Globular somatic embryos can be induced from immature cotyledons of soybean (Glycine max L. Merr. cv Jack) placed on high levels of the auxin 2,4-dichlorophenoxyacetic acid (2,4-D). Somatic embryos develop from the adaxial side of the cotyledon, whereas the abaxial side evolves into a callus. Using a 9,280-cDNA clone array, we have compared steady-state RNA from the adaxial side from which embryos develop and from the abaxial callus at five time points over the course of the 4 weeks necessary for the development of globular embryos. In a second set of experiments, we have profiled the expression of each clone in the adaxial side during the same period. A total of 495 genes differentially expressed in at least one of these experiments were grouped according to the similarity of their expression profiles using a nonhierarchical clustering algorithm. Our results indicate that the appearance of somatic embryos is preceded by dedifferentiation of the cotyledon during the first 2 weeks on auxin. Changes in mRNA abundance of genes characteristic of oxidative stress and genes indicative of cell division in the adaxial side of the cotyledons suggest that the arrangement of the new cells into organized structures might depend on a genetically controlled balance between cell proliferation and cell death. Our data also suggest that the formation of somatic globular embryos is accompanied by the transcription of storage proteins and the synthesis of gibberellic acid.Due to their ability to regenerate into full plants, somatic embryos are the tissue of choice for transformation by particle bombardment in several crop species including soybean (Glycine max L. Merr. cv Jack; Finer and McMullen, 1991). In soybean, somatic embryos are obtained by induction, or culturing, of immature cotyledons on high concentration of auxin. Globular somatic embryos originate from epidermal and subepidermal cells of the adaxial side of the cotyledon (Finer, 1988), whereas the abaxial side evolves into a callus. The adaxial side is the "flat" side of the cotyledons, closest to the axis of the embryo, whereas the abaxial side is the side of the cotyledon in contact with the endosperm and the seed coat. However, the response to tissue culture is highly genotype dependent (Meurer et al., 2001), and the ability to transform a wider range of cultivars could accelerate the production of transgenic plants.Somatic and zygotic embryos follow the same general pattern of development (Zimmerman, 1993;Goldberg et al., 1994). However, large quantities of somatic embryos can be produced in vitro, making them more amenable to experimentation than their zygotic counterparts, which are protected by fruit structures and less accessible. Therefore, somatic embryos constitute a model system to study basic aspects of embryogenesis, as well as a tool for efficient transformation.Little is known of the genes expressed in early globular stage embryos (Zimmerman, 1993). Choi evaluated that only 10% of the proteins visible on a two-dimensional gel are embryo specific (Choi ...