Switching the hypoxic response on and off in plants

Hypoxia significantly impacts plant metabolism and growth by disrupting mitochondrial respiration, and oxygen sensing plays a vital role in regulating responses to low oxygen conditions. Plants sense oxygen through the N-degron pathway, involving plant cysteine oxidases (PCOs) that oxidize the ethylene response factors belonging to group VII (ERF-VIIs), leading to their degradation under normoxia. Under hypoxic conditions, PCO activity decreases, stabilizing ERF-VII proteins and activating the transcription of hypoxia-responsive genes to adapt to oxygen limitation. Recent research highlights additional factors, including the MBR1/MED25 complex, ERF-VII phosphorylation, and the integration of energy and oxygen signals via the target of rapamicin pathway, which fine-tune the hypoxic response. Upon reoxygenation, PCOs restore activity and degrade ERF-VII, but this degradation is delayed, possibly due to reactive oxygen species inhibiting PCO function. Repressive factors such as HRA1 and ORA59 also modulate ERF-VII activity to suppress HRG expression. The response of the plant to hypoxia also involves a sophisticated network of molecular signals, including calcium signalling and the redox-modulatory role of phytoglobins and nitric oxide. Despite significant progress, much remains unknown about plant hypoxia, as its complex, spatiotemporal nature affects not only environmental adaptation but also development and plant–microbe interactions, necessitating intricate regulatory mechanisms.