Plasmodesmata as a supracellular control network in plants

Key Points Plasmodesmata are plant-unique intercellular communication channels that acquired the capacity to dilate significantly to allow the trafficking of proteins and RNA within a symplasmic domain. This property is thought to contribute to the supracellular nature of plants. A subset of proteins that regulate the signalling processes beyond the cells in which they are synthesized can traffic through plasmodesmata to function as non-cell-autonomous proteins (NCAPs). One mode of trafficking through plasmodesmata might be controlled by a gate open/gate closed (GO/GC) pathway that leads to the formation of symplasmic domains in which certain unbound molecules (up to ∼40 kDa) can diffuse between neighbouring cells. A second, selective, mode of trafficking through plasmodesmata might involve a specific interaction between each NCAP and the plasmodesmal machinery. Intercellular trafficking of transcription factors that regulate developmental patterning and/or cell-fate determination underscores the important role carried out by the NCAP pathway. RNA can function both as a local and a long-distance information macromolecule by trafficking between cells through plasmodesmata and between organs through the vascular conduit that is provided by phloem. Specific mRNA molecules that can move through phloem might enter various meristematic tissues to redirect developmental events. Phloem translocation has important roles in transmitting RNA-interference signals and systemic signals in response to wounds and pathogen attacks, and in coordinating plant nitrogen metabolism and symbiosis. The formation of a new symplasmic domain between the phloem and a nodule initial illustrates the operation of the integrated signalling system that is provided by plasmodesmata and phloem. The evolution of intercellular communication had an important role in the increasing complexity of both multicellular and supracellular organisms. Plasmodesmata, the intercellular organelles of the plant kingdom, establish an effective pathway for local and long-distance signalling. In higher plants, this pathway involves the trafficking of proteins and various forms of RNA that function non-cell-autonomously to affect developmental programmes.