Projet ANR-07-BLAN-0120 – Sexychrom : Dynamics of chromatin structure and transcriptional activity following fertilization in higher plants
Programme : Blanc (BLANC) 2007
Année de financement : 2007
Coordinateur du projet : daniel GRIMANELLI (INSTITUT DE RECHERCHE POUR LE DEVELOPPEMENT - IRD)
Partenaires : Institut de recherche pour le développement
Montant : 164000.0 euros
Date de début : 2007
Durée : 36 mois
Résumé : The maternal-to-zygotic transition is the first developmental switch of an organism's life cycle. It occurs following fertilization and corresponds to the establishment of an embryo-specific transcriptional program. In animals, there is a delay between fertilization and the maternal-to-zygotic transition: the zygotic genome remains transcriptionally quiescent, and activation occurs only after several rounds of cell division. Early steps of embryo growth are thus predominantly programmed through maternal mRNA and proteins stored in the female gamete prior to fertilization. The timing and mechanisms of the maternal-to-zygotic transition in higher plants is still controversial. Genetic analyses and expression data in maize and Arabidopsis show that early embryo and endosperm formation depends heavily on maternally inherited alleles, which is reminiscent of animal models. However, important questions remain unanswered. First, it is unclear whether maternally inherited transcripts are sufficient to early embryo growth. Second it is not known whether the maternal transcripts are gametophytic (deposited prior to fertilization), zygotic (produced after fertilization), or both. Third, it is unclear whether activation courses are temporally equivalent for the male and female genomes. Finally, we know virtually nothing regarding the mechanisms leading to transcriptional activation. -  -  In animals, large-scale epigenetic changes occurring following fertilization include DNA methylation, histone tails methylation, acetylation and phosphorilation, and structural modifications of the core nucleosome. These mechanisms target differentially maternal and paternal chromosomes, regulating the coordinated decondensation and activation of the two genomes. The best-studied parental origin effect is genomic imprinting, ie differential de novo transcription according to the parent-of-origin, which can affect either specific loci or entire chromosomes. Failure to properly define and maintain imprints leads to severe developmentalabnormalities and diseases in mammals. More generally, correct erasure and reestablishment of epigenetic marks plays an essential role in reprogramming ofthe germ lines and early embryo. In plants, recent advances show that chromatin-level regulation plays a key role incontrolling parental genomes contributions, by mechanisms that are reminiscent of animal models, albeit with plant-specific features. However, both the more global changes in chromatin structure affecting parental genomes and the genome-wide epigenetic processes occurring atfertilization in plants are still insufficiently defined, and potentially enlightening comparisons between the animal and plant models remain difficult. -  -  In this project, we will characterize parental genome activation following fertilization in the model plant Arabidopsis thaliana. Both the mechanisms controlling chromatin changes and the course of transcriptional activation of both parental genomes in the early seed will be addressed. In the first phase of the project, we will establish the dynamics of chromatin epigenetic marks. We will take a cell biology approach, based on immunolocalization of specific antibodies, to assess RNA polymerase II activity and histones marks immediately before and following fertilization. We will further address the role of the nucleosome assembly histones chaperones and the dynamics of chromatin condensation and decondensation, using in vivo imaging of fluorescent-tagged histones. In a second part, we will address the respective roles of paternal and maternal transcription during early development. First, we will take a genetic approach to discriminate between the functions of deposited maternal transcripts and de novo maternal transcription, a key factor in understanding the extent of maternal control over seeddevelopment. Additionally, we will use mRNA pyrosequencing methods designed to discriminate between maternal and paternal transcription, to eva