Abstract
Low oxygen stress often occurs during the life of green organisms, mostly due to the environmental conditions affecting oxygen availability. Both plants and algae respond to low oxygen by resetting their metabolism. The shift from mitochondrial respiration to fermentation is the hallmark of anaerobic metabolism in most organisms. This involves a modified carbohydrate metabolism coupled with glycolysis and fermentation. For a coordinated response to low oxygen, plants exploit various molecular mechanisms to sense when oxygen is either absent or in limited amounts. In Arabidopsis thaliana, a direct oxygen sensing system has recently been discovered, where a conserved N-terminal motif on some ethylene responsive factors (ERFs), targets the fate of the protein under normoxia/hypoxia. In Oryza sativa, this same group of ERFs drives physiological and anatomical modifications that vary in relation to the genotype studied. The microalga Chlamydomonas reinhardtii responses to low oxygen seem to have evolved independently of higher plants, posing questions on how the fermentative metabolism is modulated. In this review, we summarize the most recent findings related to these topics, highlighting promising developments for the future.
Highlights
A Shift towards the Fermentation PathwayPlants and algae rely on aerobic respiration to produce energy. Oxygen availability is required and changes in its accessibility lead to drastic metabolic rearrangements
Studies of several species, such as tomato, potato, Arabidopsis and rice, have shown that lactate production plays a minor role in low oxygen responses, because lactate is generally produced only during the first hours of stress and is expelled from the cell, in order to prevent an excessive accumulation of this compound in the cytosol [15]
The recent discovery of SUB1A as the gene able to confer enhanced tolerance to submergence in rice resulted in the development of new rice varieties that display an exceptional tolerance to submergence coupled to high yield [2]
Summary
Plants and algae rely on aerobic respiration to produce energy. Oxygen availability is required and changes in its accessibility lead to drastic metabolic rearrangements. Studies of several species, such as tomato, potato, Arabidopsis and rice, have shown that lactate production plays a minor role in low oxygen responses, because lactate is generally produced only during the first hours of stress and is expelled from the cell, in order to prevent an excessive accumulation of this compound in the cytosol [15] An increase in root tolerance to hypoxia has been reported when LDH was over-expressed in Arabidopsis plants [17] This resulted in a significantly higher activity of PDC, suggesting that the lactic and ethanolic fermentation pathways are inter-dependent and that the lactate metabolism is involved in the enhancement of ethanol production [17]. It will be important to deepen the current knowledge on the transcriptional regulation of metabolic genes during low oxygen responses, in relation to the recent identification of the oxygen-sensing mechanism that seems to be at the basis of ethanolic fermentation activation [41,42] (see section 2.1), which represents a challenging starting point
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