SUMMARY In eukaryotes, meiotic recombination maintains genome stability and creates genetic diversity. The conserved Ataxia‐Telangiectasia Mutated (ATM) kinase regulates multiple processes in meiotic homologous recombination, including DNA double‐strand break (DSB) formation and repair, synaptonemal complex organization, and crossover formation and distribution. However, its function in plant meiotic recombination under stressful environmental conditions remains poorly understood. In this study, we demonstrate that ATM is required for the maintenance of meiotic genome stability under heat stress in Arabidopsis thaliana. Using cytogenetic approaches we determined that ATM does not mediate reduced DSB formation but does ensure successful DSB repair, and thus meiotic chromosome integrity, under heat stress. Further genetic analysis suggested that ATM mediates DSB repair at high temperature by acting downstream of the MRE11–RAD50–NBS1 (MRN) complex, and acts in a RAD51‐independent but chromosome axis‐dependent manner. This study extends our understanding on the role of ATM in DSB repair and the protection of genome stability in plants under high temperature stress.

In eukaryotes, the conserved kinase Ataxia Telangiectasia Mutated (ATM) negatively regulates DNA double-strand break (DSB) formation and plays a central role in DSB repair. Here, by using cytogenetic approaches, we demonstrate that ATM also plays an essential role in protecting meiotic chromosome integrity in Arabidopsis thaliana at extreme high temperature. We determined the chromosome localization patterns of DSB formation proteins SPO11-1 and DFO during prophase I, both of which were disturbed by heat stress. Evaluation of the number of RAD51, DMC1, SPO11-1 and DFO protein foci in meiocytes of Arabidopsis atm mutant clarifies that ATM does not mediate the heat-induced reduction in DSB formation. Interestingly, meiotic spread analysis showed that chromosome fragmentation level was significantly increased in atm but was lowered in the mre11 and mre11 atm mutants under high temperature, indicating that ATM-dependent meiotic chromosome integrity at high temperature relies on the functional MRE1-RAD50-NBS1 (MRN) complex. Moreover, contrary to the rad51 and mnd1 mutants, which exhibited enhanced meiotic chromosome integrity under heat stress, the rad51 atm and mnd1 atm mutants retained high levels of chromosome fragmentation at extreme high temperature. Furthermore, heat stress reduced chromosome fragmentation level in the syn1 and syn1 atm mutants. Collectively, these data suggest that ATM-mediated DSB repair is required for meiotic genome stability in plants at extreme high temperature, which possibly acts in a RAD51-independent manner and relies on functional chromosome axis.