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All plants flower late in their life cycle. For example, in Arabidopsis, the shoot undergoes a transition and produces reproductive flowers after the adult phase of vegetative growth. Much is known about genetic and environmental processes that control flowering time in mature plants. However, little is understood about the mechanisms that prevent plants from flowering much earlier during embryo and seedling development. Arabidopsis embryonic flower (emf1 and emf2) mutants flower soon after germination, suggesting that a floral repression mechanism is established in wild-type plants that prevents flowering until maturity. Here, we show that polycomb group proteins play a central role in repressing flowering early in the plant life cycle. We found that mutations in the Fertilization Independent Endosperm (FIE) polycomb gene caused the seedling shoot to produce flower-like structures and organs. Flower-like structures were also generated from the hypocotyl and root, organs not associated with reproduction. Expression of floral induction and homeotic genes was derepressed in mutant embryos and seedlings. These results suggest that FIE-mediated polycomb complexes are an essential component of a floral repression mechanism established early during plant development.

The Arabidopsis LEAFY COTYLEDON2 (LEC2) gene is a central embryonic regulator that serves critical roles both early and late during embryo development. LEC2 is required for the maintenance of suspensor morphology, specification of cotyledon identity, progression through the maturation phase, and suppression of premature germination. We cloned the LEC2 gene on the basis of its chromosomal position and showed that the predicted polypeptide contains a B3 domain, a DNA-binding motif unique to plants that is characteristic of several transcription factors. We showed that LEC2 RNA accumulates primarily during seed development, consistent with our finding that LEC2 shares greatest similarity with the B3 domain transcription factors that act primarily in developing seeds, VIVIPAROUS1/ABA INSENSITIVE3 and FUSCA3. Ectopic, postembryonic expression of LEC2 in transgenic plants induces the formation of somatic embryos and other organ-like structures and often confers embryonic characteristics to seedlings. Together, these results suggest that LEC2 is a transcriptional regulator that establishes a cellular environment sufficient to initiate embryo development.

Higher plant reproduction is unique because two cells are fertilized in the haploid female gametophyte. Egg and sperm nuclei fuse to form the embryo. A second sperm nucleus fuses with the central cell nucleus that replicates to generate the endosperm, a tissue that supports embryo development. To understand mechanisms that initiate reproduction, we isolated a mutation in Arabidopsis, f644, that allows for replication of the central cell and subsequent endosperm development without fertilization. When mutant f644 egg and central cells are fertilized by wild-type sperm, embryo development is inhibited, and endosperm is overproduced. By using a map-based strategy, we cloned and sequenced the F644 gene and showed that it encodes a SET-domain polycomb protein. Subsequently, we found that F644 is identical to MEDEA (MEA), a gene whose maternal-derived allele is required for embryogenesis [Grossniklaus, U., Vielle-Calzada, J.-P., Hoeppner, M. A. & Gagliano, W. B. (1998) Science 280, 446-450]. Together, these results reveal functions for plant polycomb proteins in the suppression of central cell proliferation and endosperm development. We discuss models to explain how polycomb proteins function to suppress endosperm and promote embryo development.

We investigated soybean seed protein gene transcription during development. We found that seed protein genes are transcriptionally activated and then repressed during embryogenesis and that these genes are either inactive or transcribed at low levels in the mature plant. We further observed that genes encoding mRNAs with vastly different prevalences are transcribed at similar rates. DNA gel blot studies showed that transcriptionally active and inactive seed protein genes have indistinguishable methylation patterns. We conclude that both transcriptional and posttranscriptional processes regulate seed protein mRNA levels in the absence of detectable DNA methylation changes.

Experiments are described that characterize the organization of DNA sequences in the water mold Achlya bisexualis. These experiments demonstrate that repetitive and single copy sequences in the Achlya genome are arranged in a long-period interspersion pattern. Estimates of the spacing intervals between repetitive and single copy DNA indicate, however, that the interspersion pattern in Achlya is longer than has been previously reported in other eukaryotes. These data and measurements of structural gene expression in Achlya [Timberlake, W.E., Shumard, D. S. & Goldberg, R. B. (1977) Cell 10, 623-632] make it difficult to propose a regulatory function for repeated DNA in this eukaryote.

We transformed tobacco plants with a soybean beta-conglycinin gene that encodes the 1.7-kilobase beta-subunit mRNA. We showed that the beta-conglycinin mRNA accumulates and decays during tobacco seed development and that beta-conglycinin mRNA is undetectable in the tobacco leaf. We utilized in situ hybridization to localize beta-conglycinin mRNA within the tobacco seed. beta-Conglycinin mRNA is not detectable within the endosperm but is localized within specific embryonic cell types. The highest concentration of beta-conglycinin mRNA is found in cotyledon storage parenchyma cells. We conclude that sequences required for embryo expression, temporal control, and cell specificity are linked to the beta-conglycinin gene, and that factors regulating beta-conglycinin gene expression are compartmentalized within analogous soybean and tobacco seed regions.

The mode of induction of beta-galactosidase (EC 3.2.1.23) in synchronously growing cultures of two Hfr strains of Escherichia coli K12 was investigated. Cells can be induced to form the enzyme during any portion of the cell cycle; but when they are grown permanently in the presence of an inducer, enzyme synthesis is discontinuous. The interruption of beta-galactosidase synthesis appears to be geared to the growth cycle: it occurs when the cells are dividing actively. The observation that lac-specific messenger RNA is produced also in the absence of detectable enzyme synthesis suggests the existence of a control mechanism operating on the level of translation.

Messenger RNA was prepared from developing sea urchin gastrulae by puromycin release from polyribosomes. Approximately 60% of the total mRNA radioactivity of the postnuclear supernatant was recovered and shown to be free of any other labeled RNA species such as ribosomal and nuclear RNA. The mRNA was examined by hybridization to DNA present in great excess. The mRNA hybridizes almost exclusively with nonrepetitive DNA. Almost all of the messenger RNA molecules of sea urchin gastrulae therefore consist of transcripts from nonrepetitive sequences. It appears that the structural genes expressed at this stage are typically not repeated in the genome and the mRNA does not include recognizable repetitive sequence.