Unlike K+ and Mg2+, Ca2+ does not activate many enzymes (19), and its concentration in the cytoplasm is kept low. This calcium homeostasis is achieved by the action of membrane-bound, calcium- dependent ATPases that actively pump Ca2+ ions from the cytoplasm and into the vacuoles, the endoplasmic reticulum (ER), and the mitochondria (20). This process prevents the ion from competing with Mg2+, thereby lowering activity of some enzymes; the action prevents Ca2+ from inhibiting cytoplasmic or chloroplastic enzymes such as phosphoenol pyruvate (PEP) carboxylase (21) and prevents Ca2+ from precipitating inorganic phosphate (22).
Other sensors of calcium concentration are in the cytoplasm, for example, Ca2+-dependent (CaM-independent) protein kinases (25). The rapid increases in cytoplasmic Ca2+ concentration that occur when the channels open and let calcium out of the vacuolar store and the magnitude, duration, and precise location of these increases give a series of calcium signatures that are part of the responses of a plant to a range of environmental signals. These responses enable the plant to respond to drought, salinity, cold shock, mechanical stress, ozone and blue light, ultraviolet radiation, and other stresses (24).
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