Bricks out of the wall: polysaccharide extramural functions.

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 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. [ABSTRACT FROM AUTHOR]