Transcriptional regulation during hypoxia stress in Arabidopsis
A number of AP2/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 that is specifically induced during hypoxia. We show that under normoxic conditions the expression of AtERF73/HRE1 can be induced by exogenous addition of ACC and that a combination of hypoxia and ACC results in hyper-induction 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 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. The gas chromatography analyses showed that the production of ethylene was similar between wild-type and RNAi lines under hypoxia. Quantitative RT-PCR 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.