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DEFECTIVE KERNEL1 (DEK1), which consists of a membrane-spanning region (DEK1-MEM) and a calpain-like Cys proteinase region (DEK1-CALP), is essential for aleurone cell formation at the surface of maize (Zea mays) endosperm. Immunolocalization and FM4-64 dye incubation experiments showed that DEK1 and CRINKLY4 (CR4), a receptor kinase implicated in aleurone cell fate specification, colocalized to plasma membrane and endosomes. SUPERNUMERARY ALEURONE LAYER1 (SAL1), a negative regulator of aleurone cell fate encoding a class E vacuolar sorting protein, colocalized with DEK1 and CR4 in endosomes. Immunogold localization, dual-axis electron tomography, and diffusion of fluorescent dye tracers showed that young aleurone cells established symplastic subdomains through plasmodesmata of larger dimensions than those connecting starchy endosperm cells and that CR4 preferentially associated with plasmodesmata between aleurone cells. Genetic complementation experiments showed that DEK1-CALP failed to restore wild-type phenotypes in maize and Arabidopsis thaliana dekl mutants, and DEK1-MEM also failed to restore wild-type phenotypes in Arabidopsis dek1-1 mutants. Instead, ectopic expression of DEK1-MEM under the control of the cauliflower mosaic virus 35S promoter gave a dominant negative phenotype. These data suggest a model for aleurone cell fate specification in which DEK1 perceives and/or transmits a positional signal, CR4 promotes the lateral movement of aleurone signaling molecules between aleurone cells, and SAL1 maintains the proper plasma membrane concentration of DEK1 and CR4 proteins via endosome-mediated recycling/ degradation. [ABSTRACT FROM AUTHOR]

DNA methylation is an epigenetic modification of cytosine that is important for silencing gene transcription and transposons, gene imprinting, development, and seed viability. DNA METHYLTRANSFERASE1 (MET1) is the primary maintenance DNA methyltransferase in Arabidopsis (Arabidopsis thaliana). Reciprocal crosses between antisense MET1 transgenic and wild-type plants show that DNA hypomethylation has a parent-of-origin effect on seed size. However, due to the dominant nature of the antisense MET1 transgene, the parent with a hypomethylated genome, its gametophyte, and both the maternal and paternal genomes of the F1 seed become hypomethylated. Thus, the distinct role played by hypomethylation at each generation is not known. To address this issue, we examined F1 seed from reciprocal crosses using a loss-of-function recessive null allele, met1-6. Crosses between wild-type and homozygous met1-6 parents show that hypomethylated maternal and paternal genomes result in significantly larger and smaller F1 seeds, respectively. Our analysis of crosses between wild-type and heterozygous MET1/ met1-6 parents revealed that hypomethylation in the female or male gametophytic generation was sufficient to influence F1 seed size. A recessive mutation in another gene that dramatically reduces DNA methylation, DECREASE IN DNA METHYLATION1, also causes parent-of-origin effects on F1 seed size. By contrast, recessive mutations in genes that regulate a smaller subset of DNA methylation (CHROMOMETHYLASE3 and DOMAINS REARRANGED METHYLTRANSFERASES1 and 2) had little effect on seed size. Collectively, these results show that maternal and paternal genomes play distinct roles in the regulation of seed size in Arabidopsis. [ABSTRACT FROM AUTHOR]

DNA methylation (5-methylcytosine) in mammalian genomes predominantly occurs at CpG dinucleotides, is maintained by DNA methyltransferase1 (Dnmt1), and is essential for embryo viability. The plant genome also has 5-methylcytosine at CpG dinucleotides, which is maintained by METHYLTRANSFERASE1 (MET1), a homolog of Dnmt1. In addition, plants have DNA methylation at CpNpG and CpNpN sites, maintained, in part, by the CHROMOMETHYLASE3 (CMT3) DNA methyltransferase. Here, we show that Arabidopsis thaliana embryos with loss-of-function mutations in MET1 and CMT3 develop improperly, display altered planes and numbers of cell division, and have reduced viability. Genes that specify embryo cell identity are misexpressed, and auxin hormone gradients are not properly formed in abnormal met1 embryos. Thus, DNA methylation is critical for the regulation of plant embryogenesis and for seed viability. [ABSTRACT FROM AUTHOR]

Data from this study show that the chalazal cyst is a consistent feature of the Brassicaceae that exhibits variation of potential taxonomic value. The U-shaped seeds of mustards comprise three development chambers: micropylar (MC), central (CC), and chalazal (ChC). Early in seed development the syncytial endosperm in the ChC becomes differentiated into a cyst that remains distinct from storage endosperm. In early syncytial stages, the cyst is stratified into three zones: (1) an apical zone containing nuclei, plastids and mitochondria, (2) a mid-zone rich in endomembranes but depleted of large organelles, and (3) a basal portion that is bordered by a labyrinthine wall. The basal zone has projections that range from foot-like extensions to thread-like haustoria that penetrate the underlying maternal chalazal proliferative tissues (CPT). No cell-to-cell connections occur between endosperm and maternal tissue, but the extensive labyrinthine walls closely invest cell lysate resulting from degradation of the CPT. Structural data from light, confocal and transmission electron microscopy suggest that the cyst is active in uptake and transport of metabolites into the developing seed. Four types of cyst reflecting position and size of the ChC are recognized. In general, they correlate with major tribes of Brassicaceae. Data support recent molecular studies indicating that classification of the Lepidieae is artificial and suggest that Thlaspi is a candidate for realignment. The most divergent morphology in a tribe is seen in the two species of the Hesperidae. Erysimum has a more typical chalazal chamber and cyst morphology while the tubular chalazal chamber and filiform cyst of Chorispora is of the most extreme type encountered in the survey. This suggests that the tribe is paraphyletic and should be reinvestigated to clarify relationships. © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society, 2004, 144, 375−394. [ABSTRACT FROM AUTHOR]

Although seed plants have γ,-tubulin, a ubiquitous component of centrosomes associated with microtubule nucleation in algal and animal cells, they do not have discrete microtubule organizing centers (MTOCs) comparable to animal centrosomes, and the organization of microtubule arrays in plants has remained enigmatic. Spindle development in basal land plants has revealed a surprising variety of MTOCs that may represent milestones in the evolution of the typical diffuse a centrosomal plant spindle. We have isolated and characterized the γtubulin gene from a liverwort, one of the extant basal land plants. Sequence similarity to the γ-tubulin gene of higher plants suggests that the γ-tubulin gene is highly conserved land plants. The G9 antibody to fission yeast γ-tubulin recognized a single band of 55 kD in immunoblots from bryophytes. Immunohistochemistry with the G9 antibody clearly documented the association of γ-tubulin with various MTOC sites in basal land plants (e.g., discrete centrosomes with and without centrioles and the plastid surface in monoplastidic meiosis of bryophytes). Changes in the distribution of γ-tubulin occur in a cell cycle-specific manner during monoplastidic meiosis in the liverwort Dumortiera hirsuta. γ-tubulin changes its localization from the plastid surface in prophase I to the spindle,from the spindle to phragmoplasts and the nuclear envelope in telophase I, and back to the plastid surfaces in prophase II. In vitro experiments show that γ-tubulin is detectable on the surface of isolated plastids and nuclei of D. hirsuta, and microtubules can be repolymerized from the isolated plastids. γ-tubulin localization patterns on plastid and nuclear surfaces are not affected by the destruction of microtubules by oryzalin. We conclude that γ- tubulin is a highly conserved protein associated with microtubule nucleation in basal land plants and that it has. [ABSTRACT FROM AUTHOR]