Figure 1.

Schematic representation of seed epidermal cell differentiation and mucilage extrusion. The process of seed epidermal cell differentiation (also called MSCs) goes through a phase of cell expansion (I); massive biosynthesis and secretion of pectins from the Golgi to the apoplast leading to the formation of a cytoplasmic column (II); secondary cell wall deposition into the radial cell wall and throughout the cytoplasmic column, generating a volcano-shaped columella (III); and desiccation and apoptosis of the mature seed and mucilage (IV), as described previously by Western et al. (2000). Upon seed hydration, the hygroscopic pectic component of seed mucilage expands rapidly, breaking the outer cell wall and encapsulating the seed; a set of cellulosic rays anchor the pectic component of seed mucilage to the seed surface (V).