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

共生体是地球上生命的基础。一个这样的例子，被称为丛枝菌根（AM）共生，真菌和植物之间的进化，以促进矿物质从土壤中吸收约450百万年。这种共生安排非常普遍，影响到生态系统生产力和全球养分循环，被认为对作物生产力和可持续性至关重要。AM共生体的发展依赖于良好协调的信号交换，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，人们对真菌中允许与植物相互作用的成分知之甚少。相反，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石，这些分子过程支撑了允许这种共生的种间串扰。这个建议的中心presymbiotic信号关闭差距在我们的理解如何感知和重编程实现启动物理植物真菌参与AM symbiosization.Previously，我的小组已经确定了玉米突变体独立的丛枝菌根共生（ina），显示了一个完整的早期块对真菌根的入侵。该突变体的一个独特特征是，当真菌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时，该缺陷可以被克服。这表明，变异体分泌物缺乏一种关键成分。在BBSRC的支持下，并与工业合作伙伴Corteva合作，应用定位克隆和基于CRISPR/Cas9的反向遗传学，鉴定了一个必需的ABC转运蛋白编码基因。AM真菌是脂肪酸营养缺陷型，并且依赖于植物提供脂质。完全建立的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，植物向真菌提供脂肪酸，以换取土壤矿物质。脂质的产生是诱导在丛枝宿主植物皮层细胞和出口到真菌的异源二聚体的一半大小的ABCG转运蛋白矮化丛枝1和2（STR 1和STR 2）。与其在支持具有必需有机碳的AM真菌中不可或缺的作用相一致，脂质生物合成和递送途径的组分在植物界的菌根植物中是特别保守的。出乎意料的是，调节初始植物-真菌接合的玉米基因是STR 2。在另一方面，似乎不涉及的前共生阶段，因为在各种植物物种中的str 1突变体显示真菌定殖。基于STR 2在AM感受态植物基因组中的严格存在以及STR 2突变体对AM真菌的敏感性丧失，我们假设STR 2是定义AM宿主特异性的核心要求，这将是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.