Which plant tissues are found in animals
Oxygen in plants - how an elixir of life can be both a stress factor and a signaling substance
Research Report 2011 - Max Planck Institute for Molecular Plant Physiology
Department of Organelle Biology, Biotechnology and Molecular Ecophysiology
Low oxygen concentration as a stress factor for plants
For plants, as for many other organisms, oxygen is a vital component. It plays an essential role in breathing and is therefore directly connected to the energy supply of the cell. While the green parts of the plants themselves produce oxygen by means of photosynthesis, the oxygen supply to the roots depends on the oxygen supply from the environment. Since plants do not have an active transport mechanism for oxygen, but are dependent on diffusion, there can be a lack of oxygen in the plant tissue under certain circumstances. This condition is called hypoxia. If there is also a reduced oxygen supply from the outside - for example due to water saturation of the soil, which leads to the inhibition of gas transport in the soil - the oxygen concentration in the plant tissue can drop so much that an optimal energy supply for the cell is no longer possible. This can lead to the death of the plant, as anyone who has over watered their houseplant knows.
So that plants do not have to pay for fluctuations in the oxygen concentration immediately with death, a variety of adaptation strategies are available to them. For example, there are morphological adaptations in the form of aerial roots or aerenchymes, the formation of which improves the supply of oxygen to the cells. Such a morphological adaptation takes time, however, as new tissue structures have to be formed in some cases and cell division processes are necessary. In addition, not all plant species are able to form aerenchymes.
Another immediately effective strategy consists in adapting the metabolism to the oxygen deficiency situation and the resulting reduced energy availability. Plant species such as rice, potatoes, wheat, soy, poplar and Arabidopsis have this ability . This flexibility of the metabolism is an important prerequisite for surviving periods of low oxygen supply .
In order for plants to be able to react to fluctuations in the oxygen content, they must perceive these changes in the oxygen concentration in good time. Although efforts have been made for many years to improve the mechanism of oxygen detection (Oxygen sensing) This process was discovered in plants only last year . With this discovery, hypoxia is one of the few abiotic stress types for which a sensor is now known and the signaling pathways and adaptation strategies in plants have been described.
Regulation of the energy metabolic pathways
In order to understand the regulation of the energy metabolism during hypoxia, it is not enough to investigate respiration or oxidative phosphorylation in the mitochondria, but the entire metabolic pathways of glycolysis and the Krebs cycle must be included in the investigations.
Figure 2 shows the most important metabolic pathways schematically and shows how they are influenced by a low oxygen concentration. The lack of oxygen as a substrate for oxidative phosphorylation can result in insufficient energy being provided in the form of adenosine triphosphate (ATP). The best-known reaction of the carbon metabolism to this is the so-called Pasteur effect, named after Louis Pasteur, who was the first to describe this phenomenon in yeast . The activity of glycolysis is increased, however, not to produce substrate for the Krebs cycle, but to increase the production of ATP. This reaction also reduces nicotinamide adenine dinucleotide (NAD) to NADH. Fermentation reactions ensure that NADH is returned to NAD so that the NAD as a secondary substrate for glycolysis does not run out. Among other things, ethanol or lactate are produced in higher plants. In this way, when there is a lack of oxygen, cells are able to produce sufficient ATP for the most vital processes while consuming relatively large amounts of carbohydrates.
Interestingly, the oxygen consumption during breathing is already reduced at concentrations which do not yet suggest that oxygen is a limiting factor . From this it can be concluded that there must be additional regulatory mechanisms that ensure at an early stage that oxygen is used more sparingly.
An example of the dynamic response of the primary metabolism to a decreased oxygen concentration is a “diversion” of the carbon flow. This diversion leads to the production of the amino acid alanine and the organic acid alpha-Ketoglutarate is increased  and thus the Krebs cycle receives less substrate. As a result, less NADH is formed, with which ATP is normally produced using oxygen. The decrease in the activity of the Krebs cycle could therefore be a cause of the decrease in the respiratory rate during low oxygen conditions.
Another change in respiratory metabolism observed in experiments is the restructuring of the linkage of protein complexes in the mitochondrial membrane . The various protein complexes involved in oxidative phosphorylation normally form together in so-called super complexes. However, under low-oxygen conditions, these super-complexes fall apart and thus enable the participation of alternative metabolic pathways in breathing. For example, it is possible to use NADH, which was produced by glycolysis instead of the Krebs cycle, for oxidative phosphorylation. This reorganization of the metabolic pathways could also contribute to downregulating the activity of the Krebs cycle.
Although a wide variety of adaptation strategies are known to help minimize the effects of oxygen stress in plants, until recently there was no evidence of how plants detect the availability of oxygen in cells. Although several different sensor systems have already been described for animals and microorganisms, it was assumed that there might not be an oxygen sensor for plants, but that the effects of stress, a low energy status or over-acidification of the cell are detected and thus the reaction to low oxygen levels. Conditions is triggered.
However, during the last five years it has been described several times that proteins are involved which belong to the family of ERF transcription factors (Ethylene Response Factor) belong . These transcription factors are characterized by a characteristic amino acid sequence at the beginning of the protein, the so-called N-end, which determines the stability and lifespan of the protein. The series of reactions determined by the amino acid sequence that leads to the breakdown of the protein is known as N-end rule. A very important step in this series of reactions is the reaction of the amino acid cysteine with molecular oxygen. This means that the lifetime of the transcription factor and thus its activity as a regulator of gene expression is determined by the oxygen concentration in the cell.
A transcription factor, RAP2.12, which belongs to the ERF family, has proven to be particularly important for the regulation of the low-oxygen reaction in plants . Regardless of the oxygen concentration, it is normally located in the plasma membrane, where it cannot be active as a transcription factor (Fig. 3The binding to the membrane protein ACBP is responsible for the unusual localization of the protein. The interaction between these two proteins also ensures that RAP2.12 is protected against the degradation that actually occurs after the N-end rule would be expected. By a mechanism that has not yet been clarified, ACBP only releases the transcription factor RAP2.12 when the oxygen concentration drops. Only then is it transported to the cell nucleus in order to express the vital genes there, which become active under low-oxygen conditions. Because RAP2.12 is always present in the cell and does not have to be produced when the oxygen concentration drops, a quick reaction to changes in the cellular oxygen concentration is guaranteed. The regulation of the lifetime, and thus the activity of the protein, is then controlled by the N-end rule Fine-tuned for protein degradation so that the plant's stress reaction can be adapted to the current oxygen availability.
Although one of the most important questions regarding the regulation of the stress response to low oxygen concentrations in plants has been resolved with the discovery of the oxygen sensor, many questions remain open and new questions arise. For example, it must be clarified whether and, if so, how the reaction of molecular oxygen with cysteine in the N-terminus of the ERF transcription factor is catalyzed by another enzyme. The question also arises as to whether the oxygen sensor is able to control all metabolic adaptations. There may be not just one but several different oxygen sensor systems in plants, similar to those already known for animals. Another aspect concerns the possibility of putting the new findings into practice in order to better protect crops in the future against the effects of oxygen deficiency in waterlogging or flooding and thereby significantly reduce harvest losses.
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