Changes in tonoplast and its proteins with the development of pear fruit

To clarify the transport system of tonoplast and its roles in fruit development, we studied the changes in the nature of tonoplast during the development of pear fruit. The density of the tonoplast of young fruit was 1.12 g/ml and became lighter with fruit development. The phospholipid amount on protein basis increased with fruit development, suggesting that the change in the density of tonoplast was caused by the change in the ratio of phospholipid to protein. In addition, we determined the levels of two vacuolar proton pumps (V-ATPase and V-PPase) and a protein analogous to water channel (VM23P) by immunoblot analysis. The level of V-ATPase increased with fruit development, while, V-PPase level was particularly high at the cell-division stage in young fruit and remained almost the same at other stages. The level of VM23P was especially high during the active cell-expansion stage, and it might play an important role in the rapid expansion of cells as a vacuolar water channel. Further, we isolated the protein of 94 kDa, the major tonoplast protein in pear which function is unclear, and an antibody was raised against it. The level of TP94 was relatively low in young fruit and became higher in the tonoplasts at the middle and the end of fruit development.