No abstract

The integrity of DNA molecules is constantly challenged. All organisms have developed mechanisms to detect and repair multiple types of DNA lesions. The basic principles of DNA damage repair (DDR) in prokaryotes and unicellular and multicellular eukaryotes are similar, but the association of DNA with nucleosomes in eukaryotic chromatin requires mechanisms that allow access of repair enzymes to the lesions. This is achieved by chromatin-remodeling factors, and their necessity for efficient DDR has recently been demonstrated for several organisms and repair pathways. Plants share many features of chromatin organization and DNA repair with fungi and animals, but they differ in other, important details, which are both interesting and relevant for our understanding of genome stability and genetic diversity. In this Update, we compare the knowledge of the role of chromatin and chromatin-modifying factors during DDR in plants with equivalent systems in yeast and humans. We emphasize plant-specific elements and discuss possible implications.A DNA molecule in a eukaryotic chromosome has a diameter of 2 nm but a length in the range of centimeters. Being 10 7 times longer than it is wide would make it highly sensitive to breakage if it were not organized in compact and dynamic chromatin, with nucleosomes as basic units followed by multiple higher order levels of organization. Within this packaging, replication and transcription constitute an endogenous mechanical strain, while exposure of cells to physical or chemical hazard creates a wide range of DNA damage induced by external factors, including photoproducts, pyrimidine dimers, interstrand crosslinking, and single and double-strand breaks (DSBs). All organisms possess mechanisms that repair DNA molecules. DNA damage repair (DDR) systems for the various types of damage are overlapping as well as complementary, and their prevalence depends on the organism, the stage of the cell cycle, the site and the amount of damage, and the availability of intact templates. A coarse categorization distinguishes nucleotide excision repair, base excision repair, nonhomologous end joining (NHEJ), and homologous recombination (HR). Many schematic models for these DDR pathways describe the action and interaction of several repair components, but they are usually misleading in one aspect: DNA strands are illustrated as straight lines, neglecting the emerging role of chromatin for the location and the fate of DNA lesions. More realistically, we should envisage DNA lesions in the chromatin context like the leakage or burst of an in-ground pipe. Recognizing the defect, localizing it, excavating the broken parts, cleaning the ends, reconnecting them, and restoring the original state are mirrored in the subsequent steps of DDR: signaling, labeling, accessing, resecting, religating, and reassembling. Chromatin is expected to especially affect the access, resection, and restoration of the original arrangement, and several factors that can dissolve higher order structures, slide, evict, or exchange ...

An intron-spliced hairpin RNA approach was used for the targeted silencing of the MtTdp1α gene encoding the αisoform of tyrosyl-DNA phosphodiesterase 1 in Medicago truncatula Gaertn. Tyrosyl-DNA phosphodiesterase 1, involved in the repair of DNA topoisomerase I-mediated DNA damage, has been poorly investigated in plants. RNASeq analysis, carried out in the MtTdp1α-depleted plants, revealed different levels of transcriptional modulation (up-and down-regulation, alternative splicing, activation of alternative promoter) in genes involved in DNA damage sensing, DNA repair, and chromatin remodelling. It is suggested that the MtTdp1α gene has new, previously undetected roles in maintaining genome integrity. Up-regulation of senescence-associated genes and telomere shortening were observed. Moreover, impaired ribosome biogenesis indicated that the MtTdp1α gene is required for the nucleolar function. In agreement with the RNA-Seq data, transmission electron microscopy detected an altered nucleolar architecture in the MtTdp1α-depleted cells. Based on the reported data, a working hypothesis related to the occurrence of a nucleolar checkpoint in plant cells is proposed.