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Related Experiment Videos

Phloem loading in the tulip tree. Mechanisms and evolutionary implications.

F L Goggin1, R Medville, R Turgeon

  • 1Department of Plant Biology, Cornell University, Ithaca, New York 14853, USA.

Plant Physiology
|February 13, 2001
PubMed
Summary

This study on tulip tree phloem loading suggests an apoplastic component is involved, challenging theories of purely symplastic loading in basal angiosperms. Evidence indicates sucrose transport utilizes a sucrose-proton cotransporter.

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Area of Science:

  • Plant Biology
  • Plant Physiology
  • Angiosperm Research

Background:

  • Phloem loading mechanisms in basal angiosperms are debated, with speculation favoring entirely symplastic pathways.
  • Understanding phloem loading is crucial for plant nutrient transport and development.

Purpose of the Study:

  • To investigate the ultrastructure of minor veins and phloem loading mechanisms in Liriodendron tulipifera (tulip tree).
  • To determine if phloem loading in this species involves symplastic or apoplastic pathways.

Main Methods:

  • Microscopic analysis of minor vein ultrastructure and plasmodesmatal frequencies.
  • Sorbitol-induced plasmolysis experiments to assess solute accumulation in phloem cells.
  • Autoradiography using [(14)C]sucrose and (14)C-labeled photo-assimilates.

Related Experiment Videos

  • Inhibition studies using p-chloromercuribenzenesulfonic acid (PCMB), a sucrose-proton cotransporter inhibitor.

    • Main Results:

    • Higher plasmodesmatal frequencies were observed compared to known apoplastic loaders, but companion cells lacked specialized "intermediary cell" features.
    • Sieve elements and companion cells resisted plasmolysis in high sorbitol concentrations, indicating active solute accumulation.
    • Autoradiography confirmed [(14)C]sucrose and photo-assimilates accumulate in minor veins.
    • PCMB inhibited [(14)C]sucrose accumulation and largely, but not completely, inhibited photo-assimilate exudation.

    Conclusions:

    • The findings support the presence of an apoplastic component in phloem loading in Liriodendron tulipifera.
    • This contradicts the hypothesis that basal angiosperms exclusively use symplastic phloem loading mechanisms.
    • Sucrose-proton cotransport appears to play a role in the apoplastic loading pathway.