Effects of Aging on the ATP-and Pyrophosphate-dependent Pumping of Protons across the Tonoplast Isolated from Pumpkin Cotyledons

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Tonoplast vesicles were isolated from 7- to 26-day-old pumpkin cotyledons by an improved floating method, and the activities of pyrophosphatase (PPase) and adenosine triphosphatase (ATPase) in tonoplast vesicles, as well as rates of PPase- and ATPase-dependent pumping of pro-tons across tonoplast vesicles, were measured. PPase activity and the rate of pyrophosphate-dependent proton-pumping decreased more rapidly than loss of chlorophyll from cotyledons, and the pumping on day 14 was only 10% of that on day 7, while PPase activity was still more than 30% of that on day 7. By contrast, ATPase activity and the rate of ATP-dependent proton-pumping increased until day 14. In this latter case, the changes in both activity and pumping were not major and were parallel to one another until day 21 . However, a rapid decrease was observed only in the rate of pumping on day 26, at which time an apparent loss of fresh weight was observed in cotyledons. The relationship between the aging of pumpkin cotyledons and functional changes in vacuoles is discussed in terms of ATP- and pyrophosphate-dependent proton-pumping across the tonoplast. The two proton pumps in the tonoplast, H^+-ATPase and H^+-pyrophosphatase, ap-pear to play different roles during the growth and senescence of pumpkin cotyledons.
日本植物生理学会の論文