Differentiation of mucilage secretory cells of the Arabidopsis seed coat

In some plant species, including Arabidopsis, fertilization induces the epi dermal cells of the outer ovule integument to differentiate into a speciali zed seed coat cell type with a unique morphology and containing large quant ities of polysaccharide mucilage (pectin). Such seed coat mucilage cells ar e necessary for neither viability nor germination under normal laboratory c onditions. Thus, the Arabidopsis seed coat offers a unique system with whic h to use genetics to identify genes controlling cell morphogenesis and comp lex polysaccharide biosynthesis and secretion. As a first step in the appli cation of this system, we have used microscopy to investigate the structure and differentiation of Arabidopsis seed coat mucilage cells, including cel l morphogenesis and the synthesis, secretion, and extrusion of mucilage. Du ring seed coat development in Arabidopsis, the epidermal cells of the outer ovule integument grow and differentiate into cells that produce large quan tities of mucilage between the primary cell wall and plasma membrane. Concu rrent with mucilage production, the cytoplasm is shaped into a column in th e center of the cell. Following mucilage secretion the cytoplasmic column i s surrounded by a secondary cell wall to form a structure known as the colu mella. Thus, differentiation of the seed coat mucilage cells involves a hig hly regulated series of events including growth, morphogenesis, mucilage bi osynthesis and secretion, and secondary cell wall synthesis.