The AP2/ERF Transcription Factor AtERF73/HRE1 Modulates Ethylene Responses during Hypoxia in Arabidopsis

A number of APETALA2 (AP2)/ETHYLENE RESPONSE FACTOR (ERF) genes have been shown to function in abiotic and biotic stress responses, and these genes are often induced by multiple stresses. We report here the characterization of an AP2/ERF gene in Arabidopsis (Arabidopsis thaliana) that is specifically induced during hypoxia. We show that under normoxic conditions, the expression of AtERF73/HRE1 can be induced by exogenous addition of 1-aminocyclopropane-1-carboxylic acid and that a combination of hypoxia and 1-aminocyclopropane-1-carboxylic acid results in hyperinduction of AtERF73/HRE1 expression. In addition, hypoxic induction of AtERF73/HRE1 is reduced but not completely abolished in ethylene-insensitive mutants and in the presence of inhibitors of ethylene biosynthesis and responses. These results suggest that, in addition to ethylene, an ethylene-independent signal is also required to mediate hypoxic induction of AtERF73/HRE1. To assess the role of AtERF73/HRE1, we generated three independent RNA interference (RNAi) knockdown lines of AtERF73/HRE1. Under normoxic conditions, the AtERF73/HRE1-RNAi seedlings displayed increased ethylene sensitivity and exaggerated triple responses, indicating that AtERF73/HRE1 might play a negative regulatory role in modulating ethylene responses. Gas chromatography analyses showed that the production of ethylene was similar between wild-type and RNAi lines under hypoxia. Quantitative reverse transcription-polymerase chain reaction analyses showed that hypoxia-inducible genes could be affected by AtERF73/HRE1-RNAi lines in two different ways: hypoxic induction of glycolytic and fermentative genes was reduced, whereas induction of a number of peroxidase and cytochrome P450 genes was increased. Taken together, our results show that AtERF73/HRE1 is involved in modulating ethylene responses under both normoxia and hypoxia.