The roles of Polycomb-Group proteins in the development of Arabidopsis thaliana

Abstract

The roles of Polycomb-Group proteins in the development of Arabidopsis thaliana The Polycomb-group (Pc-G) complex determines animal cell fate by regulating the expression of the homeotic genes that specify the body pattern. Several Pc-G proteins form a complex, termed Polycomb Repressive Complex 2 (PRC2), which can methylate histone tails. In plants, cell fates are less rigidly determined, suggesting the Pc-G play minor roles in development, or that its effects are more readily reversible. In Arabidopsis, there are three homologues of the catalytic unit of the PRC2, encoded by the MEDEA (MEA), CURLY LEAF (CLF) and SWINGER (SWN) genes. CLF and SWN are expressed throughout development, whereas MEA is confined to seed development. The swn- mutants appear normal, clf- mutants are early flowering, but swn- clf- double mutants are only viable in tissue culture, and develop into immortal callus-like material. This suggests CLF and SWN function is masked by redundancy. SWN is widely conserved in flowering plants, suggesting it may have functions independent of CLF. The severity of the swn- clf- phenotype indicated the Pc-G might play broad roles in plant development, but few targets are known. The aims of this thesis were to determine whether SWN had discrete functions in development, and uncover target genes and developmental pathways controlled redundantly by CLF and SWN. Phenotypic analysis suggested that SWN is required to promote the juvenile to adult phase transition and repress leaf initiation rate. Microarray analysis was performed and combined with published “ChIP on chip” data of genome wide loci possessing histone 3 lysine 27 trimethylation (H3K27me3), a modification specifically catalyzed by the PRC2. My results suggest that there are over 1000 direct targets of Pc-G in Arabidopsis, and these tended to be the most highly over-expressed genes in Pc-G mutants. Validation of the microarray data, and phenotypic analysis revealed Pc-G complex is a global regulator of development, and SWN and CLF play novel roles in stem cell maintenance, promoting and repressing flowering, and in confining embryogenic traits to seed development.