Ectomycorrhizal Fungi and the Enzymatic Liberation of Nitrogen from Soil Organic Matter: Ecosystem Processes and Underlying Molecular Mechanisms

植物进行光合作用从而从大气中吸收碳的速度受到许多因素的限制，包括土壤中氮素的有效性。一个世纪以来的研究假定，林木可以完全使用无机氮(即铵和硝酸盐)来推动它们的生长。然而，最近的研究表明，植物可能会以土壤中也存在的更复杂的有机分子(有机氮)的形式获得更多类型的氮。这个故事的关键是，几乎所有的林木都与栖息在地下植物根部的真菌形成了共生关系。这些真菌的作用是延伸植物的根系，增加植物获得包括有机氮在内的限制性营养物质的机会。这项研究将探索某些真菌群体为林木提供有机形式氮的能力。更具体地说，这项工作将探索这类真菌利用酶分解土壤有机质的未知能力，从而释放土壤有机质中的有机氮供植物吸收。这个研究项目的主要目标是了解这些与根相关的真菌在多大程度上释放土壤中的有机氮供树木使用。该项目还将为K-12教师提供适合年龄的教学工具，以建立学生对森林生态的理解。最近的全球水平建模工作表明，与外生菌根真菌互惠共生相关的树木将通过增加大气二氧化碳浓度来受精。其他证据表明，森林土壤中大量的碳储量是外生菌根真菌(ECM)酶活性的结果。这两个模型都建立在迄今未经检验的一般主张的基础上，即ECM真菌从土壤有机质(SOM)的大池中动员N。然而，缺乏直接证据表明ECM真菌可以以这种方式发挥作用。了解这些生物通过降解SOM获得有机N的程度取决于编码介导植物碎屑和SOM腐烂的酶的基因是否在它们的进化历史中被保留，并在与植物根共生时被部署。来自全基因组测序和田间数据的多条间接证据表明，只有某些ECM真菌的进化谱系才有可能为其植物宿主提供从SOM获得的N。在一系列空间尺度和土壤N条件下解决ECM真菌的腐生生理学，有望解决控制我们可能期望植物通过其ECM互助体的活动从SOM获得限制生长的N的生态力量。因此，该项目将使用合成的全植物系统来测量编码木质纤维分解酶的基因在广泛的ECM真菌系统发育范围内的酶表达，并将有机N追踪到完整的植物中。在实验室中验证了这一机制后，将使用元转录激活的现场方法来测试这种现象的发生。总体而言，这项研究将跨越发生在地方和生态系统水平上的土壤N有效性的梯度，以提高对构成从SOM获得N的ECM真菌分布的因素的理解。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

The rate at which plants can photosynthesize, thereby capturing carbon from the atmosphere, is limited by a number of factors including the availability of nitrogen (N) in soil. A century of research has presumed that forest trees can exclusively use inorganic nitrogen (i.e., ammonium and nitrate) to fuel their growth. However, recent work has suggested that plants may have access to additional types of nitrogen in the form of more complex organic molecules (organic N) also present in the soil. The key to this story is that nearly all forest trees form symbiotic relationships with fungi that dwell on plant roots below ground. These fungi act to extend the root system of plants and increase plant access to limiting nutrients, including organic N. This research will explore the capacity for certain groups of fungi to provide forest trees with organic forms of nitrogen. More specifically, this work will explore the unknown capacity of such fungi to decompose soil organic matter using enzymes, thereby releasing organic N in soil organic matter for plant uptake. The broad goal of this research project, then is to understand the extent which these root-associated fungi are liberating organic N in soil for use by trees. This project will also provide K-12 teachers with age-appropriate teaching tools to build students' understanding of forest ecology. Recent global level modeling efforts indicate that trees that associate with ectomycorrhizal fungi mutualists will be fertilized by increasing concentrations of atmospheric CO2. Other evidence suggests that the large stocks of C in forest soils are a result of the enzymatic activity of ectomycorrhizal (ECM) fungi. Both models rest upon the hereto untested general claim that ECM fungi mobilize N from the large pools of soil organic matter (SOM). However, direct evidence that ECM fungi can function in this manner is lacking. Understanding the extent to which these organisms obtain organic N by degrading SOM is contingent on whether genes encoding enzymes that mediate both the decay of plant detritus and SOM, were retained during their evolutionary history and are deployed when in symbiosis with plant roots. Multiple lines of indirect evidence, stemming from whole genome sequencing and field data indicate that only certain evolutionary lineages of ECM fungi have the potential to provision their plant hosts with N that is obtained from SOM. Resolving the saprotrophic physiology of ECM fungi across a range of spatial scales and soil N conditions, holds promise to resolve the ecological forces governing where we may expect plants to commonly obtain growth-limiting N from SOM via the activity of their ECM mutualists. Accordingly, this project will use a synthetic whole-plant system to measure the enzymatic expression of genes encoding lignocellulolytic enzymes across a wide phylogenetic range of ECM fungi, and will also trace organic N into intact plants. After validating this mechanism in the laboratory, the occurrence of this phenomenon will be tested using meta-transcriptomic enabled field approaches. Overall, the research will span gradients in soil N availability occurring at both the local and ecosystem level scales to improve understanding of the factors that structure the distribution of ECM fungi that obtain N from SOM.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.