背景回顾：SWEET sucrose transporters play important roles in the allocation of sucrose in plants. Some SWEETs were shown to also mediate transport of the plant growth regulator gibberellin (GA). 

提出问题：The close physiological relationship between sucrose and GA raised the questions of whether there is a functional connection and whether one or both of the substrates are physiologically relevant. 

结果1-突变体：To dissect these two activities, molecular dynamics were used to map the binding sites of sucrose and GA in the pore of SWEET13 and predicted binding interactions that might be selective for sucrose or GA. Transport assays confirmed these predictions. In transport assays, the N76Q mutant had 7x higher relative GA₃ activity, and the S142N mutant only transported sucrose.

结果2-蔗糖和GA与雄性不育：The impaired pollen viability and germination in sweet13;14 double mutants were complemented by the sucrose-selective SWEET13^S142N, but not by the SWEET13^N76Q mutant, indicating that sucrose is the physiologically relevant substrate and that GA transport capacity is dispensable in the context of male fertility. 

结果3-外施GA：Therefore, GA supplementation to counter male sterility may act indirectly via stimulating sucrose supply in male sterile mutants. 

结果4-其他：These findings are also relevant in the context of the role of SWEETs in pathogen susceptibility. 

Restoration of pollen viability by the sucrose-selective SWEET13^S142N variant.

 摘 要 

蔗糖转运蛋白SWEET在植物的蔗糖分配方面发挥重要功能。一些SWEETs能够介导植物激素赤霉素（GA）的转运。蔗糖和GA之间密切的生理关系引出了二者之间是否真的存在功能联系以及一种或两种底物是否具有生理相关性的问题。为了解析SWEET蛋白的这两种转运活性，作者利用分子动态来定位SWEET13蛋白中蔗糖和GA的结合位点，从而预测其对蔗糖或GA的选择性结合。转运实验进一步验证了上述预测的准确性。在转运实验中，N76Q突变具有更高的GA₃转运活性，而S142N突变只能够转运蔗糖。sweet13;14双突的花粉活力和萌发缺陷能够通过SWEET13^S142N进行互补，但是不能通过SWEET13^N76Q进行互补，说明雄性不育SWEET蛋白对蔗糖而非GA的转运缺陷所导致的。因此，提供GA来抵消雄性不育可能是通过刺激雄性不育突变体的蔗糖供应，从而间接发挥作用。本文的研究同样可以用于研究SWEETs在病原易感性中的作用。