Phloem Loading of Sorbitol in Apple (Malus domestica Borkh.): Cloning and Sequence Analysis of Potential H+/Sorbitol Symporters from a Mature Leaf cDNA Library

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  • 1 Cornell Univ., Horticulture, Ithaca, NY 14853-5904
  • | 2 Univ. of North Texas, Biological Sciences, Denton, TX 76203-5220
  • | 3 Cornell Univ., Plant Biology, Ithaca, NY 14853-5904
  • | 4 Cornell Univ., Horticulture, Ithaca, NY 14853-5904

Sorbitol (d-glucitol) is the major end product of photosynthesis in apple (Malus domestica Borkh.), as well as the predominant phloem-translocated carbohydrate. The mechanism by which sorbitol is phloem-loaded for transport to heterotrophic sink tissues is unknown. We hypothesized that a plasma membrane-bound H+/sorbitol symporter mediates apoplastic phloem-loading of sorbitol. To discover genes potentially encoding sorbitol transporters, a cDNA library was constructed from mature `Gala' apple leaves. A homologous probe was synthesized via PCR with primers were designed against the cherry fruit sorbitol transporter, PcSot1, and using library lysate as template. From an initial plating of approximately 5 × 105 clones, twelve positives were identified after three rounds of hybridization screening. Following single-pass, 5' end sequencing, the clones were sorted into four contiguous sequences. One clone was chosen from each contig for complete sequencing. The four clones, provisionally named MdSOT1-4 (Malus domesitca Sorbitol Transporter), potentially encode full-length cDNAs for sorbitol transporters: Translated-BLAST searching (blastx) revealed that the open reading frames encode the complete Pfam sugar transporter domain, and the most significant alignments are with sequences encoding known- and putative polyol and sugar transporters.

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