花的發育決定番茄果實的產量，其中糖卸載量為花發育的關鍵，但是糖被運輸至
番茄花的分子機制仍未知。SWEET 糖轉運蛋白已被證實負責積儲器官中蔗糖輸
出及分佈，其對番茄花發育中的角色值得進一步研究。qRT-PCR 結果顯示在31
個番茄SWEET 基因中，SlSWEET5b 在花中高度表現，尤其專一表現在晚期花
苞（7mm）的雄蕊中，in situ hybridization 分析進一步發現RNA 主要累積在花粉
粒及韌皮部細胞，此分佈與SlSWEET5b-GUS 融合蛋白累積位置一致，顯示
SlSWEET5b 糖轉運蛋白在花粉有絲分裂期間可能扮演糖供應的角色。GFP 融合
蛋白分析進一步確認SlSWEET5b 表現在細胞膜，其為糖進出細胞的主要調控位
置。酵母菌生長分析和放射性追蹤分析也證實了SlSWEET5b 對葡萄糖及果糖具
有轉運活性。為了驗證SlSWEET5b 在植物中的功能，也利用RNAi 建立了表現
基因沈默的番茄轉植株。觀察發現在SlSWEET5b 沉默的植物中，花粉發育畸形
且發芽率顯著下降，而果實發育也顯著延遲使得種子總數下降。此外，
SlSWEET5b 的下降表現也顯著提高花中細胞壁轉化酶的活性，可見SlSWEET5b
活性也影響花粉內的糖訊息傳遞。綜合這些結果，証實SlSWEET5b 位在番茄花
藥的韌皮部細胞及薄壁細胞的細胞膜上輸入單糖使糖卸載至花粉腔中，同時也
位在花粉表皮細胞以輸入單糖促進花粉成熟，使得自交授粉得以成功以促使果實和種子的發育。

Flower development determines the yield of tomato fruits, where efficient sugar unloading is critical for flower development. However, the molecular mechanism to import sugars to tomato flowers is unknown. SWEET sugar transporters have been shown to be responsible for the sucrose export and distribution in sink organs. Their role in flower development deserves further research. Here, qRT-PCR showed that SlSWEET5b was highly expressed in flowers within 31 tomato SlSWEETs, particularly in stamens of the late stage of flowers buds (7mm). In situ hybridization analysis further indicated that RNA transcripts were accumulated in pollen grains and phloem cells. The pattern was consistent with accumulation of SlSWEET5b-GUS fusion proteins, indicating that SlSWEET5b may function to supply sugars during the mitosis stage of pollen development. Expression of GFP fusion proteins indicated that SlSWEET5b mainly localized on the plasma membrane, which is the major location to regulate sugar transport. The yeast growth assay and radiotracer analyses further confirmed the transport activity of SlSWEET5b to glucose and fructose. To verify SlSWEET5b function, knock-down transgenic tomato plants carrying the RNAi construct were generated. In SlSWEET5b-silenced plants, pollens were abnormal and pollen germination rate was greatly reduced. The fruit development was greatly delayed and total seed numbers were dramatically reduced. Moreover, reduced expression of SlSWEET5b significantly increased activities of cell wall invertase, indicating that SlSWEET5b transport activity affects sugar signaling pathway. Combining these results, we showed that SlSWEET5b located on the plasms membrane of phloem cells and parenchyma cells of tomato anthers to mediate sugar unloading into the pollen cavity and pollens. These sugars enable pollen maturation and make pollination successful to promote the development of fruits and seeds.

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