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(1986) Amino acid function of the LH2 (B800–850) complex from the photophosphorylation in Chlamydomonas composition of the 9 kDa phosphoprotein of pea purple photosynthetic bacterium reinhardtii. Biochim. Biophys. Acta 1413, 117–129 Plasmodesmata and plant cytoskeleton Rachid Aaziz, Sylvie Dinant and Bernard L. Epel Plant cell-to-cell communication is achieved by membranous conduits called microfilaments (polymers of tubulin and actin, plasmodesmata, which bridge the cytoplasm of neighboring cells. A growing respectively) and diverse associated proteins. body of immunolocalization data shows an association of the cytoskeleton Whereas direct cell-to-cell communication is provided machinery with plasmodesmata. The role of the cytoskeleton in the in animals and fungi by gap junctions and septal plasmodesmata-mediated transport has been well documented for virus pores, respectively, within plants, cell-to-cell movement. Because viruses are known to exploit existing host pathways and cytoplasmic trafficking takes place through because the cytoskeleton is involved in intracellular trafficking, the plasmodesmata, wall spanning co-axial membranous cytoskeleton is thought to drive and target macromolecules to organelles that bridge the cytoplasm of contiguous plasmodesmata. It is this link between plasmodesmata and the cytoskeleton cells3,4. Plasmodesmata are considered to enable both that will be described here. physiological and developmental coordination of the plant5. Major insights in plasmodesmata functions The plant cytoskeleton plays an important role in have arisen from both viral movement studies and many biological processes, including cell division and microinjection experiments4, and by the use of expansion, organogenesis, tip growth and transiently expressed green fluorescent protein (GFP) intracellular signaling1,2. The plant cytoskeleton is fusion proteins6. Functional studies have underlined composed primarily of a network of microtubules and that plasmodesmata are dynamic structures that http://plants.trends.com 1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1360-1385(01)01981-1 Review TRENDS in Plant Science Vol.6 No.7 July 2001 327 Fig. 1. A simple vesicles takes place along both microtubules and plasmodesma Endoplasmic microfilaments. This transport is mediated by (transversal section). The reticulum plasmodesma establishes Cell 1 molecular motor proteins exhibiting ATPase activity, a continuum state Plasma including members of the kinesin, dynein and myosin between adjacent cells. It membrane superfamilies. It is well accepted that microtubules, in corresponds to a channel embedded into the cell conjunction with kinesin and dynein motors, provide wall and delineated by the rails for long-distance movement of membranous plasma membranes of the vesicles, whereas microfilaments, powered by myosin two interconnected cells motors, direct their local movement12. Because (cells 1 and 2). An cytoskeletal elements were shown to be involved in the appressed form of Cell wall endoplasmic reticulum movement of intracellular vesicles and organelles, it crosses the was speculated that cytoskeleton might similarly be plasmodesma. Globular involved in plasmodesmata-mediated trafficking9. proteinaceous particles (blue and red spheres) Consistent with this is an ever-growing number of covering the inside of the reports on the close association between cytoskeletal plasmodesma are components and plasmodesmata. hypothetically represented. This review will focus on the relationship between Cell 2 the cytoskeleton and plasmodesmata and how cytoskeletal elements might be affecting the structure TRENDS in Plant Science and regulation of plasmodesmata, thus serving the plant for better or for worse. For the better by being involved in enable the intercellular transport of macromolecules, the regulation of information transfer for plant growth including endogenous and viral proteins, as well as and development, for the worse by being usurped by ribonucleoprotein complexes5. Recent data have viruses as a vehicle for the spread of viral infection. allowed non-targeted and targeted protein movement through plasmodesmata to be distinguished6,7. The Plasmodesmata: a concentrate of cytoskeletal non-targeted protein movement is considered to be a elements diffusion process and is limited only by the size of the Based on ultrastructural microscopic studies, a translocated protein. By contrast, targeted-protein simple plasmodesma appears as a coaxial movement is selective, requiring interactions between membranous conduit spanning the cell wall (Fig. 1). endogenous or exogenous (e.g. viral) factors and the The outer membrane is continuous with the plasma specific protein to direct and facilitate its translocation membranes of the interconnected cells, whereas the through plasmodesmata8. Over the past decade, the centrally tubular membranous component, termed discovery of interactions between viral proteins the desmotubule, is a derivative of and contiguous involved in virus movement and either with the cortical endoplasmic reticulum (ER). plasmodesmata or the cytoskeleton