From an enhancer trap screen for genes expressed in Arabidopsis embryos, we identified a gene expressed from the octant stage onward in the boundary between the two presumptive cotyledons and in a variety of postembryonic organ and meristem boundaries. This gene, CUP-SHAPED COTYLEDON3 ( CUC3 ), encodes a putative NAC-domain transcription factor that is homologous with CUC1 and CUC2. Analysis of a CUC3 hypomorph and a putative cuc3 null mutant indicates that CUC3 function is partially redundant with that of CUC1 and CUC2 in the establishment of the cotyledon boundary and the shoot meristem, thus revealing an even higher degree of redundancy in this class of genes than was thought previously. The CUC3 expression pattern, the cuc3 phenotypes, and CUC3 expression in a series of shoot meristem mutants and transgenes suggest a primary role for CUC 3 in the establishment of boundaries that contain cells with low proliferation and/or differentiation rates. The CUC -mediated establishment of such boundaries may be essential for the initiation of shoot meristems.

During Arabidopsis embryogenesis, the zygote divides asymmetrically in the future apical-basal axis; however, a radial axis is initiated only within the eight-celled embryo. Mutations in the GNOM, KNOLLE, and KEULE genes affect these processes: gnom zygotes tend to divide symmetrically; knolle embryos lack oriented cell divisions that initiate protoderm formation; and in keule embryos, an outer cell layer is present that consists of abnormally enlarged cells from early development. Pattern formation along the two axes is reflected by the position-specific expression of the Arabidopsis lipid transfer protein (AtLTPT) gene. In wild-type embryos, the AtLTPl gene is expressed in the protoderm and initially in all protodermal cells; later, AtLTPí expression is confined to the cotyledons and the upper end of the hypocotyl. Analysis of AtLTPT expression in gnom, knolle, and keule embryos showed that gnom embryos also can have no or reversed apical-basal polarity, whereas radial polarity is unaffected. knolle embryos initially lack but eventually form a radial pattern, and keule embryos are affected in protoderm cell morphology rather than in the establishment of the radial pattern. INTRODUCTIONIn flowering plants, the primary body plan of the seedling is laid down during embryogenesis (Steeves and Sussex, 1989). This body plan has been described as the superimposition of an apical-basal and a radial pattern . The apical-basal pattern visible in the seedling consists of distinct elements: two cotyledons, shoot meristem, hypocotyl, and root, including the root meristem. In Arabidopsis, the apical-basal polarity is already evident in the zygote, which elongates approximately threefold in the apical direction. An asymmetric division then generates a small apical cell from which all pattern elements are derived, except for part of the root, that is, the columella root cap and the quiescent center (Scheres et al., 1994), and the suspensor, which are derived from the larger basal cell. Mutations resulting in a deletion of regions of the apical-basal pattern include gurke, fackel, monopteros, and gnom (Mayer et al., , 1993 Berleth and Jürgens, 1993) and rootless, shoot meristemless, and topless (Barton and Poethig, 1993).In gnom embryos (Mayer et al., , 1993Busch et al., 1996), also called emb30 embryos (Shevell et al., 1994;Franzmann et al., 1995), the zygote tends to divide symmetrically, producing an enlarged apical cell at the expense of the basal cell. gnom embryos have no root meristem and reduced orno cotyledons, and most gnom seedlings are cone shaped, 'To whom correspondence should be addressed.retaining apical-basal polarity, although the pattern is severely compromised. Some gnom seedlings, however, are ball shaped, displaying no morphologically apparent apical-basal polarity (Mayer et al., 1993). The radial pattern is arranged in three concentric layers of tissues: the outer protoderm, the inner mass of ground tissue, and the centrally located vascular bundles. This pattern is initiated within the first eight cel...

During Arabidopsis embryogenesis, the zygote divides asymmetrically in the future apical-basal axis; however, a radial axis is initiated only within the eight-celled embryo. Mutations in the GNOM, KNOLLE, and KEULE genes affect these processes: gnom zygotes tend to divide symmetrically; knolle embryos lack oriented cell divisions that initiate protoderm formation; and in keule embryos, an outer cell layer is present that consists of abnormally enlarged cells from early development. Pattern formation along the two axes is reflected by the position-specific expression of the Arabidopsis lipid transfer protein (AtLTPT) gene. In wild-type embryos, the AtLTPl gene is expressed in the protoderm and initially in all protodermal cells; later, AtLTPí expression is confined to the cotyledons and the upper end of the hypocotyl. Analysis of AtLTPT expression in gnom, knolle, and keule embryos showed that gnom embryos also can have no or reversed apical-basal polarity, whereas radial polarity is unaffected. knolle embryos initially lack but eventually form a radial pattern, and keule embryos are affected in protoderm cell morphology rather than in the establishment of the radial pattern. INTRODUCTIONIn flowering plants, the primary body plan of the seedling is laid down during embryogenesis (Steeves and Sussex, 1989). This body plan has been described as the superimposition of an apical-basal and a radial pattern . The apical-basal pattern visible in the seedling consists of distinct elements: two cotyledons, shoot meristem, hypocotyl, and root, including the root meristem. In Arabidopsis, the apical-basal polarity is already evident in the zygote, which elongates approximately threefold in the apical direction. An asymmetric division then generates a small apical cell from which all pattern elements are derived, except for part of the root, that is, the columella root cap and the quiescent center (Scheres et al., 1994), and the suspensor, which are derived from the larger basal cell. Mutations resulting in a deletion of regions of the apical-basal pattern include gurke, fackel, monopteros, and gnom (Mayer et al., , 1993 Berleth and Jürgens, 1993) and rootless, shoot meristemless, and topless (Barton and Poethig, 1993).In gnom embryos (Mayer et al., , 1993Busch et al., 1996), also called emb30 embryos (Shevell et al., 1994;Franzmann et al., 1995), the zygote tends to divide symmetrically, producing an enlarged apical cell at the expense of the basal cell. gnom embryos have no root meristem and reduced orno cotyledons, and most gnom seedlings are cone shaped, 'To whom correspondence should be addressed.retaining apical-basal polarity, although the pattern is severely compromised. Some gnom seedlings, however, are ball shaped, displaying no morphologically apparent apical-basal polarity (Mayer et al., 1993). The radial pattern is arranged in three concentric layers of tissues: the outer protoderm, the inner mass of ground tissue, and the centrally located vascular bundles. This pattern is initiated within the first eight cel...

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