ABSTRACT
Mutation of the SCARECROW (SCR) gene results in a radial pattern defect, loss of a ground tissue layer, in the root. Analysis of the shoot phenotype of scr mutants revealed that both hypocotyl and shoot inflorescence also have a radial pattern defect, loss of a normal starch sheath layer, and consequently are unable to sense gravity in the shoot. Analogous to its expression in the endodermis of the root, SCR is expressed in the starch sheath of the hypocotyl and inflorescence stem. The SCR expression pattern in leaf bundle sheath cells and root quiescent center cells led to the identification of additional phenotypic defects in these tissues. SCR expression in a pin-formed mutant background suggested the possible origins of the starch sheath in the shoot inflorescence. Analysis of SCR expression and the mutant phenotype from the earliest stages of embryogenesis revealed a tight correlation between defective cell divisions and SCR expression in cells that contribute to ground tissue radial patterning in both embryonic root and shoot. Our data provides evidence that the same molecular mechanism regulates the radial patterning of ground tissue in both root and shoot during embryogenesis as well as postembryonically.
Subject(s)
Arabidopsis Proteins , Arabidopsis/physiology , Plant Proteins/physiology , Arabidopsis/cytology , Arabidopsis/genetics , Gene Expression Regulation, Developmental , Gene Expression Regulation, Plant , Leucine Zippers , Plant Leaves/cytology , Plant Leaves/physiology , Plant Proteins/genetics , Plant Roots/cytology , Plant Roots/physiology , Plant Stems/cytology , Plant Stems/physiology , Seeds/physiologyABSTRACT
Mutations at the SCARECROW (SCR) locus in Arabidopsis thaliana result in defective radial patterning in the root and shoot. The SCR gene product contains sequences which suggest that it is a transcription factor. A number of Arabidopsis Expressed Sequence Tags (ESTs) have been identified that encode gene products bearing remarkable similarity to SCR throughout their carboxyl-termini, indicating that SCR is the prototype of a novel gene family. These ESTs have been designated SCARECROW-LIKE (SCL). The gene products of the GIBBERELLIN-INSENSITIVE (GAI) and the REPRESSOR of ga1-3 (RGA) loci show high structural and sequence similarity to SCR and the SCLs. Sequence analysis of the products of the GRAS (GAI, RGA, SCR) gene family indicates that they share a variable amino-terminus and a highly conserved carboxyl-terminus that contains five recognizable motifs. The SCLs have distinct patterns of expression, but all of those analyzed show expression in the root. One of them, SCL3, has a tissue-specific pattern of expression in the root similar to SCR. The importance of the GRAS gene family in plant biology has been established by the functional analyses of SCR, GAI and RGA.
Subject(s)
Arabidopsis Proteins/genetics , Arabidopsis/genetics , Co-Repressor Proteins/genetics , Genes, Plant , Multigene Family , Amino Acid Sequence , Conserved Sequence , Evolution, Molecular , Gene Expression , In Situ Hybridization , Molecular Sequence Data , Mutation , Sequence Homology, Amino AcidABSTRACT
Because of its elegant simplicity, the Arabidopsis root has become a model for studying plant organogenesis. In this review we focus on recent results indicating the importance of signaling in root development. A role for positional information in root cell specification has been demonstrated by ablation analyses. Through mutational analysis, genes have been identified that play a role in radial pattern formation. The embryonic phenotypes of these mutants raised the possibility that division patterns in post-embryonic roots are dependent on signaling that originates during embryonic development. Analysis of expression of the SCARECROW gene indicates that it may play a role in this 'top-down' signaling process. Characterization of root epidermis development has led to the identification of negative regulators of root-hair formation. These appear to set up a prepattern which is reinforced by signaling by plant hormones.
Subject(s)
Arabidopsis Proteins , Arabidopsis/growth & development , Gene Expression Regulation, Plant/genetics , Plant Roots/growth & development , Arabidopsis/embryology , Arabidopsis/genetics , Cell Differentiation , Models, Biological , Mutation , Plant Proteins/genetics , Signal TransductionABSTRACT
We report the isolation and characterization of the Hirudo medicinalis homoeobox gene Lox2. Sequence analysis shows that it contains a region that has homology to Drosophila and vertebrate homoeodomains of the Antennapedia class. In addition, Lox2 shares homology with sequences in the bithorax complex Ultra-bithorax (Ubx) and abdominal A (abdA) genes in a region adjacent to the C-terminus of the homoeodomain. Whole mount in situ hybridization of embryos of various ages demonstrates that during early development this gene has temporally and spatially restricted patterns of expression that resemble those of the homoeotic genes of the Drosophila bithorax complex and of many vertebrate homoeobox genes. The largest accumulation of transcripts was seen in the posterior two-thirds of the developing leech central nervous system in 7-14-day-old embryos. Adult leeches also express Lox2. We propose that in Hirudo, Lox2 represents the ancestral gene of the Ubx and abdA genes of the bithorax complex of Drosophila.
