Genetic Control of Embryogenesis in Rice

The fundamental body organization in higher plants is established during embryogenesis, although most morphogenetic events occur after embryogenesis (Jürgens et al. 1994; Meinke 1995). Like all of the sexually reproducing organisms, vascular plants begin their existence as a single cell, the fertilized egg or zygote. This cell proliferates to become an embryo with differentiating organs and tissues. During the early stages of embryogenesis, several body axes, which form the basis for apical-basal and radial patterns, are formed. In monocotyledonous plants such as rice, the shape of the embryo is not radially symmetrical (Fig. 1). This leads to the existence of the third axis, the dorsiventral axis. In the embryo, the region where the shoot develops is defined as the ventral side, while the opposite side then becomes the dorsal side. The shoot apical meristem (SAM) is a center of morphogenesis in plants, as it produces most of the above-ground parts, including the leaves, stems, and axillary buds; the other type of meristematic tissue, the root apical meristem (RAM), generates the below-ground parts (Steeves and Sussex 1989). The SAM and RAM are first formed during early embryogenesis at fixed positions based on positional information defined by the three polarized axes, which determine the basic body organization.