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. 1990 Nov;94(3):1244–1249. doi: 10.1104/pp.94.3.1244

Phloem Loading in Coleus blumei in the Absence of Carrier-Mediated Uptake of Export Sugar from the Apoplast 1

Robert Turgeon 1, Esther Gowan 1
PMCID: PMC1077369  PMID: 16667824

Abstract

Phloem loading in Coleus blumei Benth. leaves cannot be explained by carrier-mediated transport of export sugar from the apoplast into the sieve element-companion cell complex, the mechanism by which sucrose is thought to load in other species that have been studied in detail. Uptake profiles of the export sugars sucrose, raffinose, and stachyose into leaf discs were composed of two components, one saturable and the other not. Saturable (carrier-mediated) uptake of all three sugars was almost completely eliminated by the inhibitor p-chloromercuribenzenesulfonic acid (PCMBS). However, when PCMBS was introduced by transpiration into mature leaves it did not prevent accumulation of 14C-photosynthate in minor veins or translocation of labeled photosynthate from green to nonchlorophyllous regions of the leaf following exposure to 14CO2. The efficacy of introducing inhibitor solutions in the transpiration stream was proven by observing saffranin O and calcofluor white movement in the minor veins and leaf apoplast. PCMBS introduced by transpiration completely inhibited phloem loading in tobacco leaves. Phloem loading in C. blumei was also studied in plasmolysis experiments. The carbohydrate content of leaves was lowered by keeping plants in the dark and then increased by exposing them to light. The solute level of intermediary cells increased in the light (phloem loading) in both PCMBS-treated and control tissues. A mechanism of symplastic phloem loading is proposed for species that translocate the raffinose series of oligosaccharides.

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.

  1. Bourquin S., Bonnemain J. L., Delrot S. Inhibition of Loading of C Assimilates by p-Chloromercuribenzenesulfonic Acid : Localization of the Apoplastic Pathway in Vicia faba. Plant Physiol. 1990 Jan;92(1):97–102. doi: 10.1104/pp.92.1.97. [DOI] [PMC free article] [PubMed] [Google Scholar]
  2. Giaquinta R. Evidence for Phloem loading from the apoplast: chemical modification of membrane sulfhydryl groups. Plant Physiol. 1976 Jun;57(6):872–875. doi: 10.1104/pp.57.6.872. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Hendrix J. E. Phloem loading in squash. Plant Physiol. 1977 Oct;60(4):567–569. doi: 10.1104/pp.60.4.567. [DOI] [PMC free article] [PubMed] [Google Scholar]
  4. Madore M. A. Carbohydrate Metabolism in Photosynthetic and Nonphotosynthetic Tissues of Variegated Leaves of Coleus blumei Benth. Plant Physiol. 1990 Jun;93(2):617–622. doi: 10.1104/pp.93.2.617. [DOI] [PMC free article] [PubMed] [Google Scholar]
  5. Madore M. A., Mitchell D. E., Boyd C. M. Stachyose Synthesis in Source Leaf Tissues of the CAM Plant Xerosicyos danguyi H. Humb. Plant Physiol. 1988 Jul;87(3):588–591. doi: 10.1104/pp.87.3.588. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Madore M. A., Oross J. W., Lucas W. J. Symplastic Transport in Ipomea tricolor Source Leaves : Demonstration of Functional Symplastic Connections from Mesophyll to Minor Veins by a Novel Dye-Tracer Method. Plant Physiol. 1986 Oct;82(2):432–442. doi: 10.1104/pp.82.2.432. [DOI] [PMC free article] [PubMed] [Google Scholar]
  7. Maynard J. W., Lucas W. J. A Reanalysis of the Two-Component Phloem Loading System in Beta vulgaris. Plant Physiol. 1982 Mar;69(3):734–739. doi: 10.1104/pp.69.3.734. [DOI] [PMC free article] [PubMed] [Google Scholar]
  8. Spurr A. R. A low-viscosity epoxy resin embedding medium for electron microscopy. J Ultrastruct Res. 1969 Jan;26(1):31–43. doi: 10.1016/s0022-5320(69)90033-1. [DOI] [PubMed] [Google Scholar]
  9. Sun D., Wimmers L. E., Turgeon R. Scintillation counting of 14C-labeled soluble and insoluble compounds in plant tissue. Anal Biochem. 1988 Mar;169(2):424–427. doi: 10.1016/0003-2697(88)90307-7. [DOI] [PubMed] [Google Scholar]
  10. Turgeon R., Wimmers L. E. Different Patterns of Vein Loading of Exogenous [C]Sucrose in Leaves of Pisum sativum and Coleus blumei. Plant Physiol. 1988 May;87(1):179–182. doi: 10.1104/pp.87.1.179. [DOI] [PMC free article] [PubMed] [Google Scholar]

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