Origin and co-evolution of land plant-fungal symbioses during the "greening of the Earth"

大约475年前陆生植物对陆地的殖民是地球历史上影响最深远的篇章之一。在陆地植物进化树上，Liverworts占据着关键的位置，各种各样的证据支持这些非维管植物是最基础的陆地光合生物，也是最早的陆地殖民者之一，大约在475年前。最近的分子遗传学证据支持这样的观点，即苔类菌根关联是一种基础和祖先特征，陆地植物从一个共同的祖先进化而来，在根进化之前与真菌伴侣形成共生关系。至少在过去的30年里，主流的范式一直认为AM真菌是所有与最早的陆地植物共同进化的植物-真菌共生体的祖先形式。AM关联菌是最常见的菌根类型，目前在超过70%的陆地植物物种中发现，包括简单和复杂的菌体苔类。然而，项目团队成员（MB， JGD， SP）报告的新证据表明，现存最基础的肝菌类群Haplomitriopsida与最基础的植物共生真菌类群Endogonales完全形成关联。我们现在第一次有证据表明真菌群在它们与陆地植物的营养共生初期的身份。这为研究共生如何促进陆地生物圈的出现打开了一扇非凡的新窗口。我们的建议利用这些发现，通过解决与植物“地球绿化”初始阶段有关的三个基本进化问题：(1)地物和endogo物种之间的关联在功能上是否与AM真菌伴侣形成的关联相同？(2)真菌伴侣从兼性腐养型内生内生真菌向专性生物营养型AM真菌的转变是否受到植物和真菌互惠利益增加的驱动？(3)在向植物提供矿质养分方面，土壤有机质的积累是否更有利于专性生物营养型AM真菌而不是兼性腐养型内啡肽？我们精心挑选了7种与真菌相关的苔类，提供了一个强大的模型系统光谱，适用于C交换、营养关系和真菌特异性的实验和定量功能分析。我们所有的目标生物都可以从孢子或胚芽中培养出来，为植物提供有或没有真菌共生体的植物。这为真菌定植对配子体生长、营养和生殖输出的量化提供了一个可靠的方法。我们的实验计划将在：(1)田间采集的成熟种群(和(2)从芽孢/孢子生长的共生与非共生植物上进行。田间收集的种群将用于肝草向其真菌伴侣分配c的功能研究以及通过真菌向植物相互吸收营养的功能研究。来自芽孢/孢子的种群将用于调查真菌共生体对苔类生长和繁殖产量的净成本/收益，以及真菌伙伴的生物量和范围。分子鉴定和超结构研究将在这两组植物上进行，以确定真菌内共生体的身份，植物-真菌界面的性质，以及这些与功能和宿主特异性的关系。总体而言，该项目将为陆地植物和真菌之间的古老共生关系的黎明提供基础知识和理解，这种共生关系在陆地生态系统的进化中发挥了基础性作用——这一主题与NERC地球系统科学主题的高级挑战“提高当前对生命进化与地球之间相互作用的认识”密切相关。

Colonization of the land by terrestrial plants ca. 475 Ma was one of the most far-reaching chapters in Earth's history. Liverworts occupy the pivotal position in the land plant evolutionary tree with a wide variety of evidence supporting these non-vascular plants as the most basal terrestrial photosynthetic organisms and amongst the earliest colonizers of the land ca. 475 Ma. Recent molecular genetic evidence supports the view that mycorrhiza-like associations in liverworts are a basal and ancestral trait, and land plants evolved from a single common ancestor that formed symbiotic associations with fungal partners before roots evolved. For at least the past 30 years, the prevailing paradigm has been that AM fungi are the ancestral form of all plant-fungal symbioses co-evolving with the earliest land plants. AM associations are the most common type of mycorrhiza, and are currently found in over 70% of land plant species including simple and complex thalloid liverworts. However, new evidence reported by members of the project team (MB, JGD, SP) has revealed that the most basal extant groups of liverworts, the Haplomitriopsida, exclusively form associations with the most basal group of plant-symbiotic fungi - the Endogonales. For the first time, we now have evidence pointing to the identity of the fungal group at the dawn of their nutritional symbiosis with land plants. This opens a remarkable new window for functional investigations into how the symbiosis facilitated the emergence of the terrestrial biosphere. Our proposal exploits these findings by addressing three fundamental evolutionary questions relating to the initial phase of plants 'greening of the Earth':(1) Are the associations between liverworts and species of Endogone functionally equivalent to those formed with AM fungal partners? (2) Was the switching of fungal partners from facultatively saprotrophic Endogone to obligately biotrophic AM fungi through the evolutionary advance of basal liverworts driven by increasing mutualistic benefits to the plants and fungi?(3) Did the accumulation of soil organic matter favour obligately biotrophic AM fungi over facultatively saprotrophic Endogone in supplying mineral nutrients to the plants?We have carefully selected 7 liverwort species with fungal associations that provide a powerful spectrum of model systems amenable to experimentation and quantitative functional analyses of C exchange, nutrient relationships and fungal specificity. All of our target organisms can all be cultured either from spores or gemmae to provide plants with and without fungal symbionts. This provides a robust approach for enabling quantification of fungal colonization on gametophyte growth, nutrition and reproductive output within a 3 yr project.Our experimental programme will be conducted on (1) mature field-collected populations (and (2) symbiotic vs. asymbiotic plants grown from gemmae/spores. Field collected populations will be used for functional studies of C-allocation from liverworts to their fungal partners and reciprocal uptake of nutrients into the plants via the fungi. Populations from gemmae/spores will be used for investigating the net costs/benefits of the fungal symbionts on the growth and reproductive output of the liverworts and the biomass and extent of the fungal partners. Molecular identification and ultra-structural studies will be undertaken on both sets of plants to establish the identity of the fungal endosymbionts, the nature of the plant-fungal interfaces, and how these relate to function and host specificity.Overall, this project will contribute fundamental knowledge and understanding to the dawn of an ancient symbiosis between land plants and fungi that played a founding role in the evolution of terrestrial ecosystems - a topic closely aligned to NERC's Earth System Science Theme high-level challenge "improving current knowledge of the interaction between the evolution of life and the Earth".