Biomedical subjects

Célia Baroux

Publications and source records attributed to Célia Baroux.

7 recordsLinked to original sources

PubMed2006

Dynamic regulatory interactions of Polycomb group genes: MEDEA autoregulation is required for imprinted gene expression in Arabidopsis.

The imprinted Arabidopsis Polycomb group (PcG) gene MEDEA (MEA), which is homologous to Enhancer of Zeste [E(Z)], is maternally required for normal seed development. Here we show that, unlike known mammalian imprinted genes, MEA regulates its own imprinted expression: It down-regulates the maternal allele around fertilization and maintains the paternal allele silent later during seed development. Autorepression of the maternal MEA allele is direct and independent of the MEA-FIE (FERTILIZATION-INDEPENDENT ENDOSPERM) PcG complex, which is similar to the E(Z)-ESC (Extra sex combs) complex of animals, suggesting a novel mechanism. A complex network of cross-regulatory interactions among the other known members of the MEA-FIE PcG complex implies distinct functions that are dynamically regulated during reproduction.

Alleles↗

PubMed2005

Cellular efflux of auxin catalyzed by the Arabidopsis MDR/PGP transporter AtPGP1.

Directional transport of the phytohormone auxin is required for the establishment and maintenance of plant polarity, but the underlying molecular mechanisms have not been fully elucidated. Plant homologs of human multiple drug resistance/P-glycoproteins (MDR/PGPs) have been implicated in auxin transport, as defects in MDR1 (AtPGP19) and AtPGP1 result in reductions of growth and auxin transport in Arabidopsis (atpgp1, atpgp19), maize (brachytic2) and sorghum (dwarf3). Here we examine the localization, activity, substrate specificity and inhibitor sensitivity of AtPGP1. AtPGP1 exhibits non-polar plasma membrane localization at the shoot and root apices, as well as polar localization above the root apex. Protoplasts from Arabidopsis pgp1 leaf mesophyll cells exhibit reduced efflux of natural and synthetic auxins with reduced sensitivity to auxin efflux inhibitors. Expression of AtPGP1 in yeast and in the standard mammalian expression system used to analyze human MDR-type proteins results in enhanced efflux of indole-3-acetic acid (IAA) and the synthetic auxin 1-naphthalene acetic acid (1-NAA), but not the inactive auxin 2-NAA. AtPGP1-mediated efflux is sensitive to auxin efflux and ABC transporter inhibitors. As is seen in planta, AtPGP1 also appears to mediate some efflux of IAA oxidative breakdown products associated with apical sites of high auxin accumulation. However, unlike what is seen in planta, some additional transport of the benzoic acid is observed in yeast and mammalian cells expressing AtPGP1, suggesting that other factors present in plant tissues confer enhanced auxin specificity to PGP-mediated transport.

ATP-Binding Cassette Transporters↗

PubMed2005

Predictable activation of tissue-specific expression from a single gene locus using the pOp/LhG4 transactivation system in Arabidopsis.

The pOp/LhG4 transcription factor system was used to determine whether the synthetic pOp promoter, integrated at one position in the Arabidopsis genome, could be efficiently and faithfully activated by the heterologous transcription factor, LhG4, expressed in a variety of different patterns. This is a precondition for the development and exploitation of large collections of LhG4 activation lines that direct predictable tissue-specific expression of transgenes. We selected a pOp-GUS reporter insertion that was efficiently activated after crossing to an activator line that expressed the synthetic transcription factor LhG4 from the Cauliflower Mosaic Virus 35S promoter. This reporter line, pOp-GUS(g2), was then combined with activator loci that expressed LhG4 from one of seven different promoters, each with a different tissue specificity. pOp-GUS(g2) was activated faithfully in combination with six of these seven activator constructs, but generated an unexpected expression pattern in combination with the seventh construct, a fusion to a cyclin promoter (CYC-LhG4). The aberrant expression pattern could be attributed to the pOp-GUS(g2) insertion site, as the CYC-LhG4 activator lines directed the expected pattern of expression from a second pOp-GUS insertion. These results show that it is feasible to construct an activator collection in which LhG4 is expressed from diverse promoters or enhancer traps, but that individual pOp reporter loci can vary in their competence to respond to certain activator patterns. We discuss the implications for the design and use of mis-expression technology in Arabidopsis.

Journal Article↗

PubMed2004

Nuclear fusions contribute to polyploidization of the gigantic nuclei in the chalazal endosperm of Arabidopsis.

Somatic polyploidization is recognized as a means to increase gene expression levels in highly active metabolic cells. The most common mechanisms are endoreplication, endomitosis and cell fusion. In animals and plants the nuclei of multinucleate cells are usually prevented from fusing. Here, we report that the nuclei from the syncytial cyst of the chalazal endosperm of Arabidopsis thaliana (L.) Heynh. are polyploid with some intermediate ploidy levels that cannot be attributed to endoreplication, suggesting nuclear fusion. Analysis of isolated nuclei, together with fluorescent in situ hybridization (FISH), revealed that nuclei from the chalazal endosperm are two or three times bigger than the nuclei from the peripheral endosperm and have a corresponding increase in ploidy. Together with the consistent observation of adjoined nuclei, we propose that nuclear fusion contributes, at least in part, to the process of polyploidization in the chalazal endosperm. Confocal analysis of intact seeds further suggested that free nuclei from the peripheral endosperm get incorporated into the chalazal cyst and likely participate in nuclear fusions.

Arabidopsis↗

PubMed2004

Intrachromosomal excision of a hybrid Ds element induces large genomic deletions in Arabidopsis.

Transposon activity is known to cause chromosome rearrangements in the host genome. Surprisingly, extremely little is known about Dissociation (Ds)-induced chromosome rearrangements in Arabidopsis, where Ds is intensively used for insertional mutagenesis. Here, we describe three Arabidopsis mutants with reduced fertility and propose that excision of a hybrid Ds element induced a large genomic deletion flanking Ds. In the mutants anat and haumea, the deletion mechanism consists of a local Ds transposition from replicated into unreplicated DNA followed by Ds excision, where one end of the newly transposed element and one end of the Ds transposon at the donor site served as substrate for transposase. Excision of this hybrid element reminiscent of a macrotransposon leads to loss of the chromosomal piece located between the two ends, including one full Ds element and the flanking genomic sequence. This mechanism was found to be responsible for several other deletions and occurs at a genetically trackable frequency. Thus, it could be applied to efficiently generate deletions of various sizes in the vicinity of any existing Ds element present in the genome. In the mutant tons missing, a mechanism that involves endogenous repetitive sequences caused a large flanking deletion at a position unlinked to the starter locus. Our study of Ds transposition in Arabidopsis revealed previously undescribed mechanisms that lead to large genomic deletions flanking Ds elements, which may contribute to genome dynamics and evolution.

Arabidopsis↗

PubMed2002

Evolutionary origins of the endosperm in flowering plants.

The evolutionary origin of double fertilization and the resultant endosperm tissue in flowering plants remains a puzzle, despite over a century of research. The recent resurgence of approaches to evolutionary developmental biology combining comparative biology with phylogenetics provides new understanding of endosperm origins.

Evolution, Molecular↗

PubMed2002

Genomic imprinting during seed development.

Genomic imprinting allows parent-of-origin specific control over gene expression. Although imprinted genes (or entire chromosomes) are homologous sequences that can be inherited from either parent, they are differentially marked by a heritable epigenetic modification (imprint), which can condition their behavior in term of gene expression. Imprinting-based regulation of entire chromosomes is observed in both insects (paternal genome elimination) and mammals (nonrandom X inactivation). Until recently, it was unknown whether plants possessed a similar epigenetic system discriminating between homologous chromosomes from either paternal or maternal origin. There is now experimental evidence for a genome-wide imprinting phenomenon during seed development in Arabidopsis. Genomic imprinting at the gene (or locus) level is observed in both mammals and flowering plants. In maize, only a few allelic variants of several nonessential genes expressed in the endosperm are imprinted. In Arabidopsis, gene-specific imprinting has recently been demonstrated for the MEDEA (and FIS2) gene, which is essential for normal seed development. Unlike the imprinted maize genes, so far all tested MEA alleles are subjected to regulation by imprinting. MEDEA and FIS 2 are members of the FIS class of genes (FERTILIZATION INDEPENDENT SEED) involved in regulation of growth and cell proliferation during seed development. MEDEA shares several paradigmatic features with imprinted mammalian genes. The MEDEA phenotypes provide empirical support for theories of an intragenomic parental conflict during seed development, whereby imprinting is proposed as a means to differentially balance the selfish interests of each sex's genome during the development of the progeny.

Alleles↗