Chemical modifications in DNA impact gene regulation and chromatin structure. DNA oxidation, for example, alters gene expression, DNA synthesis and cell cycle progression. Modification of telomeric DNA by oxidation is emerging as a marker of genotoxic damage and is associated with reduced genome integrity and changes in telomere length and telomerase activity. 8-oxoguanine (8-oxoG) is the most studied and common outcome of oxidative damage in DNA. The G-rich nature of telomeric DNA is proposed to make it a hotspot for oxidation, but because telomeres make up only a tiny fraction of the genome, it has been difficult to directly test this hypothesis by studying dynamic DNA modifications specific to this region in vivo. Here, we present a new, robust method to differentially enrich telomeric DNA in solution, coupled with downstream methods for determination of chemical modification. Specifically, we measure 8-oxoG in Arabidopsis thaliana telomeres under normal and oxidative stress conditions. We show that telomere length is unchanged in response to oxidative stress in three different wild-type accessions. Furthermore, we report that while telomeric DNA comprises only 0.02–0.07% of the total genome, telomeres contribute between 0.2 and 15% of the total 8-oxoG. That is, plant telomeres accumulate 8-oxoG at levels approximately 100-fold higher than the rest of the genome under standard growth conditions. Moreover, they are the primary targets of further damage upon oxidative stress. Interestingly, the accumulation of 8-oxoG in the chromosome body seems to be inversely proportional to telomere length. These findings support the hypothesis that telomeres are hotspots of 8-oxoG and may function as sentinels of oxidative stress in plants.

Edited by Patrick Sung The conversion of circular genomes to linear chromosomes during molecular evolution required the invention of telomeres. This entailed the acquisition of factors necessary to fulfill two new requirements: the need to fully replicate terminal DNA sequences and the ability to distinguish chromosome ends from damaged DNA. Here we consider the multifaceted functions of factors recruited to perpetuate and stabilize telomeres. We discuss recent theories for how telomere factors evolved from existing cellular machineries and examine their engagement in nontelomeric functions such as DNA repair, replication, and transcriptional regulation. We highlight the remarkable versatility of protection of telomeres 1 (POT1) proteins that was fueled by gene duplication and divergence events that occurred independently across several eukaryotic lineages. Finally, we consider the relationship between oxidative stress and telomeres and the enigmatic role of telomere-associated proteins in mitochondria. These findings point to an evolving and intimate connection between telomeres and cellular physiology and the strong drive to maintain chromosome integrity.

Reactive oxygen species (ROS) are natural by‐products of cellular respiration, essential for cell signaling. They are also a major threat to telomere stability and can trigger senescence. Emerging studies reveal a broad role for telomere‐associated factors in modulating the response to oxidative stress. PROTECTION OF TELOMERES 1 (POT1) is one of the most conserved telomeric proteins, critical for both chromosome‐end protection and replication. POT1 is a single‐copy gene in most species. However, the flowering plant Arabidopsis thaliana encodes two highly divergent POT1 paralogs, AtPOT1a and AtPOT1b. AtPOT1a is an essential telomerase‐associated processivity factor. In contrast, AtPOT1b is not required for telomere replication or end protection, and its overexpression cannot complement the loss of AtPOT1a. Here we explore the natural separation‐of‐function within Arabidopsis POT1 paralogs and uncover a novel role for AtPOT1b in redox homeostasis. AtPOT1b expression is highly regulated, and restricted to seeds, root tips, and gametophytes, organs whose development is controlled by endogenous ROS. Strikingly, ectopic over‐expression of AtPOT1b decreases natural ROS levels throughout the plant, while a null mutation results in high levels of cellular and genomic oxidation, telomere length changes under multiple environmental insults, decreased chromatin compaction, decreased DNA methylation and increased expression of stress‐response genes. Consequently, atpot1b mutants have reduced fitness and are hypersensitive to chemical and environmental stresses. Finally, in response to oxidative stress, AtPOT1b expression increases and the protein accumulates at telomeres, suggesting that AtPOT1b may play a direct role in mitigating telomere oxidation. We postulate that AtPOT1b functions in a molecular rheostat that both senses and regulates cellular oxidation, thereby promoting genome stability in tissues where higher ROS levels are required for development.