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Changes in oxygen (O2) availability often occur during the life of plants, mostly due to environmental conditions (flooding, monsoons…). In Arabidopsis thaliana a direct oxygen sensing mechanism has recently been discovered. Transcription factors (TFs) of the ethylene responsive factors group-VII (ERF-VII) possess a characteristic N-terminal motif that leads to specific degradation by proteasome pathway in normoxia. Under hypoxic stress, the TF is instead stabilized and can migrate to nucleus to regulate the transcription of hypoxia-responsive genes. Furthermore, it was shown that both O2 and nitric oxide (NO) are required to destabilize the group VII ERFs and that a reduction in the availability of either gas is sufficient to stabilize these proteins.
Legume crops are known for their ability to establish a symbiotic relationship with soil bacteria to reduce atmospheric nitrogen (N2) in ammonia. Increasing evidence has been reporting the presence of NO during symbiosis from early interaction steps between the plant and the bacterial partners to N2-fixing and senescence steps in mature nodules. In mature nodules, NO was notably shown to play a beneficial metabolic function for the maintenance of the energy status under microaerophilic conditions prevailing in the nodules.
 In this framework, our project is to characterize the role of MtERF-B2.1 and MtERF-B2.11 TFs in O2 sensing and adaptation to hypoxia in roots and nodules of M. truncatula, and to understand how NO interacts with oxygen in hypoxic signalization. The involvement of the N-end rule pathway in the regulation of MtERF-B2.1 activity have been analyzed by measuring the stability of a chimeric protein (MC80_MtERF-B2.1:LUC) composed by the first 80 amino acids of the MtERF-B2.1 TFs and the luciferase reporter protein in A. thaliana protoplasts. Our results indicated that N-terminal part of MtERF-B2.1 could drive the O2-dependent degradation by the N-end rule pathway. Using a RNA interference strategy to knock-down MtERF-B2.1 and MtERF-B2.11, we observed a significant decrease in the transcript level of major hypoxia responsive genes under hypoxic conditions (ADH, PDC1, PCO1; HRA1 and AlaAT). Furthermore, the knock-down transgenic roots showed a reduced capacity of nodulation indicating a role of both MtERF-B2.1 and MtERF-B2.11 in nodulation process.