植物行光合作用產生的碳水化合物除了運送到各個所需組織外，有些醣類會從根分泌出去導致喪失，但根部醣類分泌的機制尚未被了解的很透徹，目前已知SWEET轉運蛋白在醣輸出機制中扮演重要的角色於本研究中，利用(qRT)-PCR分析阿拉伯芥的所有組織顯示出SWEET16以及SWEET17在根部具有較高的表現量，藉由報導基因 (ß-Glucoronidase) 的轉植株，不論是轉錄還是轉譯的層次也顯示出SWEET16以及SWEET17主要表現在根部。藉由融合上報導螢光基因 (Green Fluorescence Protein) 表現在阿拉伯芥上，觀察SWEET16以及SWETT17其細胞內表現的位置，發現SWEET16以及SWETT17主要表現在液胞膜上。當處理醣類的逆境時，SWEET17會受到缺醣，如黑暗處理及受傷，或者醣毒害, 如1% Fructose及 5 mM Galactose等誘導表現。而這些處理皆可導致液胞膜中的醣的釋放或Fructose累積，顯示SWETT17與液胞Fructose的調節有關，同時在處理醣類的逆境時，並不會改變細胞的表現位置，仍主要表現在液胞膜上。給予高濃度的Fructose時，可以發現若將sweet16以及sweet17突變掉或是過度表達會導致阿拉伯芥的根的長度會減短或增長，而當處理過多的Galactose時， sweet17突變或是過度表達也會導致阿拉伯芥的側根數會減少或增多。這些結果指出液胞膜上的SWEET16以及SWETT17會將過多的Fructose或者Galactose儲存於液胞之中，減少對植物細胞的毒害。低溫時，植物細胞會儲存醣類在液胞之中來對抗逆境，雖然在低溫逆境下，藉由報導基因 (GUS fusion protein) 觀察到SWEET17的表現受到抑制，但從外表型來看過度表達SWEET17的轉植株以及突變株，並無顯著差異。而過度表達SWEET17的轉植株以及突變株在高濃度的Fructose的環境下並不會影響到其發芽率。因此我們推測SWEET17在根部液胞中Fructose的雙向運輸扮演一個重要的角色，來維持Fructose的恆定而也許進一步影響葉片醣的輸出。另外SWEET可能與根病原菌 (Pythium) 的感染有關，初步的(qRT)-PCR結果發現根部中的SWEET2以及SWEET10基因會受到根腐病菌(Pythium)感染誘導使表現量提高，再利用(qRT)-PCR分析不同生育階段及不同組織中的表現，顯示SWEET2表達在所有的組織，而SWEET10則主要表現在花及成熟植株的莖部。但當利用SWEET2-GUS fusion　protein分析時，則發現SWEET2主要表達在根部。且當受根腐病菌 (Pythium) 感染時，SWEET2-GUS 在側根的表現明顯受到誘導。SWEET10-GUS 的表現雖然也受到誘導，但表現量明顯較低。藉由，SWEET2-GFP的表現，觀察在其細胞內表現的位置，發現主要表現在液胞膜上，因此SWEET2也為液胞膜上的轉運蛋白。再利用SWEET2過度表達轉殖株及突變株來觀察對生長的重要性。結果發現sweet2突變株在1.5% Sucrose、3% Glucose及0.2% Galactose的毒害下，較野生型有較低的發芽率。但在過度表達SWEET2轉殖株種子中，發芽率則無差異。此外也觀察到SWEET2過度表達以及突變會分別會延後及提早開花的時間。進一步分析植物的成熟葉、幼葉、花的醣類含量，顯示在突變及過度表達的組織中，Fructose與Galactose含量皆較野生型低。缺氮或者低溫逆境下也觀察到相同的趨勢。因此我們推論SWEET2可能會影響液胞中Fructose與Galactose的雙向運送，進而影響根部sugar efflux而影響與根腐病菌(Pythium)的交互作用。

SWEET16 and SWEET17 proteins mainly expressed in root vacuoles. Mutation and ectopic expression of SWEET16/17 led to decreased and increased root growth in the presence of Fructose, respectively. Moreover, mutation and ectopic expression of SWEET17 led to decreased and increased branching roots in the presence of Galactose, respectively. These results suggest SWEET16 and SWEET17 will store excess sugars into vacuole in order to decrease toxicity for cells. We propose that SWEET17 plays a major role in facilitating bidirectional Fructose transport on root tonoplast to maintain cytosolic fructose homeostasis that in turn may regulate Fructose export from leaves. SWEET transporters may also be involved in interaction with root pathogens (Pythium). Glucose and Galactose contents in sweet2 mutants and SWEET2 overexpression lines were significantly changed in respond to cold stress and nitrogen deficiency. These results suggest that SWEET2 may mediate Glucose and Galactose uptake into vacuoles to affect Pythium infection.

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