| Regional Biophysics Meeting 2005, March 16-20, Zreče, Slovenia | [MembBiophys] |
Phloem translocation is a highly significant process which guarantees the efficient distribution of photosynthetic energy and carbon throughout vascular plants. The phloem network, assembled by nucleus- and vacuole-free interconnected living cells, is thus as essential for plants as the vascular system for humans. It represents a long-distance transport pathway for nutrients and information. According to the Münch hypothesis, sucrose and other osmolytes generate the hydrostatic pressure which drives nutrient and water flow between the source and the sink phloem. Although proton-coupled sucrose carriers have been localized in both source and sink tissues, the molecular representatives and the mechanism of the sucrose phloem efflux is still scant. In this work ZmSUT1, a maize sucrose/H+ symporter localised in the phloem, was expressed in Xenopus oocytes and the transport characteristics of the carrier were studied by electrophysiological methods. Using the double-electrode voltage-clamp and the patch-clamp techniques (in the giant inside-out patch mode) both the sucrose internal and external affinity constants of the transporter were measured. By modifying the chemical and electrochemical gradients across the sucrose carrier the reversibility of ZmSUT1 was demonstrated. It was shown that the carrier can mediate both the sucrose uptake into oocytes as well as the desorption of sugar from the oocytes. Finally, by a pure thermodynamic method, a sucrose proton 1:1 stoichiometry of the transporter was determined.
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