Osmotic stress induces phosphorylation of histone H3 at threonine 3 in pericentromeric regions of Arabidopsis thaliana

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).epigenetics | heterochromatin | histone phosphorylation | abiotic stress | protein kinase

Phosphorylation of Histone H2A at Serine 95: A Plant-Specific Mark Involved in Flowering Time Regulation and H2A.Z Deposition

“…Arabidopsis has four MLK proteins that are closely related to MUT9p. Phosphorylation activity on H3 threonine 3 was not observed for MLK1 or MLK2 in vitro; however, defects in mlk1 mLk2 result in the reduction of H3T3ph, indicating that MLK1 or MLK2 has H3T3ph activity in vivo (Wang et al, 2015). The kinase functions of MUT9p may have diverged to produce new functions during the evolution of the land plants, leading to the separation of phosphorylation activity for H3T3 and H2AS95.…”

“…In the alga Chlamydomonas, MUT9p, which is related to casein kinase I, is capable of phosphorylating H3 at threonine 3 (Casas-Mollano et al, 2008). The Arabidopsis genome encodes four proteins (MLK1, MLK2, MLK3, and MLK4) related to MUT9p (Cerutti and Casas-Mollano, 2009;Wang et al, 2015;Huang et al, 2016). These four proteins were copurified with the evening complex, which is composed of EARLY FLOWERING4 (ELF4), ELF3, and LUX ARRHYTHMO (Huang et al, 2016).…”

“…These four proteins were copurified with the evening complex, which is composed of EARLY FLOWERING4 (ELF4), ELF3, and LUX ARRHYTHMO (Huang et al, 2016). MLK1 and MLK2 also function as kinases to phosphorylate H3 at threonine 3, and the loss of MLK1 and MLK2 function results in hypersensitivity to osmotic stress (Wang et al, 2015). In rice (Oryza sativa), EARLIER FLOWERING1 encodes a casein kinase I protein that phosphorylates the DELLA protein SLR1 (Dai and Xue, 2010).…”

Phytochromes and Phytochrome Interacting Factors

“…The PPKs display sequence similarity to CK1 and have been described previously as MUT9-Like Ser/Thr Kinases (MLKs; Wang et al, 2015;Huang et al, 2016). The addition of phyB enhances the phosphorylation of PIF3 by PPK in vitro; however, this enhancement is light independent, suggesting that both the Pr and Pfr forms of phyB enhance the phosphorylation of PIF3 by PPK.…”

MLK1 and MLK2 Coordinate RGA and CCA1 Activity to Regulate Hypocotyl Elongation inArabidopsis thaliana

“…In the alga Chlamydomonas reinhardtii, MUT9p is related to casein kinase I and phosphorylates H3 at threonine 3 (Casas-Mollano et al, 2008). The Arabidopsis genome encodes four proteins (MLK1, MLK2, MLK3, and MLK4) related to MUT9p (Wang et al, 2015;Huang et al, 2016;Su et al, 2017). MLK1 and MLK2 were first identified as kinases for phosphorylation of H3 at threonine 3 and are associated with the osmotic stress response (Wang et al, 2015), while MLK4 was characterized as a kinase for phosphorylation of H2A at serine 95 and promoted flowering time under long-day conditions .…”