The significance of ericoid mycorrhizal mycelium for carbon turnover in heathland

杜鹃花科植物（Calluna，Vaccinium等）是北方森林，石楠和泥炭地的主要组成部分。这些生物群落含有大量的碳，因此在全球生物地球化学循环中发挥着关键作用。在自然界中，大多数植物都是菌根植物，并且自养生物非常迅速地将大量的光合产物分配给它们的真菌伴侣，作为回报，真菌为宿主植物提供更多的营养。杜鹃花科植物的根与杜鹃花科菌根真菌有关，但与其他类型的菌根不同，它们在碳循环中的功能重要性几乎一无所知。由于杜鹃花科植物具有低的初级生产力，有人建议，他们的细“毛”根和外部菌丝体的杜鹃花菌根真菌，殖民他们是最重要的输入相对不稳定的碳石楠土壤，虽然这一假设仍有待检验。这种知识的缺乏代表了我们对土壤生物多样性和生态系统功能的理解的一个主要差距;这是NERC研究战略的两个优先领域。除了作为石南荒原土壤重要碳源的可能作用外，外部杜鹃花菌根菌丝体在调节养分循环方面也具有积极作用。杜鹃花科植物产生的凋落物的性质是高度挥发性的，并且包含大量的酚类化合物。形成杜鹃花状菌根的真菌（例如杜鹃花膜菌（Hymenoscyphus ericae））已经适应了这些贫营养条件，并且认为它们是用于调节分解速率的微生物生物量的关键组分。简化的实验室实验表明，H. Ericae通过外部菌丝体产生胞外酶如漆酶来促进。虽然漆酶生产的直接证据，杜鹃花类菌根真菌生长在共生与宿主植物仍然有待获得，它被认为是释放，以促进养分的真菌合作伙伴从土壤有机质的收购。有一些证据表明，潜在的杜鹃花科菌根子囊菌中漆酶具有相当大的多样性，但人们对此类系统中漆酶和其他酚氧化酶的遗传多样性、作用方式和调节知之甚少。本计画将验证石南地杜鹃花菌根菌丝体在碳周转中扮演重要角色的假设。我们将1）确定在一定范围的田间条件下杜鹃花科植物菌根菌丝体的周转率，2）确定在微宇宙系统和田间中从宿主植物到毛根和外部菌丝体的光合产物转移率，3）确定不同杜鹃花科植物菌根真菌产生漆酶的潜力，并确定漆酶的产生是如何调节的，以及4）确定在向杜鹃花菌根真菌的碳供应、漆酶基因表达和向宿主植物的资源转移之间是否存在联系。该项目将使用我们实验室常规使用的一系列技术，包括稳定和放射性同位素，分子生物学和微观系统的合成，以操纵植物和杜鹃花菌根真菌的组合。利用13 C标记的杜鹃花类菌根真菌菌丝体材料研究菌丝周转。这将被引入到包含在菌根和非菌根条件下生长的植物的微观世界中，并在垃圾袋中进入田间。在该领域的真菌组合殖民分解碳源的特点是使用分子指纹技术靶向核糖体内转录间隔区（ITS）区域。本研究将利用多酚氧化酶活性测定和分子指纹技术，对参与凋落物降解的多酚氧化酶的遗传多样性和活性进行研究。

Ericaceous plants (Calluna, Vaccinium etc) are major components of boreal forest, heathlands and peatlands. These biomes contain enormous quantities of carbon and so play a key role in global biogeochemical cycles. In nature, most plants are mycorrhizal and the autotrophs allocate large amounts of photosynthate very rapidly to their fungal partners, and in return the fungi provide host plants with increased quantities of nutrients. Ericaceous plant roots associate with ericoid mycorrhizal fungi but, unlike other mycorrhizal types, virtually nothing is known of their functional importance in the carbon cycle. Because ericaceous plants have low primary productivity, it has been suggested that their fine 'hair' roots and the external mycelium of the ericoid mycorrhizal fungi that colonise them are one of the most important inputs of relatively labile carbon to heathland soils, although this hypothesis remains to be tested. This lack of knowledge represents a major gap in our understanding of soil biodiversity and ecosystem functioning; these are two priority areas of NERC's research strategy. In addition to its likely role as an important carbon source to heathland soils, external ericoid mycorrhizal mycelium also has an active role in regulating nutrient cycles. The nature of litter produced by ericaceous plants is highly recalcitrant and comprises large quantities of phenolic compounds. The fungi forming ericoid mycorrhizas (e.g. Hymenoscyphus ericae) have adapted to these oligotrophic conditions and it is thought that they are crucial components of the microbial biomass for regulating decomposition rates. Simplified laboratory experiments have demonstrated that the breakdown of complex phenolic compounds by H. ericae is facilitated by production of extracellular enzymes, such as laccases, by the external mycelium. While direct evidence for laccase production by ericoid mycorrhizal fungi when grown in symbiosis with a host plant still remains to be obtained, it is thought that such enzymes are released to facilitate nutrient acquisition by the fungal partner from soil organic matter. There is some evidence for considerable diversity in laccases among potentially ericoid mycorrhizal ascomycetes, yet little is known about the genetic diversity, mode of action, and modulation of laccases and other phenol oxidases in such systems. This project will test the hypothesis that ericoid mycorrhizal mycelium has a key role in carbon turnover in heathlands. We will 1) determine rates of turnover of ericoid mycorrhizal mycelium in a range of field conditions, 2) determine the rates of photosynthate transfer from host plant to hair roots and external mycelium in microcosm systems and in the field, 3) determine the potential for different ericoid mycorrhizal fungi to produce laccases and determine how the production of laccases is regulated, and 4) establish if there is a link between carbon supply to ericoid mycorrhizal fungi, laccase gene expression and resource transfer to the host plant. This project will use a range of techniques in routine use in our laboratories including stable and radioisotopes, molecular biology and synthesis of microcosm systems to manipulate combinations of plant and ericoid mycorrhizal fungi. Mycelial turnover will be studied using 13C labelled mycelial material from ericoid mycorrhizal fungi. This will be introduced into microcosms containing plants grown in the mycorrhizal and non-mycorrhizal condition, and into the field in litterbags. The fungal assemblages colonising decomposing carbon sources in the field will be characterised using molecular fingerprinting techniques targeting the ribosomal internal transcribed spacer (ITS) region. The genetic diversity and activity of polyphenol oxidase (PPO) enzymes involved in the degradation of litter will be studied using PPO enzyme activity assays and molecular fingerprinting techniques.