OVEREXPRESSION OF PYROPHOSPHATASE LEADS TO INCREASED SUCROSE DEGRADATION AND STARCH SYNTHESIS, INCREASED ACTIVITIES OF ENZYMES FOR SUCROSE-STARCH INTERCONVERSIONS, AND INCREASED LEVELS OF NUCLEOTIDES IN GROWING POTATO-TUBERS

Overexpression of inorganic pyrophosphatase (PPase) from Escherichia c oli in the cytosol of plants (ppal plants) leads to a decrease of inor ganic pyrophosphate (PPi; U. Sonnewald, 1992, Plant J 2. 571-581). The consequences for sucrose-starch interconversions have now been studie d in growing potato (Solanum tuberosum L. cv. Desiree) tubers. Sucrose is degraded via sucrose synthase and UDP-glucose pyrophosphorylase in growing tubers, and it was expected that the low PPi in the ppal tran sformants would restrict the mobilisation of sucrose and conversion to starch. Over-expression of PPase resulted in an accumulation of sucro se and UDP-glucose, and decreased concentrations of hexose phosphates and glycerate-3-phosphate in growing ppal tubers. Unexpectedly, the ra te of degradation of [C-14] sucrose was increased by up to 30%, the ra te of starch synthesis was increased, and the starch content was incre ased by 20-30% in ppal tubers compared to wildtype tubers. Reasons for this unexpectedly efficient conversion of sucrose to starch in the pp al tubers were investigated. (i)The transformed tubers contained incre ased activities of several enzymes required for sucrose-starch interco nversions including two- to threefold more sucrose synthase and 60% mo re ADP-glucose pyrophosphorylase. They also contained 30-100% increase d activities of several glycolytic enzymes and amylase, increased prot ein, and unaltered or slightly decreased starch phosphorylase, acid in vertase and mannosidase. (ii) The transformants contained higher pools of uridine nucleotides. As a result, although the UDP-glucose pool is increased two- to threefold, this does not lead to a decrease of UTP or UDP. (iii) The transformants contained twofold larger pools of ATP and ADP, and ADP-glucose was increased by up to threefold. In stored p pal tubers, there were no changes in the activities of glycolytic enzy mes, and nucleotides did not increase. It is concluded that in growing tubers PPi has a wider significance than just being an energy donor f or specific reactions in the cytosol. Increased rates of PPi hydrolysi s also affect general aspects of cell activity including the levels of nucleotides and protein. Possible ways in which PPi hydrolysis could affect these processes are discussed.