Abstract
Sugar transporters effect movement of sugars across cellular membranes and play a critical
role in relocalizing carbon substrates within cells, tissues, and organs to support their metabolism.
Sugar transporters are especially important for postharvest sugarbeet roots since
the export of sucrose from the vacuole of parenchymal storage cells not only fuels root
metabolism but also drives sucrose loss during storage. Despite the obvious importance of
sugar transporters to sugarbeet root postharvest metabolism, the identity and expression of
sugarbeet root sugar transporters during storage have never been examined. Therefore, the
expression of sugar transporters in harvested and stored sugarbeet roots was determined with
respect to storage duration and temperature and in genotypes that likely differed in rates of
postharvest sucrose utilization due to differences in respiration rate. Highly and differentially
expressed sugar transporters largely belonged to the SWEET (sugars will eventually be
exported transporters) and TST (tonoplast sugar transporters) classes of sugar transporters.
Eight SWEET genes and two TST genes were expressed in postharvest sugarbeet roots with
expression of SWEET and TST genes generally increasing with time in storage, but only
minimally affected by storage temperature. SWEET N3 and TST 1 were the most highly expressed
and upregulated sugar transporter genes during storage and were also differentially
expressed in lines with genetic differences in storage respiration rate. Overall, these results
highlight the likely importance of SWEET and TST genes for postharvest sugarbeet root
metabolism and identify gene candidates that may have roles in storage sucrose loss.