ATM and ATR make distinct contributions to chromosome end protection and the maintenance of telomeric DNA in Arabidopsis

Abstract

Here we examine the function of ATM and ATR at telomeres in Arabidopsis. Although plants lacking ATM or ATR display wild-type telomere length homeostasis, chromosome end protection is compromised in atm atr mutants. Moreover, atm tert Arabidopsis experience an abrupt, early onset of genome instability, arguing that ATM is required for protection of short telomeres. ATR, by contrast, is required for maintenance of telomeric DNA as telomere shortening is dramatically accelerated in atr tert mutants relative to tert plants. Thus, ATM and ATR make essential and distinct contributions to chromosome end protection and telomere maintenance in higher eukaryotes. Several factors involved in the repair of double-strand breaks in DNA are physically associated with telomeres and have essential roles in chromosome stability (d'Adda di Fagagna et al. 2004). Among them are the phosphatidyl-inositol-3-kinase-like protein kinases (PIKKs) ATM/ Tel1p and ATR/Mec1p/Rad3p, which coordinate the repair, cell-cycle checkpoint, and apoptotic responses to DNA damage (Kastan and Lim 2000;Shiloh 2003). ATM also protects against telomere fusions and participates in telomere length homeostasis as atm/tel1 mutants display shorter telomeres in both mammals and yeast (Lustig and Petes 1986;Smilenov et al. 1997). A telomeric role for ATR has not been explored in mammals because mutants are inviable (Brown and Baltimore 2000;de Klein et al. 2000). However, in yeast mec1-21 mutants, telomeres are shortened (Ritchie et al. 1999). In addition, recent data show that Mec1p facilitates loading of the single-strand telomere-binding protein Cdc13p (Takata et al. 2005). Yeast lacking both Tel1p and Mec1p fail to recruit telomerase and experience progressive telomere erosion (Naito et al. 1998;Ritchie et al. 1999;Chan et al. 2001).Here we explore the role of ATM and ATR at telomeres in Arabidopsis thaliana, a higher eukaryote that tolerates disruption of both of these genes. We find that while neither ATM nor ATR is individually required for telomere length homeostasis, chromosome end protection is impaired in atm atr mutants. Our data further indicate that ATM is involved in protecting chromosome ends in cells with short telomeres, while ATR is required for maintenance of telomeric DNA. Results and DiscussionTo examine the role of ATM and ATR in telomere length regulation, we performed telomere restriction fragment (TRF) analysis on genomic DNA extracted from atm and atr mutants that carry a homozygous T-DNA disruption in the corresponding genes (see Materials and Methods). In yeast and mammals, the absence of ATM/Tel1p leads to shortening of telomeres until they reach a new stable set point (Lustig and Petes 1986;Smilenov et al. 1997). In contrast, atm or atr Arabidopsis displayed wild-type telomere length, even after multiple generations (Fig. 1A). The same result was obtained with mutants doubly deficient in ATM and ATR (Fig. 1A). As previously noted (Culligan et al. 2004), atm atr mutants exhibited no growth defects but were completely sterile. Hence...

ArabidopsisATM and ATR Kinases Prevent Propagation of Genome Damage Caused by Telomere Dysfunction

“…It has previously been shown that tert atr mutant plants show an increased rate of telomere loss compared with tert mutants (Vespa et al, 2005). These authors suggested a role for ATR in the maintenance of telomeric DNA, and this is in accordance with the increased number of TIF-positive cells that we detect in tert atr relative to tert mutant cells.…”

Ataxia telangiectasia mutated (Atm) is not required for telomerase-mediated elongation of short telomeres

“…Recently, the role of Atm and Atr in telomere length regulation in Arabidopsis was analyzed (18). In the absence of either Atm or Atr, Arabidopsis maintains normal telomere length over multiple generations.…”

Suppression of RICE TELOMERE BINDING PROTEIN1 Results in Severe and Gradual Developmental Defects Accompanied by Genome Instability in Rice

“…Accumulating evidence in mammalian systems indicates that bridged anaphase chromosomes and unequal segregation of chromosomes are typical results from the fusion of unprotected chromosome termini due to structural defects of telomeres (Hande et al, 1999;Artandi et al, 2000). Arabidopsis mutants in which telomere-related genes are disrupted suffer genomic instability due to anaphase chromosomal fusions, as do the later generation progeny of telomerase-deficient mutants (Riha et al, 2001;Riha and Shippen, 2003b;Siroky et al, 2003;Puizina et al, 2004;Shakirov et al, 2005;Vespa et al, 2005;Vannier et al, 2006). In this regard, it should be noted that increased frequency of anaphase bridges in rtbp1 PMCs occurs in parallel with progressively increased aberrant morphology of the mutant rice plants (Table 1).…”

“…Despite limited insight into the plant telomeres compared with mammalian systems, plant telomeres have recently attracted much interest, since it is becoming increasingly apparent that the function and architectural features of telomeres as well as the patterns of telomerase regulation are largely conserved in higher plants and animals (reviewed in Riha and Shippen, 2003a;McKnight and Shippen, 2004;Gallego and White, 2005;Lamb et al, 2007;Zellinger and Riha, 2007). As in human and other animal species, the stable maintenance of telomeres is essential for growth and development in Arabidopsis, and a number of nuclear proteins, such as RAD50, KU70/80, MRE11, Pot1/2, ATM, and ATR, have been shown to be involved in telomere regulation (Gallego and White, 2001;Riha and Shippen, 2003b;Gallego et al, 2003;Puizina et al, 2004;Shakirov et al, 2005;Vespa et al, 2005;Vannier et al, 2006). Disruption of the genes encoding these telomere-related proteins causes strikingly different developmental phenotypes accompanied by an abrupt onset of genome instability.…”