Sugar beet cold-induced PMT5a and STP13 carriers are poised for taproot proton-driven plasma membrane sucrose and glucose import

As the major sugar-producing crop in Europe and North America, sugar beet (Beta vulgaris) taproots store sucrose at a concentration of about 20 %. While the TST sucrose transporter, which drives vacuolar sugar accumulation in the taproot, has already been identified, sugar transporters mediating sucrose uptake across the plasma membrane of taproot parenchyma cells remained unknown. We electrophysiologically examined taproots for proton-coupled sugar uptake and identified potentially involved transporters by transcriptomic profiling. After cloning, the transporter features were studied in the heterologous Xenopus laevis oocyte expression system using the two-electrode voltage clamp technique. Insights into the structure was gained by 3D homology modeling. As with glucose, sucrose stimulation of taproot parenchyma cells caused inward proton fluxes and plasma membrane depolarization, indicating a sugar/proton symport mechanism. As one potential candidate for sugar uploading, the BvPMT5a was characterized as a H+-driven low-affinity glucose transporter, which does not transport sucrose. BvSTP13 operated as a high-affinity H+/sugar symporter, transporting glucose and to some extent sucrose due to a binding cleft plasticity, and was more cold-resistant than BvPMT5a. Identification of BvPMT5a and BvSTP13 as taproot sugar transporters could improve breeding of cold-tolerant sugar beet to provide a sustainable energy crop.