Bricks out of the wall: polysaccharide extramural functions

A large amount of evidence has accumulated over the last decades that polysaccharides and other cell wall components such as glycoproteins can be targeted ex muro (outside the conventionally defined cell wall space). The trafficking and functions of extramural cell wall components located either intracellularly or ex planta are mostly speculative or require more attention from plant researchers. We propose that (i) the functional plasticity and extramural distribution of cell wall-related polysaccharides are based on so far overlooked or uncharacterized transport systems that bypass and/or complement conventional polysaccharide secretion routes, and that (ii) compositional variations in a cell wall polysaccharide could determine a new role, depending on its final destination. Plant polysaccharides are components of plant cell walls and/or store energy. However, this oversimplified classification neglects the fact that some cell wall polysaccharides and glycoproteins can localize outside the relatively sharp boundaries of the apoplastic moiety, where they adopt functions not directly related to the cell wall. Such polysaccharide multifunctionality (or 'moonlighting') is overlooked in current research, and in most cases the underlying mechanisms that give rise to unconventional ex muro trafficking, targeting, and functions of polysaccharides and glycoproteins remain elusive. This review highlights major examples of the extramural occurrence of various glycan cell wall components, discusses the possible significance and implications of these phenomena for plant physiology, and lists exciting open questions to be addressed by future research. Plant polysaccharides are components of plant cell walls and/or store energy. However, this oversimplified classification neglects the fact that some cell wall polysaccharides and glycoproteins can localize outside the relatively sharp boundaries of the apoplastic moiety, where they adopt functions not directly related to the cell wall. Such polysaccharide multifunctionality (or 'moonlighting') is overlooked in current research, and in most cases the underlying mechanisms that give rise to unconventional ex muro trafficking, targeting, and functions of polysaccharides and glycoproteins remain elusive. This review highlights major examples of the extramural occurrence of various glycan cell wall components, discusses the possible significance and implications of these phenomena for plant physiology, and lists exciting open questions to be addressed by future research. a highly diverse and multifunctional group of cell-surface glycoproteins found in most land plants and some algal species. They consist of a hydroxyproline-rich protein backbone and complex glycan chains of arabinose and galactose (arabinogalactan), with a minor proportion of other sugars such as glucuronic acid, fucose, and rhamnose. The glycan part in an AGP accounts for approximately 90% of its total mass. compounds that are released by plant tissues actively by secretion, passively via osmotic gradients or wounding, and due to autolysis of epidermal/cortical cells. Root exudates have various functions, such as mobilizing nutrients, allelopathy, the attraction of symbionts (chemotaxis), and/or promoting a beneficial soil microbiome. a group of plant cell wall polysaccharides that consist of unbranched or branched glycan chains with β-(1→4)- or β-(1→4)- and α-(1→3)-linked backbones in an equatorial configuration. They coat cellulose microfibrils and cross-link them. (from the Latin word murus, wall) located within the cell wall (in muro). Ex muro (extramural) means located outside the space occupied by cell walls: intracellularly or outside the plant. a viscoelastic matrix of high-molecular-weight compounds, predominantly polysaccharides, secreted to the environment by the roots, seeds, leaves, and stems of land plant and by hydroterrestrial algae. a group of galacturonic acid-rich cell wall polysaccharides that can be unbranched (α-(1→4)-linked galacturonic acid, homogalacturonan) or substituted with diverse glycosyl residues with varying degrees of complexity (xylogalacturonan, rhamnogalacturonans I and II). Together with hemicelluloses and glycoproteins, pectins form a hydrated matrix that has various functions in plant growth, development, and cell–cell adhesion. from the English word 'moonlighting' (having a secondary job), this term is used for multifunctional proteins in which one polypeptide chain exhibits more than one physiologically relevant biochemical or biophysical function. Such different functions are determined by different intercellular or extracellular localizations and can result from post-translational modifications, for example, (de-)phosphorylation. the area surrounding a plant root that is inhabited by a defined population of microorganisms which are influenced by exudates released from the plant roots. It includes the apoplastic space of the root cortex and endodermis (endorhizosphere), the root–soil interface directly adjacent to the root epidermis and mucilage (rhizoplane), and the area extending from the rhizoplane into the bulk soil (ectorhizosphere). a plant adaptation to periods of drought. It is characterized by the formation of fleshy and hydrated water storage tissue (hydrenchyma) in stems, leaves, or roots, composed of enlarged cells with prominent vacuoles.