Search

Plants express diverse nutrient use and acquisition traits, but it is unclear how trait combinations at the species level are constrained by phylogeny, trait coordination, or trade-offs in resource investment. One trait - nitrogen (N) fixation - is assumed to correlate with other traits and used to define plant functional groups, despite potential confounding effects of phylogeny. We quantified growth, carbon metabolism, fixation rate, root phosphatase activity (RPA), mycorrhizal colonization, and leaf and root morphology/chemistry across 22 species of fixing and nonfixing tropical Fabaceae trees under common conditions. Belowground trait variation was high even among closely related species, and most traits displayed a phylogenetic signal, including N-fixation rate and nodule biomass. Across species, we observed strong positive correlations between physiological traits such as RPA and root respiration. RPA increased ~ fourfold per unit increase in fixation, supporting the debated hypothesis that N-fixers 'trade' N for phosphatases to enhance phosphorus acquisition. Specific root length and root N differed between functional groups, though for other traits, apparent differences became nonsignificant after accounting for phylogenetic nonindependence. We conclude that evolutionary history, trait coordination, and fixation ability contribute to nutrient trait expression at the species level, and recommend explicitly considering phylogeny in analyses of functional groupings., (© 2024 The Author(s). New Phytologist © 2024 New Phytologist Foundation.)

Ectomycorrhizas are an intrinsic component of tree nutrition and responses to environmental variations. How epigenetic mechanisms might regulate these mutualistic interactions is unknown. By manipulating the level of expression of the chromatin remodeler DECREASE IN DNA METHYLATION 1 (DDM1) and two demethylases DEMETER-LIKE (DML) in Populus tremula × Populus alba lines, we examined how host DNA methylation modulates multiple parameters of the responses to root colonization with the mutualistic fungus Laccaria bicolor. We compared the ectomycorrhizas formed between transgenic and wild-type (WT) trees and analyzed their methylomes and transcriptomes. The poplar lines displaying lower mycorrhiza formation rate corresponded to hypomethylated overexpressing DML or RNAi-ddm1 lines. We found 86 genes and 288 transposable elements (TEs) differentially methylated between WT and hypomethylated lines (common to both OX-dml and RNAi-ddm1) and 120 genes/1441 TEs in the fungal genome suggesting a host-induced remodeling of the fungal methylome. Hypomethylated poplar lines displayed 205 differentially expressed genes (cis and trans effects) in common with 17 being differentially methylated (cis). Our findings suggest a central role of host and fungal DNA methylation in the ability to form ectomycorrhizas including not only poplar genes involved in root initiation, ethylene and jasmonate-mediated pathways, and immune response but also terpenoid metabolism., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation This article has been contributed to by U.S. Government employees and their work is in the public domain in the USA.)

Forest fungal communities are shaped by the interactions between host tree root systems and the associated soil conditions. We investigated how the soil environment, root morphological traits, and root chemistry influence root-inhabiting fungal communities in three tropical forest sites of varying successional status in Xishuangbanna, China. For 150 trees of 66 species, we measured root morphology and tissue chemistry. Tree species identity was confirmed by sequencing rbcL, and root-associated fungal (RAF) communities were determined using high-throughput ITS2 sequencing. Using distance-based redundancy analysis and hierarchical variation partitioning, we quantified the relative importance of two soil variables (site average total phosphorus and available phosphorus), four root traits (dry matter content, tissue density, specific tip abundance, and forks), and three root tissue elemental concentrations (nitrogen, calcium, and manganese) on RAF community dissimilarity. The root and soil environment collectively explained 23% of RAF compositional variation. Soil phosphorus explained 76% of that variation. Twenty fungal taxa differentiated RAF communities among the three sites. Soil phosphorus most strongly affects RAF assemblages in this tropical forest. Variation in root calcium and manganese concentrations and root morphology among tree hosts, principally an architectural trade-off between dense, highly branched vs less-dense, herringbone-type root systems, are important secondary determinants., (© 2023 The Authors New Phytologist © 2023 New Phytologist Foundation.)

To date, there has been no robust evidence for the exploitation of saprotrophic non-rhizoctonia fungi by green plants, although some fully mycoheterotrophic orchids are known to exploit them, and mycoheterotrophic evolution has probably occurred through intermediate mixotrophic stages. We investigated the physiological ecology of a fully mycoheterotrophic species Cremastra aphylla and its photosynthetic sister species Cremastra appendiculata, which putatively exploit saprotrophic fungi. Their mycorrhizal partners and ultimate nutritional sources were determined using molecular, stable isotopic, and radiocarbon analysis. Both Cremastra aphylla and Cremastra appendiculata were consistently associated with wood-decaying Psathyrellaceae. In addition, both species were highly enriched in carbon-13 ( 13 C) and, to a less degree, in nitrogen-15 ( 15 N). The δ 13 C and δ 15 N values of Cremastra appendiculata were intermediate between those of Cremastra aphylla and those of autotrophic plants. All Cremastra appendiculata samples and two Cremastra aphylla samples exhibited elevated Δ 14 C values due to the acquisition of carbon fixed in wood during the past decades ( 14 C-enriched bomb carbon). Our multifaceted evidence indicated that both species obtained carbon from deadwood via saprotrophic fungi. Our findings strongly suggest that mixotrophic relationships associated with wood-decaying fungi represent a novel evolutionary pathway for full mycoheterotrophy in orchids., (© 2021 The Authors. New Phytologist © 2021 New Phytologist Foundation.)

Water and nutrient acquisition are key drivers of plant health and ecosystem function. These factors impact plant physiology directly as well as indirectly through soil- and root-associated microbial responses, but how they in turn affect aboveground plant-microbe interactions are not known. Through experimental manipulations in the field and growth chamber, we examine the interacting effects of water stress, soil fertility, and arbuscular mycorrhizal fungi on bacterial and fungal communities of the tomato (Solanum lycopersicum) phyllosphere. Both water stress and mycorrhizal disruption reduced leaf bacterial richness, homogenized bacterial community composition among plants, and reduced the relative abundance of dominant fungal taxa. We observed striking parallelism in the individual microbial taxa in the phyllosphere affected by irrigation and mycorrhizal associations. Our results show that soil conditions and belowground interactions can shape aboveground microbial communities, with important potential implications for plant health and sustainable agriculture., (© 2021 The Authors. New Phytologist © 2021 New Phytologist Foundation.)

Phosphorus (P) is essential for plant growth. Arbuscular mycorrhizal fungi (AMF) aid its uptake by acquiring P from sources distant from roots in return for carbon. Little is known about how AMF colonise soil pore-space, and models of AMF-enhanced P-uptake are poorly validated. We used synchrotron X-ray computed tomography to visualize mycorrhizas in soil and synchrotron X-ray fluorescence/X-ray absorption near edge structure (XRF/XANES) elemental mapping for P, sulphur (S) and aluminium (Al) in combination with modelling. We found that AMF inoculation had a suppressive effect on colonisation by other soil fungi and identified differences in structure and growth rate between hyphae of AMF and nonmycorrhizal fungi. Our results showed that AMF co-locate with areas of high P and low Al, and preferentially associate with organic-type P species over Al-rich inorganic P. We discovered that AMF avoid Al-rich areas as a source of P. Sulphur-rich regions were found to be correlated with higher hyphal density and an increased organic-associated P-pool, whilst oxidized S-species were found close to AMF hyphae. Increased S oxidation close to AMF suggested the observed changes were microbiome-related. Our experimentally-validated model led to an estimate of P-uptake by AMF hyphae that is an order of magnitude lower than rates previously estimated - a result with significant implications for the modelling of plant-soil-AMF interactions., (© 2022 The Authors New Phytologist © 2022 New Phytologist Foundation.)

Mycoheterotrophic plants depend entirely on fungal associations for organic nutrients. While most mycoheterotrophic plants are associated with the mycorrhizal partners of surrounding green plants, some mycoheterotrophs are believed to obtain carbon from decaying litter or dead wood by parasitising saprotrophic fungi, based on culture experiments and 13 C and 15 N isotopic signatures. The carbon age (the time since carbon was fixed from atmospheric CO 2 by photosynthesis) can be estimated by measuring the concentration of 14 C arising from the bomb tests of the 1950s and 1960s. Given that mycorrhizal fungi obtain photosynthate from their plant partners, and saprotrophic wood-decaying fungi obtain carbon from older sources, radiocarbon could represent a new and powerful tool to investigate carbon sources of mycoheterotrophic plants. We showed that the Δ 14 C values of mycoheterotrophs exploiting ectomycorrhizal fungi were close to 0‰, similar to those of autotrophic plants. By contrast, the Δ 14 C values of mycoheterotrophs exploiting saprotrophic fungi ranged from 110.7‰ to 324.8‰, due to the 14 C-enriched bomb carbon from dead wood via saprotrophic fungi. Our study provides evidence supporting that some mycoheterotrophic orchids depend on forest woody debris. Our study also indicates that radiocarbon could be used to predict the trophic strategies of mycoheterotroph-associated fungal symbionts., (© 2020 The Authors. New Phytologist © 2020 New Phytologist Trust.)

Chronosequences at the forefront of retreating glaciers provide information about colonization rates of bare surfaces. In the northern hemisphere, forest development can take centuries, with rates often limited by low nutrient availability. By contrast, in front of the retreating Pia Glacier (Tierra del Fuego, Chile), a Nothofagus forest is in place after only 34 yr of development, while total soil nitrogen (N) increased from near zero to 1.5%, suggesting a strong input of this nutrient. We measured N-fixation rates, carbon fluxes, leaf N and phosphorus contents and leaf δ 15 N in the dominant plants, including the herb Gunnera magellanica, which is endosymbiotically associated with a cyanobacterium, in order to investigate the role of N-fixing and mycorrhizal symbionts in N-budgets during successional transition. G. magellanica presented some of the highest nitrogenase activities yet reported (potential maximal contribution of 300 kg N ha -1  yr -1 ). Foliar δ 15 N results support the framework of a highly efficient N-uptake and transfer system based on mycorrhizas, with c. 80% of N taken up by the mycorrhizas potentially transferred to the host plant. Our results suggest the symbiosis of G. magellanica with cyanobacteria, and trees and shrubs with mycorrhizas, to be the key processes driving this rapid succession., (© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)

Contents Summary 91 I. Introduction 91 II. Primary PSF mechanisms 91 III. Factors mediating the mechanisms of PSF 93 IV. Conclusions and future directions 94 Acknowledgements 95 Author contributions 95 References 95 SUMMARY: Plant-soil feedback (PSF) occurs when plants alter soil properties that influence the performance of seedlings, with consequent effects on plant populations and communities. Many processes influence PSF, including changes in nutrient availability and the accumulation of natural enemies, mutualists or secondary chemicals. Typically, these mechanisms are investigated in isolation, yet no single mechanism is likely to be completely responsible for PSF as these processes can interact. Further, the outcome depends on which resources are limiting and the other plants and soil biota in the surrounding environment. As such, understanding the mechanisms of PSF and their role within plant communities requires quantification of the interactions among the processes influencing PSF and the associated abiotic and biotic contexts., (© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust.)

Almost all plant species interact with one or more symbioses somewhere within their distribution range. Bringing together plant trait data and growth responses to symbioses spanning 552 plant species, we provide for the first time on a large scale (597 studies) a quantitative synthesis on plant performance differences between eight major types of symbiosis, including mycorrhizas, N-fixing bacteria, fungal endophytes and ant-plant interactions. Frequency distributions of plant growth responses varied considerably between different types of symbiosis, in terms of both mean effect and 'risk', defined here as percentage of experiments reporting a negative effect of symbiosis on plants. Contrary to expectation, plant traits were poor predictors of growth response across and within all eight symbiotic associations. Our analysis showed no systematic additive effect when a host plant engaged in two functionally different symbioses. This synthesis suggests that plant species' ecological strategies have little effect in determining the influence of a symbiosis on host plant growth. Reliable quantification of differences in plant performance across symbioses will prove valuable for developing general hypotheses on how species become engaged in mutualisms without a guarantee of net returns., (© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust.)

Contents Summary 982 I. Introduction 982 II. The portraits of our ancestors: a gallery of ideas from more than 100 years of mycorrhizal research 983 III. Mycorrhizal fungi in the 'omics' era: first puzzle, how to name mycorrhizal fungi 985 IV. Signalling: a central question of our time? 987 V. The colonization process: how cellular studies predicted future 'omics' data 989 VI. The genetics underlying colonization events 991 VII. Concluding thoughts: chance and needs in mycorrhizal symbioses 992 Acknowledgements 992 References 992 SUMMARY: Our knowledge of mycorrhizas dates back to at least 150 years ago, when the plant pathologists A. B. Frank and G. Gibelli described the surprisingly morphology of forest tree roots surrounded by a fungal mantle. Compared with this history, our molecular study of mycorrhizas remains a young science. To trace the history of mycorrhizal research, from its roots in the distant past, to the present and the future, this review outlines a few topics that were already central in the 19 th century and were seminal in revealing the biological meaning of mycorrhizal associations. These include investigations of nutrient exchange between partners, plant responses to mycorrhizal fungi, and the identity and evolution of mycorrhizal symbionts as just a few examples of how the most recent molecular studies of mycorrhizal biology sprouted from the roots of past research. In addition to clarifying the ecological role of mycorrhizas, some of the recent results have changed the perception of the relevance of mycorrhizas in the scientific community, and in the whole of society. Looking to past knowledge while foreseeing strategies for the next steps can help us catch a glimpse of the future of mycorrhizal research., (© 2018 The Author. New Phytologist © 2018 New Phytologist Trust.)

Content Summary 1420 I. Introduction 1421 II. Root adaptations that influence P acquisition 1422 III. Costs of P acquisition: general 1423 IV. Costs of P acquisition that are independent of soil P concentrations 1423 V. Costs of P acquisition that increase as soil P concentrations decline 1424 VI. Discussion and conclusions 1424 Acknowledgements 1425 References 1425 SUMMARY: We compare carbon (and hence energy) costs of the different modes of phosphorus (P) acquisition by vascular land plants. Phosphorus-acquisition modes are considered to be mechanisms of plants together with their root symbionts and structures such as cluster roots involved in mobilising or absorbing P. Phosphorus sources considered are soluble and insoluble inorganic and organic pools. Costs include operating the P-acquisition mechanisms, and resource requirements to construct and maintain them. For most modes, costs increase as the relevant soil P concentration declines. Costs can thus be divided into a component incurred irrespective of soil P concentration, and a component describing how quickly costs increase as the soil P concentration declines. Differences in sensitivity of costs to soil P concentration arise mainly from how economically mycorrhizal fungal hyphae or roots that explore the soil volume are constructed, and from costs of exudates that hydrolyse or mobilise insoluble P forms. In general, modes of acquisition requiring least carbon at high soil P concentrations experience a steeper increase in costs as soil P concentrations decline. The relationships between costs and concentrations suggest some reasons why different modes coexist, and why the mixture of acquisition modes differs between sites., (© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust.)

Contents Summary 507 I. Introduction 507 II. The return on investment approach 508 III. CO 2 response spectrum 510 IV. Discussion 516 Acknowledgements 518 References 518 SUMMARY: Land ecosystems sequester on average about a quarter of anthropogenic CO 2 emissions. It has been proposed that nitrogen (N) availability will exert an increasingly limiting effect on plants' ability to store additional carbon (C) under rising CO 2 , but these mechanisms are not well understood. Here, we review findings from elevated CO 2 experiments using a plant economics framework, highlighting how ecosystem responses to elevated CO 2 may depend on the costs and benefits of plant interactions with mycorrhizal fungi and symbiotic N-fixing microbes. We found that N-acquisition efficiency is positively correlated with leaf-level photosynthetic capacity and plant growth, and negatively with soil C storage. Plants that associate with ectomycorrhizal fungi and N-fixers may acquire N at a lower cost than plants associated with arbuscular mycorrhizal fungi. However, the additional growth in ectomycorrhizal plants is partly offset by decreases in soil C pools via priming. Collectively, our results indicate that predictive models aimed at quantifying C cycle feedbacks to global change may be improved by treating N as a resource that can be acquired by plants in exchange for energy, with different costs depending on plant interactions with microbial symbionts., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

The availability of global microbial diversity data, collected using standardized metabarcoding techniques, makes microorganisms promising models for investigating the role of regional and local factors in driving biodiversity. Here we modelled the global diversity of symbiotic arbuscular mycorrhizal (AM) fungi using currently available data on AM fungal molecular diversity (small subunit (SSU) ribosomal RNA (rRNA) gene sequences) in field samples. To differentiate between regional and local effects, we estimated species pools (sets of potentially suitable taxa) for each site, which are expected to reflect regional processes. We then calculated community completeness, an index showing the fraction of the species pool present, which is expected to reflect local processes. We found significant spatial variation, globally in species pool size, as well as in local and dark diversity (absent members of the species pool). Species pool size was larger close to areas containing tropical grasslands during the last glacial maximum, which are possible centres of diversification. Community completeness was greater in regions of high wilderness (remoteness from human disturbance). Local diversity was correlated with wilderness and current connectivity to mountain grasslands. Applying the species pool concept to symbiotic fungi facilitated a better understanding of how biodiversity can be jointly shaped by large-scale historical processes and recent human disturbance., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

Contents 1314 I. 1315 II. 1316 III. 1322 IV. 1323 V. 1325 VI. 1326 VII. 1326 VIII. 1327 1328 References 1328 SUMMARY: Invasions of alien plants are typically studied as invasions of individual species, yet interactions between plants and symbiotic fungi (mutualists and potential pathogens) affect plant survival, physiological traits, and reproduction and hence invasion success. Studies show that plant-fungal associations are frequently key drivers of plant invasion success and impact, but clear conceptual frameworks and integration across studies are needed to move beyond a series of case studies towards a more predictive understanding. Here, we consider linked plant-fungal invasions from the perspective of plant and fungal origin, simplified to the least complex representations or 'motifs'. By characterizing these interaction motifs, parallels in invasion processes between pathogen and mutualist fungi become clear, although the outcomes are often opposite in effect. These interaction motifs provide hypotheses for fungal-driven dynamics behind observed plant invasion trajectories. In some situations, the effects of plant-fungal interactions are inconsistent or negligible. Variability in when and where different interaction motifs matter may be driven by specificity in the plant-fungal interaction, the size of the effect of the symbiosis (negative to positive) on plants and the dependence (obligate to facultative) of the plant-fungal interaction. Linked plant-fungal invasions can transform communities and ecosystem function, with potential for persistent legacies preventing ecosystem restoration., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

Fine-root traits play key roles in ecosystem processes, but the drivers of fine-root trait diversity remain poorly understood. The plant economic spectrum (PES) hypothesis predicts that leaf and root traits evolved in coordination. Mycorrhizal association type, plant growth form and climate may also affect root traits. However, the extent to which these controls are confounded with phylogenetic structuring remains unclear. Here we compiled information about root and leaf traits for > 600 species. Using phylogenetic relatedness, climatic ranges, growth form and mycorrhizal associations, we quantified the importance of these factors in the global distribution of fine-root traits. Phylogenetic structuring accounts for most of the variation for all traits excepting root tissue density, with root diameter and nitrogen concentration showing the strongest phylogenetic signal and specific root length showing intermediate values. Climate was the second most important factor, whereas mycorrhizal type had little effect. Substantial trait coordination occurred between leaves and roots, but the strength varied between growth forms and clades. Our analyses provide evidence that the integration of roots and leaves in the PES requires better accounting of the variation in traits across phylogenetic clades. Inclusion of phylogenetic information provides a powerful framework for predictions of belowground functional traits at global scales., (© 2017 The Authors. New Phytologist © 2017 New Phytologist Trust.)

The common occurrence of mutualistic interactions between plants and root symbionts is problematic. As the delivery of benefit to hosts involves costs to symbionts, symbionts that provide reduced benefit to their host are expected to increase in frequency. Plants have been shown to allocate preferentially to the most efficient symbiont and this preferential allocation may stabilize the mutualism. I construct a general model of the interactive feedbacks of host preferential allocation and the dynamics of root symbiont populations to evaluate the stability of nutritional mutualisms. Preferential allocation can promote the evolution of mutualism even when the cost to the symbiont is very large. Moreover, the physiological plasticity of preferential allocation likely leads to coexistence of beneficial and nonbeneficial symbionts. For arbuscular mycorrhizal fungi, which facilitate plant uptake of phosphorus (P), the model predicts greater P transfer from these fungi per unit carbon invested with decreasing concentrations of soil P and with increasing concentrations of atmospheric CO2 , patterns that have been observed in laboratory and field studies. This framework connects physiological plasticity in plant allocation to population processes that determine mutualism stability and, as such, represents a significant step in understanding the stability and environmental patterns in mutualism., (© 2015 The Author. New Phytologist © 2015 New Phytologist Trust.)

Interactions between trees and microorganisms are tremendously complex and the multispecies networks resulting from these associations have consequences for plant growth and productivity. However, a more holistic view is needed to better understand trees as ecosystems and superorganisms, where many interacting species contribute to the overall stability of the system. While much progress has been made on microbial communities associated with individual tree niches and the molecular interactions between model symbiotic partners, there is still a lack of knowledge of the multi-component interactions necessary for holistic ecosystem-level understanding. We review recent studies in Populus to emphasize the importance of such holistic efforts across the leaf, stem and rooting zones, and discuss prospects for future research in these important ecosystems., (No claim to original US Government works. New Phytologist © 2014 New Phytologist Trust.)

In Brazil, the campos rupestres occur over the Brazilian shield, and are characterized by acidic nutrient-impoverished soils, which are particularly low in phosphorus (P). Despite recognition of the campos rupestres as a global biodiversity hotspot, little is known about the diversity of P-acquisition strategies and other aspects of plant mineral nutrition in this region. To explore nutrient-acquisition strategies and assess aspects of plant P nutrition, we measured leaf P and nitrogen (N) concentrations, characterized root morphology and determined the percentage arbuscular mycorrhizal (AM) colonization of 50 dominant species in six communities, representing a gradient of soil P availability. Leaf manganese (Mn) concentration was measured as a proxy for carboxylate-releasing strategies. Communities on the most P-impoverished soils had the highest proportion of nonmycorrhizal (NM) species, the lowest percentage of mycorrhizal colonization, and the greatest diversity of root specializations. The large spectrum of leaf P concentration and variation in root morphologies show high functional diversity for nutritional strategies. Higher leaf Mn concentrations were observed in NM compared with AM species, indicating that carboxylate-releasing P-mobilizing strategies are likely to be present in NM species. The soils of the campos rupestres are similar to the most P-impoverished soils in the world. The prevalence of NM strategies indicates a strong global functional convergence in plant mineral nutrition strategies among severely P-impoverished ecosystems., (© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust.)

The loss of carbon from roots (rhizodeposition) and the consequent proliferation of microorganisms in the surrounding soil, coupled with the physical presence of a root and processes associated with nutrient uptake, gives rise to a unique zone of soil called the rhizosphere. In this review, we bring together evidence to show that roots can directly regulate most aspects of rhizosphere C flow either by regulating the exudation process itself or by directly regulating the recapture of exudates from soil. Root exudates have been hypothesized to be involved in the enhanced mobilization and acquisition of many nutrients from soil or the external detoxification of metals. With few exceptions, there is little mechanistic evidence from soil-based systems to support these propositions. We conclude that much more integrated work in realistic systems is required to quantify the functional significance of these processes in the field. We need to further unravel the complexities of the rhizosphere in order to fully engage with key scientific ideas such as the development of sustainable agricultural systems and the response of ecosystems to climate change. Contents I. Introduction 460 II. What is rhizodeposition? 460 III. Regulation of rhizodeposition 460 IV. How large is the root exudation C flux? 463 V. How responsive is the root exudation C flux? 463 VI. How responsive is the microbial community to root exudation? 464 VII. The role of root exudates in nutrient acquisition 464 VIII. Mycorrhizal fungi and rhizodeposition 471 IX. Future thoughts 474 Acknowledgements 474 References 474.

•  Exotic earthworms can modify or eliminate surface organic (Oe/Oa) horizons in cold-temperate forest ecosystems and have profound effects on the forest soil environment, especially the rooting zone. •  We examined the effects of earthworm colonization of northern hardwood forest soils on the abundance and morphology of mycorrhizal fungi associated with sugar maple ( Acer saccharum ). We compared mycorrhizal associations of areas of earthworm invasion with those of reference (no-worm) areas in Arnot Forest, central New York, USA. •  The organic horizon in reference areas had higher mycorrhizal colonization rates and higher colonized root length than did surface layers in areas with active earthworm populations. Hyphal coils were more abundant and also formed a greater proportion of total fungal colonization in reference plots. Vesicles were more abundant and were a higher contribution to total colonization in earthworm plots, indicating a possible stress response to the presence of earthworms. •  By affecting mycorrhizal colonization and morphology, earthworms may influence nutrient uptake capacity of dominant forest species. Our results suggest that a profound change in the mycorrhizal system will be one component of the potential ecosystem effects of invasion of new forest habitat by nonnative earthworms.

Here, the coevolution of mycorrhizal fungi and roots is assessed in the light of evidence now available, from palaeobotanical and morphological studies and the analysis of DNA-based phylogenies. The first bryophyte-like land plants, in the early Devonian (400 million years ago), had endophytic associations resembling vesicular-arbuscular mycorrhizas (VAM) even before roots evolved. Mycorrhizal evolution would have progressed from endophytic hyphae towards balanced associations where partners were interdependent due to the exchange of limiting energy and nutrient resources. Most mycorrhizas are mutualistic, but in some cases the trend for increasing plant control of fungi culminates in the exploitative mycorrhizas of achlorophyllous, mycoheterotrophic plants. Ectomycorrhizal, ericoid and orchid mycorrhizas, as well as nonmycorrhizal roots, evolved during the period of rapid angiosperm radiation in the Cretaceous. It is hypothesised that roots gradually evolved from rhizomes to provide more suitable habitats for mycorrhizal fungi and provide plants with complex branching and leaves with water and nutrients. Selection pressures have caused the morphological divergence of roots with different types of mycorrizas. Root cortex thickness and exodermis suberization are greatest in obllgately mycorrhizal plants, while nonmycorrhizal plants tend to have fine roots, with more roots hairs and relatively advanced chemical defences. Major coevolutionary trends and the relative success of plants with different root types are discussed. Contents Summary 275 I. Introduction 276 II. Mycorrhizal Fungi 276 III. The Dawn of Mycorrhizas 279 IV. Mycorrhizal Associations of Living and Extinct Plants 282 V. Evolution of Roots 288 VI. The Root as a Habitat for Fungi 290 VII. Mycorrhizal Evolution Trends 295 Acknowledgements 298 References 298.

Dark septate root endophytes (DSE) are conidial or sterile fungi (Deuteromycotina, Fungi Imperfecti) likely to be ascomycetous and colonizing plant roots. They have been reported for nearly 600 plant species representing about 320 genera and 100 families. DSE fungi occur from the tropics to arctic and alpine habitats and comprise a heterogeneous group that functionally and ecologically overlaps with soil fungi, saprotrophic rhizoplane-inhabiting fungi, obligately and facultatively pathogenic fungi and mycorrhizal fungi. Numerous species of undescribed sterile and anamorphic taxa may also await discovery. Although DSE are abundant in washed root and soil samples from various habitats, and are easily isolated from surface-sterilized roots of ecto-, ectendo-, endo- and non-mycorrhizal host species, their ecological functions are little understood. Studies of DSE thus far have yielded inconsistent results and only poorly illustrate the role of DSE in their natural habitats. These inconsistencies are largely due to the uncertain taxonomic affinities of the strains of DSE used. In addition, because different strains of a single anamorph taxon seem to vary greatly in function, no clear generalizations on their ecological role have been drawn. This paper reviews the current literature on DSE and the ecology and discusses the need for and direction of future research.

Spatial variations in the capacity of propagules of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi to form associations in their natural habitats were investigated using bioassays with bait plants grown in intact cores of forest soil. These cores were collected from a sclerophyllous forest community dominated by Eucalyptus marginata Donn ex Smith (jarrah) and E. calophylla Lindley (marri) trees with a diverse shrub understorey in the mediterranean (winter rainfall) climatic zone of Western Australia. Small-scale (adjacent core) variations in the capacity of AM fungi to form associations were found to be as substantial as differences between locations 1 5 m apart. Comparisons of AM fungus colonization patterns within the roots of seedlings growing in the same core indicated that there was considerable spatial heterogeneity in the inoculums potential of 'individual' fungi within these 1 1 volumes of soil. A second experiment included bait plants to measure ECM formation as welt as AM formation and also considered the impact of soil disturbance. The disruption of hyphal networks reduced mycorrhizal formation somewhat, but it still remained highly variable. Some of this spatial heterogeneity could be attributed to differences in the organic matter content, length of fungal hyphae, or length of old mycorrhizal roots, measured within soil cores. In jarrah forest soil, mycelial systems of AM and ECM fungi apparently were localized in separate domains, and there were also zones where non-mycorrhizal roots (mostly cluster roots produced by members of the Proteaceae) predominated. More research is required to determine the size of domains of mycorrhizal mycelial systems in soils, how these spatial patterns change with time, and if they are associated with zones of resource utilization by different 'functional groups' of roots.

The impact of rust infection (Puccinia lagenophorae Cooke) on growth of mycorrhizal and non-mycorrhizal Senecio vulgaris L. was examined under three levels of phosphorus fertilization. Seeds were then collected from infected and control plants and germination, seed chemistry and offspring vigour were examined. Rust infection did not become established on plants which were phosphorus limited, but on hosts fed with medium or high levels of P, the proportion of total leaf number exhibiting infection was a function of leaf production. Capitula production was decreased by rust infection and increased by mycorrhizal colonization in plants fed with a medium concentration of P. In plants fed with a high P concentration, leaf number was reduced by rust infection of mycorrhizal plants, whilst rust was deleterious to both mycorrhizal and non-mycorrhizal plants grown with a medium P supply. However, the effects of the rust were less marked in the mycorrhizal plants. Seed production and weight were unaffected by fungal treatments, although percentage germination of the resultant seeds was reduced by maternal mycorrhizal colonization and enhanced in seed from rusted, mycorrhizal plants grown at high soil P. The amount of total C, N and P was also reduced by maternal mycorrhizal infection at high soil P and increased in seeds from rusted, mycorrhizal mothers. Vigour of offspring produced from treated mother plants was influenced by mycorrhizal × phosphorus and rust × phosphorus interactions with respect to bud and leaf numbers. Mycorrhizal infection of mother plants grown with a medium P supply benefited the offspring as increased leaf and bud production was observed, but rusted mothers at the same P level produced offspring with reduced leaf number and potential fecundity.

The spatial distribution of genets in the ectomycorrhizal fungus Suillus bovinus (L.: Fr.) Roussel were studied in somatic incompatibility pairings of isolates from five Scots pine (Pinus sylvestris L.) stands differing in forest history and age, With increasing forest age, the size of genets increased while the number of genets and production of sporocarps per unit area decreased. There was an estimated 700-5700 genets ha -1 in younger forests and 30-120 ha -1 in older ones. The maximum size of genets was 1.7-5.3 m in the younger forest and to 17.5 m in the older ones. The production of sporocarps per unit area decreased with increasing forest age. Furthermore, production of sporocarps increased nonlinearly with the size of the genet, suggesting that genets become fragmented. Ergosterol measurements indicated that the fungal biomass of one genet consisted of 20-45% sporocarps and 55-80% mycorrhiza, not including extramatrical mycelia. Mycorrhizal aggregations in soil were mapped in two 5 m 2 areas and, based on somatic incompatibility tests, all were round CO belong to the same genet as sporocarps present above ground. Production of spores per sporocarp was estimated to be 1.1-12.8 × 10 8 . The observations are discussed in terms of population ecology.

The symbiotic establishment of mycorrhizal fungi on plant roots is affected in various ways by the other microorganisms of the rhizosphere, and more especially by bacteria. This review discusses the case of some of these bacteria which consistently promote mycorrhizal development, leading to the concept of 'mycorrhization' helper bacteria (MHBs). Examples of MHB evidence are given from the literature, with special reference to the Douglas fir (Pseudotsuga menzeisii Mirb. Franco) Laccaria laccata Scop, ex Fr. ectomycorrhizal combination which has been more extensively studied. The fungal specificity of some MHBs and the various mechanisms underlying their effect are discussed, considering five hypotheses: effects on the receptivity of the root, effects on the root-fungus recognition, effects on the fungal growth, modification of the rhizospheric soil, and effects on the germination of the fungal propagule. MHBs are then considered for their ecological and evolutionary implications, and examples of practical applications in forest nurseries are given: when added to the fungal inoculum, MHBs can improve the success of ectomycorrhizal inoculation of planting stocks with fungi selected for their outstanding growth stimulation after outplanting. The conclusion points out a number of fundamental questions which remain unanswered about mycorrhization helper bacteria and suggests some investigation priorities in this new field of mycorrhiza research. Contents Summary 197 I. Introduction 197 II. Evidence for helper bacteria 198 III. Fungus-specificity of MHBs 200 IV. Mechanisms underlying the MHB effect 201 V. Ecological and evolutionary implications of MHBs 205 VI. Practical applications of MHBs 206 VII. Conclusions and perspectives 208 Acknowledgements 208 References 208.

Ascomycetous fungi, identified by the presence of simple septa and Woronin bodies, form a range of highly specialized associations with the unicellular rhizoids of 46 (16.2%) of the 284 species of British liverworts. The majority of these are in the two jungermannialian suborders Lepidoziineae and Cephaloziineae. Thirty three species (11.6%) also possess flagelliform axes. These extend to depths of 20-30 cm in peaty substrata and in most species bear abundant fungus-infected rhizoids. When grown in sand or water culture, isolated from their natural substrata (sandstone rocks, peat, Sphagnum, rotten wood), the hepatics are free from rhizoid infections. On returning the plants to their native substrata, the rhizoid-fungus associations are re-established. Each rhizoid or fascicle of rhizoids is infected independently and a continuous strand of fungus-containing cells is absent. Internally the flagelliform axes contain elongate parenchyma cells with abundant plasmodesmata in their transverse end walls. The apices of the axes are invested by mucilage and subapical amyloplasts appear to act as graviperceptive particles. The morphology of the flagelliform axes and the ultrastructure of the rhizoid-fungus association varies greatly between taxa. The Lepidoziaceae and Calypogeiaceae produce fascicles of rhizoids from the bases of rudimentary leaves whereas the flagelliform axes in the Cephaloziaceae are leafless. In Cladopodiella the fungus forms a pseudoparenchymatous sheath around the rips of swollen rhizoids. Development of swollen rhizoids precedes fungal infection in Lepidozia, Kurzia and Telaranea but not in other genera. Hyphae penetrating the basal walls of the rhizoids in Cephalosia, Nowellia, Calypogeia and Odantoschisma are ensheathed by host Wall material. These diverse patterns of fungal infection suggest independent, and probably relatively recent, evolution in the various genera. The flagelliform axes in hepatics appear to be major organs of assimilation and perennation structurally and functionally analogous in the former capacity to vascular plant mycorrhizas and particularly the hairs roots in Ericaceae.