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WHOLE PLANT AND ECOPHYSIOLOGY

Phloem Loading Strategies in Three Plant Species That Transport Sugar Alcohols^1,[C],[OA]

Department of Plant Biology (E.J.R., E.A.R., V.A., R.T.) and Department of Horticulture (E.J.R., L.C.), Cornell University, Ithaca, New York 14853; Research and Development Division, Nunhems USA, Inc., Brooks, Oregon 97305–9625 (E.J.R.); Department of Plant and Microbial Biology, University of California, Berkeley, California 94720 (E.A.R.); and Plant Biotechnology Unit, Agro Aquaculture Nutritional Genomic Center, National Research Agriculture Institute, CRI-Carillanca, Temuco, Chile (V.A.)

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 apple [Malus domestica]), and one that transports mannitol (Asarina scandens). Plasmodesmata are abundant at all interfaces in the minor vein phloem of apple, 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. Apple differs from the other two species in that sugar alcohol and sucrose (Suc) are present in much higher concentrations in leaves. Apple leaf tissue exposed to exogenous [¹⁴C]sorbitol, [¹⁴C]Suc, or ¹⁴CO₂ did not accumulate radiolabel in the minor veins, as determined by macroautoradiography. P. major minor veins accumulated radiolabel from [¹⁴C]Suc, [¹⁴C]sorbitol, and ¹⁴CO₂. A. scandens minor veins accumulated ¹⁴C from [¹⁴C]Suc and ¹⁴CO₂, but not from [¹⁴C]mannitol. We conclude that the movement of sugar alcohol from the mesophyll into the phloem in apple and A. scandens is symplastic and passive, but in P. major it involves an apoplastic step and is energized. We also suggest that apple 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 Suc alone. The loading pathways and the mechanisms by which hydrostatic pressure is maintained in the minor vein phloem of these species are discussed.

The author responsible for the distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Robert Turgeon (ert2{at}cornell.edu).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

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www.plantphysiol.org/cgi/doi/10.1104/pp.108.134791

^* Corresponding author; e-mail ert2{at}cornell.edu.

Received December 22, 2008; accepted December 31, 2008; published January 7, 2009.

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