Plant proton-translocating pyrophosphatases (H+-PPases) are generally thought of as tonoplast-localized proton pumps that utilize the energy released by hydrolysis of the phosphoanhydride bond of pyrophosphate (PPi) to assist in the acidification of the vacuole.It has been suggested that H+-PPases function as an additional acidification mechanism to complement the ubiquitous V-ATPase,especially in young,growing tissues where PPi is abundant and ATP is depleted (Shiratake et al.,1997).