The hypoxia responsive transcription factor genes ERF71/HRE2 and ERF73/HRE1 of Arabidopsis are differentially regulated by ethylene.

The AR2/ERF transcription factor genes ERF71/HRE2 and ERF73/HRE1 were induced at hypoxic conditions in Arabidopsis thaliana roots. ERF73/HRE1 but not its related gene ERF71/HRE2 was furthermore regulated by ethylene. Treatment with 1 ppm ethylene promoted ERF73/HRE1 expression fivefold. This induction did not occur in the presence of the ethylene receptor inhibitor 1-methylcyclopropene. ERF73/HRE1 expression positively regulated alcohol dehydrogenase (ADH) activity, which was analyzed as a marker enzyme for metabolic adaptation to hypoxic stress. The knock out lines erf73/hre1-1 and erf73/hre1-2 showed lowered ADH activity; the overexpressing lines ERF73/HRE1-ox1 and ERF73/HRE1-ox5 displayed elevated ADH activity. Treatment of wild-type Arabidopsis with 5% O₂ and 1 ppm ethylene resulted in higher induction of ADH activity than that observed with 5% O₂ or 1 ppm ethylene alone. ERF73/HRE1-ox1 and ERF73/HRE1-ox5 plants that were exposed to 5% O₂ did not show enhanced ADH activity after treatment with ethylene, indicating that the ethylene response with respect to ADH activity was saturated in the ERF73/HRE1ox lines. In contrast, erf73/hre1-1 and erf73/hre1-2 lines displayed ethylene-dependent ADH activity pointing to redundant factor(s) that can mediate ethylene regulation of ADH activity in the Arabidopsis root. Our data show that ethylene regulates metabolic adaptation to low oxygen stress in the Arabidopsis root through ERF73/ HRE1.