Abstract
In flowering plants, the precursor of the germline is a spore mother cell (SMC), formed in dedicated tissues of the flower’s sexual organs. Sporogenesis enables a somatic-toreproductive cell fate transition. This transition is accompanied by large-scale reorganisation of chromatin structure and composition in Arabidopsis thaliana. This event is initiated by the eviction of somatic linker histones (H1) which are important for genome compaction and genome function, by stabilizing nucleosome arrays and chromatin fiber folding, and by influencing transcription and epigenetic regulation, respectively. While H1 eviction is known to be crucial for pluripotency establishment in the mammalian germline, its role in plant sporogenesis remains unknown. In my PhD work, I investigated the role and mechanisms of H1 eviction in Arabidopsis. Notably, I asked whether H1 eviction could be regulated by citrullination as this is the case in the mouse germline. The work explored the role of a putative citrullination site on H1.1 and the contribution of a putative plant citrullinase, an agmatine iminohydrolase (AIH), on H1 eviction. This work generated tools for conditional inhibition of H1.1 eviction which allowed to investigate the role of H1 eviction in sporogenesis, meiosis, gametogenesis, and seed development. Moreover, attempts to generate nanobodies to validate H1 citrullination in plants are described. My work demonstrated that mutations at putative citrullination residues on H1 and the downregulation of AIH impaired H1 eviction in SMCs. Consequently, this led to an increase of heterochromatin fraction and altered levels of H3K27me3. These findings strongly support a model where H1 is citrullinated in SMCs, leading to its destabilization from chromatin and enabling downstream degradation. Additionally, this process seems dispensable for meiosis but critical for gametogenesis and subsequently plant fertility. Collectively, this work revealed a mechanism and significance of H1 eviction for plant reproduction.
Li, Yanru