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Plant and Cell Physiology Advance Access originally published online on July 27, 2007
Plant and Cell Physiology 2007 48(9):1299-1308; doi:10.1093/pcp/pcm096
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© The Author 2007. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

OeMST2 Encodes a Monosaccharide Transporter Expressed throughout Olive Fruit Maturation

Carlos Conde1, Alice Agasse1, Paulo Silva1, Rémi Lemoine2, Serge Delrot3, Rui Tavares1 and Hernâni Gerós1,*

1Departamento de Biologia, Universidade do Minho, Campus de Gualtar 4710-057 Braga, Portugal
2UMR CNRS 6161, Transport des Assimilats, Laboratoire de Physiologie, Biochimie et Biologie Moléculaires Végétales, Bâtiment Botanique, UFR Sciences, 40 Avenue du Recteur Pineau, 86022 Poitiers Cédex, France
3Institute of Vine and Wine Sciences (ISVV), University Victor Segalen Bordeaux II, Unité Mixte de Recherches Ecophysiology and Grape Functional Genomics, INRA, 71 Avenue Edouard Bourlaux, 33883, BP 81 Villenave d’Ornon, France

*Corresponding author: E-mail, geros{at}bio.uminho.pt; Fax, +351-253678980.


  Abstract

In olive fruits, sugars are the main soluble components providing energy and acting as precursors for olive oil biosynthesis. Large quantities of glucose, fructose and galactose are often found in olive pulp. To analyze sugar transport processes in Olea europaea, a cDNA encoding a monosaccharide transporter, designated OeMST2 (Olea europaea monosaccharide transporter 2) was cloned. An open reading frame of 1,569 bp codes for a protein of 523 amino acids and a calculated molecular weight of 57.6 kDa. The protein is homologous to other sugar transporters identified so far in higher plants. Expression of this cDNA in an hxt-null Saccharomyces cerevisiae strain deficient in glucose transport restored its capacity to grow on and to transport glucose. The encoded protein showed high affinity for D-glucose (Km, 25 µM) and was also able to recognize D-galactose and the analogs 3-O-methyl-D-glucose and 2-deoxy-D-glucose, but not D-fructose, D-arabinose, sucrose or D-mannitol. Maximal transport activity was high at acidic pH (5.0), and the initial D-[14C]glucose uptake rates were strongly inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone, confirming that OeMST2 is a H+/monosaccharide transporter. The expression of OeMST2 was studied during the ripening process. Transcript levels increased during fruit maturation, suggesting that OeMST2 takes part in the massive accumulation of monosaccharides in olive fruits. Monosaccharide:H+ transport system activity and OeMST2 expression were negatively regulated by glucose in suspension-cultured cells. Glucose-mediated OeMST2 repression was impaired by mannoheptulose, suggesting the involvement of a hexokinase-dependent signaling pathway.

Keywords: Glucose - Monosaccharide transport - Olea europaea - Sugar sensing

Abbreviations: CCCP, carbonyl cyanide m-chlorophenylhydrazone; 2-dG, 2-deoxy-D-glucose; DST, disaccharide transporter; HXK, hexokinase; MFS, major facilitator superfamily; 3-O-MG, 3-O-methyl-D-glucose; MST, monosaccharide transporter; RACE, rapid amplification of cDNA ends; PEG, polyethylene glycol; RT–PCR, reverse transcription–PCR; TPP+, tetraphenylphosphonium

(Received May 3, 2007; Accepted July 18, 2007)
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