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First published online October 29, 2004; 10.1104/pp.104.042036

Plant Physiology 136:3795-3803 (2004)
© 2004 American Society of Plant Biologists

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WHOLE PLANT AND ECOPHYSIOLOGY

Phloem Loading. A Reevaluation of the Relationship between Plasmodesmatal Frequencies and Loading Strategies1

Robert Turgeon* and Richard Medville

Department of Plant Biology, Cornell University, Ithaca, New York 14853 (R.T.); and Electron Microscopy Services and Consultants, Phoenix, Arizona 85022 (R.M.)

The incidence of plasmodesmata in the minor vein phloem of leaves varies widely between species. On this basis, two pathways of phloem loading have been proposed: symplastic where frequencies are high, and apoplastic where they are low. However, putative symplastic-loading species fall into at least two categories. In one, the plants translocate raffinose-family oligosaccharides (RFOs). In the other, the primary sugar in the phloem sap is sucrose (Suc). While a thermodynamically feasible mechanism of symplastic loading has been postulated for species that transport RFOs, no such mechanism is known for Suc transporters. We used p-chloromercuribenzenesulfonic acid inhibition of apoplastic loading to distinguish between the two pathways in three species that have abundant minor vein plasmodesmata and are therefore putative symplastic loaders. Clethra barbinervis and Liquidambar styraciflua transport Suc, while Catalpa speciosa transports RFOs. The results indicate that, contrary to the hypothesis that all species with abundant minor vein plasmodesmata load symplastically, C. barbinervis and L. styraciflua load from the apoplast. C. speciosa, being an RFO transporter, loads from the symplast, as expected. Data from these three species, and from the literature, also indicate that plants with abundant plasmodesmata in the minor vein phloem have abundant plasmodesmata between mesophyll cells. Thus, plasmodesmatal frequencies in the minor veins may be a reflection of overall frequencies in the lamina and may have limited relevance to phloem loading. We suggest that symplastic loading is restricted to plants that translocate oligosaccharides larger than Suc, such as RFOs, and that other plants, no matter how many plasmodesmata they have in the minor vein phloem, load via the apoplast.

1 This work was supported by the National Science Foundation (grant no. IBN–0110638 to R.T.).

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.104.042036.

* Corresponding author; e-mail ert2{at}cornell.edu; fax 607–255–5407.

Received March 9, 2004; returned for revision August 10, 2004; accepted September 3, 2004.


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