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

约475 Ma的陆地植物对陆地的殖民是地球历史上影响最深远的篇章之一。苔类植物在陆地植物进化树中占有举足轻重的地位，大量证据支持这些非维管束植物是最基本的陆地光合作用有机体，也是陆地上最早的定居者之一，约475 Ma。最近的分子遗传学证据支持这样的观点，即苔类植物中的菌根状联结是一种基本和祖先特征，而陆地植物是从单一的共同祖先进化而来的，在根进化之前与真菌伙伴形成了共生联结。至少在过去的30年里，流行的范式一直是AM真菌是所有植物-真菌共生体的祖先形式，与最早的陆地植物共同进化。丛枝菌根是最常见的菌根类型，目前在超过70%的陆地植物物种中存在，包括简单和复杂的地钱。然而，项目组成员(MB、JGD、SP)报告的新证据表明，现存的最基本的苔类群--单脉苔类--只与最基本的植物共生真菌群--Endogonales--形成联系。现在，我们第一次有证据表明，真菌群体在与陆地植物营养共生的黎明时的身份。这为共生如何促进陆地生物圈的出现打开了一个引人注目的新窗口。我们的建议通过解决与植物绿化地球的初始阶段有关的三个基本进化问题来利用这些发现：(1)地钱和Endogone物种之间的联系在功能上是否等同于与AM真菌伙伴形成的联系？(2)真菌伙伴是否通过增加对植物和真菌的互惠利益推动基础地钱的进化而从兼性营养Endogone转变为必须具有生物营养的AM真菌？(3)土壤有机质的积累是否有利于生物营养在向植物提供矿物质营养方面，真菌是否优于兼性腐生营养内毒素？我们仔细挑选了7种具有真菌组合的地钱植物，它们提供了一系列强大的模型系统，可以进行C交换的实验和定量功能分析，营养关系和真菌专一性。我们所有的目标生物都可以从孢子或生殖细胞中培养出来，为植物提供有或没有真菌共生体的植物。这为在3年项目中量化真菌定植对配子体生长、营养和生殖产出的影响提供了一种可靠的方法。我们的实验计划将在(1)成熟的田间采集种群和(2)从生殖器/孢子生长的共生植物与共生植物上进行。野外采集的种群将被用于研究从地钱到真菌伙伴的碳分配以及通过真菌向植物相互吸收营养的功能。来自孢子/孢子的种群将被用来调查真菌共生体对苔类的生长和繁殖产量以及真菌伙伴的生物量和范围的净成本/收益。将在这两组植物上进行分子鉴定和超微结构研究，以确定真菌内共生体的身份，植物-真菌界面的性质，以及这些与功能和寄主专一性的关系。总体而言，该项目将为陆地植物和真菌之间的古老共生的曙光贡献基本的知识和理解，这种共生在陆地生态系统的演化中发挥了基础作用--这一主题与国家科学研究中心的地球系统科学主题高级挑战“改善目前对生命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".