Unveiling the epigenetic and stress-responsive function of histone H1 in plants

Environmental stresses can induce epigenetic alterations, particularly in chromatin accessibility and DNA methylation patterns, which orchestrate gene expression in Arabidopsis thaliana. Many molecular mechanisms with diverse aspects have been reported to play a role, but a thorough understanding is still required. The linker protein Histone H1 (H1) plays an essential role in modulating the chromatin architecture to control gene expression. H1 stabilizes the nucleosome structure and promotes higher-order chromatin compaction. It interacts with chromatin modifiers to influence nucleosome accessibility, thereby regulating transcriptional events. H1 variants exhibit distinct spatiotemporal expression patterns and play key roles in stress adaptation. Under abiotic stress, the depletion of H1 is associated with dynamic DNA demethylation at transposable elements and altered histone marks. For instance, upon H1 depletion under drought or heat stress, shifts in H3K9me2 deposition have been observed, indicating a dual role for H1 in promoting genome stability and facilitating stress-induced transcriptional reprogramming. This review summarizes the emerging insights into H1 molecular mechanisms, crosstalk between noncoding RNAs, and phase-separated condensates. We also discuss the unresolved questions on H1 variant specificity and evolutionary conservation, as well as their potential applications in improving crop resilience through epigenetic engineering.