外生菌根菌与森林树木的相互关系

生态系统的每个过程都伴随着各种微生物的活动,其中最重要的功能群之一是菌根真菌(菌根菌)。一般认为,菌根菌是自然界多数植物生存最基本的组成部分,陆地上约90％以上的高等植物都具有菌根菌。这些菌类的菌丝体与植物根系结合形成菌根,使植物生长成为可能,使不同种类植物的根系联在一起。根据菌根菌入侵植物根系的方式及菌根的形态特征,菌根可分为外生菌根、内生菌根和内外生菌根3组共7种类型。外生菌根主要出现在松科、桦木科、壳斗科等树种的森林生态系统中,在根系表面形成菌丝鞘,部分菌丝进入根系皮层细胞间隙形成哈氏网表面。菌根菌剂在森林经营中得到广泛地应用。外生菌根菌对森林树木的作用可归纳为：1)促进造林或育苗成活与生长;2)提高森林生态系统中植物的多样性、稳定性和生产力;3)对森林生态系统的综合效应,主要表现在增加植物一土壤联结,改善土壤结构,促进土壤微生物,增强植物器官的功能;4)抗拮植物根部病害病原菌等。树木与菌根菌相互关系研究主要包括：1)菌根共生的机理;2)菌根菌在退化森林生态系统恢复与改造中的作用;3)菌根菌的分布格局与森林生态系统服务功能的关系;4)菌根菌对森林生态系统的综合效应,如菌根菌与森林植物群落结构、物种多样性以及森林系统稳定性和生产力的研究。     

外生菌根菌的分子生态学：分子标记基因、遗传个体及生态意义

外生菌根真菌与很多植物形成互利共生关系,在营养物质交换和碳循环等方面起着关键性的作用,是森林生态系统的重要组成部分。近期生物技术的发展使得人们对外生菌根菌的群体遗传学和分子生态学有了更加深入的认识。本文介绍了一些常用的鉴定外生菌根菌的分子标记,并对每种分子标记的特点及其适用范围进行了讨论。文中总结了几种常用的鉴定未知外生菌根菌的方法,指出了一些在研究外生菌根菌过程中需要克服的内在困难,其中之一就是很多外生菌根菌不可以人工培养,所以人们缺少对其地下部分分布规律和动态变化的了解。在寄主专一性、物种多样性和丰富度、遗传个体大小、繁殖方式等方面,近期对外生菌根菌的分子生物学研究已经获得了很多重要的结果。作者讨论了这些研究成果对于今后开展外生菌根菌研究的重要意义以及在森林生态系统保育方面的潜在应用价值。     

外生菌根缓解植物酸雨胁迫的机理研究进展

森林作为陆地生态系统的主体,是酸雨污染的主要受体,酸雨对生态系统产生着巨大的影响。菌根是菌根真菌与植物营养根的共生体。外生菌根真菌与宿主植物间互惠互利,在森林生态系统中,外生菌根在维持生态系统的养分平衡和改善树木营养等方面有重要的作用。本文综述了国内外关于菌根和酸雨关系的研究,酸雨能抑制外生菌根的形成,降低其活力;但另一方面,外生菌根能够缓解酸雨造成的植物危害,提高植株对酸雨的耐受力。外生菌根主要通过以下几方面缓解酸雨胁迫:(1)菌根形态结构的物理屏蔽作用;(2)增加养分吸收,增加御酸能力;(3)增强酶活性,提高植物生存能力;(4)产生有机酸或其他物质。     

东北红松纯林菌根外生菌根真菌群落与环境因子的相关性

外生菌根是木本植物根系与真菌形成的共生结构,外生菌根真菌在红松等外生菌根树种的定植与森林生态系统的保持方面起到至关重要的作用。明确菌根系统内外生菌根真菌群落组成是揭示菌根共生机制的前提条件。本研究利用Illumina Hiseq测序平台对生长季内红松纯林内根围土壤及菌根样品ITS2区进行高通量测序,分析其外生菌根真菌群落结构随季节的变化规律,同时通过统计学的方法分析了红松根系微生态中外生菌根真菌群落结构组成变化与其他生物因素、非生物因素的相关性。结果如下：（1）从6月份到10月份,5个月的菌根样品测序共得到741个真菌OTUs,利用FUNGuild数据库分析,其中85个OTUs归类为外生菌根真菌,优势属（相对丰度>5）为蜡壳菌属Sebacina、乳牛肝菌属Suillus、Meliniomyces、红菇属Russula、棉革菌属Tomentella、须腹菌属Rhizopogon和缘腺革菌属Amphinema。6月份菌根中外生菌根真菌的多样性最大,显著高于其他月份。（2）红松林外生菌根真菌群落组成受到土壤pH、有效磷含量、有效钾含量和土壤有效氮含量的影响,它们与外生菌根真菌优势属相对丰度呈现正相关或负相关。（3）根围土壤内真菌是影响红松根系外生菌根真菌相对丰度的另一重要因素,其中,包括普可尼亚属Pochonia、产丝齿菌属Hyphodontia、镰刀菌属Fusarium、Collembolispora、枝穗霉属Clonostachys、Apodus、鹅膏属Amanita在内的土壤真菌与根内外生菌根真菌的相对丰度呈线性关系。同时,超过85%的根内外生菌根真菌与同一取样地的土壤共有,可以认为侵染和扩散是红松根内外生菌根真菌群落形成的主要方式,同时兼有植物根系的选择,因为根内并不包括所有土壤中存在的外生菌根真菌,其机制需要进一步人工模拟试验验证。     

外生菌根真菌与内生细菌共生互作的研究进展

外生菌根真菌能与很多高等植物共生,广泛存在于自然界,在促进植物生长和养分吸收、增强宿主抗逆性及维持森林生态系统稳定等方面发挥着重要作用。除与寄主植物密切联系外,外生菌根真菌,在其生命周期中与细菌群落进行物理和代谢相互作用常形成共生关系。这些细菌对外生菌根真菌菌丝生长、生物量增加及子实体的形成具有积极影响。本文阐述了外生菌根真菌与内生细菌共生现象的发现、共生关系的建立、内生细菌促进外生菌根真菌生长和发育及宿主与微生物组的研究方法等,以期更好地巩固外生菌根真菌的生物学及生态学等基础性知识,并利用细菌与真菌的相互作用为可食用外生菌根真菌的生物防治、菌肥研究、人工驯化及栽培提供思路。     

病原菌及菌根菌维持森林植物多样性机制的研究进展

森林植物物种与土壤微生物之间的相互作用决定森林生物多样性。尤其是土壤中的病原菌、菌根菌等微生物在森林物种多样性保护中起到重要作用。Janzen-Connell假说和植物-土壤反馈模型是利用植物与土壤微生物之间的相互作用关系解释森林物种多样性机制的重要模型。本文针对土壤真菌解释森林物种多样性机制进行了综合评述，介绍了病原菌和菌根菌各自在森林实生苗更新的作用机制，探讨病原菌及其菌根菌两者对森林物种多样性的相对重要性，森林实生苗与土壤微生物之间相互作用的变化规律，总结了近年来国际森林物种多样性机制的研究。提出了病原菌和菌根菌两者共同在生态系统中的作用，并探讨未来的研究方向。     

川西亚高山岷江冷杉外生菌根形态随海拔梯度的分化

外生菌根真菌是一类有助于植物生长及应对环境变化的重要共生真菌类群。深入认识外生菌根特征有助于理解植物对环境变化的响应规律。本研究在四川省卧龙自然保护区巴郎山脉设置海拔2850、3000、3194、3413和3593 m 5个样地,利用中心象限法进行土块样品(10 cm×10 cm×10 cm)采集,通过分析各海拔每土块中岷江冷杉外生菌根形态个数、形态多样性及养分获取类型等指标,研究岷江冷杉外生菌根形态特性随海拔的变异特征及土壤环境因子的作用。结果表明: 1) 菌根形态为橘黄色或橘色、单轴羽状分支或不分支、菌套表面光滑、无或少见外延物、根尖直筒型或膨大的菌根类型是卧龙自然保护区岷江冷杉外生菌根的主要形态类型。该形态类型在研究区内的侵染率最高,为12.4%;2) 随着海拔的升高,岷江冷杉外生菌根形态多样性下降,每土块内的形态个数减少;3) 单因素方差分析结果表明,不同海拔上无外延菌丝或极少外延菌丝的紧密接触类型(CE)和短距离养分获取类型(SDE)外生菌根侵染率有显著差异,CE类型外生菌根侵染率随着海拔的升高而增加;4) 土壤环境因子对岷江冷杉外生菌根形态在海拔上的变异有一定的影响,冗余分析(RDA)表明,土壤全氮含量、全磷含量、土壤温度、土壤含水率、pH值及酸性磷酸酶活性对岷江冷杉外生菌根形态在海拔上的变异有显著影响,其中土壤全氮含量和土壤温度的影响最大,分别解释了岷江冷杉形态特征变异的5.4%和4.9%。本研究揭示了岷江冷杉外生菌根形态形成在海拔上的差异,为进一步研究亚高山针叶树种菌根策略对环境变化的响应机制提供了科学依据。     

内蒙古地区山杨外生菌根形态解剖学特征研究

对内蒙古地区山杨的10种外生菌根的外部形态、颜色、分枝情况、菌索及外延菌丝的有无、菌套表面质地、是否透明、有无囊状体等结构进行了描述,结合菌套菌丝排列图式、菌套表面结构及菌丝和菌索内部结构等特征对其进行了归纳总结,并参阅有关文献进行了分类.结果表明:(1)10种菌根类型的划分基础主要是依据真菌组织学特征来加以区别,同时这10种菌根类型与其它地区对杨树外生菌根分类结果相符,符合真菌区系学分布特征.(2)树种与外生菌根共生专一性是一种双向选择模式.就山杨来说,决定其形成不同共生菌根类型的主要因子是不同环境因子影响下的真菌区系组成和树木根系发育状况.     

森林土壤碳氮循环过程的新视角:丛枝与外生菌根树种的作用

几乎所有树木的根系都能与丛枝菌根(AM)真菌或外生菌根(EM)真菌形成共生关系,从而调节森林生态系统土壤碳(C)、氮(N)循环等功能过程。深入理解不同菌根类型森林土壤C、N循环的差异及其影响机制是重要的生态研究命题。该文明晰了AM与EM森林土壤C、N循环的差异;基于森林土壤C、N输入、稳定和输出等3个过程剖析了AM和EM树种对土壤C、N循环的影响机制;比较了不同菌根类型森林土壤C、N循环过程对全球变化的响应;指出了该研究领域所面临的主要挑战:(1)全面比较研究不同菌根类型森林土壤C、N循环及其相关联的生态系统结构和功能特征,为提高森林生产力、发挥生态系统服务功能提供理论基础和数据;(2)深入认知不同菌根树种地上凋落物及地下菌根与自由微生物间相互作用对土壤C、N循环的影响,以阐明不同菌根类型森林土壤C、N循环的潜在机制;(3)改进研究方法,应用新技术手段,充分考虑时空尺度效应,以便能用小尺度的研究结果合理地解释和预测生态系统C、N循环;(4)加强不同菌根类型森林土壤C、N稳定性差异的研究,以准确评价森林结构和功能对全球变化的响应。     

外生菌根真菌对科尔沁沙地樟子松人工林衰退的影响

通过对科尔沁沙地东南缘樟子松人工林气温、地温及樟子松人工林根系分布的观测,从温度对外生菌根菌存活与生长影响的角度分析了沙地樟子松人工林衰退的现象,结果表明：由于沙地表层土壤（0～5cm）高温环境,外生菌根菌无法存活;在沙地20～40cm土层内,温度条件有利于外生菌根菌的存活、生长,而13～42年生沙地樟子松人工林树木平均78％的根系分布于该层,说明外生菌根菌的存在阻止或延缓了樟子松人工林的生长衰退,不是导致沙地樟子松人工林枯叶、枯梢、枯枝、死亡的原因．表层土壤外生菌根菌的缺乏使萌发的幼苗不能形成有效根系是导致沙地樟子松人工林不能天然更新的主要原因之一．     

土壤真菌多样性研究进展

土壤真菌具有重要的生态功能,是生态系统中的重要组成部分,既包含有益真菌,也包含病原真菌,它们共同构成了土壤真菌的多样性。土壤真菌的多样性及其群落结构组成是评价其所在生态系统健康稳定的重要指标之一。近年来不仅对土壤真菌多样性和群落结构特征方面进行研究,而且对其生物功能,如土壤真菌在生物防治、有害物质吸附和降解等方面也进行了研究和探索,研究结果体现了土壤真菌的应用潜质。文中对影响土壤真菌多样性的因素进行了分析,对土壤真菌多样性的研究进行了展望,以期对未来农业生产、环境保护和科学研究提供参考。     

环境因子对AM真菌多样性的影响

土壤,气候和地理因子等环境因子对丝枝菌根（AM）真菌多样性有重要影响,本文系统地综述了最近10年来AM真菌生态学在该方面的最新研究成果,分析当前研究中所存在的问题和动向。     

Ectomycorrhizal fungi: exploring the mycelial frontier

Ectomycorrhizal (ECM) fungi form mutualistic symbioses with many tree species and are regarded as key organisms in nutrient and carbon cycles in forest ecosystems. Our appreciation of their roles in these processes is hampered by a lack of understanding of their soil-borne mycelial systems. These mycelia represent the vegetative thalli of ECM fungi that link carbon-yielding tree roots with soil nutrients, yet we remain largely ignorant of their distribution, dynamics and activities in forest soils. In this review we consider information derived from investigations of fruiting bodies, ECM root tips and laboratory-based microcosm studies, and conclude that these provide only limited insights into soil-borne ECM mycelial communities. Recent advances in understanding soil-borne mycelia of ECM fungi have arisen from the combined use of molecular technologies and novel field experimentation. These approaches have the potential to provide unprecedented insights into the functioning of ECM mycelia at the ecosystem level, particularly in the context of land-use changes and global climate change.     

Fungal soil communities in a young transgenic poplar plantation form a rich reservoir for fungal root communities

Fungal communities play a key role in ecosystem functioning. However, only little is known about their composition in plant roots and the soil of biomass plantations. The goal of this study was to analyze fungal biodiversity in their belowground habitats and to gain information on the strategies by which ectomycorrhizal (ECM) fungi form colonies. In a 2-year-old plantation, fungal communities in the soil and roots of three different poplar genotypes (Populus × canescens, wildtype and two transgenic lines with suppressed cinnamyl alcohol dehydrogenase activity) were analyzed by 454 pyrosequencing targeting the rDNA internal transcribed spacer 1 (ITS) region. The results were compared with the dynamics of the root-associated ECM community studied by morphotyping/Sanger sequencing in two subsequent years. Fungal species and family richness in the soil were surprisingly high in this simple plantation ecosystem, with 5944 operational taxonomic units (OTUs) and 186 described fungal families. These findings indicate the importance that fungal species are already available for colonization of plant roots (2399 OTUs and 115 families). The transgenic modification of poplar plants had no influence on fungal root or soil communities. Fungal families and OTUs were more evenly distributed in the soil than in roots, probably as a result of soil plowing before the establishment of the plantation. Saprophytic, pathogenic, and endophytic fungi were the dominating groups in soil, whereas ECMs were dominant in roots (87%). Arbuscular mycorrhizal diversity was higher in soil than in roots. Species richness of the root-associated ECM community, which was low compared with ECM fungi detected by 454 analyses, increased after 1 year. This increase was mainly caused by ECM fungal species already traced in the preceding year in roots. This result supports the priority concept that ECMs present on roots have a competitive advantage over soil-localized ECM fungi.     

Ectomycorrhizal fungal diversity decreases in Mediterranean pine forests adapted to recurrent fires

Fire is a major disturbance linked to the evolutionary history and climate of Mediterranean ecosystems, where the vegetation has evolved fire‐adaptive traits (e.g., serotiny in pines). In Mediterranean forests, mutualistic feedbacks between trees and ectomycorrhizal (ECM) fungi, essential for ecosystem dynamics, might be shaped by recurrent fires. We tested how the structure and function of ECM fungal communities of Pinus pinaster and Pinus halepensis vary among populations subjected to high and low fire recurrence in Mediterranean ecosystems, and analysed the relative contribution of environmental (climate, soil properties) and tree‐mediated (serotiny) factors. For both pines, local and regional ECM fungal diversity were lower in areas of high than low fire recurrence, although certain fungal species were favoured in the former. A general decline of ECM root‐tip enzymatic activity for P. pinaster was associated with high fire recurrence, but not for P. halepensis. Fire recurrence and fire‐related factors such as climate, soil properties or tree phenotype explained these results. In addition to the main influence of climate, the tree fire‐adaptive trait serotiny recovered a great portion of the variation in structure and function of ECM fungal communities associated with fire recurrence. Edaphic conditions (especially pH, tightly linked to bedrock type) were an important driver shaping ECM fungal communities, but mainly at the local scale and probably independently of the fire recurrence. Our results show that ECM fungal community shifts are associated with fire recurrence in fire‐prone dry Mediterranean forests, and reveal complex feedbacks among trees, mutualistic fungi and the surrounding environment in these ecosystems.     

Studies on Ectomycorrhiza: An Appraisal

Ectomycorrhizal (ECM) fungi are obligate symbionts of dominant vascular plants, liverworts and hornworts. There are reports of about 20,000 to 25,000 ECM fungi that promote plant growth by facilitating enhanced water and nutrient absorption, and provide tolerance to environmental stresses. These below-ground fungi play a key role in terrestrial ecosystems as they regulate plant diversity, nutrient and carbon cycles, and influence soil structure and ecosystem multifunctionality. Because ECM fungi are obligate root symbionts, host plant can have a strong effect on ECM species richness and community composition. The biogeographic pattern and detailed functioning and regulation of these mycorrhizosphere processes are still poorly understood and require detailed study. More recent researches have placed emphasis on a wider, multifunctional perspective, including the effects of ectomycorrhizal symbiosis on plant and microbial communities, and on ecosystem processes. Over the years the main focus in ECM research has been on the study of diversity and specificity of ECM strains, the role of ECM in regeneration of degraded ecosystem, the growth and establishment of seedlings through nutrient acquisition and the mediation of plant responses to various types of stress. In this review, recent progresses in ectomycorrhizal biology are presented, especially the potential role of ECM symbioses in resistance or tolerance to various biotic and abiotic stresses, and in maintinance of plant diversity for proper ecosystem functioning.     

Long‐term experimental manipulation of climate alters the ectomycorrhizal community of Betula nana in Arctic tundra

Climate warming is leading to shrub expansion in Arctic tundra. Shrubs form ectomycorrhizal (ECM) associations with soil fungi that are central to ecosystem carbon balance as determinants of plant community structure and as decomposers of soil organic matter. To assess potential climate change impacts on ECM communities, we analysed fungal internal transcribed spacer sequences from ECM root tips of the dominant tundra shrub Betula nana growing in treatments plots that had received long‐term warming by greenhouses and/or fertilization as part of the Arctic Long‐Term Ecological Research experiment at Toolik Lake Alaska, USA. We demonstrate opposing effects of long‐term warming and fertilization treatments on ECM fungal diversity; with warming increasing and fertilization reducing the diversity of ECM communities. We show that warming leads to a significant increase in high biomass fungi with proteolytic capacity, especially Cortinarius spp., and a reduction of fungi with high affinities for labile N, especially Russula spp. In contrast, fertilization treatments led to relatively small changes in the composition of the ECM community, but increased the abundance of saprotrophs. Our data suggest that warming profoundly alters nutrient cycling in tundra, and may facilitate the expansion of B. nana through the formation of mycorrhizal networks of larger size.     

Shifts in symbiotic associations in plants capable of forming multiple root symbioses across a long‐term soil chronosequence

Changes in soil nutrient availability during long‐term ecosystem development influence the relative abundances of plant species with different nutrient‐acquisition strategies. These changes in strategies are observed at the community level, but whether they also occur within individual species remains unknown. Plant species forming multiple root symbioses with arbuscular mycorrhizal (AM) fungi, ectomycorrhizal (ECM) fungi, and nitrogen‐(N) fixing microorganisms provide valuable model systems to examine edaphic controls on symbioses related to nutrient acquisition, while simultaneously controlling for plant host identity. We grew two co‐occurring species, Acacia rostellifera (N₂‐fixing and dual AM and ECM symbioses) and Melaleuca systena (AM and ECM dual symbioses), in three soils of contrasting ages (c. 0.1, 1, and 120 ka) collected along a long‐term dune chronosequence in southwestern Australia. The soils differ in the type and strength of nutrient limitation, with primary productivity being limited by N (0.1 ka), co‐limited by N and phosphorus (P) (1 ka), and by P (120 ka). We hypothesized that (i) within‐species root colonization shifts from AM to ECM with increasing soil age, and that (ii) nodulation declines with increasing soil age, reflecting the shift from N to P limitation along the chronosequence. In both species, we observed a shift from AM to ECM root colonization with increasing soil age. In addition, nodulation in A. rostellifera declined with increasing soil age, consistent with a shift from N to P limitation. Shifts from AM to ECM root colonization reflect strengthening P limitation and an increasing proportion of total soil P in organic forms in older soils. This might occur because ECM fungi can access organic P via extracellular phosphatases, while AM fungi do not use organic P. Our results show that plants can shift their resource allocation to different root symbionts depending on nutrient availability during ecosystem development.     

树种多样性对土壤微生物群落结构和元素生物地球化学循环的影响研究进展

森林是陆地生态系统的重要组成部分,其巨大的生产力和生态服务功能对人类的生存和发展至关重要。森林树种多样性增加能够显著提高森林生产力,关于树种多样性如何影响地下生物多样性及生态功能逐渐受到国内外学者的广泛关注。从土壤微生物及其介导的元素生物地球化学循环这一视角出发,综述了树种多样性对土壤细菌和真菌多样性、群落结构及功能的影响,提出需要进一步深入研究的方向。总体来说,树种多样性有利于增加土壤细菌生物量和多样性,是预测病原性真菌和菌根真菌多样性及群落结构的重要生物因子。树种多样性能增加土壤有机碳储量,增强森林土壤的甲烷氧化能力,并提高土壤磷周转速率及有效磷含量。关于树种多样性对森林土壤氮循环的影响需考虑多样性假说和质量比假说的相对贡献。今后应加强树种多样性对多个营养级之间相互作用的研究;关注树种多样性对生态系统多功能的影响;加强学科交叉,引入微生物种群动态模型和气候模型等模型预测方法,研究树种多样性对全球气候变化的应对机制,以期促进地上植物多样性与地下生态系统功能关系的研究,增强森林生态系统应对未来全球环境变化的能力。     

丛枝菌根真菌的生态分布及其影响因子研究进展

丛枝菌根(arbuscular mycorrhiza,AM)共生体系对于植物适应各种逆境胁迫具有重要积极作用。AM真菌还能够通过根外菌丝网络调节植物群落结构和演替,深刻影响生态系统结构和功能的稳定性。AM真菌生态生理功能的发挥主要取决于其生态适应性,明确AM真菌在不同环境中的多样性、生态适应性以及对各种生态因子的响应机制,是AM真菌资源管理、功能发掘与利用的前提。迄今为止,有关各种生态因子对AM真菌多样性的影响已有不少研究,但是AM真菌生态分布及其形成机制仍缺乏系统的研究和理论分析。综述了生物因子和非生物因子对AM真菌生态分布的影响,结合大型生物空间分布理论探讨了AM真菌生态分布规律和建成机制,分析了当前本研究领域所存在的问题和动向,以期推动相关研究进展。