Diurnal changes in sucrose, nucleotides, starch synthesis and AGPS transcript in growing potato tubers that are suppressed by decreased expression ofsucrose phosphate synthase

Sucrose export from leaves is high during the day and lower at night, when it depends on starch remobilisation. We have investigated the consequences of diurnal changes of photoassimilate supply for starch synthesis and other metabolic processes in growing potato tubers. Sucrose, the levels of the t ranscripts for SUS and AGPS, the levels of key metabolites, and the rate of synthesis of starch and other major end products, including protein and ce ll wall polysaccharides, increased twofold or more between the start and en d of the light period. The stimulation of starch synthesis was accompanied by an increase of UDPglucose and ADPglucose, whereas glycolytic intermediat es remained unaltered, revealing that sucrose synthase and ADP-glucose pyro phosphorylase are being co-ordinately regulated. Sucrose synthase is stimul ated via an increase of its substrates, UDP and sucrose. UDP increases due to an increase of the overall uridine nucleotide pool, and a decrease of th e UTP/UDP ratio that occurs in parallel with a decrease of the ATP/ADP rati o and adenylate energy charge when biosynthetic fluxes are high at the end of the day. Within the time frame of the diurnal changes, the changes in th e SUS and AGPS transcript levels do not lead to significant changes in the encoded enzymes. Transformants with a progressive decrease of sucrose phosp hate synthase expression, where diurnal changes in leaf sugar levels were d amped, exhibited a progressive attenuation of the diurnal changes of sucros e, nucleotide sugars and nucleotides, and fluxes in their tubers. it is con cluded that metabolic processes in tubers are tightly linked to the momenta ry supply of sucrose.