Contrasting arbuscular mycorrhizal responses of vascular and non-vascular plants to a simulated Palaeozoic CO₂ decline

Computer vision models enable mixed linear modeling to predict arbuscular mycorrhizal fungal colonization using fungal morphology

The presence of Arbuscular Mycorrhizal Fungi (AMF) in vascular land plant roots is one of the most ancient of symbioses supporting nitrogen and phosphorus exchange for photosynthetically derived carbon. Here we provide a multi-scale modeling approach to predict AMF colonization of a worldwide crop from a Recombinant Inbred Line (RIL) population derived from Sorghum bicolor and S. propinquum. The high-throughput phenotyping methods of fungal structures here rely on a Mask Region-based Convolutional Neural Network (Mask R-CNN) in computer vision for pixel-wise fungal structure segmentations and mixed linear models to explore the relations of AMF colonization, root niche, and fungal structure allocation. Models proposed capture over 95% of the variation in AMF colonization as a function of root niche and relative abundance of fungal structures in each plant. Arbuscule allocation is a significant predictor of AMF colonization among sibling plants. Arbuscules and extraradical hyphae implicated in nutrient exchange predict highest AMF colonization in the top root section. Our work demonstrates that deep learning can be used by the community for the high-throughput phenotyping of AMF in plant roots. Mixed linear modeling provides a framework for testing hypotheses about AMF colonization phenotypes as a function of root niche and fungal structure allocations.

IPD3, a master regulator of arbuscular mycorrhizal symbiosis, affects genes for immunity and metabolism of non-host Arabidopsis when restored long after its evolutionary loss

Arbuscular mycorrhizal symbiosis (AM) is a beneficial trait originating with the first land plants, which has subsequently been lost by species scattered throughout the radiation of plant diversity to the present day, including the model Arabidopsis thaliana. To explore if elements of this apparently beneficial trait are still present and could be reactivated we generated Arabidopsis plants expressing a constitutively active form of Interacting Protein of DMI3, a key transcription factor that enables AM within the Common Symbiosis Pathway, which was lost from Arabidopsis along with the AM host trait. We characterize the transcriptomic effect of expressing IPD3 in Arabidopsis with and without exposure to the AM fungus (AMF) Rhizophagus irregularis, and compare these results to the AM model Lotus japonicus and its ipd3 knockout mutant cyclops-4. Despite its long history as a non-AM species, restoring IPD3 in the form of its constitutively active DNA-binding domain to Arabidopsis altered expression of specific gene networks. Surprisingly, the effect of expressing IPD3 in Arabidopsis and knocking it out in Lotus was strongest in plants not exposed to AMF, which is revealed to be due to changes in IPD3 genotype causing a transcriptional state, which partially mimics AMF exposure in non-inoculated plants. Our results indicate that molecular connections to symbiosis machinery remain in place in this nonAM species, with implications for both basic science and the prospect of engineering this trait for agriculture.

Mycorrhizal mycelium as a global carbon pool

For more than 400 million years, mycorrhizal fungi and plants have formed partnerships that are crucial to the emergence and functioning of global ecosystems. The importance of these symbiotic fungi for plant nutrition is well established. However, the role of mycorrhizal fungi in transporting carbon into soil systems on a global scale remains under-explored. This is surprising given that ∼75% of terrestrial carbon is stored belowground and mycorrhizal fungi are stationed at a key entry point of carbon into soil food webs. Here, we analyze nearly 200 datasets to provide the first global quantitative estimates of carbon allocation from plants to the mycelium of mycorrhizal fungi. We estimate that global plant communities allocate 3.93 Gt CO₂e per year to arbuscular mycorrhizal fungi, 9.07 Gt CO₂e per year to ectomycorrhizal fungi, and 0.12 Gt CO₂e per year to ericoid mycorrhizal fungi. Based on this estimate, 13.12 Gt of CO₂e fixed by terrestrial plants is, at least temporarily, allocated to the underground mycelium of mycorrhizal fungi per year, equating to ∼36% of current annual CO₂ emissions from fossil fuels. We explore the mechanisms by which mycorrhizal fungi affect soil carbon pools and identify approaches to increase our understanding of global carbon fluxes via plant-fungal pathways. Our estimates, although based on the best available evidence, are imperfect and should be interpreted with caution. Nonetheless, our estimations are conservative, and we argue that this work confirms the significant contribution made by mycorrhizal associations to global carbon dynamics. Our findings should motivate their inclusion both within global climate and carbon cycling models, and within conservation policy and practice.

Herbivore-driven disruption of arbuscular mycorrhizal carbon-for-nutrient exchange is ameliorated by neighboring plants

Arbuscular mycorrhizal fungi colonize the roots of most plants, forming a near-ubiquitous symbiosis¹ that is typically characterized by the bi-directional exchange of fungal-acquired nutrients for plant-fixed carbon.² Mycorrhizal fungi can form below-ground networks^3,4,5,6 with potential to facilitate the movement of carbon, nutrients, and defense signals across plant communities.^7,8,9 The importance of neighbors in mediating carbon-for-nutrient exchange between mycorrhizal fungi and their plant hosts remains equivocal, particularly when other competing pressures for plant resources are present. We manipulated carbon source and sink strengths of neighboring pairs of host plants through exposure to aphids and tracked the movement of carbon and nutrients through mycorrhizal fungal networks with isotope tracers. When carbon sink strengths of both neighboring plants were increased by aphid herbivory, plant carbon supply to extraradical mycorrhizal fungal hyphae was reduced, but mycorrhizal phosphorus supply to both plants was maintained, albeit variably, across treatments. However, when the sink strength of only one plant in a pair was increased, carbon supply to mycorrhizal fungi was restored. Our results show that loss of carbon inputs into mycorrhizal fungal hyphae from one plant may be ameliorated through inputs of a neighbor, demonstrating the responsiveness and resilience of mycorrhizal plant communities to biological stressors. Furthermore, our results indicate that mycorrhizal nutrient exchange dynamics are better understood as community-wide interactions between multiple players rather than as strict exchanges between individual plants and their symbionts, suggesting that mycorrhizal C-for-nutrient exchange is likely based more on unequal terms of trade than the "fair trade" model for symbiosis.

Re-engineering a lost trait: IPD3, a master regulator of arbuscular mycorrhizal symbiosis, affects genes for immunity and metabolism of non-host Arabidopsis when restored long after its evolutionary loss

Arbuscular mycorrhizal symbiosis (AM) is a beneficial trait originating with the first land plants, which has subsequently been lost by species scattered throughout the radiation of plant diversity to the present day, including the model Arabidopsis thaliana. To explore why an apparently beneficial trait would be repeatedly lost, we generated Arabidopsis plants expressing a constitutively active form of Interacting Protein of DMI3, a key transcription factor that enables AM within the Common Symbiosis Pathway, which was lost from Arabidopsis along with the AM host trait. We characterize the transcriptomic effect of expressing IPD3 in Arabidopsis with and without exposure to the AM fungus (AMF) Rhizophagus irregularis, and compare these results to the AM model Lotus japonicus and its ipd3 knockout mutant cyclops-4. Despite its long history as a non-AM species, restoring IPD3 in the form of its constitutively active DNA-binding domain to Arabidopsis altered expression of specific gene networks. Surprisingly, the effect of expressing IPD3 in Arabidopsis and knocking it out in Lotus was strongest in plants not exposed to AMF, which is revealed to be due to changes in IPD3 genotype causing a transcriptional state which partially mimics AMF exposure in non-inoculated plants. Our results indicate that despite the long interval since loss of AM and IPD3 in Arabidopsis, molecular connections to symbiosis machinery remain in place in this nonAM species, with implications for both basic science and the prospect of engineering this trait for agriculture.

The Costs and Benefits of Plant-Arbuscular Mycorrhizal Fungal Interactions

The symbiotic interaction between plants and arbuscular mycorrhizal (AM) fungi is often perceived as beneficial for both partners, though a large ecological literature highlights the context dependency of this interaction. Changes in abiotic variables, such as nutrient availability, can drive the interaction along the mutualism-parasitism continuum with variable outcomes for plant growth and fitness. However, AM fungi can benefit plants in more ways than improved phosphorus nutrition and plant growth. For example, AM fungi can promote abiotic and biotic stress tolerance even when considered parasitic from a nutrient provision perspective. Other than being obligate biotrophs, very little is known about the benefits AM fungi gain from plants. In this review, we utilize both molecular biology and ecological approaches to expand our understanding of the plant-AM fungal interaction across disciplines.

Disruption of carbon for nutrient exchange between potato and arbuscular mycorrhizal fungi enhanced cyst nematode fitness and host pest tolerance

Plants simultaneously interact with a range of biotrophic symbionts, ranging from mutualists such as arbuscular mycorrhizal fungi (AMF), to parasites such as the potato cyst nematode (PCN). The exchange of mycorrhizal-acquired nutrients for plant-fixed carbon (C) is well studied; however, the impact of competing symbionts remains underexplored. In this study, we examined mycorrhizal nutrient and host resource allocation in potato with and without AMF and PCN using radioisotope tracing, whilst determining the consequences of such allocation. The presence of PCN disrupted C for nutrient exchange between plants and AMF, with plant C overwhelmingly obtained by the nematodes. Despite this, AMF maintained transfer of nutrients on PCN-infected potato, ultimately losing out in their C for nutrient exchange with the host. Whilst PCN exploited the greater nutrient reserves to drive population growth on AMF-potato, the fungus imparted tolerance to allow the host to bear the parasitic burden. Our findings provide important insights into the belowground dynamics of plant-AMF symbioses, where simultaneous nutritional and nonnutritional benefits conferred by AMF to hosts and their parasites are seldom considered in plant community dynamics. Our findings suggest this may be a critical oversight, particularly in the consideration of C and nutrient flows in plant and soil communities.

Agrobacterium-Mediated Transient Transformation of Marchantia Liverworts

Agrobacterium-mediated transient gene expression is a rapid and useful approach for characterizing functions of gene products in planta. However, the practicability of the method in the model liverwort Marchantia polymorpha has not yet been thoroughly described. Here we report a simple and robust method for Agrobacterium-mediated transient transformation of Marchantia thalli and its applicability. When thalli of M. polymorpha were co-cultured with Agrobacterium tumefaciens carrying β-glucuronidase (GUS) genes, GUS staining was observed primarily in assimilatory filaments and rhizoids. GUS activity was detected 2 days after infection and saturated 3 days after infection. We were able to transiently co-express fluorescently tagged proteins with proper localizations. Furthermore, we demonstrate that our method can be used as a novel pathosystem to study liverwort-bacteria interactions. We also provide evidence that air chambers support bacterial colonization.

A commercial arbuscular mycorrhizal inoculum increases root colonization across wheat cultivars but does not increase assimilation of mycorrhiza-acquired nutrients

Production and heavy application of chemical-based fertilizers to maintain crop yields is unsustainable due to pollution from run-off, high CO2 emissions, and diminishing yield returns. Access to fertilizers will be limited in the future due to rising energy costs and dwindling rock phosphate resources. A growing number of companies produce and sell arbuscular mycorrhizal fungal (AMF) inoculants, intended to help reduce fertilizer usage by facilitating crop nutrient uptake through arbuscular mycorrhizas. However, their success has been variable. Here, we present information about the efficacy of a commercially available AMF inoculant in increasing AMF root colonization and fungal contribution to plant nutrient uptake, which are critical considerations within the growing AMF inoculant industry. Summary Arable agriculture needs sustainable solutions to reduce reliance on large inputs of nutrient fertilizers while continuing to improve crop yields. By harnessing arbuscular mycorrhizal symbiosis, there is potential to improve crop nutrient assimilation and growth without additional inputs, although the efficacy of commercially available mycorrhizal inocula in agricultural systems remains controversial.Using stable and radioisotope tracing, carbon-for-nutrient exchange between arbuscular mycorrhizal fungi and three modern cultivars of wheat was quantified in a non-sterile, agricultural soil, with or without the addition of a commercial mycorrhizal inoculant.While there was no effect of inoculum addition on above-ground plant biomass, there was increased root colonization by arbuscular mycorrhizal fungi and changes in community structure. Inoculation increased phosphorus uptake across all wheat cultivars by up to 30%, although this increase was not directly attributable to mycorrhizal fungi. Carbon-for-nutrient exchange between symbionts varied substantially between the wheat cultivars.Plant tissue phosphorus increased in inoculated plants potentially because of changes induced by inoculation in microbial community composition and/or nutrient cycling within the rhizosphere. Our data contribute to the growing consensus that mycorrhizal inoculants could play a role in sustainable food production systems of the future.

Carbon for nutrient exchange between Lycopodiella inundata and Mucoromycotina fine root endophytes is unresponsive to high atmospheric CO2

Non-vascular plants associating with arbuscular mycorrhizal (AMF) and Mucoromycotina 'fine root endophyte' (MFRE) fungi derive greater benefits from their fungal associates under higher atmospheric [CO₂] (a[CO₂]) than ambient; however, nothing is known about how changes in a[CO₂] affect MFRE function in vascular plants. We measured movement of phosphorus (P), nitrogen (N) and carbon (C) between the lycophyte Lycopodiella inundata and Mucoromycotina fine root endophyte fungi using ³³P-orthophosphate, ¹⁵ N-ammonium chloride and ¹⁴CO₂ isotope tracers under ambient and elevated a[CO₂] concentrations of 440 and 800 ppm, respectively. Transfers of ³³P and ¹⁵ N from MFRE to plants were unaffected by changes in a[CO₂]. There was a slight increase in C transfer from plants to MFRE under elevated a[CO₂]. Our results demonstrate that the exchange of C-for-nutrients between a vascular plant and Mucoromycotina FRE is largely unaffected by changes in a[CO₂]. Unravelling the role of MFRE in host plant nutrition and potential C-for-N trade changes between symbionts under different abiotic conditions is imperative to further our understanding of the past, present and future roles of plant-fungal symbioses in ecosystems.

Correlative Microscopy: a tool for understanding soil weathering in modern analogues of early terrestrial biospheres

Correlative imaging provides a method of investigating complex systems by combining analytical (chemistry) and imaging (tomography) information across dimensions (2D-3D) and scales (centimetres-nanometres). We studied weathering processes in a modern cryptogamic ground cover from Iceland, containing early colonizing, and evolutionary ancient, communities of mosses, lichens, fungi, and bacteria. Targeted multi-scale X-ray Microscopy of a grain in-situ within a soil core revealed networks of surficial and internal features (tunnels) originating from organic-rich surface holes. Further targeted 2D grain characterisation by optical microscopy (OM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (SEM-EDS), following an intermediate manual correlative preparation step, revealed Fe-rich nodules within the tunnels. Finally, nanotomographic imaging by focussed ion beam microscopy (FIB-SEM) revealed coccoid and filamentous-like structures within subsurface tunnels, as well as accumulations of Fe and S in grain surface crusts, which may represent a biological rock varnish/glaze. We attribute these features to biological processes. This work highlights the advantages and novelty of the correlative imaging approach, across scales, dimensions, and modes, to investigate biological weathering processes. Further, we demonstrate correlative microscopy as a means of identifying fingerprints of biological communities, which could be used in the geologic rock record and on extra-terrestrial bodies.

Impacts of elevated atmospheric CO2 on arbuscular mycorrhizal fungi and their role in moderating plant allometric partitioning

Elevated atmospheric CO₂ concentration (eCO₂) effects on plants depend on several factors including plant photosynthetic physiology (e.g. C₃, C₄), soil nutrient availability and plants' co-evolved soil-dwelling fungal symbionts, namely arbuscular mycorrhizal (AM) fungi. Complicated interactions among these components will determine the outcomes for plants. Therefore, clearer understanding is needed of how plant growth and nutrient uptake, along with root-colonising AM fungal communities, are simultaneously impacted by eCO₂. We conducted a factorial growth chamber experiment with a C₃ and a C₄ grass species (± AM fungi and ± eCO₂). We found that eCO₂ increased plant biomass allocation towards the roots, but only in plants without AM fungi, potentially associated with an eCO₂-driven increase in plant nutrient requirements. Furthermore, our data suggest a difference in the identities of root-colonising fungal taxa between ambient CO₂ and eCO₂ treatments, particularly in the C₄ grass species, although this was not statistically significant. As AM fungi are ubiquitous partners of grasses, their response to increasing atmospheric CO₂ is likely to have important consequences for how grassland ecosystems respond to global change.

Cryptogamic ground covers as analogues for early terrestrial biospheres: Initiation and evolution of biologically mediated proto-soils

Modern cryptogamic ground covers (CGCs), comprising assemblages of bryophytes (hornworts, liverworts, mosses), fungi, bacteria, lichens and algae, are thought to resemble early divergent terrestrial communities. However, limited in situ plant and other fossils in the rock record, and a lack of CGC-like soils reported in the pre-Silurian sedimentological record, have hindered understanding of the structure, composition and interactions within the earliest CGCs. A key question is how the earliest CGC-like organisms drove weathering on primordial terrestrial surfaces (regolith), leading to the early stages of soil development as proto-soils, and subsequently contributing to large-scale biogeochemical shifts in the Earth System. Here, we employed a novel qualitative, quantitative and multi-dimensional imaging approach through X-ray micro-computed tomography, scanning electron, and optical microscopy to investigate whether different combinations of modern CGC organisms from primordial-like settings in Iceland develop organism-specific soil forming features at the macro- and micro-scales. Additionally, we analysed CGCs growing on hard rocky substrates to investigate the initiation of weathering processes non-destructively in 3D. We show that thalloid CGC organisms (liverworts, hornworts) develop thin organic layers at the surface (<1 cm) with limited subsurface structural development, whereas leafy mosses and communities of mixed organisms form profiles that are thicker (up to ~ 7 cm), structurally more complex, and more organic-rich. We term these thin layers and profiles proto-soils. Component analyses from X-ray micro-computed tomography data show that thickness and structure of these proto-soils are determined by the type of colonising organism(s), suggesting that the evolution of more complex soils through the Palaeozoic may have been driven by a shift in body plan of CGC-like organisms from flattened and appressed to upright and leafy. Our results provide a framework for identifying CGC-like proto-soils in the rock record and a new proxy for understanding organism-soil interactions in ancient terrestrial biospheres and their contribution to the early stages of soil formation.

Mucoromycotina Fungi Possess the Ability to Utilize Plant Sucrose as a Carbon Source: Evidence From Gongronella sp. w5

Mucoromycotina is one of the earliest fungi to establish a mutualistic relationship with plants in the ancient land. However, the detailed information on their carbon supply from the host plants is largely unknown. In this research, a free-living Mucoromycotina called Gongronella sp. w5 (w5) was employed to explore its effect on Medicago truncatula growth and carbon source utilization from its host plant during the interaction process. W5 promoted M. truncatula growth and caused the sucrose accumulation in M. truncatula root tissue at 16 days post-inoculation (dpi). The transportation of photosynthetic product sucrose to the rhizosphere by M. truncatula root cells seemed accelerated by upregulating the SWEET gene. A predicted cytoplasmic invertase (GspInv) gene and a sucrose transporter (GspSUT1) homology gene in the w5 genome upregulated significantly at the transcriptional level during w5–M. truncatula interaction at 16 dpi, indicating the possibility of utilizing plant sucrose directly by w5 as the carbon source. Further investigation showed that the purified GspInv displayed an optimal pH of 5.0 and a specific activity of 3380 ± 26 U/mg toward sucrose. The heterologous expression of GspInv and GspSUT1 in Saccharomyces cerevisiae confirmed the function of GspInv as invertase and GspSUT1 as sugar transporter with high affinity to sucrose in vivo. Phylogenetic tree analysis showed that the ability of Mucoromycotina to utilize sucrose from its host plant underwent a process of “loss and gain.” These results demonstrated the capacity of Mucoromycotina to interact with extant land higher plants and may employ a novel strategy of directly up-taking and assimilating sucrose from the host plant during the interaction.

Aphid Herbivory Drives Asymmetry in Carbon for Nutrient Exchange between Plants and an Arbuscular Mycorrhizal Fungus

Associations formed between plants and arbuscular mycorrhizal (AM) fungi are characterized by the bi-directional exchange of fungal-acquired soil nutrients for plant-fixed organic carbon compounds. Mycorrhizal-acquired nutrient assimilation by plants may be symmetrically linked to carbon (C) transfer from plant to fungus or governed by sink-source dynamics. Abiotic factors, including atmospheric CO2 concentration ([CO2]), can affect the relative cost of resources traded between mutualists, thereby influencing symbiotic function. Whether biotic factors, such as insect herbivores that represent external sinks for plant C, impact mycorrhizal function remains unstudied. By supplying 33P to an AM fungus (Rhizophagus irregularis) and 14CO2 to wheat, we tested the impact of increasing C sink strength (i.e., aphid herbivory) and increasing C source strength (i.e., elevated [CO2]) on resource exchange between mycorrhizal symbionts. Allocation of plant C to the AM fungus decreased dramatically following exposure to the bird cherry-oat aphid (Rhopalosiphum padi), with high [CO2] failing to alleviate the aphid-induced decline in plant C allocated to the AM fungus. Mycorrhizal-mediated uptake of 33P by plants was maintained regardless of aphid presence or elevated [CO2], meaning insect herbivory drove asymmetry in carbon for nutrient exchange between symbionts. Here, we provide direct evidence that external biotic C sinks can limit plant C allocation to an AM fungus without hindering mycorrhizal-acquired nutrient uptake. Our findings highlight the context dependency of resource exchange between plants and AM fungi and suggest biotic factors-individually and in combination with abiotic factors-should be considered as powerful regulators of symbiotic function.

Carbon for nutrient exchange between arbuscular mycorrhizal fungi and wheat varies according to cultivar and changes in atmospheric carbon dioxide concentration

Arbuscular mycorrhizal fungi (AMF) form symbioses with most crops, potentially improving their nutrient assimilation and growth. The effects of cultivar and atmospheric CO2 concentration ([CO2 ]) on wheat-AMF carbon-for-nutrient exchange remain critical knowledge gaps in the exploitation of AMF for future sustainable agricultural practices within the context of global climate change. We used stable and radioisotope tracers (15 N, 33 P, 14 C) to quantify AMF-mediated nutrient uptake and fungal acquisition of plant carbon in three wheat (Triticum aestivum L.) cultivars. We grew plants under current ambient (440 ppm) and projected future atmospheric CO2 concentrations (800 ppm). We found significant 15 N transfer from fungus to plant in all cultivars, and cultivar-specific differences in total N content. There was a trend for reduced N uptake under elevated atmospheric [CO2 ]. Similarly, 33 P uptake via AMF was affected by cultivar and atmospheric [CO2 ]. Total P uptake varied significantly among wheat cultivars and was greater at the future than current atmospheric [CO2 ]. We found limited evidence of cultivar or atmospheric [CO2 ] effects on plant-fixed carbon transfer to the mycorrhizal fungi. Our results suggest that AMF will continue to provide a route for nutrient uptake by wheat in the future, despite predicted rises in atmospheric [CO2 ]. Consideration should therefore be paid to cultivar-specific AMF receptivity and function in the development of climate smart germplasm for the future.

The distribution and evolution of fungal symbioses in ancient lineages of land plants

An accurate understanding of the diversity and distribution of fungal symbioses in land plants is essential for mycorrhizal research. Here we update the seminal work of Wang and Qiu (Mycorrhiza 16:299-363, 2006) with a long-overdue focus on early-diverging land plant lineages, which were considerably under-represented in their survey, by examining the published literature to compile data on the status of fungal symbioses in liverworts, hornworts and lycophytes. Our survey combines data from 84 publications, including recent, post-2006, reports of Mucoromycotina associations in these lineages, to produce a list of at least 591 species with known fungal symbiosis status, 180 of which were included in Wang and Qiu (Mycorrhiza 16:299-363, 2006). Using this up-to-date compilation, we estimate that fewer than 30% of liverwort species engage in symbiosis with fungi belonging to all three mycorrhizal phyla, Mucoromycota, Basidiomycota and Ascomycota, with the last being the most widespread (17%). Fungal symbioses in hornworts (78%) and lycophytes (up to 100%) appear to be more common but involve only members of the two Mucoromycota subphyla Mucoromycotina and Glomeromycotina, with Glomeromycotina prevailing in both plant groups. Our fungal symbiosis occurrence estimates are considerably more conservative than those published previously, but they too may represent overestimates due to currently unavoidable assumptions.

Dual colonization of Mucoromycotina and Glomeromycotina fungi in the basal liverwort, Haplomitrium mnioides (Haplomitriopsida)

In general, Glomeromycotina was thought to be the earliest fungi forming mycorrhiza-like structure (MLS) in land plant evolution. In contrast, because the earliest divergent lineage of extant land plants, i.e. Haplomitriopsida liverworts, associates only with Mucoromycotina mycobionts, recent studies suggested that those fungi are novel candidates for the earliest mycobionts. Therefore, Mucoromycotina-Haplomitriopsida association currently attracts attention as an ancient mycorrhiza-like association. However, mycobionts were identified in only 7 of 16 Haplomitriopsida species and the mycobionts diversity of this lineage is largely unclarified. To clarify the taxonomic composition of mycobionts in Haplomitriopsida, we observed MLSs in the rhizome of Haplomitrium mnioides (Haplomitriopsida), the Asian representative Haplomitriopsida species, and conducted molecular identification of mycobionts. It was recorded for the first time that Glomeromycotina and Mucoromycotina co-occur in Haplomitriopsida as mycobionts. Significantly, the arbuscule-like branching (ALB) of Glomeromycotina was newly described. As the Mucoromycotina fungi forming MLSs in H. mnioides, Endogonaceae and Densosporaceae were detected, in which size differences of hyphal swelling (HS) were found between the fungal families. This study provides a novel evidence in the MLS of Haplomitriopsida, i.e. the existence of Glomeromycotina association as well as the dominant Mucoromycotina association. In addition, since hyphal characteristics of the HS-type MLS were quite similar to those of fine endophytes (FE) of Endogonales in other bryophytes and vascular plants previously described, this MLS is suggested to be included in FE. These results suggest that Glomeromycotina and Mucoromycotina were acquired concurrently as the mycobionts by the earliest land plants evolved into arbuscular mycorrhizae and FE. Therefore, dual association of Haplomitriopsida, with Endogonales and Glomeromycotina will provide us novel insight on how the earliest land plants adapted to terrestrial habitats with fungi.

Contrasting Nitrogen Fertilisation Rates Alter Mycorrhizal Contribution to Barley Nutrition in a Field Trial

Controlled environment studies show that arbuscular mycorrhizal fungi (AMF) may contribute to plant nitrogen (N) uptake, but the role of these near-ubiquitous symbionts in crop plant N nutrition under natural field conditions remains largely unknown. In a field trial, we tested the effects of N fertilisation and barley (Hordeum vulgare L.) cultivar identity on the contribution of AMF to barley N uptake using 15N tracers added to rhizosphere soil compartments. AMF were shown capable of significantly increasing plant 15N acquisition from root exclusion zones, and this was influenced by nitrogen addition type, N fertiliser application rate and barley cultivar identity. Our data demonstrate a previously overlooked potential route of crop plant N uptake which may be influenced substantially and rapidly in response to shifting agricultural management practices.

Arbuscular mycorrhizal fungal inoculation and soil zinc fertilisation affect the productivity and the bioavailability of zinc and iron in durum wheat

There is a growing recognition of the role of arbuscular mycorrhizal fungi (AMF) in food security, specifically the potential for AMF to enhance the yield and mineral nutrition-including phosphorus, zinc (Zn), and iron (Fe)-of food crops. However, the bioavailability of Zn and Fe for humans in the grain of cereal crops can be overestimated by failing to consider the abundance of phytic acid (PA). This is because PA can chelate the micronutrients, making them difficult to absorb. In order to understand the effect of an AM fungus and soil Zn concentration on the productivity and nutritional quality of food parts, this study examined the growth and nutritional responses of durum wheat, with and without inoculation with Rhizophagus irregularis, at five soil Zn concentrations. Growth and nutrient responses of the plants to soil Zn amendment was stronger than responses to AMF. However, the protective effect of AMF under soil Zn toxicity conditions was observed as reduced Zn concentration in the mycorrhizal durum wheat grain at Zn50. Here, AMF inoculation increased the concentration of PA in durum wheat grain but had no effect on the concentration of Zn and Fe; this consequently reduced the predicted bioavailability of grain Zn and Fe, which could lead to a decrease in nutritional quality of the grain. This research suggests that in soil with low (available) phosphorus and Zn concentrations, AMF may reduce the food quality of durum wheat because of an increase in PA concentration, and thus, a decrease in the bioavailability of Zn and Fe.

Functional complementarity of ancient plant-fungal mutualisms: contrasting nitrogen, phosphorus and carbon exchanges between Mucoromycotina and Glomeromycotina fungal symbionts of liverworts

Liverworts, which are amongst the earliest divergent plant lineages and important ecosystem pioneers, often form nutritional mutualisms with arbuscular mycorrhiza-forming Glomeromycotina and fine-root endophytic Mucoromycotina fungi, both of which coevolved with early land plants. Some liverworts, in common with many later divergent plants, harbour both fungal groups, suggesting these fungi may complementarily improve plant access to different soil nutrients. We tested this hypothesis by growing liverworts in single and dual fungal partnerships under a modern atmosphere and under 1500 ppm [CO₂ ], as experienced by early land plants. Access to soil nutrients via fungal partners was investigated with ¹⁵ N-labelled algal necromass and ³³ P orthophosphate. Photosynthate allocation to fungi was traced using ¹⁴ CO₂ . Only Mucoromycotina fungal partners provided liverworts with substantial access to algal ¹⁵ N, irrespective of atmospheric CO₂ concentration. Both symbionts increased ³³ P uptake, but Glomeromycotina were often more effective. Dual partnerships showed complementarity of nutrient pool use and greatest photosynthate allocation to symbiotic fungi. We show there are important functional differences between the plant-fungal symbioses tested, providing new insights into the functional biology of Glomeromycotina and Mucoromycotina fungal groups that form symbioses with plants. This may explain the persistence of the two fungal lineages in symbioses across the evolution of land plants.

The origin and evolution of mycorrhizal symbioses: from palaeomycology to phylogenomics

Contents Summary 1012 I. Introduction 1013 II. The mycorrhizal symbiosis at the dawn and rise of the land flora 1014 III. From early land plants to early trees: the origin of roots and true mycorrhizas 1016 IV. The diversification of the AM symbiosis 1019 V. The ECM symbiosis 1021 VI. The recently evolved ericoid and orchid mycorrhizas 1023 VII. Limits of paleontological vs genetic approaches and perspectives 1023 Acknowledgements 1025 References 1025 SUMMARY: The ability of fungi to form mycorrhizas with plants is one of the most remarkable and enduring adaptations to life on land. The occurrence of mycorrhizas is now well established in c. 85% of extant plants, yet the geological record of these associations is sparse. Fossils preserved under exceptional conditions provide tantalizing glimpses into the evolutionary history of mycorrhizas, showing the extent of their occurrence and aspects of their evolution in extinct plants. The fossil record has important roles to play in establishing a chronology of when key fungal associations evolved and in understanding their importance in ecosystems through time. Together with calibrated phylogenetic trees, these approaches extend our understanding of when and how groups evolved in the context of major environmental change on a global scale. Phylogenomics furthers this understanding into the evolution of different types of mycorrhizal associations, and genomic studies of both plants and fungi are shedding light on how the complex set of symbiotic traits evolved. Here we present a review of the main phases of the evolution of mycorrhizal interactions from palaeontological, phylogenetic and genomic perspectives, with the aim of highlighting the potential of fossil material and a geological perspective in a cross-disciplinary approach.

Unity in diversity: structural and functional insights into the ancient partnerships between plants and fungi

Contents Summary 996 I. Introduction 996 II. An ancient, and diverse, symbiosis 998 III. Structural diversity in ancient plant-fungal partnerships 1000 IV. Mycorrhizal unity in host plant nutrition 1002 V. Plant-to-fungus carbon transfer 1003 VI. From individuals to networks 1003 VII. Diverse responses of mycorrhizal functioning to dynamic environments 1006 VIII. Summary of future research direction 1007 Acknowledgements 1006 References 1006 SUMMARY: Mycorrhizal symbiosis is an ancient and widespread mutualism between plants and fungi that facilitated plant terrestrialisation > 500 million years ago, with key roles in ecosystem functioning at multiple scales. Central to the symbiosis is the bidirectional exchange of plant-fixed carbon for fungal-acquired nutrients. Within this unifying role of mycorrhizas, considerable diversity in structure and function reflects the diversity of the partners involved. Early diverging plants form mutualisms not only with arbuscular mycorrhizal Glomeromycotina fungi, but also with poorly characterised Mucoromycotina, which may also colonise the roots of 'higher' plants as fine root endophytes. Functional diversity in these symbioses depends on both fungal and plant life histories and is influenced by the environment. Recent studies have highlighted the roles of lipids/fatty acids in plant-to-fungus carbon transport and potential contributions of Glomeromycotina fungi to plant nitrogen nutrition. Together with emerging appreciation of mycorrhizal networks as multi-species resource-sharing systems, these insights are broadening our views on mycorrhizas and their roles in nutrient cycling. It is crucial that the diverse array of biotic and abiotic factors that together shape the dynamics of carbon-for-nutrient exchange between plants and fungi are integrated, in addition to embracing the unfolding and potentially key role of Mucoromycotina fungi in these processes.

Impacts of Atmospheric CO2 and Soil Nutritional Value on Plant Responses to Rhizosphere Colonization by Soil Bacteria

Concerns over rising atmospheric CO₂ concentrations have led to growing interest in the effects of global change on plant-microbe interactions. As a primary substrate of plant metabolism, atmospheric CO₂ influences below-ground carbon allocation and root exudation chemistry, potentially affecting rhizosphere interactions with beneficial soil microbes. In this study, we have examined the effects of different atmospheric CO₂ concentrations on Arabidopsis rhizosphere colonization by the rhizobacterial strain Pseudomonas simiae WCS417 and the saprophytic strain Pseudomonas putida KT2440. Rhizosphere colonization by saprophytic KT2440 was not influenced by sub-ambient (200 ppm) and elevated (1,200 ppm) concentrations of CO₂, irrespective of the carbon (C) and nitrogen (N) content of the soil. Conversely, rhizosphere colonization by WCS417 in soil with relatively low C and N content increased from sub-ambient to elevated CO₂. Examination of plant responses to WCS417 revealed that plant growth and systemic resistance varied according to atmospheric CO₂ concentration and soil-type, ranging from growth promotion with induced susceptibility at sub-ambient CO₂, to growth repression with induced resistance at elevated CO₂. Collectively, our results demonstrate that the interaction between atmospheric CO₂ and soil nutritional status has a profound impact on plant responses to rhizobacteria. We conclude that predictions about plant performance under past and future climate scenarios depend on interactive plant responses to soil nutritional status and rhizobacteria.

Ancient plants with ancient fungi: liverworts associate with early-diverging arbuscular mycorrhizal fungi

Arbuscular mycorrhizas are widespread in land plants including liverworts, some of the closest living relatives of the first plants to colonize land 500 million years ago (MYA). Previous investigations reported near-exclusive colonization of liverworts by the most recently evolved arbuscular mycorrhizal fungi, the Glomeraceae, indicating a recent acquisition from flowering plants at odds with the widely held notion that arbuscular mycorrhizal-like associations in liverworts represent the ancestral symbiotic condition in land plants. We performed an analysis of symbiotic fungi in 674 globally collected liverworts using molecular phylogenetics and electron microscopy. Here, we show every order of arbuscular mycorrhizal fungi colonizes early-diverging liverworts, with non-Glomeraceae being at least 10 times more common than in flowering plants. Arbuscular mycorrhizal fungi in liverworts and other ancient plant lineages (hornworts, lycopods, and ferns) were delimited into 58 taxa and 36 singletons, of which at least 43 are novel and specific to liverworts. The discovery that early plant lineages are colonized by early-diverging fungi supports the hypothesis that arbuscular mycorrhizas are an ancestral symbiosis for all land plants.

Marchantia liverworts as a proxy to plants' basal microbiomes

Microbiomes influence plant establishment, development, nutrient acquisition, pathogen defense, and health. Plant microbiomes are shaped by interactions between the microbes and a selection process of host plants that distinguishes between pathogens, commensals, symbionts and transient bacteria. In this work, we explore the microbiomes through massive sequencing of the 16S rRNA genes of microbiomes two Marchantia species of liverworts. We compared microbiomes from M. polymorpha and M. paleacea plants collected in the wild relative to their soils substrates and from plants grown in vitro that were established from gemmae obtained from the same populations of wild plants. Our experimental setup allowed identification of microbes found in both native and in vitro Marchantia species. The main OTUs (97% identity) in Marchantia microbiomes were assigned to the following genera: Methylobacterium, Rhizobium, Paenibacillus, Lysobacter, Pirellula, Steroidobacter, and Bryobacter. The assigned genera correspond to bacteria capable of plant-growth promotion, complex exudate degradation, nitrogen fixation, methylotrophs, and disease-suppressive bacteria, all hosted in the relatively simple anatomy of the plant. Based on their long evolutionary history Marchantia is a promising model to study not only long-term relationships between plants and their microbes but also the transgenerational contribution of microbiomes to plant development and their response to environmental changes.

Marchantia liverworts as a proxy to plants' basal microbiomes

Microbiomes influence plant establishment, development, nutrient acquisition, pathogen defense, and health. Plant microbiomes are shaped by interactions between the microbes and a selection process of host plants that distinguishes between pathogens, commensals, symbionts and transient bacteria. In this work, we explore the microbiomes through massive sequencing of the 16S rRNA genes of microbiomes two Marchantia species of liverworts. We compared microbiomes from M. polymorpha and M. paleacea plants collected in the wild relative to their soils substrates and from plants grown in vitro that were established from gemmae obtained from the same populations of wild plants. Our experimental setup allowed identification of microbes found in both native and in vitro Marchantia species. The main OTUs (97% identity) in Marchantia microbiomes were assigned to the following genera: Methylobacterium, Rhizobium, Paenibacillus, Lysobacter, Pirellula, Steroidobacter, and Bryobacter. The assigned genera correspond to bacteria capable of plant-growth promotion, complex exudate degradation, nitrogen fixation, methylotrophs, and disease-suppressive bacteria, all hosted in the relatively simple anatomy of the plant. Based on their long evolutionary history Marchantia is a promising model to study not only long-term relationships between plants and their microbes but also the transgenerational contribution of microbiomes to plant development and their response to environmental changes.

On plant defense signaling networks and early land plant evolution

All land plants must cope with phytopathogens. Algae face pathogens, too, and it is reasonable to assume that some of the strategies for dealing with pathogens evolved prior to the origin of embryophytes – plant terrestrialization simply changed the nature of the plant-pathogen interactions. Here we highlight that many potential components of the angiosperm defense toolkit are i) found in streptophyte algae and non-flowering embryophytes and ii) might be used in non-flowering plant defense as inferred from published experimental data. Nonetheless, the common signaling networks governing these defense responses appear to have become more intricate during embryophyte evolution. This includes the evolution of the antagonistic signaling pathways of jasmonic and salicylic acid, multiple independent expansions of resistance genes, and the evolution of resistance gene-regulating microRNAs. Future comparative studies will illuminate which modules of the streptophyte defense signaling network constitute the core and which constitute lineage- and/or environment-specific (peripheral) signaling circuits.

What have we learnt from studying the evolution of the arbuscular mycorrhizal symbiosis?

The arbuscular mycorrhizal (AM) symbiosis is a nearly ubiquitous association formed by most land plants. Numerous insights into the molecular mechanisms governing this symbiosis have been obtained in recent years leading to the identification of a core set of plant genes essential for successful formation of the AM symbiosis by angiosperm hosts. Recent phylogenetic analyses indicate that while the origin of some of these symbiotic genes predated the first land plants, the rest appeared through processes including de novo evolution and gene duplication that occurred specifically in the land plants. Purifying selection on this core gene set has been maintained over millions of years of plant evolution to conserve the AM symbiosis. However, several independent losses of this association have been recorded in numerous embryophyte lineages. In these lineages, potential compensatory mechanisms have been identified that could have helped these plants overcome the adversities imposed by the loss of the AM symbiosis. This review will focus on the processes governing the conservation of the AM symbiosis in the land plant lineage.

A mycorrhizal revolution

It has long been postulated that symbiotic fungi facilitated plant migrations onto land through enhancing the scavenging of mineral nutrients and exchanging these for photosynthetically fixed organic carbon. Today, land plant-fungal symbioses are both widespread and diverse. Recent discoveries show that a variety of potential fungal associates were likely available to the earliest land plants, and that these early partnerships were probably affected by changing atmospheric CO₂ concentrations. Here, we evaluate current hypotheses and knowledge gaps regarding early plant-fungal partnerships in the context of newly discovered fungal mutualists of early and more recently evolved land plants and the rapidly changing views on the roles of plant-fungal symbioses in the evolution and ecology of the terrestrial biosphere.

History and contemporary significance of the Rhynie cherts-our earliest preserved terrestrial ecosystem

The Rhynie cherts Unit is a 407 million-year old geological site in Scotland that preserves the most ancient known land plant ecosystem, including associated animals, fungi, algae and bacteria. The quality of preservation is astonishing, and the initial description of several plants 100 years ago had a huge impact on botany. Subsequent discoveries provided unparalleled insights into early life on land. These include the earliest records of plant life cycles and fungal symbioses, the nature of soil microorganisms and the diversity of arthropods. Today the Rhynie chert (here including the Rhynie and Windyfield cherts) takes on new relevance, especially in relation to advances in the fields of developmental genetics and Earth systems science. New methods and analytical techniques also contribute to a better understanding of the environment and its organisms. Key discoveries are reviewed, focusing on the geology of the site, the organisms and the palaeoenvironments. The plants and their symbionts are of particular relevance to understanding the early evolution of the plant life cycle and the origins of fundamental organs and tissue systems. The Rhynie chert provides remarkable insights into the structure and interactions of early terrestrial communities, and it has a significant role to play in developing our understanding of their broader impact on Earth systems.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.

A novel experimental system using the liverwort Marchantia polymorpha and its fungal endophytes reveals diverse and context-dependent effects

Fungal symbioses are ubiquitous in plants, but their effects have mostly been studied in seed plants. This study aimed to assess the diversity of fungal endophyte effects in a bryophyte and identify factors contributing to the variability of outcomes in these interactions. Fungal endophyte cultures and axenic liverwort clones were isolated from wild populations of the liverwort, Marchantia polymorpha. These collections were combined in a gnotobiotic system to test the effects of fungal isolates on the growth rates of hosts under laboratory conditions. Under the experimental conditions, fungi isolated from M. polymorpha ranged from aggressively pathogenic to strongly growth-promoting, but the majority of isolates caused no detectable change in host growth. Growth promotion by selected fungi depended on nutrient concentrations and was inhibited by coinoculation with multiple fungi. The M. polymorpha endophyte system expands the resources for this model liverwort. The experiments presented here demonstrate a wealth of diversity in fungal interactions even in a host reported to lack standard mycorrhizal symbiosis. In addition, they show that some known pathogens of vascular plants live in M. polymorpha and can confer benefits to this nonvascular host. This highlights the importance of studying endophyte effects across the plant tree of life.

From rhizoids to roots? Experimental evidence of mutualism between liverworts and ascomycete fungi

Background and Aims

The rhizoids of leafy liverworts (Jungermanniales, Marchantiophyta) are commonly colonized by the ascomycete fungus Pezoloma ericae. These associations are hypothesized to be functionally analogous to the ericoid mycorrhizas (ErMs) formed by P. ericae with the roots of Ericaceae plants in terms of bi-directional phosphorus for carbon exchange; however, this remains unproven. Here, we test whether associations between the leafy liverwort Cephalozia bicuspidata and P. ericae are mutualistic.

Methods

We measured movement of phosphorus and carbon between C. bicuspidata and P. ericae using [33P]orthophosphate and 14CO2 isotope tracers in monoxenic cultures. We also measured leafy liverwort growth, with and without P. ericae.

Key Results

We present the first demonstration of nutritionally mutualistic symbiosis between a non-vascular plant and an ErM-forming fungus, showing transfer of fungal-acquired P to the liverwort and of liverwort-fixed C to the fungus alongside increased growth in fungus-colonized liverworts.

Conclusions

Thus, this ascomycete-liverwort symbiosis can now be described as mycorrhiza-like, providing further insights into ericoid mycorrhizal evolution and adding Ascomycota fungi to mycorrhizal fungal groups engaging in mutualisms with plants across the land plant phylogeny. As P. ericae also colonizes the rhizoids of Schistochilaceae liverworts, which originated in the Triassic and are sister to all other jungermannialean liverworts associated with fungi, our findings point toward an early origin of ascomycete-liverwort symbioses, possibly pre-dating their evolution in the Ericales by some 150 million years.

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.

Resolving the 'nitrogen paradox' of arbuscular mycorrhizas: fertilization with organic matter brings considerable benefits for plant nutrition and growth

Arbuscular mycorrhizal fungi (AMF) can transfer nitrogen (N) to host plants, but the ecological relevance is debated, as total plant N and biomass do not generally increase. The extent to which the symbiosis is mutually beneficial is thought to rely on the stoichiometry of N, phosphorus (P) and carbon (C) availability. While inorganic N fertilization has been shown to elicit strong mutualism, characterized by improved plant and fungal growth and mineral nutrition, similar responses following organic N addition are lacking. Using a compartmented microcosm experiment, we determined the significance to a mycorrhizal plant of placing a (15) N-labelled, nitrogen-rich patch of organic matter in a compartment to which only AMF hyphae had access. Control microcosms denied AMF hyphal access to the patch compartment. When permitted access to the patch compartment, the fungus proliferated extensively in the patch and transferred substantial quantities of N to the plant. Moreover, our data demonstrate that allowing hyphal access to an organic matter patch enhanced total plant N and P contents, with a simultaneous and substantial increase in plant biomass. Furthermore, we demonstrate that organic matter fertilization of arbuscular mycorrhizal plants can foster a mutually beneficial symbiosis based on nitrogen transfer, a phenomenon previously thought irrelevant.

High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician

It has been hypothesized that predecessors of today's bryophytes significantly increased global chemical weathering in the Late Ordovician, thus reducing atmospheric CO₂ concentration and contributing to climate cooling and an interval of glaciations. Studies that try to quantify the enhancement of weathering by non-vascular vegetation, however, are usually limited to small areas and low numbers of species, which hampers extrapolating to the global scale and to past climatic conditions. Here we present a spatially explicit modelling approach to simulate global weathering by non-vascular vegetation in the Late Ordovician. We estimate a potential global weathering flux of 2.8 (km³ rock) yr⁻¹, defined here as volume of primary minerals affected by chemical transformation. This is around three times larger than today's global chemical weathering flux. Moreover, we find that simulated weathering is highly sensitive to atmospheric CO₂ concentration. This implies a strong negative feedback between weathering by non-vascular vegetation and Ordovician climate.

Early non-vascular vegetation may have caused an interval of glaciations in the Late Ordovician by enhancing global chemical weathering. Here, by simulating the organisms with a spatially explicit, process-based model, the authors propose that Ordovician vegetation had a high potential for chemical weathering.

Chemotypic Variation of Conocephalum salebrosum in the Southeastern Appalachian Range: A Search for Cryptic Plant Biodiversity Around the Tennessee River Valley

The chemotaxonomy of the Conocephalum spp. complex, based on GC-MS analysis of the volatile compositions, has helped to reveal cryptic biodiversity and delineate actual distribution patterns of chemotypes. In the Appalachian Mountains, two samples from eastern central part of the range were previously shown to be C. salebrosum. Additionally, it has recently come to light that stress can alter the volatile composition of C. conicum. This study address a previously unsampled region of the southeastern Appalachians, a region that is a biodiversity epicenter, to determine if more chemotypic diversity remains to be seen for the Conocephalum spp. complex. A common garden experiment was performed, but yielded more of a common stress experiment, and significantly altered the volatile compositions. Wild-collected controls and a meta-analysis of these data and those from previous works suggest that the common garden experiment caused stress and that the liverworts sampled belong to the C. salebrosum clade of of the Conocephalum spp. complex.

Functional analysis of liverworts in dual symbiosis with Glomeromycota and Mucoromycotina fungi under a simulated Palaeozoic CO2 decline

Most land plants form mutualistic associations with arbuscular mycorrhizal fungi of the Glomeromycota, but recent studies have found that ancient plant lineages form mutualisms with Mucoromycotina fungi. Simultaneous associations with both fungal lineages have now been found in some plants, necessitating studies to understand the functional and evolutionary significance of these tripartite associations for the first time. We investigate the physiology and cytology of dual fungal symbioses in the early-diverging liverworts Allisonia and Neohodgsonia at modern and Palaeozoic-like elevated atmospheric CO2 concentrations under which they are thought to have evolved. We found enhanced carbon cost to liverworts with simultaneous Mucoromycotina and Glomeromycota associations, greater nutrient gain compared with those symbiotic with only one fungal group in previous experiments and contrasting responses to atmospheric CO2 among liverwort-fungal symbioses. In liverwort-Mucoromycotina symbioses, there is increased P-for-C and N-for-C exchange efficiency at 440 p.p.m. compared with 1500 p.p.m. CO2. In liverwort-Glomeromycota symbioses, P-for-C exchange is lower at ambient CO2 compared with elevated CO2. No characteristic cytologies of dual symbiosis were identified. We provide evidence of a distinct physiological niche for plant symbioses with Mucoromycotina fungi, giving novel insight into why dual symbioses with Mucoromycotina and Glomeromycota fungi persist to the present day.

Friends or foes? Emerging insights from fungal interactions with plants

Fungi interact with plants in various ways, with each interaction giving rise to different alterations in both partners. While fungal pathogens have detrimental effects on plant physiology, mutualistic fungi augment host defence responses to pathogens and/or improve plant nutrient uptake. Tropic growth towards plant roots or stomata, mediated by chemical and topographical signals, has been described for several fungi, with evidence of species-specific signals and sensing mechanisms. Fungal partners secrete bioactive molecules such as small peptide effectors, enzymes and secondary metabolites which facilitate colonization and contribute to both symbiotic and pathogenic relationships. There has been tremendous advancement in fungal molecular biology, omics sciences and microscopy in recent years, opening up new possibilities for the identification of key molecular mechanisms in plant-fungal interactions, the power of which is often borne out in their combination. Our fragmentary knowledge on the interactions between plants and fungi must be made whole to understand the potential of fungi in preventing plant diseases, improving plant productivity and understanding ecosystem stability. Here, we review innovative methods and the associated new insights into plant-fungal interactions.

Algal ancestor of land plants was preadapted for symbiosis

Colonization of land by plants was a major transition on Earth, but the developmental and genetic innovations required for this transition remain unknown. Physiological studies and the fossil record strongly suggest that the ability of the first land plants to form symbiotic associations with beneficial fungi was one of these critical innovations. In angiosperms, genes required for the perception and transduction of diffusible fungal signals for root colonization and for nutrient exchange have been characterized. However, the origin of these genes and their potential correlation with land colonization remain elusive. A comprehensive phylogenetic analysis of 259 transcriptomes and 10 green algal and basal land plant genomes, coupled with the characterization of the evolutionary path leading to the appearance of a key regulator, a calcium- and calmodulin-dependent protein kinase, showed that the symbiotic signaling pathway predated the first land plants. In contrast, downstream genes required for root colonization and their specific expression pattern probably appeared subsequent to the colonization of land. We conclude that the most recent common ancestor of extant land plants and green algae was preadapted for symbiotic associations. Subsequent improvement of this precursor stage in early land plants through rounds of gene duplication led to the acquisition of additional pathways and the ability to form a fully functional arbuscular mycorrhizal symbiosis.

Evolutionary signals of symbiotic persistence in the legume-rhizobia mutualism

Understanding the origins and evolutionary trajectories of symbiotic partnerships remains a major challenge. Why are some symbioses lost over evolutionary time whereas others become crucial for survival? Here, we use a quantitative trait reconstruction method to characterize different evolutionary stages in the ancient symbiosis between legumes (Fabaceae) and nitrogen-fixing bacteria, asking how labile is symbiosis across different host clades. We find that more than half of the 1,195 extant nodulating legumes analyzed have a high likelihood (>95%) of being in a state of high symbiotic persistence, meaning that they show a continued capacity to form the symbiosis over evolutionary time, even though the partnership has remained facultative and is not obligate. To explore patterns associated with the likelihood of loss and retention of the N2-fixing symbiosis, we tested for correlations between symbiotic persistence and legume distribution, climate, soil and trait data. We found a strong latitudinal effect and demonstrated that low mean annual temperatures are associated with high symbiotic persistence in legumes. Although no significant correlations between soil variables and symbiotic persistence were found, nitrogen and phosphorus leaf contents were positively correlated with legumes in a state of high symbiotic persistence. This pattern suggests that highly demanding nutrient lifestyles are associated with more stable partnerships, potentially because they "lock" the hosts into symbiotic dependency. Quantitative reconstruction methods are emerging as a powerful comparative tool to study broad patterns of symbiont loss and retention across diverse partnerships.

The early evolution of land plants, from fossils to genomics: a commentary on Lang (1937) 'On the plant-remains from the Downtonian of England and Wales'

During the 1920s, the botanist W. H. Lang set out to collect and investigate some very unpromising fossils of uncertain affinity, which predated the known geological record of life on land. His discoveries led to a landmark publication in 1937, 'On the plant-remains from the Downtonian of England and Wales', in which he revealed a diversity of small fossil organisms of great simplicity that shed light on the nature of the earliest known land plants. These and subsequent discoveries have taken on new relevance as botanists seek to understand the plant genome and the early evolution of fundamental organ systems. Also, our developing knowledge of the composition of early land-based ecosystems and the interactions among their various components is contributing to our understanding of how life on land affects key Earth Systems (e.g. carbon cycle). The emerging paradigm is one of early life on land dominated by microbes, small bryophyte-like organisms and lichens. Collectively called cryptogamic covers, these are comparable with those that dominate certain ecosystems today. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.

From mycoheterotrophy to mutualism: mycorrhizal specificity and functioning in Ophioglossum vulgatum sporophytes

Mycorrhizal functioning in the fern Ophioglossum is complex and poorly understood. It is unknown whether mature O. vulgatum sporophytes form mutualistic associations with fungi of the Glomeromycota and with what specificity. Are green sporophytes able to 'repay' fungal carbon (C) invested in them by mycorrhizal partners during the initially heterotrophic gametophyte and early sporophyte stages of the lifecycle? We identified fungal partners of O. vulgatum sporophytes using molecular techniques and supplied them with (33) P-orthophosphate and O. vulgatum sporophytes with (14) CO2 . We traced the movement of fungal-acquired nutrients and plant-fixed C between symbionts and analysed natural abundance (13) C and (15) N isotope signatures to assess nutritional interactions. We found fungal specificity of O. vulgatum sporophytes towards a mycorrhizal fungus closely related to Glomus macrocarpum. Our radioisotope tracers revealed reciprocal C-for-phosphorus exchange between fern sporophytes and fungal partners, despite competition from surrounding vegetation. Monocultures of O. vulgatum were enriched in (13) C and (15) N, providing inconclusive evidence of mycoheterotrophy when experiencing competition from the surrounding plant community. We show mutualistic and specific symbiosis between a eusporangiate fern and fungi of the Glomeromycota. Our findings suggest a 'take now, pay later' strategy of mycorrhizal functioning through the lifecycle O. vulgatum, from mycoheterotrophic gametophyte to mutualistic aboveground sporophyte.