© 2004 Oxford University Press
Identification of Sorbitol Transporters Expressed in the Phloem of Apple Source Leaves
1 Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya, 464-8601 Japan
2 Laboratory of Biophysics, School of Medicine, Teikyo University, Hachioji, Tokyo, 192-0395 Japan
3 Department of Biological Production, Akita Prefectural University, Shimoshinjo-nakano, Akita, 010-0195 Japan
4 Faculty of Agriculture, Tamagawa University, 6-1-1 Tamagawa Gakuen, Machida, Tokyo, 194-8610 Japan
5 Department of Molecular Plant Physiology, Institute of Plant Biology, University of Zurich, Zollikerstrasse 107, CH-8008 Zurich, Switzerland
Sorbitol is a major photosynthetic product and a major phloem-translocated component in Rosaceae (e.g. apple, pear, peach, and cherry). We isolated the three cDNAs, MdSOT3, MdSOT4, and MdSOT5 from apple (Malus domestica) source leaves, which are homologous to plant polyol transporters. Yeasts transformed with the MdSOTs took up sorbitol significantly. MdSOT3- and MdSOT5-dependent sorbitol uptake was strongly inhibited by xylitol and myo-inositol, but not or only weakly by mannitol and dulcitol. Apparent Km values of MdSOT3 and MdSOT5 for sorbitol were estimated to be 0.71 mM and 3.2 mM, respectively. The protonophore, carbonyl cyanide m-chlorophenylhydrazone (CCCP), strongly inhibited the sorbitol transport. MdSOT3 was expressed specifically in source leaves, whereas MdSOT4 and MdSOT5 were expressed in source leaves and also in some sink organs. MdSOT4 and MdSOT5 expressions were highest in flowers. Fruits showed no or only weak MdSOT expression. Although MdSOT4 and MdSOT5 were also expressed in immature leaves, MdSOT expressions increased with leaf maturation. In addition, in situ hybridization revealed that all MdSOTs were expressed to high levels in phloem of minor veins in source leaves. These results suggest that these MdSOTs are involved in sorbitol loading in Rosaceae.
6 Corresponding author: E-mail, shira{at}agr.nagoya-u.ac.jp; Fax, +81-52-789-4025.
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