A possible role for pyrophosphate in the coordination of cytosolic and plastidial carbon metabolism within the potato tuber

The early stages of tuber development are characterized by cell division, h igh metabolic activity, and the predominance of invertase as the sucrose (S uc) cleaving activity. However, during the subsequent phase of starch accum ulation the cleavage of Suc occurs primarily by the action of Suc synthase. The mechanism that is responsible for this switch in Suc cleaving activiti es is currently unknown. One striking difference between the invertase and Suc synthase mediated cleavage of Suc is the direct involvement of inorgani c pyrophosphate (PPi) in the latter case. There is presently no convincing explanation of how the PPi required to support this process is generated in potato (Solanum tuberosum) tubers. The major site of PPi production in a m aturing potato tubers is likely to be the reaction catalyzed by ADP-glucose pyrophosphorylase, the first committed step of starch biosynthesis in amyl oplasts. We present data based on the analysis of the PPi levels in various transgenic plants altered in starch and Suc metabolism that support the hy pothesis that PPi produced in the plastid is used to support cytosolic Suc breakdown and that PPi is an important coordinator of cytosolic and plastid ial metabolism in potato tubers.