Seasonal Changes in the Activities of Vacuolar H⁺-pumps and their Gene Expression in the Developing Japanese Pear Fruit

Abstract

Vacuolar H⁺-ATPase (V-ATPase) and vacuolar H⁺-inorganic pyrophosphatase (V-PPase) generate an electrochemical gradient across the vacuolar membrane, which is indispensable for accumulating sugars into the vacuole. We investigated the enzymatic and transcriptional changes of V-ATPase and V-PPase in Japanese pear fruit to establish their roles in fruit development. As fruit developed, the V-ATPase and V-PPase activities increased on the total protein basis. The V-PPase activity was higher than that of V-ATPase throughout the developmental period. Immunoblot analyses indicated that the changes in the polypeptides for the catalytic subunit of V-ATPase and V-PPase generally paralleled the enzymatic activities. Furthermore, seasonal changes in the mRNA levels generally corresponded with those of polypeptides except at the cell division stage. Our findings suggest that V-PPase, more than V-ATPase, plays an important role throughout the development of Japanese pear fruit and that V-ATPase and V-PPase are regulated at the transcriptional level during the cell elongation stage.