Chromatin reprogramming in Arabidopsis thaliana pollen

Buttress, Toby (2021) Chromatin reprogramming in Arabidopsis thaliana pollen. Doctoral thesis, University of East Anglia.

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Chromatin is extensively reprogrammed through male sexual lineage development in flowering plants. Such reprogramming results in highly dimorphic chromatin architectures in the nuclei of mature pollen. Sperm is highly compacted whilst the accompanying vegetative nucleus is globally decondensed. The pollen chromatin dimorphism phenomenon has long been characterised, but the underlying mechanisms that drive such differences are largely unknown. Here, the nuclear proteomes of sperm and vegetative nuclei were generated and mined for novel determinants of pollen chromatin structure.
In Chapter 2, H2B.8 is introduced as a novel sperm-expressed histone variant which is responsible for sperm chromatin compaction and is important for male fertility. Using super-resolution microscopy, H2B.8 is revealed to achieve compaction by aggregating euchromatin into small foci. Such foci form by phase separation and are dependent on an intrinsically disordered region in the histone tail. H2B.8 is deposited to inactive euchromatic regions and achieves compaction without affecting transcription. Evolutionarily, H2B.8 is specific to flowering plants and provides a mechanism by which sperm compacts in eukaryotes in the absence of protamines. Generally, these results demonstrate a novel mechanism of nuclear condensation via aggregation of euchromatic regions that are transcriptionally inactive.
Chapter 3 concerns the vegetative nucleus which undergoes active DNA demethylation by DEMETER (DME), preferentially targeting heterochromatic transposons. How DME gains access to such loci remains unknown. This work characterises Histone Acetyltransferase of the MYST family (HAM) proteins, which are present specifically in the vegetative cell nuclei and are required for normal DME-mediated cytosine demethylation, particularly in heterochromatin. Greater chromatin accessibility is provided at DME targets by HAM proteins, likely by putative H4 lysine 5 acetylation activity. HAM proteins facilitate activation of a subset of pollen expressed transposons but do not affect gene transcription. Collectively, this work demonstrates a developmental switch whereby an active chromatin modification mark, which usually targets genes in soma, targets transposons in the vegetative nucleus and relaxes heterochromatin for further epigenetic reprogramming at the DNA methylation level.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Nicola Veasy
Date Deposited: 07 Oct 2021 12:31
Last Modified: 07 Oct 2021 12:31

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