The roles of Polycomb-Group proteins in the development of Arabidopsis thaliana
View/ Open
Date
2009Author
Thorpe, Frazer
Metadata
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.