Plasmodesmata as regulators of the growth-defence trade-off

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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.