Emerging roles of chromatin in the maintenance of genome organization and function in plants

Genome-wide excision repair in Arabidopsis is coupled to transcription and reflects circadian gene expression patterns

Plants are exposed to numerous DNA-damaging stresses including the exposure to ultraviolet (UV) component of solar radiation. They employ nucleotide excision repair to remove DNA-bulky adducts and to help eliminate UV-induced DNA lesions, so as to maintain their genome integrity and their fitness. Here, we generated genome-wide single-nucleotide resolution excision repair maps of UV-induced DNA damage in Arabidopsis at different circadian time points. Our data show that the repair of UV lesions for a large fraction of the genome is controlled by the joint actions of the circadian clock and transcription by RNA polymerase II. Our findings reveal very strong repair preference for the transcribed strands of active genes in Arabidopsis, and 10–30% of the transcription-coupled repair is circadian time-dependent. This dynamic range in nucleotide excision repair levels throughout the day enables Arabidopsis to cope with the bulky DNA lesion-inducing environmental factors including UV.

Plants use nucleotide excision repair to maintain genome integrity in response to damage caused by stresses such as UV radiation. Here Oztas et al. use genome-wide profiling to show that excision repair in Arabidopsis is strongly coupled to transcription and reflects circadian patterns of gene expression.

The long and short of doubling down: polyploidy, epigenetics, and the temporal dynamics of genome fractionation

We consider the rapidly advancing discipline of plant evolutionary genomics, with a focus on the evolution of polyploid genomes. In many lineages, polyploidy is followed by 'biased fractionation', the unequal loss of genes from ancestral progenitor genomes. Mechanistically, it has been proposed that biased fractionation results from changes in the epigenetic landscape near genes, likely mediated by transposable elements. These epigenetic changes result in unequal gene expression between duplicates, establishing differential fitness that leads to biased gene loss with respect to ancestral genomes. We propose a unifying conceptual framework and a set of testable hypotheses based on this model, relating genome size, the proximity of transposable elements to genes, epigenetic reprogramming, chromatin accessibility, and gene expression.

Not just gene expression: 3D implications of chromatin modifications during sexual plant reproduction

DNA methylation and histone modifications are epigenetic changes on a DNA molecule that alter the three-dimensional (3D) structure locally as well as globally, impacting chromatin looping and packaging on a larger scale. Epigenetic marks thus inform higher-order chromosome organization and placement in the nucleus. Conventional epigenetic marks are joined by chromatin modifiers like cohesins, condensins and membrane-anchoring complexes to support particularly 3D chromosome organization. The most popular consequences of epigenetic modifications are gene expression changes, but chromatin modifications have implications beyond this, particularly in actively dividing cells and during sexual reproduction. In this opinion paper, we will focus on epigenetic mechanisms and chromatin modifications during meiosis as part of plant sexual reproduction where 3D management of chromosomes and re-organization of chromatin are defining features and prime tasks in reproductive cells, not limited to modulating gene expression. Meiotic chromosome organization, pairing and synapsis of homologous chromosomes as well as distribution of meiotic double-strand breaks and resulting crossovers are presumably highly influenced by epigenetic mechanisms. Special mobile small RNAs have been described in anthers, where these so-called phasiRNAs seem to direct DNA methylation in meiotic cells. Intriguingly, many of the mentioned developmental processes make use of epigenetic changes and small RNAs in a manner other than gene expression changes. Widening our approaches and opening our mind to thinking three-dimensionally regarding epigenetics in plant development holds high promise for new discoveries and could give us a boost for further knowledge.