Molecular mechanism and regulation of development of vacuoles during elongation of plant cells.

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In the cells of higher plants, vacuoles function as storage organelles, as hydrolytic compartments, and as controllers of turgor pressure. The space-filling role of the vacuoles is also important and is essential to the growth of plant cells. The increase in cell volume is accounted for by enlargement of the vacuole. Thus it is impossible to discuss the cell growth without a full understanding of the biogenesis and development of vacuoles. In this study, the vacuolar H^+-ATPase and H^+-pyrophosphatase were investigated as markers of the vacuolar membranes. These two proton pumps transport protons into vacuoles and maintain their acidity.Vacuolar H^+-ATPase of mung bean consists of nine subunits, while the H^+-pyrophosphatase consists of a single polypeptide of 73,000 daltons. The following results have been obtained.(1) The vacuolar ATPase and H^+-pyrophosphatase were detected in all plant species investigated including green alga, moss and fern. The molecular masses and molecular activities of the pyrophosphatase were similar to the mung bean enzyme.(2) The content of the pyrophosphatase in the vacuolar membrane decreased after maturation of cells, although the amount of the ATPase was constant. Therefore, the regulatory mechanisms of synthesis may be different between the ATPase and H^+-pyrophosphatase.(3) A major integral protein (VM23) which is very hydrophobic was found in vacuolar membrane. From the data concerning the amino- terminal sequence and reactivity to inhibitor such as DCCD (dicylohexylcarbodiimide), VM23 has been estimated as a water channel or ion transporter.