The completion of the Arabidopsis thaliana genome sequence allows a comparative analysis of transcriptional regulators across the three eukaryotic kingdoms. Arabidopsis dedicates over 5% of its genome to code for more than 1500 transcription factors, about 45% of which are from families specific to plants. Arabidopsis transcription factors that belong to families common to all eukaryotes do not share significant similarity with those of the other kingdoms beyond the conserved DNA binding domains, many of which have been arranged in combinations specific to each lineage. The genome-wide comparison reveals the evolutionary generation of diversity in the regulation of transcription.

In plants, resistance to pathogens is frequently determined by dominant resistance genes, whose products are proposed to recognize pathogen-encoded avirulence gene (Avr) products. The tomato resistance locus Cf-2 was isolated by positional cloning and found to contain two almost identical genes, each conferring resistance to isolates of tomato leaf mould (C. fulvum) expressing the corresponding Avr2 gene. The two Cf-2 genes encode protein products that differ from each other by only three amino acids and contain 38 leucine-rich repeat (LRR) motifs. Of the LRRs, 20 show extremely conserved alternating repeats. The C-terminus of Cf-2 carries regions of pronounced homology to the protein encoded by the unlinked Cf-9 gene. We suggest that this conserved region interacts with other proteins involved in activating plant defense mechanisms.

The defective chloroplasts and leaves‐mutable (dcl‐m) mutation of tomato was identified in a Ds mutagenesis screen. This unstable mutation affects both chloroplast development and palisade cell morphogenesis in leaves. Mutant plants are clonally variegated as a result of somatic excision of Ds and have albino leaves with green sectors. Leaf midribs and stems are light green with sectors of dark green tissue but fruit and petals are wild‐type in appearance. Within dark green sectors of dcl‐m leaves, palisade cells are normal, whereas in albino areas of dcl‐m leaves, palisade cells do not expand to become their characteristic columnar shape. The development of chloroplasts from proplastids in albino areas is apparently blocked at an early stage. DCL was cloned using Ds as a tag and encodes a novel protein of approximately 25 kDa, containing a chloroplast transit peptide and an acidic alpha‐helical region. DCL protein was imported into chloroplasts in vitro and processed to a mature form. Because of the ubiquitous expression of DCL and the proplastid‐like appearance of dcl‐affected plastids, the DCL protein may regulate a basic and universal function of the plastid. The novel dcl‐m phenotype suggests that chloroplast development is required for correct palisade cell morphogenesis during leaf development.

The defective chloroplast and leaf-mutable (dcl-m) mutation of tomato blocks chloroplast differentiation in leaf mesophyll cells and a signaling system that appears to be required for morphogenesis of palisade cells during leaf growth. To dissect the function of DCL, mutants with stable dcl alleles (dcl-s) were generated and examined for their phenotype. DCL/dcl-s plant produce dcl-s/dcl-s seeds with embryos arrested at the globular stage of development. The levels of several chloroplast- and nuclear-encoded proteins are strongly reduced in dcl-m mutant leaf sectors without significant changes in their corresponding mRNAs. The 4.5S rRNA fails to be processed efficiently, however, suggesting that DCL has a direct or indirect function in rRNA processing or correct ribosome assembly. Accordingly, chloroplasts in dcl-m sectors are impaired in polysome assembly, which can explain the reduced accumulation of chloroplast-encoded proteins. These results suggest that DCL is required for chloroplast rRNA processing, and emphasize the importance of plastid function during embryogenesis.

myo-Inositol monophosphatase (IMP) is a soluble, Li(+)-sensitive protein that catalyzes the removal of a phosphate from myo-inositol phosphate substrates. IMP is required for de novo inositol synthesis from glucose 6-phosphate and for breakdown of inositol trisphosphate, a second messenger generated by the phosphatidylinositol signaling pathway. We cloned the IMP gene from tomato (LeIMP) and show that the plant enzyme is encoded by a small gene family. Three different LeIMP cDNAs encode distinct but highly conserved IMP enzymes that are catalytically active in vitro. Similar to the single IMP from animals, the activities of all three LeIMPs are inhibited by low concentrations of LiCl. LeIMP mRNA levels are developmentally regulated in seedlings and fruit and in response to light. Immunoblot analysis detected three proteins of distinct molecular masses (30, 29, and 28 kD) in tomato; these correspond to the predicted molecular masses of the LeIMPs encoded by the genes. Immunoreactive proteins in the same size range are also present in several other plants. Immunolocalization studies indicated that many cell types within seedlings accumulate LeIMP proteins. In particular, cells associated with the vasculature express high levels of LeIMP protein; this may indicate a coordinate regulation between phloem transport and synthesis of inositol. The presence of three distinct enzymes in tomato most likely reflects the complexity of inositol utilization in higher plants.

myo-Inositol monophosphatase (IMP) is a soluble, Li(+)-sensitive protein that catalyzes the removal of a phosphate from myo-inositol phosphate substrates. IMP is required for de novo inositol synthesis from glucose 6-phosphate and for breakdown of inositol trisphosphate, a second messenger generated by the phosphatidylinositol signaling pathway. We cloned the IMP gene from tomato (LeIMP) and show that the plant enzyme is encoded by a small gene family. Three different LeIMP cDNAs encode distinct but highly conserved IMP enzymes that are catalytically active in vitro. Similar to the single IMP from animals, the activities of all three LeIMPs are inhibited by low concentrations of LiCl. LeIMP mRNA levels are developmentally regulated in seedlings and fruit and in response to light. Immunoblot analysis detected three proteins of distinct molecular masses (30, 29, and 28 kD) in tomato; these correspond to the predicted molecular masses of the LeIMPs encoded by the genes. Immunoreactive proteins in the same size range are also present in several other plants. Immunolocalization studies indicated that many cell types within seedlings accumulate LeIMP proteins. In particular, cells associated with the vasculature express high levels of LeIMP protein; this may indicate a coordinate regulation between phloem transport and synthesis of inositol. The presence of three distinct enzymes in tomato most likely reflects the complexity of inositol utilization in higher plants.

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 ...

In Brassica napus, oleosins are expressed at high levels in the seed during the latter stages of embryo development. The cis-acting regulatory properties of an 872 bp promoter fragment of a B. napus oleosin gene were examined by analysis of beta-glucuronidase (GUS) expression in transgenic tobacco plants containing an oleosin promoter-GUS transcriptional fusion. The reporter gene was expressed at high levels only in seeds, specifically in embryo and endosperm tissue and regulated throughout seed development. These data demonstrate that oleosin gene transcription is regulated in a tissue-specific and temporally regulated manner and clearly indicate that oleosin protein expression is co-ordinated primarily at the transcriptional level. Oleosin mRNA was shown to be abscisic acid (ABA) inducible and an ABA-response element in the oleosin promoter was shown to be bound by a protein factor in a sequence-specific manner. Sequence analysis of the oleosin promoter has identified several other putative cis-acting sequences which may direct oleosin gene expression. The presence of a large open reading frame in the bottom strand of the oleosin promoter (ORF2) which encodes a polypeptide similar to the ethylene-induced E4 gene of tomato is reported. A PCR-generated DNA probe containing the ORF2 sequence hybridised with a 1.4 kb transcript in total RNA extracts of a variety of tissues, including leaves and germinated seed cotyledons. This finding suggests that the oleosin gene promoter directs transcription in both directions. It is the first report of a bi-directional nuclear gene promoter in plants.