Functional Characterization and RNAi-Mediated Suppression Reveals Roles for Hexose Transporters in Sugar Accumulation by Tomato Fruit
Functional Characterization and RNAi-Mediated Suppression Reveals Roles for Hexose Transporters in Sugar Accumulation by Tomato Fruit
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DOI：
10.1093/mp/ssq050
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发表时间：
2010-11-01
期刊：
影响因子：
27.5
通讯作者：
Patrick, John W.
中科院分区：
文献类型：
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作者：
McCurdy, David W.;Dibley, Stephen;Patrick, John W.
己糖在番茄果实的贮藏薄壁细胞中积累到高浓度（类似于200 mM）。己糖来源于果实质外体，是韧皮部引入的蔗糖的水解产物。水果贮藏薄壁细胞中表达的三种己糖转运蛋白（LeHT 1、LeHT 2、LeHT 3）可能有助于这些细胞对己糖的吸收。它们的全长序列的分析表明，所有三个转运蛋白属于定位于质膜的单糖转运蛋白的STP亚家族。LeHT 1的异源表达（以及之前的LeHT 2，Gear等人，2000），而不是LeHT 3，拯救了一个己糖转运受损的酵母突变体，当葡萄糖或果糖作为唯一的碳源时。在生物化学上，LeHT 1，类似于LeHT 2，表现出与高亲和力葡萄糖/H+同向转运体一致的运输特性。值得注意的是，LeHT 1和LeHT 2也作为低亲和力果糖/H+同向转运蛋白，其表观Km值与果实组织的Km值相当。通过RNAi介导的敲低，所有LeHT基因的果实表达水平显著降低（80-90%），导致果实己糖积累减少55%。与此相反，源叶和韧皮部运输能力的光合产物生产的水果不受转运敲除。总的来说，这些研究结果表明，LeHTs在番茄果实发育过程中驱动己糖积累到贮藏薄壁细胞中发挥关键作用。
Hexoses accumulate to high concentrations (similar to 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 cells, may contribute to hexose uptake by these cells. An analysis of their full-length sequences demonstrated that all three transporters belong to the STP sub-family 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 transport-impaired 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 K-m 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.