A citrullination-ubiquitination pathway controls pre-meiotic H1 eviction and is essential for gametogenesis inArabidopsis

In flowering plants, the transition from somatic to reproductive cell fate is marked by the differentiation of spore mother cells (SMCs), the functional equivalent of animal primordial germ cells (PGCs). Like their animal counterparts, SMCs undergo extensive structural and compositional changes in chromatin, initiated by the loss of linker histones, in a stepwise process. To investigate the role and mechanism of H1 eviction, we focused on two candidate pathways. Our findings show that specific point mutations, affecting either a putative citrullination site or ubiquitination site, prevent the eviction of H1.1 in the SMCs. Additionally, downregulation or chemical inhibition of the AIH citrulinase (Agmatine Imino Hydrolase) or of the E3-Ubiqtuitin ligase CULLIN4 also results in the persistence of H1.1 in SMCs. Notably, SMCs with impaired H1.1 eviction exhibit mild defects in chromatin reorganization and cell elongation. Despite these defects, the SMCs proceed through meiosis but fail to complete gametogenesis. Collectively, we propose a citrullination-ubiquitination pathway governing pre-meiotic H1 eviction as a critical mechanism for establishing post-meiotic competence in theArabidopsisgermline.