Mycorrhizas: Gene to Function

Substantial progress has been made toward development of molecular tools for identification and quantification of mycorrhizal fungi in roots and evaluation of the diversity of ectomycorrhizal (ECM) fungi and the phylogeny and genetic structure of arbuscular mycorrhizal (AM) fungi. rDNA analysis confirms high diversity of ECM fungi on their hosts, and for AM fungi has revealed considerable genetic variation within and among morphologically similar AM fungal species. The fungal and plant genes, regulation of their expression, and biochemical pathways for nutrient exchange between symbiotic partners are now coming under intense study and will eventually be used to define the ecological nutritional role of the fungi. While molecular biological approaches have increased understanding of the mycorrhizal symbiosis, such knowledge about these lower-scale processes has yet to influence our understanding of larger-scale responses to any great extent.     

Ultrastructural localization of acid phosphatase in arbusculate coils of mycorrhizal Phoenix canariensis roots

Acid phosphatase (ACP) activity has been detected in roots of mycorrhizal and non-mycorrhizal Phoenix canariensis. This enzyme was ultrastructurally localized in arbusculate coils for the first time. This localization was carried out using a cerium-based method, which minimizes non-specific precipitation. The ACP was localized in inter- and intracellular hyphae, in the fungal cytoplasm as well as at the interface and the fungal cell wall and the periarbuscular membrane limiting it. The novel localization of an ACP in the arbuscular mycorrhizal (AM) interface of arbusculate coils suggests that this enzyme may be involved in the phosphorus efflux from the mycorrhizal fungus to the host. The results presented in this article indicate that the role played by ACP in AM symbiosis may be more important than was previously thought and that arbusculate coils are highly relevant when considering nutrient transfer through AM symbiosis.     

亚热带草地中植物优势种与从属种对AM真菌的差异性生长反应

AM真菌能够影响植物生态系统的群落结构.以亚热带草地生态系统为研究对象,调查了两块草地中优势种和从属种的菌根,并在盆栽试验中比较了优势种和从属种对AM真菌的土著菌种和外源菌种Glomus mosseae的生长反应、养分吸收.结果表明,两块草地各自的优势种藿香蓟和两耳草对土著菌种的菌根依赖性分别是41.5%和77.4%,远远高于从属种莎草和毛蓼（16.0%和7.9%）;但是它们对Glomus mosseae的菌根依赖性有所变化,分别是79.6%、44.2%、74.1%和24.9%.这表明,土著菌种是优势种和从属种的形成机制之一,而外源菌种可能改变基于土著菌种而形成的植物群落结构.植株磷营养的分析结果表明,AM真菌对优势种和从属种生长的促进与对磷吸收的促进高度相关,表明AM真菌促进养分吸收是其影响植物群落结构的机制之一.     

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

Given that arbuscular mycorrhizal (AM) fungi are not consistently beneficial to their host plants, it is difficult to explain the evolutionary persistence of this relationship. We tested the hypothesis that increasing either fungal or host biodiversity allows an AM fungus to persist on a host where it shows little benefit. We found that growing such a fungus (an isolate of Glomus custos associating with Plantago laceolata) in combination with certain fungi improved its success as measured by mtLSU DNA abundance. Increasing plant species richness facilitated the spread of this fungus as measured by spore density and fungal colonization; the role of host species richness was not as clear when looking at measures of root abundance. These results indicate that diversity in the AM symbiosis, both plant and fungal, can promote the persistence of low-quality fungi. By existing within a complex mycelial network fungal strains that show little growth benefit to their hosts have a better chance of persisting on that same host. This has the potential to promote selection for heterogeneous AM fungal communities on a small spatial scale.     

菌根真菌与植物共生营养交换机制研究进展

菌根是陆地生态系统普遍存在的、由土壤中的菌根真菌侵染宿主植物根系形成的联合共生体.菌根的建立是以共生体双方的营养交换为基础的:菌根真菌从土壤中吸收氮、磷等营养物质并转运给宿主植物,供其生长;作为交换,植物则以脂质或糖的形式向菌根真菌提供其生长所必需的碳水化合物.近年来,菌根真菌与宿主植物间的营养交换机制一直是研究的热点,国内外对菌根真菌介导的植物营养物质吸收和转运机制的研究也取得了巨大进展.本文综述了丛枝和外生两种菌根真菌与宿主植物间营养交换的最新研究进展,尤其是碳、氮、磷等几种重要营养物质的吸收与双向转运机制,以及营养交换在菌根形成中的潜在调控作用,并对目前存在的关键问题和未来研究方向进行了分析和展望,这对菌根模型的建立及菌根效益的优化具有重要意义.     

At the Root of the Wood Wide Web: Self Recognition and Non-Self Incompatibility in Mycorrhizal Networks

Arbuscular mycorrhizal (AM) fungi are mutualistic symbionts living in the roots of 80% of land plant species, and developing extensive, below-ground extraradical hyphae fundamental for the uptake of soil nutrients and their transfer to host plants. Since AM fungi have a wide host range, they are able to colonize and interconnect contiguous plants by means of hyphae extending from one root system to another. Such hyphae may fuse due to the widespread occurrence of anastomoses, whose formation depends on a highly regulated mechanism of self recognition. Here, we examine evidences of self recognition and non-self incompatibility in hyphal networks formed by AM fungi and discuss recent results showing that the root systems of plants belonging to different species, genera and families may be connected by means of anastomosis formation between extraradical mycorrhizal networks, which can create indefinitely large numbers of belowground fungal linkages within plant communities.Key Words: arbuscular mycorrhizal symbiosis, extraradical mycelium, anastomosis, plant interconnectedness, self recognition, non-self incompatibility, mycorrhizal networks     

Plant hormones as signals in arbuscular mycorrhizal symbiosis

Abstract

Arbuscular mycorrhizal (AM) fungi are non-specific symbionts developing mutual and beneficial symbiosis with most terrestrial plants. Because of the obligatory nature of the symbiosis, the presence of the host plant during the onset and proceeding of symbiosis is necessary. However, AM fungal spores are able to germinate in the absence of the host plant. The fungi detect the presence of the host plant through some signal communications. Among the signal molecules, which can affect mycorrhizal symbiosis are plant hormones, which may positively or adversely affect the symbiosis. In this review article, some of the most recent findings regarding the signaling effects of plant hormones, on mycorrhizal fungal symbiosis are reviewed. This may be useful for the production of plants, which are more responsive to mycorrhizal symbiosis under stress.     

植物菌根共生磷酸盐转运蛋白

大多数植物能和丛枝菌根（arbuscular mycorrhiza, AM）真菌形成菌根共生体。AM能够促进植物对土壤中矿质营养的吸收,尤其是磷的吸收。磷的吸收和转运由磷酸盐转运蛋白介导。总结了植物AM磷酸盐转运蛋白及其结构特征,分析其分类及系统进化,并综述了AM磷酸盐转运蛋白介导的磷的吸收和转运过程及其基因的表达调控。植物AM磷酸盐转运蛋白属于Pht1家族成员,它不仅对磷的吸收和转运是必需的,而且对AM共生也至关重要,为进一步了解菌根形成的分子机理及信号转导途径提供了理论基础。     

Phosphate in the arbuscular mycorrhizal symbiosis: transport properties and regulatory roles

In response to the colonization by arbuscular mycorrhizal (AM) fungi, plants reprioritize their phosphate (Pi)-uptake strategies to take advantage of nutrient transfer via the fungus. The mechanisms underlying Pi transport are beginning to be understood, and recently, details of the regulation of plant and fungal Pi transporters in the AM symbiosis have been revealed. This review summarizes recent advances in this area and explores current data and hypotheses of how the plant Pi status affects the symbiosis. Finally, suggestions of an interrelationship of Pi and nitrogen (N) in the AM symbiosis are discussed.     

AM真菌群落结构与功能研究进展

在总结最近10 a来有关丛枝菌根(AM)真菌研究的基础上,讨论了AM真菌群落结构的概念、特征、功能以及寄主植物、土壤条件、其他土壤微生物、农业技术等因素对AM真菌群落结构的影响及其调控途径;介绍了研究AM真菌群落结构的方法;探讨了今后研究的方向与前景。     

丛枝菌根共生体中碳、氮代谢及其相互关系

丛枝菌根共生体(arbuscular mycorrhiza, AM)是丛枝菌根真菌(arbuscular mycorrhizal fungi, AMF)与宿主植物之间形成的互惠共生形式.共生体中的碳、氮交换和代谢影响着宿主植物和共生真菌之间的营养平衡和资源重新分配,在物质和能量循环中发挥着重要作用.宿主植物光合固定的碳输送到真菌内,并且分解和释放真菌所需的生命物质和能量,包括促进孢子萌发、菌丝生长和提高氮等营养元素的吸收;而菌根真菌利用宿主植物提供的碳骨架和能量,发生氮的转化和运输,最终传递给宿主植物供其利用.本文综述了丛枝菌根共生体中碳、氮传输和代谢的主要模式,碳、氮的交互影响和调控机制,以促进丛枝菌根在可持续农业和生态系统中的应用.     

丛枝菌根真菌脂类代谢对共生信号调控的响应和反馈

丛枝菌根(AM)真菌是自然生态系统中分布最为广泛的真菌之一,在自然界物质循环和能量流动中发挥着重要作用。经过长期的协同进化,AM真菌和宿主植物之间形成了完美的互惠互利的共生关系,而真菌的脂类代谢可能是揭示共生秘密的关键所在。本文综述了AM真菌脂类代谢在共生关系建立和维持中关键作用的最新研究进展,重点探讨了AM真菌脂类代谢对共生信号调控的响应和反馈机制,主要包括:AM真菌脂类存储和释放对共生和非共生状态的响应,以及脂类代谢产物变化与共生营养传递之间的关系;脂类分解过程在共生建立初期对信号分子调控发生的响应,以及相应的物质转化和能量代谢;菌根共生互惠互利关系维持中,真菌脂类代谢与信号分子交流通道的相互渗透和影响。本文对于理解菌根共生机制,促进菌根在生产中的应用具有促进作用。     

Soil nutritional status, not inoculum identity, primarily determines the effect of arbuscular mycorrhizal fungi on the growth of Knautia arvensis plants

Arbuscular mycorrhizal (AM) symbiosis is among the factors contributing to plant survival in serpentine soils characterised by unfavourable physicochemical properties. However, AM fungi show a considerable functional diversity, which is further modified by host plant identity and edaphic conditions. To determine the variability among serpentine AM fungal isolates in their effects on plant growth and nutrition, a greenhouse experiment was conducted involving two serpentine and two non-serpentine populations of Knautia arvensis plants grown in their native substrates. The plants were inoculated with one of the four serpentine AM fungal isolates or with a complex AM fungal community native to the respective plant population. At harvest after 6-month cultivation, intraradical fungal development was assessed, AM fungal taxa established from native fungal communities were determined and plant growth and element uptake evaluated. AM symbiosis significantly improved the performance of all the K. arvensis populations. The extent of mycorrhizal growth promotion was mainly governed by nutritional status of the substrate, while the effect of AM fungal identity was negligible. Inoculation with the native AM fungal communities was not more efficient than inoculation with single AM fungal isolates in any plant population. Contrary to the growth effects, a certain variation among AM fungal isolates was revealed in terms of their effects on plant nutrient uptake, especially P, Mg and Ca, with none of the AM fungi being generally superior in this respect. Regardless of AM symbiosis, K. arvensis populations significantly differed in their relative nutrient accumulation ratios, clearly showing the plant’s ability to adapt to nutrient deficiency/excess.     

丛枝菌根真菌对大气CO2浓度升高和增温响应研究进展

全球变化深刻影响着陆地生态系统生物多样性及生态功能。丛枝菌根(AM)真菌可与绝大多数陆生植物根系形成互惠共生体,在协助宿主养分吸收、促进植物生长、维持植物多样性等方面发挥着重要作用。本文主要分析了大气CO₂浓度升高(eCO₂)和增温对森林和草地生态系统AM真菌群落组成及其功能的影响。eCO₂主要通过影响宿主植物、土壤碳(C)输入等方式间接影响AM真菌,可增加AM真菌的多度和活性,影响AM真菌的多样性与群落组成。增温可直接或间接地(通过宿主植物和土壤途径)影响AM真菌,显著改变森林土壤AM真菌的群落组成,但对草地土壤AM真菌群落组成的影响尚无定论。我们提出了当前研究中存在的主要问题及未来应重点关注的内容。本文旨在明晰AM真菌对eCO₂和增温的响应和适应,增进对AM真菌介导的土壤生态功能的认识,为利用AM真菌缓解全球变化、增强土壤功能的韧性和全球变化的生态系统适应性提供依据。     

Plant–fungus competition for nitrogen erases mycorrhizal growth benefits of Andropogon gerardii under limited nitrogen supply

Considered to play an important role in plant mineral nutrition, arbuscular mycorrhizal (AM) symbiosis is a common relationship between the roots of a great majority of plant species and glomeromycotan fungi. Its effects on the plant host are highly context dependent, with the greatest benefits often observed in phosphorus (P)‐limited environments. Mycorrhizal contribution to plant nitrogen (N) nutrition is probably less important under most conditions. Moreover, inasmuch as both plant and fungi require substantial quantities of N for their growth, competition for N could potentially reduce net mycorrhizal benefits to the plant under conditions of limited N supply. Further compounded by increased belowground carbon (C) drain, the mycorrhizal costs could outweigh the benefits under severe N limitation. Using a field AM fungal community or a laboratory culture of Rhizophagus irregularis as mycorrhizal inoculants, we tested the contribution of mycorrhizal symbiosis to the growth, C allocation, and mineral nutrition of Andropogon gerardii growing in a nutrient‐poor substrate under variable N and P supplies. The plants unambiguously competed with the fungi for N when its supply was low, resulting in no or negative mycorrhizal growth and N‐uptake responses under such conditions. The field AM fungal communities manifested their potential to improve plant P nutrition only upon N fertilization, whereas the R. irregularis slightly yet significantly increased P uptake of its plant host (but not the host's growth) even without N supply. Coincident with increasing levels of root colonization by the AM fungal structures, both inoculants invariably increased nutritional and growth benefits to the host with increasing N supply. This, in turn, resulted in relieving plant P deficiency, which was persistent in non‐mycorrhizal plants across the entire range of nutrient supplies.     

Branched absorbing structures (BAS): a feature of the extraradical mycelium of symbiotic arbuscular mycorrhizal fungi

The present work describes the morphogenesis and cytological characteristics of 'branched absorbing structures' (BAS, formely named arbuscule-like structures, ALS), small groups of dichotomous hyphae formed by the extraradical mycelium of arbuscular mycorrhizal (AM) fungi. Monoxenic cultures of the AM fungus Glomus intraradices Smith & Schenck and tomato ( Lycopersicum esculentum Mill.) roots allowed the continuous, non-destructive study of BAS development. These structures were not observed in axenic cultures of the fungus under different nutritional conditions or in unsuccessful (asymbiotic) monoxenic cultures. However, extraradical mycelium of G. intraradices formed BAS immediately after fungal penetration of the host root and establishment of the symbiosis. The average BAS development time was 7 d under our culture conditions, after which they degenerated, becoming empty septate structures. Certain BAS were closely associated with spore formation, appearing at the spore's substending hypha. Branches of these spore-associated BAS (spore-BAS) usually formed spores. Electron microscopy studies revealed that BAS and arbuscules show several ultrastructural similarities. The possible role of BAS in nutrient uptake by the mycorrhizal plant is discussed.     

Mycorrhizal fungal growth responds to soil characteristics,but not host plant identity,during a primary lacustrine dune succession

Soil factors and host plant identity can both affect the growth and functioning of mycorrhizal fungi. Both components change during primary succession, but it is unknown if their relative importance to mycorrhizas also changes. This research tested how soil type and host plant differences among primary successional stages determine the growth and plant effects of arbuscular mycorrhizal (AM) fungal communities. Mycorrhizal fungal community, plant identity, and soil conditions were manipulated among three stages of a lacustrine sand dune successional series in a fully factorial greenhouse experiment. Late succession AM fungi produced more arbuscules and soil hyphae when grown in late succession soils, although the community was from the same narrow phylogenetic group as those in intermediate succession. AM fungal growth did not differ between host species, and plant growth was similarly unaffected by different AM fungal communities. These results indicate that though ecological filtering and/or adaptation of AM fungi occurs during this primary dune succession, it more strongly reflects matching between fungi and soils, rather than interactions between fungi and host plants. Thus, AM fungal performance during this succession may not depend directly on the sequence of plant community succession.     

Sex-specific responses to mycorrhiza in a dioecious species

In most studies about dioecious plants, the role of arbuscular mycorrhizae (AM) and the potential sex-specific differences between the plant hosts have been overlooked. Because plant sexes frequently differ in drought tolerance and AM fungal colonization provides higher resistance to drought, we investigated whether the relation of mycorrhizal fungi with either male or female Antennaria dioica plants differs using a factorial experiment. We hypothesized that because AM usually increase growth rate and male plants usually grow larger than females, males should gain more benefit from the mycorrhizal symbiosis in terms of mineral nutrition and water supply. Because of higher demands of carbohydrates (C) in males, we expected males to allocate less C resources to the mycorrhizal fungus so that the associated fungi should benefit less of the association with males. In contrast to our initial hypothesis, the male plants, although faster growing under drought, did not gain more symbiosis-mediated benefits than did the females, and both sexes seemed to provide resources equally to their fungal symbiont. Therefore, we conclude that the two plant sexual morphs provide equal amounts of C to their fungal root symbionts and that they can gain specific benefits from the symbiosis, which, however, depend on soil water availability.     

Reflections on the role of environmentally-governed reproductive versatility in the adaptation of plant populations

Summary

Mycorrhizal associations vary widely in structure and function, but the commonest interaction is the Arbuscular Mycorrhizal (AM) symbiosis which forms between the roots of over 80% of all terrestrial plant species and Zygomycete fungi of the Order Glomales. These are obligate symbionts which colonise plant root cells. This symbiosis confers benefits directly to the host plants through the acquisition of phosphate and other mineral nutrients from the soil by the fungus while the fungus receives a carbon source from the host. In addition, the symbiosis may also enhance the plants resistance to biotic and abiotic stresses. The beneficial effects of AM symbioses occur as a result of a complex molecular dialogue between the two symbiotic partners. Identifying the molecules and genes involved in the dialogue is necessary for a greater understanding of the symbiosis. This paper reviews the process of AM fungal colonisation of plant roots and the underlying molecular mechanisms associated with the formation and functioning of an AM symbiosis.     

丛枝菌根真菌形态结构、物种多样性和群落组成对氮沉降响应研究进展

工业革命以来,人类活动输入到生态系统中的氮迅速增加,已突破地球所能承受的氮循环阈值。过量氮沉降会造成生物多样性丧失等一系列危害,严重影响生态系统结构和功能。丛枝菌根(AM)真菌能够与大约70%-80%的陆地植物种类形成共生关系,在宿主植物养分吸收、抵抗外界不良环境压力、群落动态和物种共存、生物地球化学循环等方面具有重要的作用。探究AM真菌对氮沉降的响应对认识和把握菌根真菌缓解氮沉降的负面后果,维持生态系统的结构和功能具有重要意义。本文综述了AM真菌的形态结构、物种多样性和群落组成等对氮沉降的响应机制。前人研究表明氮沉降通常降低AM真菌的根系定殖率,减少根外菌丝密度和土壤孢子密度,改变菌丝生长的时间动态;降低AM真菌多样性,改变AM真菌群落组成。氮沉降主要通过缓解植物氮限制、降低植物对菌根的依赖性、减少植物对菌根的碳分配、改变根系和土壤中菌根生物量比率、在植物根内维持稳定的菌根真菌组成作为应对未来扰动的“保险”、改变土壤资源有效性及土壤酸度等直接和间接途径影响AM真菌结构和功能。我们建议在未来研究中整合多组学手段、开展学科交叉,聚焦复杂的生物互作体系对氮沉降的响应机制,以及AM真菌对氮沉降响应的生态后果。