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Plant and Cell Physiology Advance Access originally published online on September 6, 2008
Plant and Cell Physiology 2008 49(10):1607-1613; doi:10.1093/pcp/pcn134
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© The Author 2008. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

A Maltose Transporter from Apple is Expressed in Source and Sink Tissues and Complements the Arabidopsis Maltose Export-Defective Mutant

Edwin J. Reidel1,2,3, Robert Turgeon2 and Lailiang Cheng1,*

1 Department of Horticulture, Cornell University, Ithaca, NY 14853, USA
2 Department of Plant Biology, Cornell University, Ithaca, NY 14853, USA

*Corresponding author: E-mail, LC89{at}cornell.edu; Fax, +1-607-255-0599.


  Abstract

Prior to the cytosolic synthesis of transport sugars during transitory starch utilization, intermediate products of starch breakdown, such as maltose, must be exported from chloroplasts. Recent work in Arabidopsis indicates that a novel transporter mediates maltose transfer across the chloroplast inner envelope membrane. We cloned a gene from an apple cDNA library that is highly homologous with the Arabidopsis maltose transporter, MEX1. Expression levels of MdMEX determined by real-time PCR were low in the tips of growing shoots, higher in expanding leaves and maximal in mature leaves. Expression was also detected in fruits and roots, indicating a role for MdMEX in starch mobilization in sink tissues. The cDNA from apple was subcloned into an expression cassette between the cauliflower mosaic virus 35S promoter and the sGFP (green fluorescent protein) coding sequence. Plants of the Arabidopsis maltose excess1-1 mutant, which is homozygous for a defective MEX1 allele, were transformed with the 35S:MdMEX:GFP construct. Fluorescence of GFP was localized to chloroplasts, indicating that Arabidopsis recognized the predicted 55 amino acid chloroplast transit peptide in the apple protein. The phenotypes of several independently transformed lines were analyzed. The complemented plants were relieved of the severe stunting and chlorosis characteristic of mex1-1 plants. Furthermore, starch levels and concentrations of soluble sugars, leaf chlorophyll content and maximum quantum efficiency of PSII were restored to wild-type levels. MdMEX (Malus domestica maltose transporter) is the second member of the unique maltose transporter gene family.

Keywords: Apple - Maltose transporter - Malus domestica - MEX

Abbreviations: CaMV, cauliflower mosaic virus; EST, expressed sequence tag; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GFP, green fluorescent protein; LSCM, laser scanning confocal microscopy; MdMEX, Malus domestica maltose transporter; MEX1, Arabidopsis maltose transporter; ORF, open reading frame; qRT-PCR, quantitative real-time PCR; RCP1, root cap protein; TSU, transitory starch utilization; TPT, triose phosphate/phosphate translocator.

3Present address: Nunhems USA, Inc., Brooks, OR 97305, USA

The nucleotide sequence reported in this paper has been submitted to GenBank under accession number DQ648082.

(Received June 14, 2008; Accepted September 2, 2008)
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