Studies on Pi homeostasis by artificial manipulation of intracellular Pi levels.

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Inorganic phosphate is one of the essential molecules for plant growth. In natural environment, however, Pi level in soil is extremely low. Even under Pi deficiency, it is well-known that the cytoplasmic Pi level was kept at constant by changing Pi transport activities across the membrane, although the vacuolar Pi level changed related to Pi supply. We call it is Pi homeostasis in plants. However, we have no information, yet how the plant cell detects changes in Pi supply and controls Pi status in the cell. Until now, most of works on the cellular Pi transport mechanisms related to Pi supply were carried out using growing plants or proliferating cultured cells as experimental materials. In these situation, since the cellular physiological status is always changing, it is difficult to analyze Pi homeostatic mechanisms only. In the present study, we tried to control Pi situation independent of cell growth.First we measured the Pi distribution of barley plants under different Pi situation in order to know the usual experimental conditions. Then we used an isolated and matured Chara internodal cells as material. They do not grow, divide and of course differentiate. Using such cells, we have found first that the removal of Pi from the vacuole by perfusion could not induce any changes in Pi transport activity of the tonoplast. Furthermore, we found that the Pi uptake activity of the plasma membrane also changes independent of both the cytoplasmic and the vacuolar Pi pools.Besides above, we found that the acid treatment of suspension cultured Catharanthus cells induced changes in the Pi metabolisms. We are now analyzing the relationship of all these phenomena.