The shoot and root apical meristems (SAMs and RAMs, respectively) of higher plants are mechanistically and structurally similar. This has led previously to the suggestion that the SAM and RAM represent modifications of a fundamentally homologous plan of organization. Despite recent interest in plant development, especially in the areas of meristem regulation, genes specifically required for the function of both the SAM and RAM have not yet been identified. Here, we report on a novel gene, Defective embryo and meristems ( Dem ), of tomato. This gene is required for the correct organization of shoot apical tissues of developing embryos, SAM development, and correct cell division patterns and meristem maintenance in roots. Dem was cloned using transposon tagging and shown to encode a novel protein of 72 kD with significant homology to YNV2, a protein of unknown function of Saccharomyces cerevisiae. Dem is expressed in root and shoot meristems and organ primordia but not in callus. The expression pattern of Dem mRNA in combination with the dem mutant phenotype suggests that Dem plays an important role within apical meristems. INTRODUCTIONIn plants, organogenesis is continuous and occurs in apices throughout the entire life cycle. This process is achieved by the action of apical meristems, which are groups of stem cells that are established early in embryogenesis and maintained in the tips of shoots and roots. Because apical meristems are almost entirely responsible for the elaboration of plant architecture, they have been a major subject of observational, experimental, and genetic studies (described in Steeves and Sussex, 1991;Meyerowitz, 1997). We are now beginning to elucidate the genes involved in meristem regulation and to understand their function (Meyerowitz, 1997).In angiosperms, the shoot apical meristem (SAM) is usually a small dome of cells that consists of a peripheral zone in which leaves are initiated and a central zone in which the peripheral zone cells are replenished. The central zone contains cells that divide slowly, whereas the peripheral zone contains cells that divide rapidly (Lyndon, 1990;Steeves and Sussex, 1991). Superimposed upon this zonation are three clonally distinct cell layers (Poethig, 1987): L1 (forming the epidermis), L2 (forming the mesoderm), and L3 (forming the pith and vascular tissue). These cell layers generate the whole shoot. The L1 and L2 layers in the SAM are maintained by anticlinal cell divisions. Occasional cell divisions occur that result in the insertion of cells derived from one layer into the adjacent layer. These cells adopt a fate appropriate to their new layer, thus suggesting that positional information, rather than cell lineage, is the major factor influencing cell fate decisions during plant development. How cells in meristems communicate with each other has not yet been determined; however, recent results indicate roles for protein trafficking (Lucas et al., 1995) and extracellular signaling (Clark et al., 1997).The root apical meristem (RAM), in contrast to ...

The shoot and root apical meristems (SAMs and RAMs, respectively) of higher plants are mechanistically and structurally similar. This has led previously to the suggestion that the SAM and RAM represent modifications of a fundamentally homologous plan of organization. Despite recent interest in plant development, especially in the areas of meristem regulation, genes specifically required for the function of both the SAM and RAM have not yet been identified. Here, we report on a novel gene, Defective embryo and meristems (Dem), of tomato. This gene is required for the correct organization of shoot apical tissues of developing embryos, SAM development, and correct cell division patterns and meristem maintenance in roots. Dem was cloned using transposon tagging and shown to encode a novel protein of 72 kD with significant homology to YNV2, a protein of unknown function of Saccharomyces cerevisiae. Dem is expressed in root and shoot meristems and organ primordia but not in callus. The expression pattern of Dem mRNA in combination with the dem mutant phenotype suggests that Dem plays an important role within apical meristems.

Most flowering plant species contain at least two copies of the DEFECTIVE EMBRYO AND MERISTEMS (DEM) gene with the encoded DEM proteins lacking homology to proteins of known biochemical function. In tomato (Sl; Solanum lycopersicum), stable mutations in the SlDEM1 locus result in shoot and root meristem defects with the dem1 mutant failing to progress past the cotyledon stage of seedling development. Generation of a Somatic Mutagenesis of DEM1 (SMD) transformant line in tomato allowed for the characterization of SlDEM1 gene function past the seedling stage of vegetative development with SMD plants displaying a range of leaf development abnormalities. Further, the sectored or stable in planta expression of specific regions of the SlDEM1 coding sequence also resulted in the generation of tomato transformants that displayed a range of vegetative development defects, which when considered together with the dem1 mutant seedling and SMD transformant line phenotypic data, allowed for the assignment of SlDEM1 gene function to early embryo development, adaxial epidermis cell development, lateral leaf blade expansion, and mesophyll cell proliferation and differentiation.

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V. bicolor, V. trifolia s. str. and V. rotundifolia are part of a species complex that has recorded medicinal use in the Philippines. We assembled the first chloroplast genome of V. bicolor through next-generation sequencing and compared this to earlier established chloroplast genomes of V. trifolia s. str. and V. rotundifolia to provide additional insights into their genotypic differences. To ensure the continued utility of the research outputs in case of future taxonomic revisions, we characterized the morphology of PBN 2018-674, the reference germplasm utilized to generate the plastome. The complete chloroplast genome sequence of V. bicolor was 154,460 bp long with 131 coding genes comprising 87 mRNA genes, 36 tRNA genes and 8 rRNA genes. Using a separate accession from a different type locality, an identical chloroplast genome was equally established, indicating its conserved nature. When compared to V. trifolia s. str. and V. rotundifolia, slight variations were observed in genome features between these species; however, single nucleotide polymorphisms were exhibited in 13 protein-coding genes that often have a conserved nature. A phylogenetic analysis of the assembled genome, together with 12 other Lamiaceae species, exhibited high bootstrap support (>88%) within the species complex, and associated V. trifolia as the closest relative of V. bicolor. The identified variations in the plastomes can be utilized as markers that could distinguish the three closely related genotypes which can help the Philippine herbal industry provide a more stable source of quality herbal medicines.