Chromatin alterations during pollen development in Hordeum vulgare.

The dynamics of posttranslational histone modifications in relation to nuclear architecture has been analyzed during pollen development in Hordeum vulgare L. cv. Igri. Notwithstanding the asymmetry of cytokinesis associated with pollen mitosis I, immunolabeling revealed that the vegetative and generative nuclei initially display identical chromatin modification patterns. Yet, differential chromatin modification patterns between vegetative and generative nuclei emerge with the development of conspicuous differences in nuclear morphology as visualized by 4',6-diamidino-2-phenylindole staining. The temporal and spatial distribution of most histone modifications observed is in agreement with reduced gene activity in the generative nucleus and increased expression in the vegetative nucleus as indicated by immunolabeling of active RNA polymerase II. Signals of trimethylation of histone H3 lysine 27 proved to be particularly enriched in euchromatic domains of subtelomeric regions. In the context of nuclear differentiation in bicellular pollen, this modification became restricted to the vegetative nucleus, indicating a role in activating rather than suppressing gene expression. The presence of acetylated histone H3 at lysine 9 in the cytoplasm of the generative cell is indicative of a more complex, still unknown function of this particular modification.