Functional Characterization and RNAi-Mediated Suppression Reveals Roles for Hexose Transporters in Sugar Accumulation by Tomato Fruit

• Published in: Molecular plant 2010-11, Vol.3 (6), p.1049-1063
• Main Authors: McCurdy, David W., Dibley, Stephen, Cahyanegara, Ricky, Martin, Antony, Patrick, John W.
• Format: Article
• Language: English
• Subjects: Biomass; Carbohydrate Metabolism - genetics; Cloning, Molecular; Enzymes; Fruit; Fruit - metabolism; hexose transporter; Hexoses - metabolism; Lycopersicon esculentum - metabolism; Microbiology; Monosaccharide Transport Proteins - deficiency; Monosaccharide Transport Proteins - genetics; Monosaccharide Transport Proteins - metabolism; Phylogeny; Pigmentation; RNA Interference; RNAi; Saccharomyces cerevisiae - genetics; sink; tomato; Tomatoes; 介导; 功能特性; 己糖; 番茄果实; 糖分积累; 薄壁细胞; 转运体
• ISSN: 1674-2052; 1752-9867
• DOI: 10.1093/mp/ssq050

Hexoses accumulate to high concentrations （-200 mM） in storage parenchyma cells of tomato fruit. Hexoses are sourced from the fruit apoplasm as hydrolysis products of phloem-imported sucrose. Three hexose transporters （LeHT1, LeHT2, LeHT3）, expressed in fruit storage parenchyma ceils, may contribute to hexose uptake by these cells. An analysis of their full-length sequences demonstrated that all three transporters belong to the STP subfamily of monosaccharide transporters that localize to plasma membranes. Heterologous expression of LeHT1 （and previously LeHT2, Gear et al., 2000）, but not LeHT3, rescued a hexose transportimpaired yeast mutant when raised on glucose or fructose as the sole carbon source. Biochemically, LeHT1, similarly to LeHT2, exhibited transport properties consistent with a high- affinity glucose/H^＋ symporter. Significantly, LeHT1 and LeHT2 also functioned as low-affinity fructose/H^＋ symporters with apparent Km values commensurate with those of fruit tissues. A substantial reduction （80-90%） in fruit expression levels of all LeHT genes by RNAi-mediated knockdown caused a 55% decrease in fruit hexose accumulation. In contrast, photoassimilate production by source leaves and phloem transport capacity to fruit were unaffected by transporter knockdown. Collectively, these findings demonstrate that LeHTs play key roles in driving accumulation of hexoses into storage parenchyma cells during tomato fruit development.