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Published on January 7, 2009; 10.1104/pp.108.134791

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Received December 22, 2008
Accepted December 31, 2008

Phloem Loading Strategies in Three Plant Species that Transport Sugar Alcohols

Edwin J. Reidel , Emilie Rennie , Veronique Amiard , Lailiang Cheng , and Robert Turgeon *

Department of Plant Biology; Department of Horticulture; Cornell University; Ithaca, New York 14853

* Corresponding author; email: ert2{at}cornell.edu.

Many plants translocate sugar alcohols in the phloem. However, the mechanism(s) of sugar alcohol loading in the minor veins of leaves are debated. We characterized the loading strategies of two species that transport sorbitol (Plantago major and Malus domestica), and one that transports mannitol (Asarina scandens). Plasmodesmata are abundant at all interfaces in the minor vein phloem of M. domestica, and in one of two types of phloem in the minor veins of A. scandens. Few plasmodesmata are present in the minor veins of P. major. M. domestica differs from the other two species in that sugar alcohol and sucrose are present in much higher concentrations in leaves. M. domestica leaf tissue exposed to exogenous [14C]sorbitol, [14C]sucrose, or 14CO2, did not accumulate radiolabel in the minor veins, as determined by macroautoradiography. P. major minor veins accumulated radiolabel from [14C]sucrose, [14C]sorbitol, and 14CO2. A. scandens minor veins accumulated 14C from [14C]sucrose and 14CO2, but not from [14C]mannitol. We conclude that the movement of sugar alcohol from the mesophyll into the phloem in M. domestica and A. scandens is symplastic and passive, but in P. major it involves an apoplastic step and is energized. We also suggest that apples leaves transport sorbitol in high concentrations to avoid the feedback limitation of photosynthesis that would result from driving passive movement of solute into the phloem with high levels of sucrose alone. The loading pathways and the mechanisms by which hydrostatic pressure is maintained in the minor vein phloem of these species are discussed.




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