Compromised telomere maintenance in hypomethylated Arabidopsis thaliana plants

Abstract

Telomeres, nucleoprotein structures at the ends of linear eukaryotic chromosomes, are important for the maintenance of genomic stability. Telomeres were considered as typical heterochromatic regions, but in light of recent results, this view should be reconsidered. Asymmetrically located cytosines in plant telomeric DNA repeats may be substrates for a DNA methyltransferase enzyme and indeed, it was shown that these repeats are methylated. Here, we analyse the methylation of telomeric cytosines and the length of telomeres in Arabidopsis thaliana methylation mutants (met 1-3 and ddm 1-8), and in their wild-type siblings that were germinated in the presence of hypomethylation drugs. Our results show that cytosine methylation in telomeric repeats depends on the activity of MET1 and DDM1 enzymes. Significantly shortened telomeres occur in later generations of methylation mutants as well as in plants germinated in the presence of hypomethylation drugs, and this phenotype is stably transmitted to the next plant generation. A possible role of compromised in vivo telomerase action in the observed telomere shortening is hypothesized based on telomere analysis of hypomethylated telomerase knockout plants. Results are discussed in connection with previous data in this field obtained using different model systems.

Novel features of telomere biology revealed by the absence of telomeric DNA methylation

“…In contrast, the reads corresponding to ITSs or to the telomere-subtelomere junction of Chromosome I exhibited high levels of cytosines, which is in agreement with the high levels of DNA methylation previously reported for these loci ( Fig. 7; Cokus et al 2008;Vrbsky et al 2010;Vaquero-Sedas et al 2011Ogrocká et al 2014). Thus, the analysis of the complete 100-bp reads from SRR578938 confirms the virtual absence of DNA methylation in Arabidopsis telomeres.…”

“…Mammalian DNA methylation is primarily found in the CG context (Ramsahoye et al 2000;Lister et al 2009). In contrast, plants have significant levels of DNA methylation in all sequence contexts (CG, CHG, and CHH, where H can be A, C, or T) (Law and Jacobsen 2010).Although subtelomeric DNA methylation has been reported in animals and plants, the presence of DNA methylation at telomeres remains an open question in both kingdoms (Blasco 2007;Vrbsky et al 2010;Vaquero-Sedas et al 2011;Ogrocká et al 2014). The methylation status of mammalian telomeres has not been investigated because, as mentioned above, mammals have low levels of non-CG methylation, which is the type of DNA methylation that should be associated with telomeric sequences (CCCTAA in mammals and CCCTAAA in plants).…”

“…The methylation status of mammalian telomeres has not been investigated because, as mentioned above, mammals have low levels of non-CG methylation, which is the type of DNA methylation that should be associated with telomeric sequences (CCCTAA in mammals and CCCTAAA in plants). In turn, although the methylation levels of plant telomeres have been studied by different groups, they remain controversial (Vrbsky et al 2010; Majerová et al 2011a,b; Vaquero-Sedas and Vega-Palas 2011a,b;Vaquero-Sedas et al 2011Ogrocká et al 2014). Therefore, it is important to settle the methylation status of telomeres.…”

“…This DNA modification regulates multiple processes in plants and animals, including the homeostasis of telomere length (Blasco 2007;Suzuki and Bird 2008;Ooi et al 2009;Law and Jacobsen 2010;Castel and Martienssen 2013;Ogrocká et al 2014;Vaquero-Sedas and Vega-Palas 2014). Mammalian DNA methylation is primarily found in the CG context (Ramsahoye et al 2000;Lister et al 2009).…”

“…Although subtelomeric DNA methylation has been reported in animals and plants, the presence of DNA methylation at telomeres remains an open question in both kingdoms (Blasco 2007;Vrbsky et al 2010;Vaquero-Sedas et al 2011;Ogrocká et al 2014). The methylation status of mammalian telomeres has not been investigated because, as mentioned above, mammals have low levels of non-CG methylation, which is the type of DNA methylation that should be associated with telomeric sequences (CCCTAA in mammals and CCCTAAA in plants).…”