Arabidopsis Seed Mucilage: A Specialised Extracellular Matrix that Demonstrates the Structure–Function Versatility of Cell Wall Polysaccharides

Abstract Arabidopsis seed coat mucilage is a specialised extracellular matrix composed of the same broad classes of polysaccharides found in a primary cell wall (cellulose, pectin, and hemicellulose), arranged in a distinct structure with special properties. It is deposited by the seed coat epidermal cells in the apoplast in a polar manner to produce a doughnut‐shaped mucilage pocket on the outside of the cell. Following exposure to water the mucilage extrudes from the outer surface of the cell. A portion of this mucilage remains strongly adherent to the seed. The unique features of mucilage make it amenable to study, and genetic, cytological, and biochemical analyses have been used to determine its composition, synthesis, and structure, and establish how these combine to determine its properties. Lessons learned from these studies strengthen and extend our understanding of plant extracellular matrices, especially the cell wall. In this article, we summarise and contextualise what is currently known about mucilage by focusing on its main features – extrusion and adherence.