Expression of genes coding for sugar transporters and sugar metabolizing enzymes during dynamic fructan metabolism in chicory (Cichorium intybus L.).

Carbohydrate partitioning and intracellular sugar transport are central to plant energy management, stress adaptation, and storage metabolism. In fructan-accumulating species such as Cichorium intybus, however, the underlying transport processes remain poorly understood. Because fructan metabolism occurs in the vacuole, coordinated regulation of tonoplast transporters and fructan-metabolizing enzymes is required to control sugar fluxes across the vacuolar membrane. Here, we analyzed the expression of genes encoding sugar transporters, including TSTs, ERDL6, SUC4, SWEETs, and SUT1, during vegetative growth, cold stress, and flowering, together with key genes involved in fructan metabolism. The predicted sucrose transporter CiTST2.1 was consistently upregulated and showed expression patterns closely associated with fructan biosynthetic genes. Their vacuolar localization and sucrose-transport activity of CiTST2.1 and CiTST2.2 were experimentally confirmed. Notably, CiTST2.1 expression closely paralleled that of the vacuolar hexose exporters CiERDL6.1 and CiERDL6.2, providing now a mechanistic framework for sugar fluxes across the tonoplast during fructan accumulation in the vegetative growth phase. In contrast, cold exposure and floral transition induced the expression of genes associated with fructan degradation (1-FEH I and II) and sucrose allocation (SWEET16, SUC4, and SUT1) in roots, suggesting carbon remobilization to support reproductive sink tissues. Together, these findings reveal the coordinated regulation of sugar transport and fructan metabolism underlying fructan accumulation and remobilization in chicory roots across developmental and environmental transitions.