Although landscapes of several histone marks are now available for Arabidopsis thaliana and Oryza sativa, such profiles remain static and do not provide information about dynamic changes of plant epigenomes in response to developmental or environmental cues. Here, we analyzed the effects of light on four histone modifications (acetylation and trimethylation of lysines 9 and 27 on histone H3: H3K9ac, H3K9me3, H3K27ac, and H3K27me3, respectively). Our genome-wide profiling of H3K9ac and H3K27ac revealed that these modifications are nontransposable element gene-specific. By contrast, we found that H3K9me3 and H3K27me3 target nontransposable element genes, but also intergenic regions and transposable elements. Specific light conditions affected the number of modified regions as well as the overall correlation strength between the presence of specific modifications and transcription. Furthermore, we observed that acetylation marks not only ELONGATED HYPOCOTYL5 and HY5-HOMOLOG upon deetiolation, but also their downstream targets. We found that the activation of photosynthetic genes correlates with dynamic acetylation changes in response to light, while H3K27ac and H3K27me3 potentially contribute to light regulation of the gibberellin metabolism. Thus, this work provides a dynamic portrait of the variations in histone modifications in response to the plant's changing light environment and strengthens the concept that histone modifications represent an additional layer of control for light-regulated genes involved in photomorphogenesis.

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Lysine residue 9 of histone H3 can either be acetylated or mono-, di-, or tri-methylated. These epigenetic states have a diverse impact on regulating gene transcriptional activity and chromatin organization. H3K9ac is invariably correlated with transcriptional activation, whereas H3K9me2 has been reported to be mainly located in constitutive heterochromatin in Arabidopsis. Here, we present epigenetic landscapes for histone H3 lysine 9 acetylation (H3K9ac) and dimethylation (H3K9me2) in Arabidopsis seedlings. The results show that H3K9ac targeted 5,206 non-transposable element (non-TE) genes and 321 transposable elements (TEs), whereas H3K9me2 targeted 2,281 TEs and 1,112 non-TE genes. H3K9ac was biased towards the 5' end of genes and peaked at the ATG position, while H3K9me2 tended to span the entire gene body. H3K9ac correlated with high gene expression, while H3K9me2 correlated with low expression. Analyses of H3K9ac and H3K9me2 with the available datasets of H3K27me3 and DNA methylation revealed a correlation between the occurrence of multiple epigenetic modifications and gene expression. Genes with H3K9ac alone were actively transcribed, while genes that were also modified by either H3K27me3 or DNA methylation showed a lower expression level, suggesting that a combination of repressive marks weakened the positive regulatory effect of H3K9ac. Furthermore, we observed a significant increase of the H3K9ac modification level of selected target genes in hda19 (histone deacetylase 19) mutant seedlings, which indicated that HDA19 plays an important role in regulating the level of H3K9ac and thereby influencing the transcriptional activity in young seedlings.

SUMMARY The COP10-DET1-DDB1 (CDD) complex is an evolutionarily conserved protein complex discovered for its role in the repression of photomorphogenesis in Arabidopsis, and is important in many cellular and developmental processes in both plants and animals. However, its molecular mode of action remains poorly understood. Here, we show that the CDD component, DET1, possesses transcriptional repression activity and physically interacts with two closely related MYB transcription factors, CCA1 and LHY, which are core components of the plant circadian clock. DET1 associates with the promoter of CCA1/LHY target genes, such as TOC1, in a CCA1/LHY-dependent manner and is required for their repression, suggesting a recruitment of DET1 by the central oscillator components to regulate the clock. Our results reveal DET1 as a core transcriptional repression factor important for clock progression. Overall, the CDD complex may function as a transcriptional co-repressor in diverse processes through direct interaction with distinct transcription factors.

Plant growth-promoting bacteria (PGB) induce positive effects in plants, for instance, increased growth and reduced abiotic stresses susceptibility. The mechanisms by which these bacteria impact the host plant are numerous, diverse and often specific. Here, we studied the agronomical, molecular and biochemical effects of the endophytic PGB Bacillus subtilis B26 on the full life cycle of Brachypodium distachyon Bd21, an established model species for functional genomics in cereal crops and temperate grasses. Inoculation of Brachypodium with B. subtilis strain B26 increased root and shoot weights, accelerated growth rate and seed yield as compared to control plants. B. subtilis strain B26 efficiently colonized the plant and was recovered from roots, stems and blades as well as seeds of Brachypodium, indicating that the bacterium is able to migrate, spread systemically inside the plant, establish itself in the aerial plant tissues and organs, and is vertically transmitted to seeds. The presence of B. subtilis strain B26 in the seed led to systemic colonization of the next generation of Brachypodium plants. Inoculated Brachypodium seedlings and mature plants exposed to acute and chronic drought stress minimized the phenotypic effect of drought compared to plants not harbouring the bacterium. Protection from the inhibitory effects of drought by the bacterium was linked to upregulation of the drought-response genes, DREB2B-like, DHN3-like and LEA-14-A-like and modulation of the DNA methylation genes, MET1B-like, CMT3-like and DRM2-like, that regulate the process. Additionally, total soluble sugars and starch contents increased in stressed inoculated plants, a biochemical indication of drought tolerance. In conclusion, we show a single inoculation of Brachypodium with a PGB affected the whole growth cycle of the plant, accelerating its growth rates, shortening its vegetative period, and alleviating drought stress effects. These effects are relevant to grasses and cereal crops.