A new presymbiotic recognition mechanism from cereals enabling root invasion by arbuscular mycorrhizal fungi.

共生是地球上生命的基础。其中一个例子，被称为丛枝菌根（AM）共生，在真菌和植物之间进化，以促进从土壤中吸收矿物质。这种共生安排是如此广泛，它影响生态系统的生产力和全球营养循环，并被认为是作物生产力和可持续性的根本重要性。AM共生的发展依赖于信号的良好协调交换，以实现两个相互作用的生物体的感知和重编程。然而，由于AM真菌复杂的遗传结构，人们对真菌中存在的允许与植物相互作用的成分知之甚少。相反，对植物突变体的鉴定和分析已经形成了我们对分子过程的理解的基础，这些过程支持物种间的串扰，从而允许这种共生。本研究以共生前信号为中心，填补了我们对AM共生中感知和重编程如何启动植物-真菌物理参与的理解空白。在此之前，我的研究小组已经发现了一个独立于丛枝菌根共生（ina）的玉米突变体，该突变体显示出对真菌根入侵的完全早期阻断。该突变体的一个独特特征是，通过与野生型植物的菌丝连接或通过添加野生型根分泌物来支持真菌，可以克服这种缺陷。这表明突变体渗出物缺乏一种关键成分。在BBSRC的支持下，与工业合作伙伴Corteva合作，应用定位克隆和基于CRISPR/ cas9的反向遗传学，鉴定了一个重要的ABC转运蛋白编码基因。AM真菌是脂肪酸营养不良者，依靠植物提供脂质。充分建立AM共生体的标志是在内皮层组织中形成细胞内真菌丛枝。正是在这里，植物将脂肪酸输送给真菌，以换取土壤矿物质。脂质产生在寄主丛枝植物皮质细胞中被诱导，并通过半大小的ABCG转运蛋白受阻丛枝1和2 （STR1和STR2）的异源二聚体输出到真菌中。与其在支持AM真菌必需有机碳方面不可或缺的作用一致，脂质生物合成和传递途径的组成部分在植物界的菌根植物中是特别保守的。出乎意料的是，决定最初植物与真菌接触的玉米基因是STR2。另一方面，STR1似乎不参与共生前阶段，因为STR1突变体在各种植物物种中都表现出真菌定植。基于STR2在AM敏感植物基因组中的严格存在以及STR2突变体对AM真菌的敏感性丧失，我们假设STR2是定义AM宿主特异性的核心要求，这将是对这种全球普遍存在的共生关系的突破性发现。

Symbioses are fundamental to life on the Earth. One such example, known as arbuscular mycorrhizal (AM) symbiosis, evolved between fungi and plants to facilitate mineral uptake from the soil around 450 mya. This symbiotic arrangement is so widespread that it impacts on ecosystem productivity and global nutrient cycles, and is considered of fundamental importance for crop productivity and sustainability. The development of AM symbioses relies on the well-coordinated exchange of signals to achieve perception and reprogramming of the two interacting organisms. However, due to the complex genetic architecture of the AM fungi, little is known about the components present in the fungus that allow interaction with the plant. Instead, identification and analysis of plant mutants have formed the bedrock of our understanding of molecular processes that underpin the inter-species crosstalk allowing this symbiosis. This proposal centers on presymbiotic signalling to close the gap in our understanding on how perception and reprogramming is achieved to initiate physical plant-fungal engagement in AM symbiosis.Previously, my group had identified the maize mutant independent of arbuscular mycorrhizal symbiosis (ina) that showed a complete early block against fungal root invasion. A unique feature of this mutant is that the defect could be overcome when the fungus was either supported through hyphal connections with wild type plants, or through the addition of wild type root exudates. This suggested that the mutant exudates lacked a critical component. Supported by the BBSRC and in collaboration with the industrial partner Corteva, applying positional cloning and CRISPR/Cas9-based reverse genetics, led to the identification of an essential ABC transporter encoding gene. AM fungi are fatty acid auxotrophs and rely on plants for the provision of lipids. Fully established AM symbioses are marked by the formation of intracellular fungal arbuscules in the inner cortex tissue. It is here where the plant delivers fatty acids to the fungus in exchange for soil minerals. Lipid production is induced in arbuscule-hosting plant cortex cell and exported towards the fungus by a heterodimer of the half-size ABCG transporters Stunted Arbuscule1 and 2 (STR1 and STR2). Consistent with its indispensable role in supporting AM fungi with essential organic carbon, the components of the lipid biosynthetic and delivery pathway are specifically conserved across the mycorrhizal plants in the plant kingdom. Unexpectedly, the maize gene that conditions initial plant-fungal engagement is STR2. STR1 on the other hand, appears not to be involved with the presymbiotic stage since str1 mutants in a variety of plant species show fungal colonisation. On the basis of the strict occurrence of STR2 in genomes of AM-competent plants and the loss of susceptibility of str2 mutants to AM fungi, we hypothesise that STR2 is a core requirement for defining AM host specificity, which would be a ground breaking discovery for this globally prevalent symbiosis.