Nuclear endosperm development in cereals and Arabidopsis thaliana.

The nuclear endosperm of monocots, including the cereal species maize, rice, barley, and wheat, represents humankind’s most important renewable source of food, feed, and industrial raw materials. In addition, the endosperm is an attractive system for developmental biology studies. Similar to the embryo, the endosperm is the result of a fertilization process and therefore may be considered an organism in its own right. Endosperm development has been the subject of several recent reviews (DeManson, 1997; Becraft et al., 2000; Becraft, 2001; Olsen, 2001; Brown et al., 2002; Berger, 2003). This review emphasizes the main developmental aspects of nuclear endosperm development in cereals and Arabidopsis thaliana, including evolutionary origin, coenocyte development, endosperm cellularization, cell fate specification, and differentiation. After fertilization in what is the most common type of endosperm development, the nuclear type, the initial endosperm nucleus divides repeatedly without cell wall formation, resulting in a characteristic coenocyte-stage endosperm. Cellularization occurs via the formation of radial microtubule systems (RMS) and alveolation. Many dicot species, including commercially important crop plants such as soybean, cotton, and Arabidopsis, also have a nuclear endosperm, although the endosperm is consumed in part by the embryo during seed maturation. During its development, striking similarities exist between the endosperm in these groups, particularly with respect to the cellularization phase. Insight into the genetic specification of the different processes in endosperm development (e.g., coenocyte formation, alveolation, cell fate specification, and differentiation) may ultimately reveal how developmental subprograms are integrated into a master program specifying the entire endosperm body plan. This knowledge will not only benefit basic science but also strengthen efforts to improve cereal grain quality.