A coordinated switch in sucrose and callose metabolism enables enhanced symplastic unloading in potato tubers

Abstract

One of the early changes upon tuber induction is the switch from apoplastic to symplastic unloading. Whether and how this change in unloading mode contributes to sink-strength has remained unclear. In addition, developing tubers also change from energy to storage-based sucrose metabolism. Here we investigated the coordination between changes in unloading mode and sucrose metabolism and their relative role in tuber sink strength by looking into callose and sucrose metabolism gene expression combined with a model of apoplastic and symplastic unloading. Decreased callose deposition in tubers is driven by decreased callose synthase activity. Furthermore, changes in callose metabolism and sucrose metabolism are strongly correlated, indicating a well-coordinated developmental switch. Modelling indicates that symplastic unloading is not the most efficient unloading mode per se. Instead, it is the concurrent metabolic switch that provides the physiological conditions necessary to potentiate symplastic transport and thereby enhance tuber sink strength.