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

植物光合作用从而从大气中捕获碳的速率受到许多因素的限制，包括土壤中氮(N)的有效性。一个世纪以来的研究一直认为，森林树木可以完全利用无机氮（即铵和硝酸盐）来促进其生长。然而，最近的研究表明，植物可能以土壤中更复杂的有机分子（有机氮）的形式获得其他类型的氮。这个故事的关键在于，几乎所有的森林树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长植物的根系，增加植物对有限营养物质的获取，包括有机氮。本研究将探索某些真菌群为森林树木提供有机氮的能力。更具体地说，本工作将探索这种真菌利用酶分解土壤有机质，从而释放土壤有机质中的有机氮供植物吸收的未知能力。这个研究项目的总体目标是了解这些与根有关的真菌在多大程度上释放土壤中的有机氮供树木利用。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，与共生外生菌根真菌相关的树木将通过增加大气二氧化碳浓度而受精。其他证据表明，森林土壤中大量的碳储量是外生菌根（ECM）真菌酶活性的结果。这两种模型都基于未经检验的一般说法，即ECM真菌从大量土壤有机质（SOM）中动员N。然而，ECM真菌可以以这种方式发挥作用的直接证据缺乏。了解这些生物通过降解SOM获得有机氮的程度取决于编码介导植物腐质和SOM腐烂的酶的基因是否在其进化史中被保留，并在与植物根系共生时被部署。来自全基因组测序和野外数据的多个间接证据表明，只有某些进化谱系的ECM真菌有可能向其植物宿主提供从SOM获得的氮。解决ECM真菌在一系列空间尺度和土壤N条件下的腐坏营养生理，有望解决控制生态力量的问题，我们可以期望植物通常通过其ECM共生体的活动从SOM获得生长限制N。因此，该项目将使用一个合成的全植物系统来测量在ECM真菌的广泛系统发育范围内编码木质纤维素水解酶的基因的酶表达，并将有机氮追踪到完整的植物中。在实验室验证了这一机制后，将使用启用元转录组学的现场方法测试这一现象的发生。总体而言，该研究将跨越在局部和生态系统水平尺度上发生的土壤氮有效性梯度，以提高对从SOM中获取氮的ECM真菌分布结构因素的理解。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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.