Plant Physiology Preview
Published on June 24, 2005; 10.1104/pp.104.058511
Received January 28, 2005
Returned for revision April 12, 2005
Accepted April 12, 2005
Thermodynamic Battle for Photosynthate Acquisition between Sieve Tubes and Adjoining Parenchyma in Transport Phloem
Jens B. Hafke , Jan-Kees van Amerongen , Frits Kelling , Alexandra C.U. Furch , Frank Gaupels , and Aart J.E. van Bel *
Plant Cell Biology Research Group, Institute of General Botany, Justus-Liebig University, 35390 Giessen, Germany
Transport Physiology Research Group, Department of Plant Ecology and Evolutionary Biology, University of Utrecht, 3584 CA Utrecht, The Netherlands
Plant Cell Biology Research Group, Institute of General Botany, Justus-Liebig University, 35390 Giessen, Germany; Institute of Phytopathology, Justus-Liebig University, Interdisciplinary Research Center for Environmental Sciences, 35392 Giessen, Germany
Plant Cell Biology Research Group, Institute of General Botany, Justus-Liebig University, 35390 Giessen, Germany; Transport Physiology Research Group, Department of Plant Ecology and Evolutionary Biology, University of Utrecht, 3584 CA Utrecht, The Netherlands
* Corresponding author; email: aart.v.bel{at}bot1.bio.uni-giessen.de.
In transport phloem, photoassimilates escaping from the sieve tubes are released into the apoplasmic space between sieve element (SE)/companion cell (CC) complexes (SE/CCs) and phloem parenchyma cells (PPCs). For uptake respective retrieval, PPCs and SE/CCs make use of plasma membrane translocators energized by the proton motive force (PMF). Their mutual competitiveness, which essentially determines the amount of photoassimilates translocated through the sieve tubes, therefore depends on the respective PMFs. We measured the components of the PMF, membrane potential and  pH, of SE/CCs and PPCs in transport phloem. Membrane potentials of SE/CCs and PPCs in tissue slices as well as in intact plants fell into two categories. In the first group including apoplasmically phloem-loading species (e.g. Vicia, Solanum), the membrane potentials of the SEs are more negative than those of the PPCs. In the second group including symplasmically phloem-loading species (e.g. Cucurbita, Ocimum), membrane potentials of SEs are equal to or slightly more positive than those of PPCs. Pure sieve tube sap collected from cut aphid stylets was measured with H+-selective microelectrodes. Under our experimental conditions, pH of the sieve tube saps was around 7.5, which is comparable to the pH of cytoplasmic compartments in parenchymatous cells. In conclusion, only the membrane potential appears to be relevant for the PMF-determined competition between SE/CCs and PPCs. The findings may imply that the axial sinks along the pathway withdraw more photoassimilates from the sieve tubes in symplasmically loading species than in apoplasmically loading species.
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