Plasmodesmata

Abstract

Plasmodesmata are plasma membrane‐lined pores that span the adjoining walls of plant cells. They permit the intercellular passage of molecules and signals and play a central role in plant physiology and development. Evidence suggests that small molecules can pass from cell to cell by passive diffusion and that this is controlled by regulation of the pore itself. This contributes to processes such as the transport of sugars between cells and tissues. Many endogenous plant proteins and ribonucleic acid (RNA)‐based signals also utilise plasmodesmata for cell‐to‐cell and long‐distance movement. Recent data suggests that plasmodesmata are tightly controlled during development and in response to environmental changes. There is increasing evidence that this regulation is controlled by proteins that have specific plasmodesmata‐associated functions in stimulus perception and signalling. In addition, many viruses exploit plasmodesmata for cell‐to‐cell spread during infection and their encoded viral movement proteins manipulate the pores to facilitate this process. Key Concepts Plant cells are connected to each other by cytoplasmic bridges called plasmodesmata. The continuous interconnected cytoplasm in plants is referred to as the symplasm. A tube of endoplasmic reticulum passes through plasmodesmata and connects the endoplasmic reticulum of neighbouring cells, thus providing endomembrane continuity between cells. Primary plasmodesmata are formed at cytokinesis when strands of endoplasmic reticulum are trapped between fusing vesicles in the developing cell wall. Secondary plasmodesmata are formed across existing cell walls, including those at graft unions, and usually arise immediately adjacent to existing plasmodesmata. Molecules smaller than the size exclusion limit (SEL) of plasmodesmata are able to move freely through the cytoplasmic channel of plasmodesmata by simple diffusion. The SEL of plasmodesmata may increase or decrease to allow changes in plasmodesmatal conductance. This may occur under different conditions, for example, in response to intracellular factors such as cytoplasmic calcium levels or pathogen perception. Some endogenous proteins and some movement proteins encoded by plant viruses are able to increase the SEL of plasmodesmata to facilitate their own passage into neighbouring cells. This enables these proteins to function in cells in which they are not normally expressed. Groups of cells may be connected by plasmodesmata that share an SEL different to that of neighbouring cells. These regions of cells are called symplasmic domains. Some proteins and RNA molecules pass into the plant's translocation stream and move over long distances. These macromolecules traffic through the plasmodesmata that join sieve elements (SE) and companion cells within the phloem. These macromolecules may have a site of action distant to their site of expression and synthesis.