AbstractOxidative stress can lead to plant growth retardation, yield loss, and death. Theatr7mutant ofArabidopsis thalianaexhibits pronounced tolerance to oxidative stress. Using positional cloning, confirmed by knockout and RNA interference (RNAi) lines, we identified theatr7mutation and revealed thatATR7is a previously uncharacterized gene with orthologs in other seed plants but with no homology to genes in lower plants, fungi or animals. Expression ofATR7-GFPfusion shows that ATR7 is a nuclear-localized protein. RNA-seq analysis reveals that transcript levels of genes encoding abiotic- and oxidative stress-related transcription factors (DREB19, HSFA2, ZAT10), chromatin remodelers (CHR34), and unknown or uncharacterized proteins (AT5G59390, AT1G30170, AT1G21520) are elevated inatr7. This indicates thatatr7is primed for an upcoming oxidative stress via pathways involving genes of unknown functions. Collectively, the data revealATR7as a novel seed plants-specific nuclear regulator of oxidative stress response.