The significance of ericoid mycorrhizal mycelium for carbon turnover in heathland

白垩系植物（愈伤草、牛头草等）是北方森林、石南地和泥炭地的主要组成部分。这些生物群系含有大量的碳，因此在全球生物地球化学循环中起着关键作用。在自然界中，大多数植物都是菌根植物，自养生物将大量的光合作用非常迅速地分配给它们的真菌伴侣，而真菌则为寄主植物提供更多的营养。埃里克森植物的根与埃里克森菌根真菌有关，但与其他菌根类型不同，几乎没有人知道它们在碳循环中的功能重要性。由于毛蕊属植物的初级生产力较低，有人认为它们的细“毛”根和定植在它们上的毛蕊类菌根真菌的外部菌丝体是向荒原土壤提供相对不稳定碳的最重要的输入之一，尽管这一假设仍有待验证。这种知识的缺乏表明我们对土壤生物多样性和生态系统功能的理解存在重大差距；这是NERC研究战略的两个优先领域。外菌根菌丝体除了可能是荒原土壤的重要碳源外，还具有调节养分循环的积极作用。白垩系植物产生的凋落物具有很强的顽固性，含有大量的酚类化合物。形成ericoid菌根的真菌（例如Hymenoscyphus ericae）已经适应了这些寡营养条件，并且被认为是调节分解速率的微生物生物量的关键组成部分。简化的实验室实验已经证明，由外部菌丝产生的细胞外酶，如漆酶，促进了埃里卡菌对复杂酚类化合物的分解。虽然与寄主植物共生生长的ericoid菌根真菌产生漆酶的直接证据仍有待获得，但人们认为这些酶的释放是为了促进真菌伴侣从土壤有机质中获取营养。有一些证据表明，漆酶在潜在的ericoid菌根子囊菌中具有相当大的多样性，但对漆酶和其他酚氧化酶在这种系统中的遗传多样性、作用方式和调节知之甚少。本项目将验证石楠荒原菌根菌丝体在碳循环中起关键作用的假设。我们将1)确定在一系列田间条件下ericoid菌根菌丝体的周转率，2)确定光合作用从宿主植物到微观系统和田间的发根和外部菌丝体的转移率，3)确定不同ericoid菌根真菌产生漆酶的潜力，并确定漆酶的产生是如何被调节的，4)确定ericoid菌根真菌的碳供应之间是否存在联系。漆酶基因表达及资源向寄主植物转移。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合成，以操纵植物和ericoid菌根真菌的组合。菌丝周转将用13C标记的菌丝材料进行研究。这将被引入含有在菌根和非菌根条件下生长的植物的微型环境中，并在垃圾袋中进入田间。真菌组合定殖在分解碳源领域将使用分子指纹技术针对核糖体内转录间隔（ITS）区域进行表征。利用多酚氧化酶活性测定和分子指纹图谱技术研究凋落物降解过程中多酚氧化酶的遗传多样性和活性。

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.