Phytohormone signaling in arbuscular mycorrhiza development

丛枝菌根(AM)是一种广泛存在的植物与球孢菌门真菌的共生现象，其基础是共生体之间的营养交换：植物受益于增加矿质养分的吸收，而生物营养真菌则接受光合作用固定的碳。AM真菌对植物根部的定殖包括通过可扩散的信号分子相互识别，然后菌丝对接到根表面，穿透外层细胞层，以及在皮层细胞内形成分枝丛枝。细胞内定植是由一种称为共同SYM通路的古老的进化信号转导程序控制的。植物还根据环境条件和由此产生的生理状态，通过未知的机制动态控制定殖程度，优化共生相互作用。最近的报告强调了植物激素在AM定殖化控制中的重要作用，植物激素是植物发育的调节器。然而，这些知识仍然是初级的，需要机械方法来揭开AM和整个植物发育之间明显的相互联系的分子基础。这项建议旨在研究植物激素信号在AM发育中的机制和结果。它建立在我之前的发现之上，即Karrikin受体复合体的两个组成部分，即α/β折叠水解酶D14L和F-box蛋白Max2，对于AM真菌在根上的定植是必不可少的。卡瑞金是从烟雾中分离出来的一种新的丁烯内酯，最初发现在火灾后的环境中会引发植物萌发。它们还在植物中表现出独立于火萌发的激素活性，D14L的广泛系统发育分布和Karrikin反应表明植物产生一种内源Karrikin样物质。我令人惊讶的发现是，Karrikin受体模块参与了植物与共生真菌的日常相互作用，这表明它在调控植物生命方面的影响比之前假设的更广泛。在这里，我的目标是：1.阐明D14L Max2模块在AM发育过程中的动力学、感知和信号机制；2.揭示D14L信号特异性是如何决定的；3.确定AM发育所需的D14L Max2信号的下游靶点；4.揭示D14L Max2模块与其他植物激素信号通路的相互作用。拟议的研究将结合反向遗传学、生理学和AM分析、转录学、蛋白质相互作用研究，以及通过荧光实时成像对AM发育过程中的D14L Max2信号组件和下游靶标进行时空定位。它将使用公认的根部共生模式植物莲花。我期望这项工作不仅将揭示通过激素信号控制AM共生的机制，而且将有助于全面理解激素在植物发育中的作用，而不是AM领域。

Arbuscular mycorrhiza (AM) is a widespread symbiosis of plants with fungi of the Glomeromycota that is based on the exchange of nutrients between the symbionts: The plant benefits from increased mineral nutrient uptake while the biotrophic fungus receives photosynthetically fixed carbon. Colonization of plant roots by AM fungi involves mutual recognition through diffusible signal molecules, followed by hyphal docking to the root surface, penetration of the outer cell layers, and formation of branched arbuscules inside cortex cells. Intracellular colonization is steered by an evolutionary ancient signal transduction program called common SYM pathway. The plant also dynamically controls the extent of colonization through unknown mechanisms to optimize the symbiotic interaction according to environmental conditions and its resulting physiological state. Recent reports highlight a prominent role of phytohormones, regulators of plant development, in the control of AM colonization. However, this knowledge is still rudimentary and mechanistic approaches are needed to unravel the molecular basis for the obvious interconnections between AM and whole plant development. This proposal is designed to study mechanisms and outcomes of plant hormone signaling in AM development. It builds on my previous discovery that two components of the karrikin receptor complex, the alpha/beta-fold hydrolase D14L and the F-box protein MAX2, are essential for the colonization of roots by AM fungi. Karrikins are newly described butenolides isolated from smoke and originally found to trigger plant germination in post-fire environments. They also display hormonal activity in plants that germinate independently of fire and the wide phylogenetic distribution of D14L and karrikin-responsiveness suggests that plants produce an endogenous karrikin-like substance. My surprising finding that the karrikin-receptor module is involved in the everyday interaction of plants with symbiotic fungi shows that it is more broadly implicated in regulating plant life than previously assumed. Here I aim to 1. elucidate the dynamics, perception and signaling mechanisms of the D14L MAX2 module during AM development; 2. unravel how D14L signaling specificity is determined; 3. identify downstream targets of D14L MAX2 signaling required for AM development; 4. uncover interactions of the D14L MAX2 module with other phytohormone signaling pathways. The proposed research will combine reverse genetics, physiological and AM assays, transcriptomics, protein interaction studies, and spatiotemporal localization by fluorescence live imaging of D14L MAX2 signaling components and downstream targets during AM development. It will use the well-established root symbiosis model plant Lotus japonicus. I expect that this work not only will reveal mechanisms that control AM symbiosis via hormone signaling but will contribute to the general understanding of hormone function in plant development beyond the AM field.