Plasmodesmata as regulators of the growth-defence trade-off

植物细胞与它们的邻居相连，以进行通信和资源交换，这种相互连接使多细胞组织和器官之间能够协调生长和反应。胞间连丝是一种连接相邻细胞穿过细胞壁并在相邻细胞之间建立质膜和细胞质连续性的通道，胞间连丝使细胞间的连接成为可能。胞间连丝有效地起到“闸门”的作用，调节可溶性分子的流量，对一系列刺激做出反应，关闭和打开。排列在胞间连丝上的质膜在功能上与膜的其余部分不同，允许对它们的反应进行离散和独立的功能控制。胞间连丝通过启动信号级联反应许多应激信号，导致它们关闭，这被观察到在细胞感受到微生物威胁时对最佳免疫信号至关重要。除了促进免疫反应，胞间连丝关闭导致细胞隔离，这必须限制资源的交换，如碳，以及与其他细胞过程相关的信号。事实上，胞间连丝是糖在植物中分配的关键通道，在需要糖维持生长的情况下，胞间连丝的关闭可能会对碳分配产生重大影响。我们最近发现，与成熟叶片相比，年轻叶片不会对微生物的感知做出反应，关闭胞间连丝。这支持了一种假设，即年轻的组织优先于生长而不是防御，保持胞间连丝开放，以维持来源组织的糖流，以维持生长过程。这一点，以及胞间连丝反应受到独立调控的观察结果，表明胞间连丝是平衡生长和防御之间的权衡机制的核心组成部分。该提案将测试这一模型，确定胞间连丝关闭是否在一系列发育阶段、生理状态和应激环境中受到不同的调节。我们将利用我们的知识，即胞间连丝调节免疫反应的幅度和时间，以描述胞间连丝操作如何扰乱汇组织中的生长-防御权衡。胞间连丝如何做出不同的反应必须取决于执行反应的机制，因此我们将表征胞间连丝在关闭和不关闭胞间连丝的组织中的不同蛋白质组成。此外，由于胞间连丝的结构已被证明与汇或源的状态有一定的相关性，我们将对胞间连丝的结构进行分析，以确定哪些特征定义胞间连丝的反应性。因此，我们将定义胞间连丝对压力作出反应所需的机械要素。这项提案将详细说明胞间连丝如何在免疫信号背景下调节碳分配，以及它们如何作为控制生长-防御权衡的信号中枢的组成部分发挥作用。除了免疫反应，碳分配的调节还与一系列生理背景有关，因此这一提议将建立对碳资源如何分配和消耗的一般理解，并在植物生长、发育和环境反应中找到应用。

Plant cells are connected to their neighbours for communication and resource exchange, with this interconnectivity enabling co-ordinated growth and responses across multicellular tissues and organs. Cell-to-cell connectivity is enabled by plasmodesmata, plasma membrane-lined 'tunnels' that connect neighbouring cells across the cell wall and establish both plasma membrane and cytoplasmic continuity between neighbouring cells. Plasmodesmata effectively act as 'sluice gates' to regulate the flux of soluble molecules, closing and opening in response to a range of stimuli. The plasma membrane that lines plasmodesmata is functionally distinct from the rest of the membrane, allowing discrete and independent functional control of their responses. Plasmodesmata respond to many stress signals by initiating a signalling cascade that results in their closure, and this was observed to be critical for optimal immune signalling when cells perceive microbe threats. In addition to contributing to immune responses, plasmodesmal closure induces cellular isolation that must limit the exchange of resources such as carbon, and signals relevant to other cellular processes. Indeed, plasmodesmata act as a key passage through which sugar is distributed through a plant, raising the possibility that plasmodesmal closure carries significant consequences for carbon allocation in a context in which sugars are required to maintain growth. We recently discovered that by contrast with mature leaves, young leaves do not close their plasmodesmata in response to microbe perception. This supports the hypothesis that young tissues prioritise growth over defence, keeping plasmodesmata open to maintain the flow of sugar from source tissues to sustain growth processes. This, and the observation that plasmodesmal responses are independently regulated, suggests plasmodesmata are a central component of the mechanism by which the trade-off between growth and defence is balanced.The proposal will test this model, determining whether plasmodesmal closure is differently regulated across a range of developmental stages, physiological states and stress contexts. We will exploit our knowledge that plasmodesmata regulate the amplitude and timing of immune responses, to profile how plasmodesmal manipulation perturbs the growth-defence trade-off in sink tissues.How plasmodesmata can differentially respond must depend on the machinery present to execute a response and therefore we will characterise the different protein composition of plasmodesmata in tissues that do and do not close their plasmodesmata. Further, as the structure of plasmodesmata has been shown to have some correlation with the sink or source status, we will profile plasmodesmal structure to identify what features define plasmodesmal responsiveness. Thus, we will define the mechanistic elements required for a plasmodesma to respond to stress. This proposal will detail how plasmodesmata regulate carbon allocation in an immune signalling context and how they function as a component of the signalling hub that controls the growth-defence trade-off. Beyond immune responses, regulation of carbon allocation is relevant to a range of physiological contexts and this proposal will therefore establish general understanding of how carbon resources are partitioned and consumed, finding application across plant growth, development and environmental responses.