Histone H1.1, Meiotic Chromosome Axis and Recombination

DNA associates with proteins to form the chromatin. This structure is essential for the compaction of the DNA into the cell nucleus but needs to be highly dynamic in order to allow the different biological processes of the DNA to occur. The linker histone H1 has been predicted to play a important role in the establishment of the Higher Order Chromatin fibre in vitro, but it is not very clear its role in vivo. In this study, we have investigated the role of the different histone H1 isoforms in Arabidopsis thaliana. Analysis of T-DNA insertion mutant lines for the different histone H1 genes showed significant delay in flowering time in some of these lines indicating a developmental role. Nevertheless, in most cases the absence of one of these H1 isoforms could be compensated by other isoforms.

Nevertheless, one T-DNA mutant line for a histone H1, h1.1-1, showed a plants with dwarf size due to defects during mitosis and severe fertility reduction due to defects during meiosis. Cytological analysis confirmed defects in homologous chromosome synapsis at pachytene, early univalents with incomplete chromosome compaction through diakinesis, and a great reduction in chiasma frequency at metaphase I (1.31 vs 10 in WT). As a consequence, univalents appeared at metaphase I, leading to chromosome missegregation through the second meiotic division and ending up in non-viable gametes (reduced fertility). Moreover, analysis of an RNAi line for the same histone h1.1 showed a similar but less severe semi-sterile phenotype. Chiasma frequency was also reduced compare to the wild type to 7.58. Immunolocalisation analysis of meiotic chromosome axis proteins ASY1 and ZYP1 have shown a defect in Synaptonemal Complex polymerization in this mutant line indicating that histone H1.1 is vital for proper chromatin axis organization, hence mediates physical meiotic homologous recombination in Arabidopsis.