Zhen Wang^a, Juan Armando Casas-Mollano^a,¹, Jianping Xu^a,², Jean-Jack M. Riethoven^b, Chi Zhang^a, and Heriberto Cerutti^a,³

Significance

Histone phosphorylation and chromatin reorganization are emerging as critical factors in eukaryotic responses to environmental stimuli. Phosphorylation of histone H3 at Thr 3 is evolutionarily conserved but its role(s) in interphase cells has remained unexplored. In Arabidopsis, we found that H3T3ph increases in pericentromeric regions upon drought-like treatments and it might be required for maintaining proper heterochromatin/chromosome structure. In active genes the distribution of H3T3ph appears to be antagonistic to that of H3K4me3, suggesting that H3T3ph may have a repressive function. Interestingly, H3T3 phosphorylation depends on several protein kinase families, including some that are lineage specific. Understanding the dynamics of H3T3 phosphorylation and the kinases involved in its deposition may provide insights into epigenetic aspects of stress responses.

Abstract

Histone phosphorylation plays key roles in stress-induced transcriptional reprogramming in metazoans but its function(s) in land plants has remained relatively unexplored. Here we report that an Arabidopsis mutant defective in At3g03940 and At5g18190, encoding closely related Ser/Thr protein kinases, shows pleiotropic phenotypes including dwarfism and hypersensitivity to osmotic/salt stress. The double mutant has reduced global levels of phosphorylated histone H3 threonine 3 (H3T3ph), which are not enhanced, unlike the response in the wild type, by drought-like treatments. Genome-wide analyses revealed increased H3T3ph, slight enhancement in trimethylated histone H3 lysine 4 (H3K4me3), and a modest decrease in histone H3 occupancy in pericentromeric/knob regions of wild-type plants under osmotic stress. However, despite these changes in heterochromatin, transposons and repeats remained transcriptionally repressed. In contrast, this reorganization of heterochromatin was mostly absent in the double mutant, which exhibited lower H3T3ph levels in pericentromeric regions even under normal environmental conditions. Interestingly, within actively transcribed protein-coding genes, H3T3ph density was minimal in 5′ genic regions, coincidental with a peak of H3K4me3 accumulation. This pattern was not affected in the double mutant, implying the existence of additional H3T3 protein kinases in Arabidopsis. Our results suggest that At3g03940 and At5g18190 are involved in the phosphorylation of H3T3 in pericentromeric/knob regions and that this repressive epigenetic mark may be important for maintaining proper heterochromatic organization and, possibly, chromosome function(s).

See: http://www.pnas.org/content/early/2015/06/18/1423325112

PNAS June 22, 2015