Pattern Formation in the Arabidopsis Embryo Revealed by Position-Specific Lipid Transfer Protein Gene Expression

Abstract

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...

Expression Analysis of a Family ofnsLTPGenes Tissue Specifically Expressed throughout the Plant and during Potato Tuber Life Cycle

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The TRANSPARENT TESTA12 Gene of Arabidopsis Encodes a Multidrug Secondary Transporter-like Protein Required for Flavonoid Sequestration in Vacuoles of the Seed Coat Endothelium

“…Nonradioactive in situ detection of RNA expression using digoxigenin-labeled RNA probes was performed as described by Vroemen et al (1996). The TT12 antisense and sense RNA probes were transcribed from the pBluescript-SKϩ plasmid (Stratagene) containing the TT12 cDNA using the T7 (antisense) or the T3 (sense) promoter.…”

Between the sheets: inter–cell–layer communication in plant development

“…Indeed, studies of the effects of a GNOM variant engineered to be insensitive to the inhibitor Brefeldin A (BFA) on the transport of membrane proteins other than PIN1 in BFA-treated plants, suggest that GNOM function in localizing/recycling membrane proteins may be more specific to auxin efflux carriers than previously thought (Geldner et al 2003). Although cells on the outside of the developing gnom embryo still express L1-specific markers, internal cells show disruption in cell layer organization with a partial loss of controlled cell division and specification of internal cell identities (Shevell et al 1994;Vroemen et al 1996). However, GNOM, in addition to being required for localization of auxin efflux carriers, also appears to play a role in the secretion and deposition of cell-wall components (Shevell et al 2000) making phenotypic interpretation difficult.…”

“…Mutations in the guanosine exchangefactor protein EMB30/GNOM effectively abolish cell polarity in the developing embryo as early as the singlecelled zygote stage (Shevell et al 1994;Vroemen et al 1996). Mutant embryos lose characteristic polar localization of the presumptive auxin efflux carrier AtPIN1, and possibly other auxin efflux carriers, thus compromising polar auxin transport (Galweiler et al 1998;Steinmann et al 1999).…”