![]() A number of other cell wall-localized proteins, such as ESB1 have also been identified to both localize to the CS and to be involved in its formation ( Hosmani et al., 2013 Reyt et al., 2020), indicating that a complex machinery is involved in locally synthesising CS lignin. These CASP platforms are thought to bring together reactive oxygen species (ROS)-producing NADPH oxidases with ROS-utilising peroxidases, both of which have been shown to be crucial for CS formation ( Lee et al., 2013 Rojas-Murcia et al., 2020). CASPs form central, longitudinal rings in each endodermal cell, with their coordinated localisation leading to a net-like appearance at a tissue-wide level that precisely coincides with the lignin impregnations of the CS themselves. We then discovered the CASPARIAN STRIP MEMBRANE DOMAIN PROTEINs (CASPs) - the first proteins localised to the CS – and showed that their function is to form an extended, highly stable transmembrane platform ( Roppolo et al., 2011). Work from our lab initially established that the CS is made of lignin - a hydrophobic, polyphenolic polymer, impregnating cellulosic plant cell walls ( Naseer et al., 2012). The CS is an illustrative example of such a tight membrane-wall barrier. ![]() We are only beginning to understand the complex molecular identity of this plasma membrane-cell wall nexus ( Barbosa et al., 2019). Instead, a more complex interplay between transmembrane proteins and cell wall enzymes must generate a localised, coordinated membrane-wall adhesion, guiding hydrophobic cell wall modifications between cells, thus creating a membrane-to-cell-wall-to-membrane continuum. In animals, extracellular diffusion barriers are formed by tight and adherence junctions, polymeric assemblies of transmembrane proteins that enter in direct contact between neighbouring epithelial cells, forming a network of joined rings ( Cereijido, 2004).īecause of their cell walls, plants cannot achieve extracellular (apoplastic) diffusion barriers by membrane protein-mediated cell-to-cell contacts. As a consequence, epithelia have to maintain two functionally distinct plasma membrane surfaces, facing the different extracellular environments, allowing them to generate selective and vectorial transport. Polarised epithelia restrict extracellular diffusion between different extracellular spaces by generating specific, extracellular diffusion barriers. A particularly straightforward difference arising from this, is the way polarised epithelia can be formed. Thus, many of the profound differences between animal and plant development can be seen as resulting from the fact that plants have generated complex multicellularity from walled cells. Yet, cell walls necessarily immobilise and isolate cells, not allowing for cell-to-cell contact of plasma membranes, something which animals rely on extensively during development and cellular signalling. One fundamental feature of plants is the presence of a cell wall that allows plant cells to resist high internal pressure, enabling rapid cell growth through a dominating central vacuole ( Somerville et al., 2004). Plants acquired multicellularity independently of animals and thus evolved independent solutions to the many challenges that arose with multicellularity ( Knoll, 2011). Our work reveals that CASPs enforce displacement of initial secretory foci through exclusion of vesicle tethering factors, thereby ensuring rapid fusion of microdomains and effective sealing of the cell wall space. We confirm their localisation and function at the CSD, similar to exocyst subunits, known Rab effectors. ![]() ![]() Biotin proximity labelling identifies RabA-GTPases as potential CASP-interactors. Ultra-structurally, however, these spots appear as highly disorganized secretory foci, with neither exclusion zone nor membrane attachment and excessive cell wall growth. A full CASP knock-out ( caspQ) now reveals that CASPs are not needed for localization of lignification or lignin-polymerizing enzymes, since correctly aligned spots still form in the mutant. The CS membrane domain (CSD) proteins 1-5 (CASP1-5) define and accumulate at the CS associated membrane domains displaying matrix adhesion and protein exclusion. Casparian strips (CS), the main extracellular diffusion barrier in plant roots, are precisely localized cell wall lignin-impregnations, contrasting animal tight-junctions. ![]()
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