The apoplast of higher terrestrial plants represents an essential storage and reaction space that connects cellular tissues, engages in transport of matter, enzyme catalysis and defense reactions, and also in gas exchange and storage of oxygen . Since the apoplast is that extracellular space within the plant through which energy-rich compounds have to pass before they are taken up into the cells through H+-symport. A fast and typical response to anoxia/hypoxia is the rapid acidification of the cytoplasm, which in many cases ranges around half a pH unit. For instance, the carbohydrate reserves that are recycled to be broken down in glycolysis and fermentation have to pass the apoplast from the site of storage to the cytoplasm. In case the apoplastic ionic milieu (especially the pH) has changed, the scarce energy that could be gained from fermentation cannot be harvested accordingly, with the consequence that the time span of anoxia tolerance will decline rapidly. Apoplastic pH is one of the most important parameters for transmembrane transport of organic matter. Organic compounds like sugars, amino acids, etc. are driven from the apoplast into the cytoplasm by H+ symport which critically depends on the so-called proton motive force (pmf).
The evolutionary origin of membranes propitiated the capability for regulating the cytosol pH, and so, cytosol proteins could evolve to a optimized pH. pH regulation is done by a membrane ATPase, which primary role would be this internal pH regulation. The pH homeostasis is obtained with the expense of ATP, and the consequence is the cell wall acidification: cytosol has a pH usually 7.2-7.4 and cell wall something near 5.5. This implicates in a concentration difference near 100X, and generates the proton motive force. Another consequence is that other protons, like K and Na will be attracted to the cytosol by this difference in the electric charge. Both are small molecules and can cross the membrane accumulating in the cytosol. K will accumulate up to 74X, but Na cannot, and it will be necessary to be carried out, expending cell energy. So, one important effect of salinization is the expenditure of cell energy resources for driving Na out. In anoxia, as ATP disappears, pH cannot be controlled anymore. Cytosol pH decreases and cell wall ph increases. PMF disappears and K leak.
In general cell wall acidification is so crucial for cell elongation (i.e., acid growth hypothesis), and I think the case is hold true for plant growth under salinity stress. But, as far as I know no publications correlate wall acidification with salt tolerance. Plasma membrane proton pump causes wall acidification via its pumping of hydrogen from the cytosol into the cell wall. You can read more on that on Mansour MMF 2014. The plasma membrane transport systems and adaptation to salinity. J. Plant Physiol. in press. The review is currently published online.
Thanks for reply but the question is : Cytosol has a pH usually 7.2-7.4 and cell wall something near 5.5. Q: What regulates cytosolic pH and which enzymes are involved in stress response
What regulates the cytosolic pH is the activity of the plasma membrane ATPase (proton pump), since its main function is to transport H+ from the cytosol into the cell wall. Thus, it maintains the cytosolic pH around neutral. This is one key role of the plasma membrane ATPase in the cell. For enzymes involved in stress response, I refer you to the review article by Mansour MMF 2014. The plasma membrane transport systems and adaptation to salinity. J. Plant Physiol. in press, but the review is currently published on line.
If you are asking about other cellular enzymes, Wyn Jones and others since seventies of the last century refuted the determinant role of enzymes in adaptation to saline conditions. They isolated several different enzymes in saline and non saline conditions and compare their responses to salinity in plants contrasting in salt tolerance. No difference in their response to salinity has been found in salt sensitive and tolerant plants.
Greetings!
Since according to acid growth considerations apoplastic pH is an important factor in elongation growth, we suggest that this pH increase is a main cause for reduced leaf growth under salt stress conditions.
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