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Updated: Jun 5, 2026
11:23
Phloem Sap Sampling from Brassica napus for 3D-PAGE of Protein and Ribonucleoprotein Complexes
Published on: January 9, 2018
Phloem loading--not metaphysical, only complex: towards a unified model of phloem loading
Journal of Experimental Botany
|January 20, 2011
Summary
Phloem loading in Ricinus seedlings involves both direct uptake and mesophyll transfer, with the apoplast acting as a sucrose reservoir. This process, regulated by sink activity and mediated by sucrose carriers, drives mass flow in the phloem.
Area of Science:
- Plant Physiology
- Molecular Biology
- Biochemistry
Background:
- Phloem loading is a complex process involving symplastic and apoplastic transport of solutes from source to sink.
- Understanding phloem loading mechanisms is crucial for plant development and nutrient transport.
Purpose of the Study:
- To elucidate the mechanisms of phloem loading in Ricinus seedlings.
- To investigate the role of the apoplast and sucrose carriers in phloem transport.
Main Methods:
- Radioactive labeling of sucrose and analysis of sieve tube sap.
- Anatomical and cytological studies.
- cDNA cloning and gene expression analysis.
- Non-invasive NMR imaging.
Main Results:
- Approximately 50% of sucrose is loaded directly from the external medium, while the other 50% is transferred via mesophyll cells.
- The apoplast serves as a transit reservoir for sucrose.
- Glycolytic enzymes are sequestered in companion cells, and sieve tube sap contains higher glycolytic metabolite concentrations.
- A sucrose-H(+) symporter is present in most cotyledonary cells, and its cDNA was isolated.
- Recirculation of solutes from sink to source regulates phloem loading.
- NMR imaging confirmed internal circulating solution flow.
Conclusions:
- Phloem loading in Ricinus is a species-specific process involving both direct and indirect sucrose uptake pathways.
- Active sucrose carriers and a pump-and-leak model explain solute accumulation and phloem pressure buildup.
- Sink activity plays a key role in regulating phloem loading through solute recirculation.
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