School of Environmental and Life Sciences, University of Newcastle, Callaghan, NSW 2308, Australia.
Mol Plant. 2010 Nov;3(6):1049-63. doi: 10.1093/mp/ssq050. Epub 2010 Sep 10.
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 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.
己糖在番茄果实的贮藏组织细胞中积累到高浓度（约 200mM）。己糖来源于果实质外体，作为韧皮部导入蔗糖的水解产物。三种己糖转运蛋白（LeHT1、LeHT2、LeHT3）在果实贮藏组织细胞中表达，可能有助于这些细胞摄取己糖。对其全长序列的分析表明，这三种转运蛋白都属于单糖转运蛋白的 STP 亚家族，定位于质膜上。异源表达 LeHT1（先前也表达 LeHT2，Gear 等人，2000 年），而不是 LeHT3，当酵母突变体以葡萄糖或果糖作为唯一碳源时，可恢复其己糖转运缺陷。从生化角度来看，LeHT1 与 LeHT2 相似，表现出与高亲和力葡萄糖/H+同向转运体一致的转运特性。重要的是，LeHT1 和 LeHT2 还作为低亲和力果糖/H+同向转运体发挥作用，其表观 K(m) 值与果实组织的 K(m) 值相当。通过 RNAi 介导的敲低，所有 LeHT 基因在果实中的表达水平降低 80-90%，导致果实中己糖积累减少 55%。相比之下，源叶的光合产物产生和韧皮部向果实的运输能力不受转运蛋白敲低的影响。总的来说，这些发现表明 LeHTs 在番茄果实发育过程中，在将己糖驱动积累到贮藏组织细胞中方面发挥着关键作用。
