Fig. 4.

Two models for ATM control of telomere length in Arabidopsis. (A) ATM ameliorates the consequences of mitotic TRD by promoting elongation of the short telomeres through ALT or telomerase. (B) ATM promotes resetting of telomeric length on homologous chromosomes in meiosis. ATM senses the size difference between the two telomeres when they are aligned in meiotic prophase I. Cells with homologous telomeres that are in a similar size range (≤1 kb difference) proceed through meiosis. If telomeres differ in size by >1 kb, an ATM-dependent checkpoint is activated (ATM*) that blocks meiotic progression. The block could be alleviated through ATM signaling to telomerase or to recombinational machinery (ALT) to elongate the shorter telomere of the pair. Alternatively, the longer telomere could be subjected to TRD.