Mitochondrial thioredoxin-o(AtTrxo1) was characterized and its expression examined in different organs ofArabidopsis thaliana.AtTrxo1transcript levels were particularly high in dry seeds and cotyledons where they reached a maximum 36 h after imbibition with water, coinciding with 50% germination. Expression was lower in seeds germinating in 100 mM NaCl. To gain insight into the transcriptional regulation of theAtTrxo1gene, a phylogenomic analysis was coupled with the screening of an arrayed library of Arabidopsis transcription factors in yeast. The basic leucine zipper AtbZIP9 and the zinc finger protein AZF2 were identified as putative transcriptional regulators. Transcript regulation of AtbZIP9 and AtAFZ2 during germination was compatible with the proposed role in transcriptional regulation ofAtTrxo1. Transient over-expression of AtbZIP9 and AtAZF2 inNicotiana benthamianaleaves demonstrated an activation effect of AtbZIP9 and a repressor effect of AtAZF2 onAtTrxo1promoter-driven reporter expression. Although moderate concentrations of salt delayed germination in Arabidopsis wild-type seeds, those of two differentAtTrxo1knock-out mutants germinated faster and accumulated higher H2O2 levels than the wild-type. All these data indicate that AtTrxo1 has a role in redox homeostasis during seed germination under salt conditions.