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

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.