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Ratio of sugar concentrations in the phloem sap and the cytosol of mesophyll cells in different tree species as an indicator of the phloem loading mechanism

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Abstract

Main conclusion

Sucrose concentration in phloem sap was several times higher than in the cytosol of mesophyll cells. The results suggest that phloem loading involves active steps in the analyzed tree species.

Phloem loading in source leaves is a key step for carbon partitioning and passive symplastic loading has been proposed for several tree species. However, experimental evidence to prove the potential for sucrose diffusion from mesophyll to phloem is rare. Here, we analyzed three tree species (two angiosperms, Fagus sylvatica, Magnolia kobus, and one gymnosperm, Gnetum gnemon) to investigate the proposed phloem loading mechanism. For this purpose, the minor vein structure and the sugar concentrations in phloem sap as well as in the subcellular compartments of mesophyll cells were investigated. The analyzed tree species belong to the open type minor vein subcategory. The sucrose concentration in the cytosol of mesophyll cells ranged between 75 and 165 mM and was almost equal to the vacuolar concentration. Phloem sap could be collected from F. sylvatica and M. kobus and the concentration of sucrose in phloem sap was about five- and 11-fold higher, respectively, than in the cytosol of mesophyll cells. Sugar exudation of cut leaves was decreased by p-chloromercuribenzenesulfonic acid, an inhibitor of sucrose–proton transporter. The results suggest that phloem loading of sucrose in the analyzed tree species involves active steps, and apoplastic phloem loading seems more likely.

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Abbreviations

BSC:

Bundle sheath cell

CC:

Companion cell

MC:

Mesophyll cell

SE:

Sieve element

PCMBS:

p-Chloromercuribenzenesulfonic acid

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Acknowledgements

The authors would like to thank Elisabeth Wesbuer and Tim Kreutzer for technical assistance, as well as Sarah Rau for help with the electron micrographs. We are grateful to Rosi Ritter and Christoph Senges for critical reading of the manuscript.

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  1. Daniel Fink and Elena Dobbelstein contributed equally to this work.

Authors and Affiliations

  1. Molecular Plant Science/Plant Biochemistry, University of Wuppertal, Wuppertal, Germany

    Daniel Fink, Elena Dobbelstein & Gertrud Lohaus

  2. Core Facility Electron Microscopy, UKD, Heinrich Heine University Düsseldorf, Düsseldorf, Germany

    Andreas Barbian

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  1. Daniel Fink
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  2. Elena Dobbelstein
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  3. Andreas Barbian
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  4. Gertrud Lohaus
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Correspondence to Gertrud Lohaus.

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Fink, D., Dobbelstein, E., Barbian, A. et al. Ratio of sugar concentrations in the phloem sap and the cytosol of mesophyll cells in different tree species as an indicator of the phloem loading mechanism. Planta 248, 661–673 (2018). https://doi.org/10.1007/s00425-018-2933-7

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