Endosperm Cell Walls: Formation, Composition, and Functions

Abstract

This chapter deals with the mechanisms by which endosperm cell walls arise, with the composition and synthesis of endosperm cell wall polysaccharides, and with their specific functions during seed development and germination. The endosperm cell walls differ from somatic cell walls in polysaccharide composition. Endosperm walls often contain massive amounts of storage hemicelluloses such as mannan, galactomannan, glucomannans, xyloglucans, (1,3;1,4)-β-glucans but very little of cellulose. These hemicelluloses act as carbohydrate stores for the germinating embryo and, depending on their specific physico-chemical properties, may also play additional roles in water retention during post-imbibition drought, mechanical protection of the embryo, and signaling. In recent years, many of the genes responsible for the synthesis of endosperm cell wall polysaccharides have been identified. Endosperm cell walls have also evolved special mechanisms of formation. In the nuclear-type of endosperm development, which is the most common type in all analyzed species, the endosperm becomes multinucleate (syncytial) before cellularizing. Cellularization proceeds by the simultaneous formation of syncytial-type cell plates between sister and non-sister nuclei. Syncytial-type cell plates form at the boundaries of nuclear cytoplasmic domains, which are defined by radial systems of microtubules that organize on the envelope of endosperm nuclei. These cell plates are associated with clusters of microtubules called mini-phragmoplasts that transport the cell plate-forming vesicles to the division plane. In contrast to phragmoplasts in dividing somatic cells, mini-phragmoplasts never form a coherent cytoskeletal structure around the expanding cell plate. Syncytial-type cell plates undergo several developmental stages before completion of the new cell wall: vesicle fusion intermediates, wide tubules and wide tubular networks, convoluted fenestrated sheets, and planar fenestrated sheets.