Embryo formation is the first patterning process during vegetative plant growth. Using transposons as insertional mutagens in Arabidopsis, we identified the mutant edd1 that shows embryo-defective development. The insertion mutation is lethal, arresting embryo growth between the globular and heart stages of embryonic development. The mutant phenotype cosegregates with a transposed Dissociation element. Sequences flanking the transposed element were isolated and used to isolate a full-length cDNA clone representing the wild-type EDD1 gene. Complementation of the mutant through Agrobacterium-mediated gene transfer of an EDD1 wild-type copy as well as loss of the transposon concomitant with phenotypic reversion demonstrated that the transposon had caused the mutation. Based on homology to Escherichia coli , the EDD1 gene is predicted to encode a novel glycyl-tRNA synthetase (GlyRS) that has not been identified previously in higher plants. An N-terminal portion of the plant protein is able to direct a marker protein into pea chloroplasts. Thus, the gene identified by the embryo-defective insertion mutation encodes a GlyRS homolog, probably acting within the plastidic compartment. INTRODUCTIONPattern formation presents one of the most challenging aspects of plant development. It addresses the organizational aspect of development, namely, the origin of diverse cell types, tissues, and organs at specific locations to form a structurally and functionally meaningful context. In flowering plants, most pattern-forming events occur in the postembryonic sporophyte after seed germination. However, the primary plant architecture, which is fairly uniform among flowering plants, is generated during early embryogenesis (reviewed in Jürgens et al., 1991;Jürgens and Mayer, 1994).Formation of the embryo body can be divided conceptually into three overlapping phases (West and Harada, 1993;Jürgens and Mayer, 1994). During the first morphogenetic phase, an apical-basal axis is established throughout the embryo body, with the root apical meristem at one end and the shoot apical meristem at the other. A radial pattern is created by the formation of three primordial tissue layers, that is, protoderm, procambium, and ground meristem, and bilateral symmetry occurs throughout the development of the cotyledons Meinke, 1995).The second phase is characterized by embryo growth and the accumulation of storage reserves (Souèges, 1919;Müller, 1963;Goldberg et al., 1989). During the final phase, the embryo prepares for desiccation, dehydrates, and enters a period of developmental arrest. The ovule develops into the seed, which is surrounded by an elaborated coat that enables the differentiated embryo to survive harsh environmental conditions until germination and sporophytic development are favorable.Higher plant embryogenesis was investigated extensively during the past century (Hanstein, 1870;Vandendries, 1909;Schnarf, 1929;Maheshwari, 1950). Studies using light and electron microscopy have provided detailed descriptions of the morphological and a...

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Embryo formation is the first patterning process during vegetative plant growth. Using transposons as insertional mutagens in Arabidopsis, we identified the mutant edd1 that shows embryo-defective development. The insertion mutation is lethal, arresting embryo growth between the globular and heart stages of embryonic development. The mutant phenotype cosegregates with a transposed Dissociation element. Sequences flanking the transposed element were isolated and used to isolate a full-length cDNA clone representing the wild-type EDD1 gene. Complementation of the mutant through Agrobacterium-mediated gene transfer of an EDD1 wild-type copy as well as loss of the transposon concomitant with phenotypic reversion demonstrated that the transposon had caused the mutation. Based on homology to Escherichia coli , the EDD1 gene is predicted to encode a novel glycyl-tRNA synthetase (GlyRS) that has not been identified previously in higher plants. An N-terminal portion of the plant protein is able to direct a marker protein into pea chloroplasts. Thus, the gene identified by the embryo-defective insertion mutation encodes a GlyRS homolog, probably acting within the plastidic compartment. INTRODUCTIONPattern formation presents one of the most challenging aspects of plant development. It addresses the organizational aspect of development, namely, the origin of diverse cell types, tissues, and organs at specific locations to form a structurally and functionally meaningful context. In flowering plants, most pattern-forming events occur in the postembryonic sporophyte after seed germination. However, the primary plant architecture, which is fairly uniform among flowering plants, is generated during early embryogenesis (reviewed in Jürgens et al., 1991;Jürgens and Mayer, 1994).Formation of the embryo body can be divided conceptually into three overlapping phases (West and Harada, 1993;Jürgens and Mayer, 1994). During the first morphogenetic phase, an apical-basal axis is established throughout the embryo body, with the root apical meristem at one end and the shoot apical meristem at the other. A radial pattern is created by the formation of three primordial tissue layers, that is, protoderm, procambium, and ground meristem, and bilateral symmetry occurs throughout the development of the cotyledons Meinke, 1995).The second phase is characterized by embryo growth and the accumulation of storage reserves (Souèges, 1919;Müller, 1963;Goldberg et al., 1989). During the final phase, the embryo prepares for desiccation, dehydrates, and enters a period of developmental arrest. The ovule develops into the seed, which is surrounded by an elaborated coat that enables the differentiated embryo to survive harsh environmental conditions until germination and sporophytic development are favorable.Higher plant embryogenesis was investigated extensively during the past century (Hanstein, 1870;Vandendries, 1909;Schnarf, 1929;Maheshwari, 1950). Studies using light and electron microscopy have provided detailed descriptions of the morphological and a...

Using a two-component Ac/Ds system consisting of a stabilized Ac element (Acc1) and a non-autonomous element (DsA), 650 families of plants carrying independent germinal DsA excisions/transpositions were isolated. Progenies of 559 of these Acc1/DsA families, together with 43 families of plants selected for excision/transposition of wild-type (wt) Ac, were subjected to a broad screening program for mutants exhibiting visible alterations. This resulted in the identification of 48 mutants showing a wide variety of mutant phenotypes, including embryo lethality (24 mutants), chlorophyll defects (5 mutants), defective seedlings (2 mutants), reduced fertility (5 mutants), reduced size (3 mutants), altered leaf morphology (2 mutants), dark green, unexpanded rosette leaves (3 mutants), and aberrant flower or shoot morphology (4 mutants). To whether these mutants were due to transposon insertions, a series of Southern blot experiments was performed on 28 families, comparing in each case several mutant plants with others showing the wild-type phenotype. A preliminary analysis revealed in 4 of the 28 families analyzed a common, novel DsA fragment in all mutant plants, which was present only in heterozygous plants with wt phenotype, as expected for DsA insertion mutations. These four mutants included two showing embryo lethality, one with dark green, unexpanded rosette leaves and stunted inflorescences, and one with curly growth of stems, leaves and siliques. Further evidence for DsA insertion mutations was obtained for one embryo lethal mutant and for the stunted mutant, while in case of the second embryo lethal mutant, the DsA insertion could be separated from the mutant locus by genetic recombination.

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