Decrease in diversity and changes in community composition of arbuscular mycorrhizal fungi in roots of apple trees with increasing orchard management intensity across a regional scale

Understanding which factors drive the diversity and community composition of arbuscular mycorrhizal fungi (AMF) is important due to the role of these soil micro‐organisms in ecosystem functioning and current environmental threats to AMF biodiversity. Additionally, in agro‐ecosystems, this knowledge may help to evaluate their use in making agriculture more sustainable. Here, we used 454‐pyrosequencing of small subunit rRNA gene amplicons to quantify AMF diversity and community composition in the roots of cultivated apple trees across 24 orchards in central Belgium. We aimed at identifying the factors (soil chemical variables, organic vs. conventional farming, and geographical location) that affect AMF diversity and community composition. In total, 110 AMF OTUs were detected, of which the majority belonged to the Glomeraceae (73%) and the Claroideoglomeraceae (19%). We show that soil characteristics and farming system, rather than the geographical location of the orchards, shape AMF communities on apple trees. Particularly, plant‐available P content of the soil was associated with lower AMF diversity. In orchards with a lower plant‐available P content of the soil (P < 100 mg/kg soil), we also found a significantly higher AMF diversity in organically managed orchards as compared to conventionally managed orchards. Finally, the degree of nestedness of the AMF communities was related to plant‐available P and N content of the soil, pointing at a progressive loss of AMF taxa with increasing fertilization. Overall, we conclude that a combination of organic orchard management and moderate fertilization may preserve diverse AMF communities on apple trees and that AMF in the roots of apple trees appear not to be dispersal limited at the scale of central Belgium.     

陆地农业生态系统丛枝菌根真菌物种多样性研究进展

丛枝菌根真菌(AMF)是一种古老的、在自然界中普遍存在的土壤微生物,能与大部分陆生植物形成互惠互利的菌根共生体.在这种共生关系中,AMF从植物获取自身生长所需碳源的同时,帮助宿主吸收氮、磷等营养物质.AMF在农业生态系统中具有重要作用,能够促进植物生长、改善作物品质、提高植物抗逆性、稳定土壤结构、维护生态平衡和维持农业可持续发展.本文总结了近几年来陆地农业生态系统AMF的研究进展,着重从我国陆地农业生态系统AMF物种多样性、AMF生物多样性时空分布特征及影响AMF多样性的因素等几个方面,综述了陆地农业生态系统AMF的物种多样性,并对以后的研究进行了展望.     

Role of soil microbes in the rhizospheres of plants growing on trace metal contaminated soils in phytoremediation

This article reviews recent developments in in situ bioremediation of trace metal contaminated soils, with particular reference to the microbial dynamics in the rhizospheres of plants growing on such soils and their significance in phytoremediation. In non-agricultural conditions, the natural role of plant growth promoting rhizobacteria (PGPR), P-solubilizing bacteria, mycorrhizal-helping bacteria (MHB) and arbuscular mycorrhizal fungi (AMF) in maintaining soil fertility is more important than in conventional agriculture, horticulture, and forestry where higher use of agrochemicals minimize their significance. These microbes initiate a concerted action when a particular population density is achieved, i.e. quorum sensing. AMF also recognize their host by signals released by host roots, allowing a functional symbiosis. AM fungi produce an insoluble glycoprotein, glomalin, which sequester trace elements and it should be considered for biostabilization leading to remediation of contaminated soils. Conclusions drawn from studies of metal uptake kinetics in solution cultures may not be valid for more complex field conditions and use of some combination of glasshouse and field experiments with organisms that occur within the same plant community is suggested. Phytoextraction strategies, such as inoculation of plants to be used for phytoremediation with appropriate heavy metal adapted rhizobial microflora, co-cropping system involving a non-mycorrhizal hyperaccumulator plant and a non-accumulator but mycorrhizal with appropriate AMF, or pre-cropping with mycotrophic crop systems to optimize phytoremediation processes, merit further field level investigations. There is also a need to improve our understanding of the mechanisms involved in transfer and mobilization of trace elements by rhizosphere microbiota and to conduct research on selection of microbial isolates from rhizosphere of plants growing on heavy metal contaminated soils for specific restoration programmes. This is necessary if we are to improve the chances of successful phytoremediation.     

Interaction between Earthworms and Arbuscular Mycorrhizal Fungi in Plants: A Review

Different kinds of soil animals and microorganisms inhabit the plant rhizosphere, which function closely to plant roots. Of them, arbuscular mycorrhizal fungi (AMF) and earthworms play a critical role in sustaining the soil-plant health. Earthworms and AMF belong to the soil community and are soil beneficial organisms at different trophic levels. Both of them improve soil fertility and structural development, collectively promoting plant growth and nutrient acquisition capacity. Earthworm activities redistribute mycorrhizal fungi spores and give diversified effects on root mycorrhizal fungal colonization. Dual inoculation with both earthworms and AMF strongly magnifies the response on plant growth through increased soil enzyme activities and changes in soil nutrient availability, collectively mitigating the negative effects of heavy metal pollution in plants and soils. This thus enhances phytoremediation and plant disease resistance. This review simply outlines the effects of earthworms and AMF on the soil-plant relationship. The effects of earthworms on root AMF colonization and activities are also analyzed. This paper also summarizes the interaction between earthworms and AMF on plants along with suggested future research.     

Role of soil microbes in the rhizospheres of plants growing on trace metal contaminated soils in phytoremediation

This article reviews recent developments in in situ bioremediation of trace metal contaminated soils, with particular reference to the microbial dynamics in the rhizospheres of plants growing on such soils and their significance in phytoremediation. In non-agricultural conditions, the natural role of plant growth promoting rhizobacteria (PGPR), P-solubilizing bacteria, mycorrhizal-helping bacteria (MHB) and arbuscular mycorrhizal fungi (AMF) in maintaining soil fertility is more important than in conventional agriculture, horticulture, and forestry where higher use of agrochemicals minimize their significance. These microbes initiate a concerted action when a particular population density is achieved, i.e. quorum sensing. AMF also recognize their host by signals released by host roots, allowing a functional symbiosis. AM fungi produce an insoluble glycoprotein, glomalin, which sequester trace elements and it should be considered for biostabilization leading to remediation of contaminated soils. Conclusions drawn from studies of metal uptake kinetics in solution cultures may not be valid for more complex field conditions and use of some combination of glasshouse and field experiments with organisms that occur within the same plant community is suggested. Phytoextraction strategies, such as inoculation of plants to be used for phytoremediation with appropriate heavy metal adapted rhizobial microflora, co-cropping system involving a non-mycorrhizal hyperaccumulator plant and a non-accumulator but mycorrhizal with appropriate AMF, or pre-cropping with mycotrophic crop systems to optimize phytoremediation processes, merit further field level investigations. There is also a need to improve our understanding of the mechanisms involved in transfer and mobilization of trace elements by rhizosphere microbiota and to conduct research on selection of microbial isolates from rhizosphere of plants growing on heavy metal contaminated soils for specific restoration programmes. This is necessary if we are to improve the chances of successful phytoremediation.     

丛枝菌根真菌在外来植物入侵演替中的作用与机制

外来植物入侵不仅是环境、经济和社会问题,也是一个生理学和生态学问题,尤其是入侵植物与本地植物、入侵植物和本地土壤生物之间的相互作用决定外来植物入侵程度。丛枝菌根真菌（AMF）作为土壤中一类极为重要的功能生物,在外来植物入侵演替过程中发挥多种不同作用。文章系统总结了AMF对入侵植物个体和群体的影响,入侵植物与本地植物竞争中AMF发挥的促进和抑制作用;探讨了AMF与入侵植物的相互作用关系,以及环境因子对AMF一入侵植物关系的影响：对AMF在外来植物入侵演替中的作用机制进行了讨论。旨在为探索控制生物入侵的新途径、为我国开展外来植物入侵研究与防控实践提供新思路。     

Molecular biodiversity of arbuscular mycorrhizal fungi in trace metal-polluted soils

We assessed the indigenous arbuscular mycorrhizal fungi (AMF) community structure from the roots and associated soil of Plantago major (plantain) plants growing on sites polluted with trace metals (TM) and on unpolluted sites. Uncontaminated and TM-contaminated sites containing As, Cd, Cu, Pb, Sn and Zn were selected based on a survey of the TM concentration in soils of community gardens in the City of Montréal. Total genomic DNA was extracted directly from these samples. PCR followed by denaturing gradient gel electrophoresis (PCR-DGGE), augmented by cloning and sequencing, as well as direct sequencing techniques, was all used to investigate AMF community structure. We found a decreased diversity of native AMF (assessed by the number of AMF ribotypes) in soils and plant roots harvested from TM-polluted soils compared with unpolluted soils. We also found that community structure was modified by TM contamination. Various species of Glomus, Scutellospora aurigloba and S. calospora were the most abundant ribotypes detected in unpolluted soil; ribotypes of G. etunicatum, G. irregulare/G. intraradices and G. viscosum were found in both polluted and unpolluted soils, while ribotypes of G. mosseae and Glomus spp. (B9 and B13) were dominant in TM-polluted soils. The predominance of G. mosseae in metal-polluted sites suggests the tolerance of this species to TM stress, as well as its potential use for phytoremediation. These data are relevant for our understanding of how AMF microbial communities respond to natural environments that contain a broad variety of toxic inorganic compounds and will substantially expand our knowledge of AMF ecology and biodiversity.     

Arbuscular Mycorrhizal Fungi as Components of Sustainable Soil-Plant Systems

Abstract

Increased pressure for food production has, in recent years, led to the development of intensive agricultural systems that use significant quantities of inorganic fertilizers and pesticides. However, there is now substantial evidence for the environmental costs of this high-input strategy and this has led to demands for agricultural systems to be modified in order to make them more sustainable. Arbuscular mycorrhizal fungi (AMF) play a key role in natural and agricultural ecosystems through major functions in the enhancement of plant phosphorus and nitrogen nutrition, nutrient and soil conservation, and the biological control of plant pathogens. They are essential to the sustainability of systems and their importance in agricultural ecosystems is likely to increase as inputs are reduced and/or rationalized. In order to maximize their benefits it is essential to ensure that management practices include minimum tillage, reduced use of inappropriate fertilizer, appropriate crop rotations with minimal fallow, and rationalized pesticide use. Furthermore, crop breeders should take full account of the symbiosis in selection. Future research should be targeted to understanding the functional ecology of AMF in agroecosystems.     

Edaphic and host plant factors are linked to the composition of arbuscular mycorrhizal fungal communities in the root zone of endangered Ulmus chenmoui Cheng in China

Despite the importance of arbuscular mycorrhizal fungi (AMF) within deciduous forest ecosystems, we know little about how natural AMF communities are structured in the root zone of the endangered elm species Ulmus chenmoui. In this study, three U. chenmoui sampling sites, differing with respect to plant health, age, and growth status, were selected in Anhui Province, China. AMF biodiversity in the root zones of individual U. chenmoui trees was investigated using high‐throughput sequencing. In total, 61 AMF operational taxonomic units were detected. Five genera, namely Glomus (62.82%), Paraglomus (17.82%), Rhizophagus (4.29%), Septoglomus (4.06%) and Funneliformis (2.35%), and 29 species of AMF were identified. Correlation analysis indicated that available soil phosphorus and potassium concentrations were the main edaphic factors influencing AMF community structure. There was a difference in AMF species richness among the three U. chenmoui stands. Our study showed that soil nutrient concentrations and plant health, age, and growth status can exert a selective effect on the composition of the AMF population in the soil in the root zones of U. chenmoui trees.     

Mycorrhizal fungi as mediators of defence against insect pests in agricultural systems

• 1 Below‐ground organisms influence above‐ground interactions in both natural and agricultural ecosystems. Among the most important below‐ground organisms are mycorrhizal fungi, comprising ubiquitous and ancient plant mutualists that have significant effects on plant growth and fitness mediated by resource exchange with plants. In the present study, we focus on the effects of arbuscular mycorrhizal fungi (AMF) on crop defence against insect pests.
• 2 AMF alter the availability of resources used by crop plants to manufacture defences against pests and to compensate for pest damage. However, AMF also provide plants with nutrients that are known to increase insect performance. Through potentially opposing effects on plant nutritional quality and defence, mycorrhizal fungi can positively or negatively affect pest performance.
• 3 Additionally, AMF may directly affect gene expression and plant defence signalling pathways involved in the construction and induction of plant defences, and these effects are apparently independent of those caused by nutrient availability. In this way, AMF may still influence plant defences in the fertilized and highly managed systems typical of agribusiness.
• 4 Because AMF can affect plant tolerance to pest damage, they may have a significant impact on the shape of damage–yield relationships in crops. Potential mechanisms for this effect are suggested.
• 5 We highlight the need for continuing research on the effects of AMF identity and the abundance on crop defences and tolerance to pest attack. Much work is needed on the potential effects of mycorrhizal colonization on plant signalling and the induction of direct and indirect defences that may protect against pest damage.

     

Soil,But Not Cultivar,Shapes the Structure of Arbuscular Mycorrhizal Fungal Assemblages Associated with Strawberry

Arbuscular mycorrhizal fungi are widespread plant symbionts occurring in most agricultural crops, where they can play key roles in the growth and health of their plant hosts. Plant benefits can depend on the identity of the associated arbuscular mycorrhizal fungi (AMF), but little is known about the identity of the fungal partners in most agricultural systems. In this study, we describe the AMF assemblages associated with four cultivars of strawberry in an outdoor experiment using two field soils with different origin and management history. Assemblages were characterised by clone library sequencing of 18S rRNA gene fragments. Soil dramatically influenced the degree of mycorrhizal colonisation and AMF assemblage structure in the roots. No differences were observed between cultivars. Fungi belonging to the genus Acaulospora dominated the AMF assemblages in one soil, but they were not detected in the other. These results suggest that physicochemical soil characteristics and management can play a role in determining the identity and structure of microbial communities associated with particular hosts in agricultural systems.     

Independent effects of arbuscular mycorrhiza and earthworms on plant diversity and newcomer plant establishment

Questions: How do arbuscular mycorrhiza and earthworms affect the structure and diversity of a ruderal plant community? Is the establishment success of newcomer plants enhanced by these soil organisms and their interactions? Methods: We grew a native ruderal plant community composed of different functional groups (grasses, legumes and forbs) in the presence and absence of arbuscular mycorrhizal fungi (AMF) and endogeic earthworms in mesocosms. We introduced seeds of five, mainly exotic, plant species from the same functional groups after a disturbance simulating mowing. The effects of the soil organisms on the native ruderal plant community and seedling establishment of the newcomer plants were assessed. Results: After disturbance, the total above‐ground regrowth of the native plant community was not affected by the soil organisms. However, AMF increased plant diversity and shoot biomass of forbs, but decreased shoot biomass of grasses of the native plant community. Earthworms led to a reduction in total root biomass. Establishment of the introduced newcomer plants increased in the presence of AMF and earthworms. Especially, seedling establishment of the introduced non‐native legume Lupinus polyphyllus and the native forb Plantago lanceolata was promoted in the presence of AMF and earthworms, respectively. The endogeic earthworms gained more weight in the presence of AMF and led to increased extraradical AMF hyphal length in soil. However, earthworms did not seem to modify the effect of AMF on the plant community. Conclusion: The present study shows the importance of mutualistic soil organisms in mediating the establishment success of newcomer plants in a native plant community. Mutualistic soil organisms lead to changes in the structure and diversity of the native plant community and might promote newcomer plants, including exotic species.     

Enhanced phytoremediation of soils contaminated with PAHs by arbuscular mycorrhiza and rhizobium

Greenhouse experiment was conducted to evaluate the potential effectiveness of a legume (Sesbania cannabina), arbuscular mycorrhizal fungi (AMF) (Glomus mosseae), and rhizobia (Ensifer sp.) symbiosis for remediation of Polycyclic aromatic hydrocarbons (PAHs) in spiked soil. AMF and rhizobia had a beneficial impact on each other in the triple symbiosis. AMF and/or rhizobia significantly increased plant biomass and PAHs accumulation in plants. The highest PAHs dissipation was observed in plant + AMF + rhizobia treated soil, in which >97 and 85–87% of phenanthrene and pyrene, respectively, had been degraded, whereas 81–85 and 72–75% had been degraded in plant-treated soil. During the experiment, a relatively large amount of water-soluble phenolic compounds was detected in soils of AMF and/or rhizobia treatment. It matches well with the high microbial activity and soil enzymes activity. These results suggest that the mutual interactions in the triple symbiosis enhanced PAHs degradation via stimulating both microbial development and soil enzyme activity. The mutual interactions between rhizobia and AMF help to improve phytoremediation efficiency of PAHs by S. cannabina.     

The Role of Mycorrhizae Associated with Vetiver Grown in Pb-/Zn-Contaminated Soils: Greenhouse Study

The effects of mycorrhizae on growth and uptake of N, P, Zn, and Pb by plants were investigated in a greenhouse trial using vetiver grass (Vetiveria zizanioides) as host. Inoculation of the host plants with arbuscular mycorrhizal fungi (AMF), Glomus mosseae and G. intraradices spores, significantly increased the growth and P uptake. Mycorrhizal colonization increased Pb and Zn uptake by plants under low soil metal concentrations (at 0 and 10 mg/kg of Pb or Zn), whereas under higher concentrations (at 100 and 1,000 mg/kg of Pb or Zn), it decreased Pb and Zn uptake. P concentration in soil was negatively correlated with mycorrhizal colonization as well as Zn or Pb concentrations. The results showed that inoculation of the host plants with AMF protects them from the potential toxicity caused by increased uptake of Pb and Zn, but the degree of protection varied according to the fungus and host plant combination. The potential of arbuscular mycorrhizae in phytoremediation of the Zn‐ or the Pb‐contaminated soils is discussed in this article.     

丛枝菌根在植物修复重金属污染土壤中的作用

丛枝菌根（Arbuscular mycorrhizae,AM）是自然界中分布最广的一类菌根,AM真菌能与陆地上绝大多数的高等植物共生,常见于包括重金属污染土壤在内的各种生境中。在重金属污染条件下,AM真菌可以减轻重金属对植物的毒害,影响植物对重金属的吸收和转运,在重金属污染土壤的植物修复中显示出极大的应用潜力。重点介绍了AM真菌对植物重金属耐性的影响及其在植物提取和植物稳定中的应用等方面的进展,讨论了未来研究所面临的任务和挑战。     

Colonization of Tanacetum vulgare by aphids is reduced by earthworms

Soil organisms affect plant growth and chemistry and have subsequent effects on aboveground herbivore performance. However, whether herbivores discriminate between plants exposed to different soil organisms when colonizing their host plants is largely unexplored. In a greenhouse study, Tanacetum vulgare L. (Asteraceae) growing in a ruderal plant community in the presence and absence of arbuscular mycorrhizal fungi (AMF) and earthworms [Aporrectodea spp. (Haplotaxida: Lumbricidae)] was colonized by aphids [Myzus persicae Sulzer (Hemiptera: Aphididae)]. The aphids preferred to colonize plants without earthworms in the soil, and the numbers of aphids remained lower on the plants with earthworms, irrespective of the presence of AMF. Although the N, C, and P concentrations of the shoots were not affected by the soil organisms, AMF increased total aboveground biomass, total N, C, and P content, and photosynthetic activity (measured as electron transport rate) in the leaves under high light intensity. These results suggest that earthworms affect chemical cues that are used by aphids to judge host quality prior to feeding. Discrimination between plants with and without exposure to earthworms by aboveground herbivores is a novel aspect of plant‐mediated interactions between below‐ and aboveground organisms.     

Arbuscular mycorrhizal fungi in national parks,nature reserves and protected areas worldwide: a strategic perspective for their in situ conservation

Soil fungi play a crucial role in producing fundamental ecosystem services such as soil fertility, formation and maintenance, nutrient cycling and plant community dynamics. However, they have received little attention in the field of conservation biology. Arbuscular mycorrhizal fungi (AMF) are beneficial soil symbionts fulfilling a key function in the complex networks of belowground/aboveground biotic interactions as they live in association with the roots of most (80%) land plant families and influence not only soil fertility but also plant nutrition, diversity and productivity. The diversity of AMF communities can decline due to habitat loss and anthropogenic disturbance, especially in agro-ecosystems, and many valuable ecotypes could become extinct before they are even discovered. Consequently, long-term strategies are urgently needed to ensure their conservation in habitats where they naturally occur and have evolved. Protected areas, where living organisms are under the care of national and international authorities, represent an appropriate place for the in situ conservation of AMF, providing them with adapted situations together with established complex networks of interactions with different components within each specific ecosystem. Here, we review data available about the main present-day threats to AMF and the current state of knowledge about their occurrence in protected sites worldwide, providing a checklist of national parks and nature reserves where they have been reported. The aim was to offer a strategic perspective to increase awareness of the importance of conserving these beneficial plant symbionts and of preserving their biodiversity in the years to come.     

The effect of agricultural practices on the development of indigenous arbuscular mycorrhizal fungi. I. Field studies in an Indonesian ultisol

Two pre-established agricultural field trials were assessed for the abundance of arbuscular mycorrhizal fungi (AMF) in the soil (density of spores, species richness and lengths of extra-radical mycelium [ERM]) in association with one of three tropical plant species (Gliricidia sepium, Peltophorum dasyrachis and Zea mays). The trials were managed by one of three agricultural practices: soil disturbance in a monoculture system, a root barrier to prevent interactions between plants in an agroforestry system or the addition of organic matter (OM) in an agroforestry and a monoculture system. The lengths of ERM of AMF in the soil were greater in the agroforestry system than the monoculture system. These were greater when a root barrier was present, but decreased when OM was added. Soil disturbance reduced the density of spores, species richness and the lengths of ERM of AMF compared with the undisturbed soil. This work indicates that agricultural trials may provide a useful tool to monitor the abundance of AMF in the field. Clearly, there is potential to increase the abundance of AMF, from different genera, in the soil through the management of agricultural practices. The significance of the abundance of AMF for subsequent benefits to plant growth and development and ultimately the sustainability of tropical agro-ecosystems are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Contribution of the arbuscular mycorrhizal symbiosis to heavy metal phytoremediation

High concentrations of heavy metals (HM) in the soil have detrimental effects on ecosystems and are a risk to human health as they can enter the food chain via agricultural products or contaminated drinking water. Phytoremediation, a sustainable and inexpensive technology based on the removal of pollutants from the environment by plants, is becoming an increasingly important objective in plant research. However, as phytoremediation is a slow process, improvement of efficiency and thus increased stabilization or removal of HMs from soils is an important goal. Arbuscular mycorrhizal (AM) fungi provide an attractive system to advance plant-based environmental clean-up. During symbiotic interaction the hyphal network functionally extends the root system of their hosts. Thus, plants in symbiosis with AM fungi have the potential to take up HM from an enlarged soil volume. In this review, we summarize current knowledge about the contribution of the AM symbiosis to phytoremediation of heavy metals.     

Mycorrhiza‐mediated interference between cover crop and weed in organic winter cereal agroecosystems: The mycorrhizal colonization intensity indicator

The mycorrhizal fungi are symbiotic organisms able to provide many benefits to crop production by supplying a set of ecosystem functions. A recent ecological approach based on the ability of the fungi community to influence plant–plant interactions by extraradical mycelium development may be applied to diversified, herbaceous agroecosystems. Our hypothesis is that the introduction of a winter cereal cover crop (CC) as arbuscular mycorrhizal fungi (AMF)–host plant in an organic rotation can boosts the AMF colonization of the other plants, influencing crop–weed interference. In a 4‐years organic rotation, the effect of two winter cereal CC, rye and spelt, on weed density and AMF colonization was evaluated. The AMF extraradical mycelium on CC and weeds roots was observed by scanning electron microscopy analysis. By joining data of plant density and mycorrhization, we built the mycorrhizal colonization intensity of the Agroecosystem indicator (MA%). Both the CC were colonized by soil AMF, being the mycorrhizal colonization intensity (M%) affected by environmental conditions. Under CC, the weed density was reduced, due to the increase of the reciprocal competition in favor of CC, which benefited from mycorrhizal colonization and promoted the development of AMF extraradical mycelium. Even though non‐host plants, some weed species showed an increased mycorrhizal colonization in presence of CC respect to the control. Under intense rainfall, the MA% was less sensitive to the CC introduction. On the opposite, under highly competitive conditions, both the CC boosted significantly the mycorrhization of coexistent plants in the agroecosystem. The proposed indicator measured the agroecological service provided by the considered CCs in promoting or inhibiting the overall AMF colonization of the studied agroecosystems, as affected by weed selection and growth: It informs about agroecosystem resilience and may be profitably applied to indicate the extent of the linkage of specific crop traits to agroecosystem services, contributing to further develop the functional biodiversity theory.