at disentangling the effects of environmental and edaphic conditions, namely the availability of light, phosphorus and nitrogen, and the implications for competitive interactions between Hieracium pilosella and Corynephorus canescens for the outcome of the AMF symbiosis. Both species were planted in single, intraspecific and interspecific combinations using a target-neighbor approach with six treatments distributed along a gradient simulating conditions for the interaction between plants and AMF ranking from mutualistic to parasitic. Across all treatments we found mycorrhizal association of H. pilosella being consistently mutualistic, while pronounced parasitism was observed in C. canescens, indicating that environmental and edaphic conditions did not markedly affect the cost:benefit ratio of the mycorrhizal symbiosis in both species. Competitive interactions between both species were strongly affected by AMF, with the impact of AMF on competition being modulated by colonization. Biomass in both species was lowest when grown in interspecific competition, with colonization being increased in the less mycotrophic C. canescens, while decreased in the obligate mycotrophic H. pilosella. Although parasitism-promoting conditions negatively affected MGD in C. canescens, these effects were small as compared to growth decreases related to increased colonization levels in this species. Thus, the lack of plant control over mycorrhizal colonization was identified as a possible key factor for the outcome of competition, while environmental and edaphic conditions affecting the mutualism-parasitism continuum appeared to be of minor importance.

The Use of Arbuscular Mycorrhizal Fungi to Improve Strawberry Production in Coir Substrate

Strawberry is an important fruit crop within the UK. To reduce the impact of soil-borne diseases and extend the production season, more than half of the UK strawberry production is now in substrate (predominantly coir) under protection. Substrates such as coir are usually depleted of microbes including arbuscular mycorrhizal fungi (AMF) and consequently the introduction of beneficial microbes is likely to benefit commercial cropping systems. Inoculating strawberry plants in substrate other than coir has been shown to increase plants tolerance to soil-borne pathogens and water stress. We carried out studies to investigate whether AMF could improve strawberry production in coir under low nitrogen input and regulated deficit irrigation. Application of AMF led to an appreciable increase in the size and number of class I fruit, especially under either deficient irrigation or low nitrogen input condition. However, root length colonization by AMF was reduced in strawberry grown in coir compared to soil and Terragreen. Furthermore, the appearance of AMF colonizing strawberry and maize roots grown in coir showed some physical differences from the structure in colonized roots in soil and Terragreen: the colonization structure appeared to be more compact and smaller in coir.

Molecular diversity and distribution of indigenous arbuscular mycorrhizal communities colonizing roots of two different winter cover crops in response to their root proliferation

A clear understanding of how crop root proliferation affects the distribution of the spore abundance of arbuscular mycorrhizal fungi (AMF) and the composition of AMF communities in agricultural fields is imperative to identify the potential roles of AMF in winter cover crop rotational systems. Toward this goal, we conducted a field trial using wheat (Triticum aestivum L.) or red clover (Trifolium pratense L.) grown during the winter season. We conducted a molecular analysis to compare the diversity and distribution of AMF communities in roots and spore abundance in soil cropped with wheat and red clover. The AMF spore abundance, AMF root colonization, and abundance of root length were investigated at three different distances from winter crops (0 cm, 7.5 cm, and 15 cm), and differences in these variables were found between the two crops. The distribution of specific AMF communities and variables responded to the two winter cover crops. The majority of Glomerales phylotypes were common to the roots of both winter cover crops, but Gigaspora phylotypes in Gigasporales were found only in red clover roots. These results also demonstrated that the diversity of the AMF colonizing the roots did not significantly change with the three distances from the crop within each rotation but was strongly influenced by the host crop identity. The distribution of specific AMF phylotypes responded to the presence of wheat and red clover roots, indicating that the host crop identity was much more important than the proliferation of crop roots in determining the diversity of the AMF communities.

Regulation of resource exchange in the arbuscular mycorrhizal symbiosis

Arbuscular mycorrhizal (AM) fungi are one of the most important groups of plant symbionts. These fungi provide mineral nutrients to plants in exchange for carbon. Although substantial amounts of resources are exchanged, the factors that regulate trade in the AM symbiosis are poorly understood. Recent evidence for the reciprocally regulated exchange of resources by AM fungi and plants has led to the suggestion that these symbioses operate according to biological market dynamics, in which interactions are viewed from an economic perspective, and the most beneficial partners are favoured. Here we present five arguments that challenge the importance of reciprocally regulated exchange, and thereby market dynamics, for resource exchange in the AM symbiosis, and suggest that such reciprocity is only found in a subset of symbionts, under specific conditions. We instead propose that resource exchange in the AM symbiosis is determined by competition for surplus resources, functional diversity and sink strength.

Impact of water regimes on an experimental community of four desert arbuscular mycorrhizal fungal (AMF) species, as affected by the introduction of a non-native AMF species

Field studies have revealed the impact of changing water regimes on the structure of arbuscular mycorrhizal fungal (AMF) communities, but it is not known what happens to the abundance of individual AMF species within the community when the water conditions in the rhizosphere change. The behavior of four AMF species isolated from the Arabian desert (Diversispora aurantia, Diversispora omaniana, Septoglomus africanum, and an undescribed Paraglomus species) was investigated when assembled in microcosms containing Sorghum bicolor as host plant, and treated with various water regimes. Furthermore, the impact of invasion of these assemblages by Rhizophagus irregularis, an AMF species widely used in commercial inocula, was studied. The abundance of each AMF species in sorghum roots was measured by determining the transcript numbers of their large ribosomal subunit (rLSU) by real-time PCR, using cDNA and species-specific primers. Plant biomass and length of AMF extraradical hyphae were also measured. The abundance of each AMF species within the sorghum roots was influenced by both the water regime and the introduction of R. irregularis. Under dry conditions, the introduction of R. irregularis reduced the total abundance of all native AMF species in roots and also led to a reduction in the amount of extraradical mycelium, as well as to a partial decrease in plant biomass. The results indicate that both water regime and the introduction of an invasive AMF species can strongly alter the structure of an AMF native assemblage with a consequent impact on the entire symbiotic mycorrhizal relationship.

Abundant toxin-related genes in the genomes of beneficial symbionts from deep-sea hydrothermal vent mussels

Bathymodiolus mussels live in symbiosis with intracellular sulfur-oxidizing (SOX) bacteria that provide them with nutrition. We sequenced the SOX symbiont genomes from two Bathymodiolus species. Comparison of these symbiont genomes with those of their closest relatives revealed that the symbionts have undergone genome rearrangements, and up to 35% of their genes may have been acquired by horizontal gene transfer. Many of the genes specific to the symbionts were homologs of virulence genes. We discovered an abundant and diverse array of genes similar to insecticidal toxins of nematode and aphid symbionts, and toxins of pathogens such as Yersinia and Vibrio. Transcriptomics and proteomics revealed that the SOX symbionts express the toxin-related genes (TRGs) in their hosts. We hypothesize that the symbionts use these TRGs in beneficial interactions with their host, including protection against parasites. This would explain why a mutualistic symbiont would contain such a remarkable 'arsenal' of TRGs.

The role of community and population ecology in applying mycorrhizal fungi for improved food security

The global human population is expected to reach ∼9 billion by 2050. Feeding this many people represents a major challenge requiring global crop yield increases of up to 100%. Microbial symbionts of plants such as arbuscular mycorrhizal fungi (AMF) represent a huge, but unrealized resource for improving yields of globally important crops, especially in the tropics. We argue that the application of AMF in agriculture is too simplistic and ignores basic ecological principals. To achieve this challenge, a community and population ecology approach can contribute greatly. First, ecologists could significantly improve our understanding of the determinants of the survival of introduced AMF, the role of adaptability and intraspecific diversity of AMF and whether inoculation has a direct or indirect effect on plant production. Second, we call for extensive metagenomics as well as population genomics studies that are crucial to assess the environmental impact that introduction of non-local AMF may have on native AMF communities and populations. Finally, we plead for an ecologically sound use of AMF in efforts to increase food security at a global scale in a sustainable manner.

Inoculation of drought-stressed strawberry with a mixed inoculum of two arbuscular mycorrhizal fungi: effects on population dynamics of fungal species in roots and consequential plant tolerance to water deficiency

The effect of inoculation with two arbuscular mycorrhizal fungi (AMF) on growth and drought tolerance of cultivated strawberry (Fragaria × ananassa) was studied. Three treatments (a single treatment either of Funneliformis mosseae BEG25, Funneliformis geosporus BEG11 or a 50:50 mixed inoculation treatment of both species) were compared to uninoculated plants. Species-specific primers for qPCR quantification of F. geosporus and F. mosseae DNA were developed to quantify the relative abundance of each fungus in roots of strawberry under different conditions of water stress. Co-occupation of the same root by both species was shown to commonly occur, but their relative abundance varied with water stress (reduced irrigation of up to 40%). Greater root colonisation was observed microscopically under water stress, but this increased colonisation was often accompanied with decreased amounts of fungal DNA in the root. F. mosseae tended to become more abundant under water stress relative to F. geosporus. There was significant correlation in the fungal colonisation measurements from the microscopic and qPCR methods under some conditions, but the nature of this relationship varied greatly with AMF inoculum and abiotic conditions. Single-species inoculation treatments gave similar benefits to the host to the mixed inoculation treatment regardless of irrigation regime; here, amount of colonisation was of greater importance than functional diversity. The addition of AMF inocula to plants subjected to reduced irrigation restored plant growth to the same or higher values as the non-mycorrhizal, fully-watered plants. The water use efficiency of plants was greater under the regulated deficit irrigation (RDI) regime and in AMF-inoculated plants, but there were no significant differences between plants inoculated with the single or combined inoculum. This study demonstrated that the increase in plant growth was directly influenced by an increase in root colonisation by AMF when individual plants were examined.

Order of arrival structures arbuscular mycorrhizal colonization of plants

Priority effects - the impact of a species' arrival on subsequent community development - have been shown to influence species composition in many organisms. Whether priority effects among arbuscular mycorrhizal fungi (AMF) structure fungal root communities is not well understood. Here, we investigated whether priority effects influence the success of two closely related AMF species (Rhizophagus irregularis and Glomus aggregatum), hypothesizing that a resident AMF suppresses invader success, this effect is time-dependent and a resident will experience reduced growth when invaded. We performed two glasshouse experiments using modified pots, which permitted direct inoculation of resident and invading AMF on the roots. We quantified intraradical AMF abundances using quantitative PCR and visual colonization percentages. We found that both fungi suppressed the invading species and that this effect was strongly dependent on the time lag between inoculations. In contrast to our expectations, neither resident AMF was negatively affected by invasion. We show that order of arrival can influence the abundance of AMF species colonizing a host. These priority effects can have important implications for AMF ecology and the use of fungal inocula in sustainable agriculture.

Host diversity affects the abundance of the extraradical arbuscular mycorrhizal network

Arbuscular mycorrhizal fungi (AMF) can form complex networks in the soil that connect different host plants. Previous studies have focused on the effects of these networks on individual hosts and host communities. However, very little is known about how different host species affect the success of the fungal network itself. Given the potentially strong selection pressure against hosts that invest in a fungal network which benefits their competitors, we predict that the presence of multiple host species negatively affects the growth of the extraradical network. We designed an experiment using an in vitro culture approach to investigate the effect of different hosts (carrot, chichory and medicago) on the formation of a common mycelial network. In vitro root cultures, each inoculated with their own fungal network, were grown in a double split plate design with two host compartments and a common central compartment where fungal networks could form. We found that the size of fungal networks differs depending on the social environment of the host. When host species were propagated in a mixed species environment, the fungal abundance was significantly reduced compared to monoculture predictions. Our work demonstrates how host-to-host conflict can influence the abundance of the fungal partner.

Partner selection in the mycorrhizal mutualism

Partner selection in the mycorrhizal symbiosis is thought to be a key factor stabilising the mutualism. Both plant hosts and mycorrhizal fungi have been shown to preferentially allocate resources to higher quality partners. This can help maintain underground cooperation, although it is likely that different plant species vary in the spatial precision with which they can select partners. Partner selection in the mycorrhizal symbiosis is presumably context-dependent and can be mediated by factors like (relative) resource abundance and resource fluctuations, competition among mycorrhizas, arrival order and cultivation history. Such factors complicate our current understanding of the importance of partner selection and its effectiveness in stimulating mutualistic cooperation.

Species richness of arbuscular mycorrhizal fungi: associations with grassland plant richness and biomass

Although experiments show a positive association between vascular plant and arbuscular mycorrhizal fungal (AMF) species richness, evidence from natural ecosystems is scarce. Furthermore, there is little knowledge about how AMF richness varies with belowground plant richness and biomass. We examined relationships among AMF richness, above- and belowground plant richness, and plant root and shoot biomass in a native North American grassland. Root-colonizing AMF richness and belowground plant richness were detected from the same bulk root samples by 454-sequencing of the AMF SSU rRNA and plant trnL genes. In total we detected 63 AMF taxa. Plant richness was 1.5 times greater belowground than aboveground. AMF richness was significantly positively correlated with plant species richness, and more strongly with below- than aboveground plant richness. Belowground plant richness was positively correlated with belowground plant biomass and total plant biomass, whereas aboveground plant richness was positively correlated only with belowground plant biomass. By contrast, AMF richness was negatively correlated with belowground and total plant biomass. Our results indicate that AMF richness and plant belowground richness are more strongly related with each other and with plant community biomass than with the plant aboveground richness measures that have been almost exclusively considered to date.

Sequencing and comparison of the mitochondrial COI gene from isolates of Arbuscular Mycorrhizal Fungi belonging to Gigasporaceae and Glomeraceae families

Arbuscular Mycorrhizal Fungi (AMF) are well known for their ecological importance and their positive influence on plants. The genetics and phylogeny of this group of fungi have long been debated. Nuclear markers are the main tools used for phylogenetic analyses, but they have sometimes proved difficult to use because of their extreme variability. Therefore, the attention of researchers has been moving towards other genomic markers, in particular those from the mitochondrial DNA. In this study, 46 sequences of different AMF isolates belonging to two main clades Gigasporaceae and Glomeraceae have been obtained from the mitochondrial gene coding for the Cytochrome c Oxidase I (COI), representing the largest dataset to date of AMF COI sequences. A very low level of divergence was recorded in the COI sequences from the Gigasporaceae, which could reflect either a slow rate of evolution or a more recent evolutionary divergence of this group. On the other hand, the COI sequence divergence between Gigasporaceae and Glomeraceae was high, with synonymous divergence reaching saturated levels. This work also showed the difficulty in developing valuable mitochondrial markers able to effectively distinguish all Glomeromycota species, especially those belonging to Gigasporaceae, yet it represents a first step towards the development of a full mtDNA-based dataset which can be used for further phylogenetic investigations of this fungal phylum.

Intense competition between arbuscular mycorrhizal mutualists in an in vitro root microbiome negatively affects total fungal abundance

The root microbiome is composed of an incredibly diverse microbial community that provides services to the plant. A major question in rhizosphere research is how species in root microbiome communities interact with each other and their host. In the nutrient mutualism between host plants and arbuscular mycorrhizal fungi (AMF), competition often leads to certain species dominating host colonization, with the outcome being dependent on environmental conditions. In the past, it has been difficult to quantify the abundance of closely related species and track competitive interactions in different regions of the rhizosphere, specifically within and outside the host. Here, we used an artificial root system (in vitro root organ cultures) to investigate intraradical (within the root) and extraradical (outside the root) competitive interactions between two closely related AMF species, Rhizophagus irregularis and Glomus aggregatum, under different phosphorus availabilities. We found that competitive interactions between AMF species reduced overall fungal abundance. R. irregularis was consistently the most abundant symbiont for both intraradical and extraradical colonization. Competition was the most intense for resources within the host, where both species negatively affected each other's abundance. We found the investment ratio (i.e. extraradical abundance/intraradical abundance) shifted for both species depending on whether competitors were present or not. Phosphorus availability did not change the outcome of these interactions. Our results suggest that studies on competitive interactions should focus on intraradical colonization dynamics and consider how changes in investment ratio are mediated by fungal species interactions.

Context-dependency of arbuscular mycorrhizal fungi on plant-insect interactions in an agroecosystem

Plants interact with a variety of other community members that have the potential to indirectly influence each other through a shared host plant. Arbuscular mycorrhizal fungi (AMF) are generally considered plant mutualists because of their generally positive effects on plant nutrient status and growth. AMF may also have important indirect effects on plants by altering interactions with other community members. By influencing plant traits, AMF can modify aboveground interactions with both mutualists, such as pollinators, and antagonists, such as herbivores. Because herbivory and pollination can dramatically influence plant fitness, comprehensive assessment of plant-AMF interactions should include these indirect effects. To determine how AMF affect plant-insect interactions, we grew Cucumis sativus (Cucurbitaceae) under five AMF inoculum treatments and control. We measured plant growth, floral production, flower size, and foliar nutrient content of half the plants, and transferred the other half to a field setting to measure pollinator and herbivore preference of wild insects. Mycorrhizal treatment had no effect on plant biomass or floral traits but significantly affected leaf nutrients, pollinator behavior, and herbivore attack. Although total pollinator visitation did not vary with AMF treatment, pollinators exhibited taxon-specific responses, with honey bees, bumble bees, and Lepidoptera all responding differently to AMF treatments. Flower number and size were unaffected by treatments, suggesting that differences in pollinator preference were driven by other floral traits. Mycorrhizae influenced leaf K and Na, but these differences in leaf nutrients did not correspond to variation in herbivore attack. Overall, we found that AMF indirectly influence both antagonistic and mutualistic insects, but impacts depend on the identity of both the fungal partner and the interacting insect, underscoring the context-dependency of plant-AMF interactions.