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Phytohistology. Phytocytology --- Plasmodesmata.

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ANA Anatomy & Morphology --- cell physiology --- plant anatomy --- plasma membrane --- plasmodesmata

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Plasmodesmata (PD) are plant-specific intercellular nanopores defined by specialised domains of the plasma membrane (PM) and the endoplasmic reticulum (ER), both of which contain unique proteins, and probably different lipid compositions than the surrounding bulk membranes. The PD membranes form concentric tubules with a minimal outer diameter of only 50 nm, and the central ER strand constricted to ~10-15 nm, representing one of the narrowest stable membrane tubules in nature. This unique membrane architecture poses many biophysical, structural and functional questions. PM continuity across PD raises the question as to how a locally confined membrane site is established and maintained at PD. There is increasing evidence that the PM within PD may be enriched in membrane ‘rafts’ or TET web domains. Lipid rafts often function as signalling platforms, in line with the emerging view of PD as central players in plant defense responses. Lipid-lipid immiscibility could also provide a mechanism for membrane sub- compartmentalisation at PD. Intricate connections of the PM to the wall and the underlying cytoskeleton and ER may anchor the specialised domains locally. The ER within PD is even more strongly modified. Its extreme curvature suggests that it is stabilised by densely packed proteins, potentially members of the reticulon family that tubulate the cortical ER. The diameter of the constricted ER within PD is similar to membrane stalks in dynamin-mediated membrane fission during endocytosis and may need to be stabilised against spontaneous rupture. The function of this extreme membrane constriction, and the reasons why the ER is connected between plant cells remain unknown. Whilst the technically challenging search for the protein components of PD is ongoing, there has been significant recent progress in research on biological membranes that could benefit our understanding of PD function. With this Research Topic, we therefore aim to bring together researchers in the PD field and those in related areas, such as membrane biophysics, membrane composition and fluidity, protein-lipid interactions, lateral membrane heterogeneity, lipid rafts, membrane curvature, and membrane fusion/fission.

Plant cells and tissues. --- Plant cell culture. --- Plasmodesmata. --- lipid rafts --- membrane curvature --- plasmodesmata --- membrane microdomains --- plasma membrane --- protein-lipid interaction --- endoplasmic reticulum --- super-resolution microscopy

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Plasmodesmata are minuscule plasma corridors between plant cells which are of paramount importance for transport, communication and signalling between cells. These nano-channels are responsible for the integrated action of cells within tissues and for the subdivision of the plant body into working symplast units. This book updates the wealth of new information in this rapidly expanding field. Reputed workers in the field discuss major techniques in plasmodesmatal research and describe recent discoveries on the ultrastructure, the functioning and the role of plasmodesmata in intracellular transport and communication, in cell differentiation, plant development and virus translocation.

Plasmodesmata. --- Plasmodesmata --- Botany --- Earth & Environmental Sciences --- Plant Physiology --- Cell interaction --- Cell junctions --- Plant cells and tissues --- Plant science. --- Botany. --- Plant Sciences. --- Botanical science --- Phytobiology --- Phytography --- Phytology --- Plant biology --- Plant science --- Biology --- Natural history --- Plants --- Floristic botany --- Cell Communication. --- Cell Communication

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All plant biologists in research history observed the paramount role of cell-to-cell interconnections, so-called plasmodesmata. The plasmodesmata ultra-structure and the elements involved in its regulation are not fully known yet. However, the new viewpoint sees plasmodesmata as specialized membrane contact sites (MCS) – where the endoplasmic reticulum and the plasma membrane get tethered together within the pores – brings us an original hypothesis on the importance of the spoke-like tethering elements. The identity of such elements was not uncovered yet, but recent research has proved plasmodesmata localization of the plant Multiple C2 domains and Transmembrane region Proteins (MCTPs). As members of this family possess all the required features for membrane tethering, they are perfect candidates for being membrane tethers. Indeed, they display a transmembrane domain, that could bind the endoplasmic reticulum membrane, and several C2 domains – known in mammal MCS tethers to bind lipids and/or calcium ions – that could bind the plasma membrane. The objective of our work was to predict the membrane and/or calcium interactions of the C2 domains of two plasmodesmata-located MCTPs: QUIRKY/AtMCTP5, from Arabidopsis thaliana, which is involved in morphogenesis regulation and NbMCTP11, from Nicotiana benthamiana. For this, we had to establish a series of bioinformatics protocols to delimit the C2 domain sequences, make 3D models and, finally, predict their docking to model plant membrane and possible calcium dependence. The results of the modeling simulations suggested interactions with the membrane, as most of the studied domains were able to interact with the lipids polar heads, but also on more specific features such as phosphatidylinositol phosphate binding sites in NbMCTP11 C2D and calcium binding sites in AtMCTP5 C2D domains. In vivo assays on the full length NbMCTP11 proved that the protein localizes at plasmodesmata and truncated mutants showed different phenotypes, revealing different roles of the different parts of the protein. Further analysis has to be carried out on this part but preliminary results showed convergence between mutant localization and domains behavior. Les chercheurs en biologie végétale ont toujours observé le rôle primordial des connections intercellulaires chez les cellules de plante, autrement appelées plasmodesmes. La structure détaillée des plasmodesmes ainsi que les éléments impliqués dans leur régulation ne sont pas encore entièrement connus. Cependant, le nouveau point de vue qui perçoit les plasmodesmes comme des sites de contacts membranaires particuliers amène d'intéressantes hypothèses sur l'importance des attaches moléculaires qui lient les deux membranes. La nature de ces éléments n'est pas encore complètement élucidé à ce jour, mais de récentes recherches ont décelé la présence de protéines MCTP (Multiple C2 domains and Transmembrane region Proteins) au niveau des plasmodesmes. Ces dernières semblent posséder toutes les caractéristiques nécessaires au pontage des membranes et sont donc des candidates de choix. En effet, les membres de cette famille de protéines sont constitués d'une région transmembranaire, qui pourrait se lier à la membrane du réticulum endoplasmique, et de plusieurs domaines C2 – connus chez les mammifères comme étant des éléments qui se lient aux membranes et qui peuvent dépendre du calcium – qui pourraient interagir avec la membrane plasmique. L'objectif du présent travail a été de prédire les capacités d'interaction membranaire et de dépendance au calcium des domaines C2 de deux MCTPs associées aux plasmodesmes: QUIRKY/AtMCTP5, du modèle Arabidopsis thaliana, qui est également impliqué dans des mécanismes de morphogenèse, et NbMCTP11, du modèle Nicotiana benthamiana. Nous avons préalablement établi une série de protocoles bioinformatiques visant à délimiter les domaines C2, modéliser leur structure 3D et enfin prédire leurs comportements. Les résultats de ces simulations nous ont permis de prédire les modes d'interactions des domaines, notamment l'importance des interactions électrostatiques entre les domaines C2 et les têtes polaires des lipides. Nous avons également mis en évidence des caractéristiques plus spécifiques comme la liaison potentielle de C2D de N. benthamiana aux phosphatidylinositol phosphates ou bien la coordination d'ions calcium du C2D de A. thaliana. Les prédictions d'interaction membranaire ont ensuite été comparés avec les résultats des expériences in vivo sur des mutants tronqués de la protéine NbMCTP11, afin de trouver une possible corrélation entre la localisation cellulaire et le comportement moléculaire. Les résultats préliminaires semblent indiquer une certaine cohérence entre la localisation cytosolique et la faible interaction membranaire de certains domaines. L'utilisation d'autres mutants basés sur les prédictions doivent cependant confirmer ces allégations.

Plant biology, Plasmodesmata, MCTP, C2 domains, Molecular modeling, Molecular dynamics, Calcium-binding, Membrane docking, Lipid interaction --- Biologie végétale, Plasmodesmes, MCTP, Domaines C2, Modélisation moléculaire, Dynamique moléculaire, Interaction calcium, Interaction lipides membranaires --- Sciences du vivant > Biologie végétale (sciences végétales, sylviculture, mycologie...) --- Ingénierie, informatique & technologie > Sciences informatiques --- Sciences du vivant > Biochimie, biophysique & biologie moléculaire