Early events in the life of apple roots: variation in root growth rate is linked to mycorrhizal and nonmycorrhizal fungal colonization

Early events of mycorrhizal and nonmycorrhizal fungal colonization in newly-emerging roots of mature apple (Malus domestica Borkh) trees were characterized to determine the relationship of these events to fine root growth rate and development. New roots were traced on root windows to measure growth and then collected and stained to quantify microscopically the presence of mycorrhizal and nonmycorrhizal fungal structures. Most new roots were colonized by either mycorrhizal or nonmycorrhizal fungi but none less 25 days old were ever internally colonized by both. Compared to nonmycorrhizal colonization, mycorrhizal colonization was associated with faster growing roots and roots that grew for a longer duration, leading to longer roots. While either type of fungi was observed in roots as soon as 3 days after root emergence, intraradical colonization by mycorrhizal fungi was generally faster (peaking at 7 to 15 days) than that by nonmycorrhizal fungi and often occurred more frequently in younger roots. Only 15 to 35% of the roots had no fungal colonization by 30 days after emergence. This study provides the first detailed examination of the early daily events of mycorrhizal and nonmycorrhizal fungal colonization in newly emerging roots under field conditions. We observed marked discrimination of roots between mycorrhizal and nonmycorrhizal fungi and provide evidence that mycorrhizal fungi may select for faster growing roots and possibly influence the duration of root growth by non-nutritional means.     

An ecological view of the formation of VA mycorrhizas

In spite of the major advances in understanding the functioning of symbioses between plants and arbuscular mycorrhizal fungi, details of the ecology of mycorrhizal fungi are not well documented. The benefits of the association are related to the timing and extent of colonization of roots, and fungi differ in their contribution to plant growth and presumably to soil aggregation. Knowledge of the processes that lead to successful colonization of roots by beneficial fungi at appropriate times for the host plants will form the basis of guidelines for soil management to maximize the benefits from the symbiosis. Fungi differ in the manner and extent to which they colonize roots. They also differ in their capacity to form propagules. The importance of hyphae, spores and propagules within living or dead mycorrhizal roots also differs among species and for the same species in different habitats. The relationships between colonization of roots and propagule formation, and between propagule distribution and abundance and subsequent mycorrhiza formation, for different fungi in field environments, are not well understood. Methods for quantifying mycorrhizal fungi are not especially suitable for distinguishing among different fungi within roots. Consequenctly, the dynamics of colonization of roots by different fungi, within and between seasons, have been little studied. Research is required that focuses on the dynamics of fungi within roots as well as on changes in the abundance of propagules of different fungi within soil. Interactions between fungi during the colonization of roots, the colonization of soil by hyphae and sporulation are all poorly understood. Without knowledge of these processes, it will by difficult to predict the likely success of inoculation with introduced fungi. Such knowledge is also required for selecting soil management procedures to enhance growth and survival of key species within the population. The relative tolerance of various fungi to perturbations in their surroundings will provide a basis for identifying those fungi that are likely to persist in specific environments. The processes that influence mycorrhizal fungi in field soils can be identified in controlled studies. However, greater emphasis is required on studying these processes with mixed populations of fungi. The role played by diversity within populations of mycorrhizal fungi is virtually unexplored.     

Defoliation and mycorrhizal symbiosis: a functional balance between carbon sources and below-ground sinks

Herbivory is generally assumed to negatively influence mycorrhizal fungi because of reduced photosynthate to support mycorrhizae following defoliation. We examined effects of 60% and 100% defoliation (excluding current year needles) on tree growth and ectomycorrhizal associations of 10–15 year old Scots pines ( Pinus sylvestris ). Over 98% of short roots were colonized by mycorrhizal fungi, and contrary to expectation, defoliation did not decrease the proportion of living fungi in fine roots. Furthermore, defoliation did not alter the ratios of produced needle biomass to the biomass of fine roots or living fungi in fine roots. The composition of mycorrhizal morphotypes was changed, however, which suggests competition among different mycorrhizal growth forms owing to their carbon demands. We propose that these outcomes are a consequence of a functional balance between carbon sources in plant foliage and below-ground sinks, i.e. growing roots and mycorrhizal associates.     

丛枝菌根真菌菌丝体吸附重金属的潜力及特征

应用玻璃珠分室培养系统获得丛枝菌根真菌材料,研究了离体真菌菌丝体对pH缓冲体系中Zn、Cd和Mn等金属离子的吸附特征。试验结果表明,真菌菌丝体对各金属离子吸附能力差异显著,对Cd最强,Zn次之,Mn最弱。试验条件下,菌体可分别吸附相当于自身干物重1.6%的Mn、2.8%的Zn和13.3%的Cdo吸附于菌丝体的Cd2+绝大部分可以被Ca2+交换吸附。另外研究了宿主植物根系对Cd的吸附作用,证实菌根真菌侵染改变了根系的吸附特性,相对于非菌根根系,菌根的CEC较高,对Cd的吸附能力较强。试验结果为重金属污染条件下丛枝菌根强化根系的屏障作用提供了直接证据。     

Monotropa uniflora plants of eastern Massachusetts form mycorrhizae with a diversity of russulacean fungi

Plant species in the subfamily Monotropoideae are mycoheterotrophs; they obtain fixed carbon from photosynthetic plants via a shared mycorrhizal network. Previous findings show mycoheterotrophic plants exhibit a high level of specificity to their mycorrhizal fungi. In this study we explore the association of mycorrhizal fungi and Monotropa uniflora (Monotropoideae: Ericaceae) in eastern North America. We collected M. uniflora roots and nearby basidiomycete sporocarps from four sites within a 100 km2 area in eastern Massachusetts. We analyzed DNA sequences of the internal transcribed spacer region (ITS) from the fungal nuclear ribosomal gene to assess the genetic diversity of fungi associating with M. uniflora roots. In this analysis we included 20 ITS sequences from Russula sporocarps collected nearby, 44 sequences of Russula or Lactarius species from GenBank and 12 GenBank sequences of fungi isolated from M. uniflora roots in previous studies. We found that all 56 sampled M. uniflora mycorrhizal fungi were members of the Russulaceae, confirming previous research. The analysis showed that most of the diversity of mycorrhizal fungi spreads across the genus Russula. ITS sequences of the mycorrhizal fungi consisted of 20 different phylotypes: 18 of the genus Russula and two of Lactarius, based on GenBank searches. Of the sampled plants, 57% associated with only three of the 20 mycorrhizal fungi detected in roots, and of the 25 sporocarp phylotypes collected three, were associated with M. uniflora. Furthermore the results indicate that the number of different fungal phylotypes associating with M. uniflora of eastern North America is higher than that of western North America but patterns of fungal species abundance might be similar between mycorrhizae from the two locations.     

锦绣杜鹃菌根真菌rDNA ITS序列分析及接种效应研究

利用rDNA ITS序列对锦绣杜鹃菌根真菌的16个菌株进行了分类分析。根据菌株ITS序列全长计算各菌株间序列相似度和遗传距离,并与GenBank中最相似菌株序列构建系统发育树。结果表明：16个菌株在系统树上聚为3个大分支。其中7个菌株在支持率为100%的基础上与树粉孢属真菌Oidiodendron sp.聚为一类;2个菌株与未鉴定的杜鹃花科植物根系真菌unidentified root associated fungi聚为一类,支持率为100%;其他7个菌株在98%的支持率上与几种未命名的欧石楠类菌根真菌     

Diversity and Spatial Structure of Belowground Plant–Fungal Symbiosis in a Mixed Subtropical Forest of Ectomycorrhizal and Arbuscular Mycorrhizal Plants

Plant–mycorrhizal fungal interactions are ubiquitous in forest ecosystems. While ectomycorrhizal plants and their fungi generally dominate temperate forests, arbuscular mycorrhizal symbiosis is common in the tropics. In subtropical regions, however, ectomycorrhizal and arbuscular mycorrhizal plants co-occur at comparable abundances in single forests, presumably generating complex community structures of root-associated fungi. To reveal root-associated fungal community structure in a mixed forest of ectomycorrhizal and arbuscular mycorrhizal plants, we conducted a massively-parallel pyrosequencing analysis, targeting fungi in the roots of 36 plant species that co-occur in a subtropical forest. In total, 580 fungal operational taxonomic units were detected, of which 132 and 58 were probably ectomycorrhizal and arbuscular mycorrhizal, respectively. As expected, the composition of fungal symbionts differed between fagaceous (ectomycorrhizal) and non-fagaceous (possibly arbuscular mycorrhizal) plants. However, non-fagaceous plants were associated with not only arbuscular mycorrhizal fungi but also several clades of ectomycorrhizal (e.g., Russula) and root-endophytic ascomycete fungi. Many of the ectomycorrhizal and root-endophytic fungi were detected from both fagaceous and non-fagaceous plants in the community. Interestingly, ectomycorrhizal and arbuscular mycorrhizal fungi were concurrently detected from tiny root fragments of non-fagaceous plants. The plant–fungal associations in the forest were spatially structured, and non-fagaceous plant roots hosted ectomycorrhizal fungi more often in the proximity of ectomycorrhizal plant roots. Overall, this study suggests that belowground plant–fungal symbiosis in subtropical forests is complex in that it includes “non-typical” plant–fungal combinations (e.g., ectomycorrhizal fungi on possibly arbuscular mycorrhizal plants) that do not fall within the conventional classification of mycorrhizal symbioses, and in that associations with multiple functional (or phylogenetic) groups of fungi are ubiquitous among plants. Moreover, ectomycorrhizal fungal symbionts of fagaceous plants may “invade” the roots of neighboring non-fagaceous plants, potentially influencing the interactions between non-fagaceous plants and their arbuscular-mycorrhizal fungal symbionts at a fine spatial scale.     

Relationship between mycorrhizal fungi and functional traits in absorption roots: research progress and synthesis

吸收根(absorption root)一般是指根枝系统末端少数几级具有初生结构、负责物质吸收的根。吸收根功能性状被广泛用于评价和预测植物个体到生态系统水平上的一系列功能和过程。菌根真菌侵染是吸收根的一个关键性状, 它可以深刻影响吸收根的形态、结构, 以及功能性状之间的关系。该文针对与吸收功能密切相关的菌根真菌与根毛和根直径之间的关系进行了研究综述, 提出了真菌侵染、根毛和化学防御之间关系的一个假说; 探讨了温带和热带不同类型的吸收根如何通过菌根真菌影响根的功能性状, 从而适应不同的水热条件、养分状况和能量消耗; 提出一些需要关注的议题和研究方向, 以期为菌根真菌与吸收根功能性状之间关系的研究提供借鉴。     

亚热带典型树种根毛特征及其与共生真菌的关系

根毛和共生真菌增加了吸收面积,提高了植物获取磷等土壤资源的能力。由于野外原位观测根表微观结构较为困难,吸收细根、根毛、共生真菌如何相互作用并适应土壤资源供应,缺乏相应的数据和理论。该研究以受磷限制的亚热带森林为对象,选取了21种典型树种,定量了根毛存在情况、属性变异,分析了根毛形态特征与共生真菌侵染率、吸收细根功能属性之间的关系,探讨了根表结构对低磷土壤的响应和适应格局。结果表明:1)在亚热带森林根毛不是普遍存在的, 21个树种中仅发现7个树种存有根毛, 4个为丛枝菌根(AM)树种, 3个为外生菌根(ECM)树种。其中,马尾松(Pinus massoniana)根毛出现率最高,为86%;2)菌根类型是理解根-根毛-共生真菌关系的关键,AM树种根毛密度与共生真菌侵染率正相关,但ECM树种根毛直径与共生真菌侵染率负相关; 3) AM树种根毛长度和根毛直径、ECM树种根毛出现率与土壤有效磷含量呈负相关关系。该研究揭示了不同菌根类型树种根毛-共生真菌-根属性的格局及相互作用,为精细理解养分获取策略奠定了基础。     

Mycorrhizal symbioses found in roots of fern and its relatives in Korea

Mycorrhizal symbioses were found in the roots of 45 out of 59 species of pteridophytes collected in Korea. The mycorrhizal fungi were colonized in the root cortical cells, primarily in terrestrial species, but rarely in epiphytic or aquatic pteridophytes. Mycorrhizae that are typically found in orchid colonized the roots of the epiphytic pteridophytes, but not in other species. These were the first observations of orchid mycorrhizae in pteridophytes. Arbuscular mycorrhizal fungi were examined after staining, then confirmed with PCR, using a specific primer. This is the first report of arbuscular mycorrhizal colonization in the roots of pteridophyte species in Asia.     

The developmental ecology of mycorrhizal associations in mayapple, Podophyllum peltatum, Berberidaceae

Associations between plants and arbuscular mycorrhizal (AM) fungi are widespread and well-studied. Yet little is known about the pattern of association between clonal plants and AM fungi. Here we report on the pattern of mycorrhizal association within the rhizome systems of mayapple, Podophyllum peltatum. Mayapple is a long-lived understory clonal herb that is classified as obligately mycorrhizal. We found that while all mayapple rhizome systems maintained mycorrhizal associations, the percent colonization of roots by AM fungi differed among ramets of different age. The highest concentrations of AM fungi were in the roots of intermediate-aged ramets, while roots beneath the youngest ramet were not colonized. This pattern of ramet age or position-dependent colonization was observed in two separate studies; each conducted in a different year and at a different site. The pattern of AM fungal colonization of mayapple rhizome systems suggests that the mycorrhizal relationship is facultative at the ramet level. This conclusion is reinforced by our observation that augmentation of soil phosphate lowers root colonization by AM fungi. We also found that soil phosphate concentrations were depleted by ca. 1% under the same ramet positions where roots bore the highest AM fungal loads. Three non-exclusive hypotheses are proposed regarding the mechanisms that might cause this developmentally dependent pattern of mycorrhizal association.     

Communities, populations and individuals of arbuscular mycorrhizal fungi

Arbuscular mycorrhizal fungi in the phylum Glomeromycota are found globally in most vegetation types, where they form a mutualistic symbiosis with plant roots. Despite their wide distribution, only relatively few species are described. The taxonomy is based on morphological characters of the asexual resting spores, but molecular approaches to community ecology have revealed a considerable unknown diversity from colonized roots. Although the lack of genetic recombination is not unique in the fungal kingdom, arbuscular mycorrhizal fungi are probably ancient asexuals. The long asexual evolution of the fungi has resulted in considerable genetic diversity within morphologically recognizable species, and challenges our concepts of individuals and populations. This review critically examines the concepts of species, communities, populations and individuals of arbuscular mycorrhizal fungi.     

Mycorrhizal status and diversity of fungal endophytes in roots of common buckwheat (Fagopyrum esculentum) and tartary buckwheat (F. tataricum)

To determine the mycorrhizal status and to identify the fungi colonising the roots of the plants, common buckwheat (Fagopyrum esculentum) and tartary buckwheat (F. tataricum) were inoculated with an indigenous fungal mixture from a buckwheat field. Root colonisation was characterised by the hyphae and distinct microsclerotia of dark septate endophytes, with occasional arbuscules and vesicles of arbuscular mycorrhizal fungi. Sequences of arbuscular mycorrhizal fungi colonising tartary buckwheat clustered close to the Glomus species group A. Sequences with similarity to the Ceratobasidium/Rhizoctonia complex, a putative dark septate endophyte fungus, were amplified from the roots of both common and tartary buckwheat. To the best of our knowledge, this is the first report of arbuscular mycorrhizal colonisation in tartary buckwheat and the first molecular characterisation of these fungi that can colonise both of these economically important plant species.     

Wood-decay fungi in fine living roots of conifer seedlings

The mycorrhizal basidiomycetes are known to have multiple, independent evolutionary origins from saprotrophic ancestors. To date, a number of studies have revealed functional resemblance of mycorrhizal fungi to free-living saprotrophs, but information on the ability of saprotrophic fungi to perform as mycorrhizal symbionts is scarce. Here, the objective was to investigate the ability of three wood-decay fungi, Phlebiopsis gigantea, Phlebia centrifuga and Hypholoma fasciculare, to colonize fine roots of conifer seedlings. For each fungus, mycorrhizal syntheses were attempted with Picea abies and Pinus sylvestris. After 24 wk, isolation of fungi and direct sequencing of fungal internal transcribed spacer (ITS) rDNA were carried out from healthy-looking surface-sterilized root tips that yielded both pure cultures and ITS sequences of each inoculated strain. Mycelial mantle of P. gigantea was frequently formed on root tips of P. abies, and microscopical examination has shown the presence of intercellular hyphae inside the roots. The results provide evidence of the ability of certain wood-decay fungi to colonise fine roots of tree seedlings.     

Mycorrhizal fungi of Vanilla: diversity, specificity and effects on seed germination and plant growth

Mycorrhizal fungi are essential for the germination of orchid seeds. However, the specificity of orchids for their mycorrhizal fungi and the effects of the fungi on orchid growth are controversial. Mycorrhizal fungi have been studied in some temperate and tropical, epiphytic orchids, but the symbionts of tropical, terrestrial orchids are still unknown. Here we study diversity, specificity and function of mycorrhizal fungi in Vanilla, a pantropical genus that is both terrestrial and epiphytic. Mycorrhizal roots were collected from four Vanilla species in Puerto Rico, Costa Rica and Cuba. Cultured and uncultured mycorrhizal fungi were identified by sequencing the internal transcribed spacer region of nuclear rDNA (nrITS) and part of the mitochondrial ribosomal large subunit (mtLSU), and by counting number of nuclei in hyphae. Vanilla spp. were associated with a wide range of mycorrhizal fungi: Ceratobasidium, Thanatephorus and Tulasnella. Related fungi were found in different species of Vanilla, although at different relative frequencies. Ceratobasidium was more common in roots in soil and Tulasnella was more common in roots on tree bark, but several clades of fungi included strains from both substrates. Relative frequencies of genera of mycorrhizal fungi differed significantly between cultured fungi and those detected by direct amplification. Ceratobasidium and Tulasnella were tested for effects on seed germination of Vanilla and effects on growth of Vanilla and Dendrobium plants. We found significant differences among fungi in effects on seed germination and plant growth. Effects of mycorrhizal fungi on Vanilla and Dendrobium were similar: a clade of Ceratobasidium had a consistently positive effect on plant growth and seed germination. This clade has potential use in germination and propagation of orchids. Results confirmed that a single orchid species can be associated with several mycorrhizal fungi with different functional consequences for the plant.     

Effects of cadmium and mycorrhizal fungi on growth, fitness, and cadmium accumulation in flax (Linum usitatissimum; Linaceae)

? Premise of the study: Agricultural soils have become contaminated with a variety of heavy metals, including cadmium. The degree to which soil contaminants affect plants may depend on symbiotic relationships between plant roots and soil microorganisms. We examined (1) whether mycorrhizal fungi counteract the potentially negative effects of cadmium on the growth and fitness of flax (Linum usitatissimum) and (2) whether mycorrhizal fungi affect the accumulation of cadmium within plant parts. ? Methods: Two flax cultivars (Linott and Omega) were grown in three soil cadmium environments (0, 5, and 15 ppm). Within each cadmium environment, plants were grown in either the presence or absence of mycorrhizal fungi. Upon senescence, we measured growth and fitness and quantified the concentration of cadmium within plants. ? Key results: Soil cadmium significantly decreased plant fitness, but did not affect plant growth. Mycorrhizal fungi, which were able to colonize roots of plants growing in all cadmium levels, significantly increased plant growth and fitness. Although mycorrhizal fungi counteracted the negative effects of cadmium on fruit and seed production, they also enhanced the concentration of cadmium within roots, fruits, and seeds. ? Conclusions: The degree to which soil cadmium affects plant fitness and the accumulation of cadmium within plants depended on the ability of plants to form symbiotic relationships with mycorrhizal fungi. The use of mycorrhizal fungi in contaminated agricultural soils may offset the negative effects of metals on the quantity of seeds produced, but exacerbate the accumulation of these metals in our food supply.     

Ectomycorrhizal colonization slows root decomposition: the post-mortem fungal legacy

The amount of carbon plants allocate to mycorrhizal symbionts exceeds that emitted by human activity annually. Senescent ectomycorrhizal roots represent a large input of carbon into soils, but their fate remains unknown. Here, we present the surprising result that, despite much higher nitrogen concentrations, roots colonized by ectomycorrhizal (EM) fungi lost only one-third as much carbon as non-mycorrhizal roots after 2 years of decomposition in a piñon pine ( Pinus edulis ) woodland. Experimentally excluding live mycorrhizal hyphae from litter, we found that live mycorrhizal hyphae may alter nitrogen dynamics, but the afterlife (litter-mediated) effects of EM fungi outweigh the influences of live fungi on root decomposition. Our findings indicate that a shift in plant allocation to mycorrhizal fungi could promote carbon accumulation in soil by this pathway. Furthermore, EM litters could directly contribute to the process of stable soil organic matter formation, a mechanism that has eluded soil scientists.     

Transport processes at the plant-fungus interface in mycorrhizal associations: physiological studies

The roots of most plants form symbiotic associations with mycorrhizal fungi. The net flux of nutrients, particularly phosphorus (P), from the soil into the plant is greater in mycorrhizal than in comparable non-mycorrhizal plants. However despite the widespread occurrence of mycorrhizal associations the processes controlling the transfer of solutes between the symbionts are poorly understood. To understand the mechanisms regulating the transfer of solutes information about conditions at the interface between plant and fungus is needed.Measurements of apoplastic and intracellular electrical potential difference in leek roots colonised by mycorrhizal fungi and estimates of cytosolic pH in fungal hyphae are presented. These and the implications for plant/fungal mineral nutrition in vesicular-arbuscular mycorrhizas are discussed.     

Arbuscular mycorrhizal fungi of the Glomus-group A lineage (Glomerales; Glomeromycota) detected in myco-heterotrophic plants from tropical Africa

We amplified and sequenced partial 18S rDNA of fungi in the roots of 11 African myco-heterotrophic plants out of four angiosperm families (Burmanniaceae, Thismiaceae, Triuridaceae, and Gentianaceae). The sequences were cladistically analyzed with published sequences of arbuscular mycorrhizal fungi. We show that all investigated African myco-heterotrophic plants are associated with arbuscular mycorrhizal fungi within a clade of Glomus (Glomus-group A). We reveal a fine-level mycorrhizal specificity for a particular set of arbuscular mycorrhizal fungi within Glomus-group A by Afrothismia hydra (Thismiaceae). Furthermore, we show that the roots of two myco-heterotrophic plant individuals, besides being colonized by representatives of Glomus-group A, also contain DNA of Acaulospora sp. Consequently, Acaulospora is interpreted as a facultative mycorrhizal associate.     

Soil fungal-arthropod responses to Populus tremuloides grown under enriched atmospheric CO2 under field conditions

We investigated the influence of elevated CO₂ and soil N availability on the growth of arbuscular mycorrhizal and non-mycorrhizal fungi, and on the number of mycophagous soil microarthropods associated with the roots of Populus tremuloides . CO₂ concentration did not significantly affect percentage infection of Populus roots by mycorrhizal or non-mycorrhizal fungi. However, the extra-radical hyphal network was altered both qualitatively and quantitatively, and there was a strong interaction between CO₂ and soil N availability. Under N-poor soil conditions, elevated CO₂ stimulated hyphal length by arbuscular mycorrhizal fungi, but depressed growth by non-mycorrhizal fungi. There was no CO₂ effect at high N availability. High N availability stimulated growth by opportunistic saprobic/pathogenic fungi. Soil mites were not affected by any treatment, but collembolan numbers were positively correlated with the increase in non-mycorrhizal fungi. Results indicate a strong interaction between CO₂ concentration and soil N availability on mycorrhizal functioning and on fungal-based soil food webs.