Oxygen sensing mechanisms in plants.
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Oxygen sensing mechanisms are important for organisms to adapt their survival strategy when faced with low oxygen conditions. These survival strategies are morphological, physiological and biochemical adaptations to the lack or reduction in oxygen. Oxygen is a major electron acceptor that is needed for the aerobic metabolism. For plants during floods, oxygen diffuses slower in water than in air, creating an oxygen deficit. This results in the plant’s biochemical adaptation from an aerobic to an anaerobic respiration. This is achieved through the glycolysis and the fermentation pathway. During flooding, ethylene also accumulates and serves as a signal to activate hypoxic responsive-genes. Other organisms such as bacteria, fungi and animals have oxygen sensing mechanisms. In these mechanisms, the heme protein plays a central role as an oxygen sensor. For plants, the group VII of ethylene response factor (ERF) transcription factors seems to have an important role as a mediator for oxygen sensing. The group VII of ERF transcription factors are regulated through the N-end rule proteolysis pathway. The N-end rule pathway has an oxygen dependent sequence which would be responsible for oxygen sensing. The conserved sequence of the N-end rule pathway is responsible for the degradation of the ERF under aerobic conditions. When there is no more oxygen available, the oxidation of cysteine in the N-end rule pathway cannot occur. This leads to the inhibition of the degradation pathway. The research around direct oxygen sensing mechanisms for plants still needs to be completed, but I believe that recent studies are close to find the answer behind the ‘unknown’ factor that is behind the activation and regulation group VII of ERF transcription factors and/or N-end rule proteolysis pathway.