Sugar beet PMT5a and STP13 carriers suitable for proton‐driven plasma membrane sucrose and glucose import in taproots

Sugar beet (Beta vulgaris) is the major sugar-producing crop in Europe and Northern America, as the taproot stores sucrose at a concentration of around 20%. Genome sequence analysis together with biochemical and electrophysiological approaches led to the identification and characterization of the TST sucrose transporter driving vacuolar sugar accumulation in the taproot. However, the sugar transporters mediating sucrose uptake across the plasma membrane of taproot parenchyma cells remained unknown. As with glucose, sucrose stimulation of taproot parenchyma cells caused inward proton fluxes and plasma membrane depolarization, indicating a sugar/proton symport mechanism. To decipher the nature of the corresponding proton-driven sugar transporters, we performed taproot transcriptomic profiling and identified the cold-induced PMT5a and STP13 transporters. When expressed in Xenopus laevis oocytes, BvPMT5a was characterized as a voltage- and H