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Symplasmic transport and phloem loading in gymnosperm leaves
Johannes Liesche1, Helle Juel Martens, Alexander Schulz
1Department of Plant Biology and Biotechnology, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Denmark.
Protoplasma
|November 26, 2010
Summary
Phloem loading in gymnosperms remains poorly understood due to unique leaf structures. This review highlights the symplasmic pathway and the bundle sheath
Area of Science:
- Plant Biology
- Plant Physiology
- Botany
Background:
- Phloem loading in gymnosperms is complex and not fully understood after over 130 years of research.
- Gymnosperm leaves possess a unique architecture, including a transfusion tissue, distinguishing them from angiosperm leaves.
- The precise mechanisms of photoassimilate transport and loading in gymnosperms require further investigation.
Purpose of the Study:
- To review and synthesize existing literature on pre-phloem transport and phloem loading in gymnosperms.
- To identify critical control points and experimental approaches for understanding gymnosperm phloem loading.
- To compare gymnosperm phloem loading with known mechanisms in angiosperms.
Main Methods:
- Literature review of accessible and neglected studies on gymnosperm leaf anatomy and transport.
- Analysis of structural characteristics of the pre-phloem pathway, including plasmodesmata and cell connections.
- Identification of key cellular structures, such as the bundle sheath and transfusion tissue, involved in transport.
Main Results:
- Gymnosperm pre-phloem transport involves a structurally rich symplasmic pathway with numerous plasmodesmata from mesophyll to sieve elements.
- The bundle sheath in gymnosperms acts as a controlled barrier, similar to an endodermis, regulating apoplasmic transport.
- Structural modifications like median cavities and branching suggest secondary alterations to plasmodesmata during growth.
Conclusions:
- The extensive symplasmic pathway in gymnosperms suggests a potential for symplasmic loading, but functional tests are needed.
- The bundle sheath's endodermal properties represent a crucial control point for nutrient and photoassimilate flow.
- Further experimental research is essential to definitively elucidate the mode of phloem loading in gymnosperms.
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