Whether deposited maternal products are important during early seed development in flowering plants remains controversial. Here, we show that RNA interference-mediated downregulation of transcription is deleterious to endosperm development but does not block zygotic divisions. Furthermore, we show that RNA POLYMERASE II is less active in the embryo than in the endosperm. This dimorphic pattern is established late during female gametogenesis and is inherited by the two products of fertilization. This juxtaposition of distinct transcriptional activities correlates with differential patterns of histone H3 lysine 9 dimethylation, LIKE HETEROCHROMATIN PROTEIN1 localization, and Histone H2B turnover in the egg cell versus the central cell. Thus, distinct epigenetic and transcriptional patterns in the embryo and endosperm are already established in their gametic progenitors. We further demonstrate that the non-CG DNA methyltransferase CHROMOMETHYLASE3 (CMT3) and DEMETER-LIKE DNA glycosylases are required for the correct distribution of H3K9 dimethylation in the egg and central cells, respectively, and that plants defective for CMT3 activity show abnormal embryo development. Our results provide evidence that cell-specific mechanisms lead to the differentiation of epigenetically distinct female gametes in Arabidopsis thaliana. They also suggest that the establishment of a quiescent state in the zygote may play a role in the reprogramming of the young plant embryo.

The extended classic ternary complex model predicts that a G protein-coupled receptor (GPCR) exists in only two interconvertible states: an inactive R, and an active R*. However, different structural active R* complexes may exist in addition to a silent inactive R ground state (Rg). Here we demonstrate, in a cellular context, that several R* states of 5-hydroxytryptamine-4 (5-HT 4 ) receptors involve different side-chain conformational toggle switches. Using site-directed mutagenesis and molecular modeling approaches, we show that the basal constitutive receptor (R*basal) results from stabilization of an obligatory double toggle switch (Thr3.36 from inactive gϪ to active gϩ and Trp6.48 from inactive gϩ to active t

In order to better understand the developmental processes that govern the formation of somatic embryos in oil palm (Elaeis guineensis Jacq.), we investigated the transcription factor genes expressed during embryogenesis in this species. The AP2/EREBP transcription factor family includes the AP2 subgroup, which contains several proteins that play important roles in plant development. We identified and characterized EgAP2-1, which codes for a protein that contains two AP2 domains similar to those of the transcription factor BABYBOOM (BBM) and more generally AINTEGUMENTA-like (AIL) proteins of the AP2 subgroup. In a similar way to related genes from eudicots, ectopic expression of EgAP2-1 in transgenic Arabidopsis plants alters leaf morphology and enhances regeneration capacity. In oil palm, EgAP2-1 transcripts accumulate to the greatest extent in zygotic embryos. This expression pattern was investigated in more detail by in-situ hybridization, revealing that in both zygotic and somatic embryos, EgAP2-1 expression is concentrated in proliferating tissues associated with the early development of leaf primordia, root initials and provascular tissues.

G protein-coupled receptors (GPCRs) have been found to trigger G protein-independent signalling. However, the regulation of G protein-independent pathways, especially their desensitization, is poorly characterized. Here, we show that the G protein-independent 5-HT 4 receptor (5-HT 4 R)-operated Src/ERK (extracellular signal-regulated kinase) pathway, but not the G s pathway, is inhibited by GPCR kinase 5 (GRK5), physically associated with the proximal region of receptor' C-terminus in both human embryonic kidney (HEK)-293 cells and colliculi neurons. This inhibition required two sequences of events: the association of b-arrestin1 to a phosphorylated serine/ threonine cluster located within the receptor C-t domain and the phosphorylation, by GRK5, of b-arrestin1 (at Ser 412 ) bound to the receptor. Phosphorylated b-arrestin1 in turn prevented activation of Src constitutively bound to 5-HT 4 Rs, a necessary step in receptor-stimulated ERK signalling. This is the first demonstration that b-arrestin1 phosphorylation by GRK5 regulates G protein-independent signalling.

General transcription factor IIB (TFIIB) and TFIIB-related factor (BRF), are conserved RNA polymerase II/III (RNAPII/III) selectivity factors that are involved in polymerase recruitment and transcription initiation in eukaryotes. Recent findings have shown that plants have evolved a third type of B-factor, plant-specific TFIIB-related protein 1 (pBRP1), which seems to be involved in RNAPI transcription. Here, we extend the repertoire of B-factors in plants by reporting the characterization of a novel TFIIB-related protein, plant-specific TFIIB-related protein 2 (pBRP2), which is found to date only in the Brassicacea family. Unlike other B-factors that are ubiquitously expressed, PBRP2 expression is restricted to reproductive organs and seeds as shown by RT-PCR, immunofluorescence labelling and GUS staining experiments. Interestingly, pbrp2 loss-of-function specifically affects the development of the syncytial endosperm, with both parental contributions required for wild-type development. pBRP2, is the first B-factor to exhibit cell-specific expression and regulation in eukaryotes, and might play a role in enforcing bi-parental reproduction in angiosperms.

In sexually reproducing organisms, the fusion of two highly specialized cells, the gametes, gives rise to a totipotent embryo. In animals, both gametes are transcriptionally silent and the early embryo remains in a relative quiescent status. This period of transcriptional quiescence is absolutely crucial for reprogramming the genome and conferring totipotency to the embryo.1,2 Unlike animals, plant germ cells arise from somatic cells in the adult and fertilization involves two pairs of gametes: two sperm cells fuse with the egg and central cells to produce the embryo and the endosperm nurturing tissue, respectively. We recently showed 3 that in Arabidopsis, as in animals, the early embryo was transcriptionally quiescent and that this transcriptional quiescence was first established in the egg cell. We further demonstrated that this transcriptional pattern correlated with both H3K9 dimethylation (H3K9me2) and TERMINAL FLOWER2 (TFL2) localization, respectively associated with silencing in heterochromatic and euchromatic regions. Finally, we found that a CHROMOMETHYLASE 3 (CMT3) mutation resulted in a dramatic loss of H3K9me2 in the egg cell and abnormal divisions of the zygote following fertilization, revealing a potential role of CMT3 in the reprogramming of the young plant embryo. Here, we show that CMT3 loss-of-function induces transcriptional reactivation of transposable elements and alters TFL2 localization in the female gametophyte. We also propose a model in which the CMT3 egg cell-specific silencing effect could rely on small interfering RNAs (siRNAs).During embryogenesis there is a major switch from dependence upon maternally-deposited products to reliance on products of the zygotic genome. in animals, this so-called maternal-to-zygotic transition occurs following a period of transcriptional quiescence. recently, we have shown that the early embryo in arabidopsis is also quiescent, a state inherited from the female gamete and linked to specific patterns of h3K9 dimethylation and tErminaL FLOWEr2 (tFL2) localization. We also demonstrated that ChrOmOmEthYLaSE 3 (Cmt3) is required for h3K9 dimethylation in the egg cell and for normal embryogenesis during the first few divisions of the zygote. Subsequent analysis of Cmt3 mutants points to a key role in egg cell reprogramming by controlling silencing in both transposon and euchromatic regions. a speculative model of the Cmt3-induced egg cell silencing is presented here, based on these results and current data from the literature suggesting the potential involvement of small rnas targeted to the egg cell, a process conceptually similar to the division of labor described in the male gametophyte for which we show that h3K9 modifications and tFL2 localization are reminiscent of the female gametophyte. CMT3 Plays a Gametophyte-Specific Role in Transposon SilencingCMT3 is known for its role in maintenance of methylation at transposon-related sequences 4 and is necessary for immobilization of transposons. 5,6 To better understand its potential implication in the rep...