AtHaspin phosphorylates histone H3 at threonine 3 during mitosis and contributes to embryonic patterning in Arabidopsis.

Post-translational histone modifications regulate many aspects of chromosome activity. Threonine 3 of histone H3 is highly conserved, but the significance of its phosphorylation is unclear, and the identity of the corresponding kinase in plants is unknown. Therefore, we characterized the candidate kinase in Arabidopsis thaliana, called AtHaspin. Recombinant AtHaspin in vitro phosphorylates histone H3 at threonine 3. Reduction of H3 threonine 3 phosphorylation level and reduced chromatin condensation in interphase nuclei by AtHaspin RNAi supports the proposition that this kinase is involved in histone H3 phosphorylation in vivo in mitotic cells. In addition, we provide a developmental function for a Haspin kinase. At the whole plant level, altered expression of the kinase induced pleiotropic phenotypes with defects in floral organs and vascular tissue. It reduced fertility and modified adventitious shoot apical meristems that then gave rise to plants with multi-rosettes and multi-shoots. Haspin mutant embryos frequently showed alteration in division plane orientation that could be traced back to the earliest divisions of embryo development, thus Haspin contributes to embryonic patterning.

Additive inheritance of histone modifications in Arabidopsis thaliana intra-specific hybrids.

Plant genomes are earmarked with defined patterns of chromatin marks. Little is known about the stability of these epigenomes when related, but distinct genomes are brought together by intra-species hybridization. Arabidopsis thaliana accessions and their reciprocal hybrids were used as a model system to investigate the dynamics of histone modification patterns. The genome-wide distribution of histone modifications H3K4me2 and H3K27me3 in the inbred parental accessions Col-0, C24 and Cvi and their hybrid offspring was compared by chromatin immunoprecipitation in combination with genome tiling array hybridization. The analysis revealed that, in addition to DNA sequence polymorphisms, chromatin modification variations exist among accessions of A. thaliana. The range of these variations was higher for H3K27me3 (typically a repressive mark) than for H3K4me2 (typically an active mark). H3K4me2 and H3K27me3 were rather stable in response to intra-species hybridization, with mainly additive inheritance in hybrid offspring. In conclusion, intra-species hybridization does not result in gross changes to chromatin modifications.

Aurora1 phosphorylation activity on histone H3 and its cross-talk with other post-translational histone modifications in Arabidopsis.

The enzymological properties of AtAurora1, a kinase responsible for the cell cycle-dependent phosphorylation of histone H3 at S10, and its cross-talk with other post-translational histone modifications, were determined. In vitro phosphorylation of H3S10 by AtAurora1 is strongly increased by K9 acetylation, and decreased by K14 acetylation and T11 phosphorylation. However, S10 phosphorylation activity is unaltered by mono-, di- or trimethylation of K9. An interference of H3K9 dimethylation by SUVR4 occurs by a pre-existing phosphorylation at S10. Hence, cross-talk in plants exists between phosphorylation of H3S10 and methylation, acetylation or phosphorylation of neighbouring amino acid residues. AtAurora1 undergoes autophosphorylation in vivo regardless of the presence of substrate, and forms dimers in planta. Of the three ATP-competitive Aurora inhibitors tested, Hesperadin was most effective in reducing the in vivo kinase activity of AtAurora1. Hesperadin consistently inhibited histone H3S10 phosphorylation during mitosis in Arabidopsis cells, but did not affect other H3 post-translational modifications, suggesting a specific inhibition of AtAurora in vivo. Inactivation of AtAurora also caused lagging chromosomes in a number of anaphase cells, but, unlike the situation in mammalian cells, Hesperadin did not influence the microtubule dynamics in dividing cells.