Linker histones are fine-scale chromatin architects modulating developmental decisions in Arabidopsis.

BACKGROUND: Chromatin provides a tunable platform for gene expression control. Besides the well-studied core nucleosome, H1 linker histones are abundant chromatin components with intrinsic potential to influence chromatin function. Well studied in animals, little is known about the evolution of H1 function in other eukaryotic lineages for instance plants. Notably, in the model plant Arabidopsis, while H1 is known to influence heterochromatin and DNA methylation, its contribution to transcription, molecular, and cytological chromatin organization remains elusive. RESULTS: We provide a multi-scale functional study of Arabidopsis linker histones. We show that H1-deficient plants are viable yet show phenotypes in seed dormancy, flowering time, lateral root, and stomata formation-complemented by either or both of the major variants. H1 depletion also impairs pluripotent callus formation. Fine-scale chromatin analyses combined with transcriptome and nucleosome profiling reveal distinct roles of H1 on hetero- and euchromatin: H1 is necessary to form heterochromatic domains yet dispensable for silencing of most transposable elements; H1 depletion affects nucleosome density distribution and mobility in euchromatin, spatial arrangement of nanodomains, histone acetylation, and methylation. These drastic changes affect moderately the transcription but reveal a subset of H1-sensitive genes. CONCLUSIONS: H1 variants have a profound impact on the molecular and spatial (nuclear) chromatin organization in Arabidopsis with distinct roles in euchromatin and heterochromatin and a dual causality on gene expression. Phenotypical analyses further suggest the novel possibility that H1-mediated chromatin organization may contribute to the epigenetic control of developmental and cellular transitions.

A Specialized Histone H1 Variant Is Required for Adaptive Responses to Complex Abiotic Stress and Related DNA Methylation in Arabidopsis.

Linker (H1) histones play critical roles in chromatin compaction in higher eukaryotes. They are also the most variable of the histones, with numerous nonallelic variants cooccurring in the same cell. Plants contain a distinct subclass of minor H1 variants that are induced by drought and abscisic acid and have been implicated in mediating adaptive responses to stress. However, how these variants facilitate adaptation remains poorly understood. Here, we show that the single Arabidopsis (Arabidopsis thaliana) stress-inducible variant H1.3 occurs in plants in two separate and most likely autonomous pools: a constitutive guard cell-specific pool and a facultative environmentally controlled pool localized in other tissues. Physiological and transcriptomic analyses of h1.3 null mutants demonstrate that H1.3 is required for both proper stomatal functioning under normal growth conditions and adaptive developmental responses to combined light and water deficiency. Using fluorescence recovery after photobleaching analysis, we show that H1.3 has superfast chromatin dynamics, and in contrast to the main Arabidopsis H1 variants H1.1 and H1.2, it has no stable bound fraction. The results of global occupancy studies demonstrate that, while H1.3 has the same overall binding properties as the main H1 variants, including predominant heterochromatin localization, it differs from them in its preferences for chromatin regions with epigenetic signatures of active and repressed transcription. We also show that H1.3 is required for a substantial part of DNA methylation associated with environmental stress, suggesting that the likely mechanism underlying H1.3 function may be the facilitation of chromatin accessibility by direct competition with the main H1 variants.

Germination and proliferation of emetic Bacillus cereus sensu lato strains in milk.

The Bacillus cereus sensu lato group includes potentially pathogenic bacteria that are ubiquitous in the environment and, importantly, could also be present in food products. This study focuses on emetic isolates which presumably could cause acute food poisoning and emetic syndrome. Here, we evaluate the ability of psychrotolerant Bacillus weihenstephanensis MC118 (isolated from soil) and mesophilic B. cereus BOD3/9 isolated from milk to germinate and multiply at 7 and 30 °C. Whereas the rates of germination at 30 °C in milk and nutrient broth of MC118 and BOD3/9 were similar, MC118, but not BOD3/9, proliferated to achieve relatively high numbers (∼10(6) colony-forming units/g) within 7 days of incubation at 7 °C. Mesophilic BOD3/9 showed a slight decrease of cell concentration in similar studies at 7 °C. Genotyping with repetitive extragenic palindromic sequence-based PCR and pulsed field gel electrophoresis revealed significant similarities between BOD3/9 and emetic reference B. cereus F4810/72 strain, while the B. weihenstephanensis MC118 isolate was more similar to the B. weihenstephanensis non-emetic reference DSMZ11821 strain. Our data suggest that emetic isolates that are also psychrotolerant, such as MC118, could constitute a hazard in the dairy industry, where milk could be a suitable medium for germination and growth.