First published online March 27, 2003; 10.1104/pp.012054
Plant Physiol, April 2003, Vol. 131, pp. 1518-1528
Symplastic Continuity between Companion Cells and the
Translocation Stream: Long-Distance Transport Is Controlled by
Retention and Retrieval Mechanisms in the
Phloem1
Brian G.
Ayre,*
Felix
Keller, and
Robert
Turgeon
Department of Plant Biology, Cornell University, Ithaca, New York
14853 (B.G.A., R.T.); and Institute of Plant Biology, University of
Zurich, CH-8008 Zurich, Switzerland (F.K.).
Substantial symplastic continuity appears to exist between
companion cells (CCs) and sieve elements of the phloem, which suggests that small solutes within the CC are subject to indiscriminate long-distance transport via the translocation stream. To test this
hypothesis, the distributions of exotic and endogenous solutes synthesized in the CCs of minor veins were studied. Octopine, a charged
molecule derived from arginine and pyruvate, was efficiently transported through the phloem but was also transferred in substantial amounts to the apoplast, and presumably other non-phloem compartments. The disaccharide galactinol also accumulated in non-phloem
compartments, but long-distance transport was limited. Conversely,
sucrose, raffinose, and especially stachyose demonstrated reduced
accumulation and efficient transport out of the leaf. We conclude that
small metabolites in the cytosol of CCs do enter the translocation
stream indiscriminately but are also subject to distributive forces, such as nonselective and carrier-mediated membrane transport and symplastic dispersal, that may effectively clear a compound from the
phloem or retain it for long-distance transport. A model is proposed in
which the transport of oligosaccharides is an adaptive strategy to
improve photoassimilate retention, and consequently translocation
efficiency, in the phloem.
1
This work was supported by the U.S. Department
of Agriculture/Cooperative State Research, Education, and Extension
Service/National Research Initiative Competitive Grants Program
(proposal no. 2001-35318-10893 to R.T.).
*
Corresponding author; e-mail bga2{at}cornell.edu; fax
607-255-5407.
© 2003 American Society of Plant Biologists
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