Journal Article

Identification of Sorbitol Transporters Expressed in the Phloem of Apple Source Leaves

Junya Watari, Yoshihiro Kobae, Shohei Yamaki, Kunio Yamada, Kyoko Toyofuku, Toshihito Tabuchi and Katsuhiro Shiratake

in Plant and Cell Physiology

Published on behalf of Japanese Society of Plant Physiologists

Volume 45, issue 8, pages 1032-1041
Published in print August 2004 | ISSN: 0032-0781
Published online August 2004 | e-ISSN: 1471-9053 | DOI: http://dx.doi.org/10.1093/pcp/pch121
Identification of Sorbitol Transporters Expressed in the Phloem of Apple Source Leaves

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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.

Keywords: Keywords: Apple (Malus domestica) — Phloem — Polyol — Rosaceae — Sorbitol transporter — Sugar alcohol.; Abbreviations: CCCP, carbonylcyanide m-chlorophenylhydrazone; DIG, digoxigenin; MFS, major facilitator superfamily; PCMBS, p-chloromercuribenzensulfonic acid; pI, isoelectric point; RFOs, raffinose family of oligosaccharides; TMD, transmembrane domain

Journal Article.  6753 words.  Illustrated.

Subjects: Biochemistry ; Molecular and Cell Biology ; Plant Sciences and Forestry

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