Update on Legume Seed Development Using Genomics to Study Legume Seed Development

Seeds are essential for flowering plant reproduction because they protect, nourish, and contain the developing embryo that represents the next sporophytic generation. In addition, seeds contain energy resources that sustain the young sporophyte during germination before photosynthesis begins. In legumes, food reserves stored in embryonic cotyledons make seeds important as a food source for both human and animal consumption. For example, soybean (Glycine max) is now one of the most important seed crops in the world (Wilcox, 2004). Research on legume seed development has led to direct applications, such as seeds with more nutrients (Kinney, 1998; Wang et al., 2003; Krishnan, 2005), reduced allergens (Herman et al., 2003), and novel constituents, such as edible vaccines (Moravec et al., 2007). In the current genomic era, it is now possible to begin to understand what genes are required to make a legume seed and how regulatory networks are interconnected in legume genomes to program seed formation. In the future, this information should permit novel approaches to breed and engineer legume seeds with new agronomic traits and, most importantly, help provide a sustainable food supply for a growing human population. This Update outlines how our laboratories have been using legumes and functional genomics to identify genes that program legume seed development. Seed development is triggered by a novel doublefertilization process that leads to the differentiation of the embryo, endosperm, and seed coat, which are the major compartments of the seed (Fig. 1, A–C; Goldberg et al., 1994; Miller et al., 1999; Gehring et al., 2004; Laux et al., 2004; Moise et al., 2005). These compartments have different origins and play distinct roles in seed formation. The maternally derived seed coat differentiates from the ovule integuments that surround the embryo sac and plays a major role in protecting the embryo and transferring nutrients from the maternal plant to the developing embryo (Fig. 1, A and C; Murray, 1987; Borisjuk et al., 2004; Moise et al., 2005). By contrast, the embryo and endosperm are direct descendents of the fertilized egg and central cell, respectively. The endosperm proliferates to occupy most of the postfertilization embryo sac and nourishes the embryo early in development (Gehring et al., 2004). In many flowering plants, such as legumes, the endosperm is absorbed by the embryo during development and is not present in the mature seed (Fig. 1, A–C; Goldberg et al., 1994). After fertilization, the zygote divides asymmetrically, giving rise to a small apical cell that develops into the embryo proper and a large basal cell that forms the suspensor. The suspensor is a terminally differentiated structure that supports and nourishes the embryo proper and degenerates later in development (Yeung and Meinke, 1993). The embryo proper, on the other hand, represents the new sporophytic generation and contains the shoot and root meristems that are responsible for generating organ systems of the mature plant after seed germination (Fig. 1C; Goldberg et al., 1994; Laux et al., 2004).