Nitric oxide's (NO) involvement in breaking seed dormancy has been demonstrated in previous research but its action mechanism remains to be clarified. We observed that a rapid accumulation of NO induces an equally rapid decrease of abscisic acid (ABA) that is required for the NO's action in Arabidopsis. In addition, the NO-induced ABA decrease correlates with the regulation of CYP707A2 transcription and the (+)-abscisic acid 8'-hydroxylase (encoded by CYP707A2) protein expression. By analyzing cyp707a1, cyp707a2 and cyp707a3 mutants, we found that CYP707A2 plays a major role in ABA catabolism during the first stage of imbibition. Fluorescent images demonstrate that NO is released rapidly in the early hours at the endosperm layer during imbibition. Evidently such response precedes the enhancement of ABA catabolism which is required for subsequent seed germination.