Regulation of sucrose to starch conversion in growing potato tubers.

Growing potato tubers have been used as a model system to investigate the regulation of starch synthesis. Results indicate that sucrose degradation and starch synthesis are controlled via regulatory signals in response to sucrose and oxygen availability. (i) Sucrose leads to a co-ordinated up-regulation of sucrose synthase and ADP-glucose phosphorylase at the transcriptional and post-transcriptional level. Transcriptional regulation of ADP-glucose phosphorylase leads to rapid changes in transcript levels, but relatively slow changes in protein levels. The rapid regulation of this enzyme in response to sucrose is mediated by a novel mechanism, involving redox-activation of ADPGlc pyrophosphorylase. Sucrose synthase is regulated via transcriptional regulation, but again the resulting changes in enzyme activity occur relatively slowly. More rapid changes in the flux of this enzyme follow due to rapid changes in the levels of uridine nucleotides. (ii) Internal oxygen concentrations fall to low levels in growing tubers, triggering a restriction of respiration, a decrease in the adenylate energy status, and a widespread decrease in metabolic and biosynthetic activity. These metabolic adaptations will allow oxygen consumption to be decreased and prevent the tissue from becoming anoxic. It will be discussed how these factors interact at different levels and different time-scales of control to regulate tuber metabolism in response to physiological and environmental inputs.