Caractérisations fonctionnelles de la cycline, Nicta;CYCA3;2, et des protéines à domaine SET chez les plantes

The development of plants, compared with that of animals, is largely post-embryonic and influenced much more by the environment, suggesting that the plants have evolved and distinct mechanisms to control plant development and growth. Cell division plays a vital role in plant development and growth. During the cell division, the G1-S transition is a crucial crossroad at the interface between cell proliferation and differentiation. By genetic, physiological, and molecular approaches, we demonstrate that Nicta;CYCA3;2 has important functions, analogous to those of cyclin E (an important regulator involved in G1-S transition) in animals, in the control of plant cell division and differentiation. Once S phase is initiated, DNA is replicated and chromatin is assembled. The establishment and maintenance of specific chromatin states provides an epigenetic mechanism for inheriting expression states throughout plant development. Our studies show that the tobacco SET domain protein NtSET1 methylates H3K9, which marks primarily heterochromatin. Ectopic expression of NtSET1 increases the amount of H3K9 dimethylation and induces chromosome segregation defects in tobacco BY2 cells. Furthermore, NtSET1 is shown to bind LHP1, the only Arabidopsis homologue of animal heterochromatin protein 1. By immunolocalization and in vivo target analysis, it shows that both NtSET1-YFP and LHP1-YFP bind heterochromatic regions, suggesting a mechanism of function in heterochromatin formation. Study of Arabidopsis SET domain group gene SDG8 reveals that SDG8 encodes a major enzyme controlling methylation of H3K36 in Arabidopsis, which positively regulates FLC transcription to prevent early flowering. In conclusion, my thesis work allows me to acquire a better understanding of plant development. The knowledge and techniques acquired in molecular and cellular biology will not only enable me to continue the research in plants but also adapt with the research on other organisms