1] Department of Biology, Stanford University, Stanford, California 94305, USA [2] Carnegie Institution for Science, 260 Panama Street, Stanford, California 94305, USA.
Carnegie Institution for Science, 260 Panama Street, Stanford, California 94305, USA.
Nature. 2014 Apr 24;508(7497):546-9. doi: 10.1038/nature13082. Epub 2014 Mar 16.
Angiosperms developed floral nectaries that reward pollinating insects. Although nectar function and composition have been characterized, the mechanism of nectar secretion has remained unclear. Here we identify SWEET9 as a nectary-specific sugar transporter in three eudicot species: Arabidopsis thaliana, Brassica rapa (extrastaminal nectaries) and Nicotiana attenuata (gynoecial nectaries). We show that SWEET9 is essential for nectar production and can function as an efflux transporter. We also show that sucrose phosphate synthase genes, encoding key enzymes for sucrose biosynthesis, are highly expressed in nectaries and that their expression is also essential for nectar secretion. Together these data are consistent with a model in which sucrose is synthesized in the nectary parenchyma and subsequently secreted into the extracellular space via SWEET9, where sucrose is hydrolysed by an apoplasmic invertase to produce a mixture of sucrose, glucose and fructose. The recruitment of SWEET9 for sucrose export may have been a key innovation, and could have coincided with the evolution of core eudicots and contributed to the evolution of nectar secretion to reward pollinators.
被子植物发育出了花内蜜腺来奖励传粉昆虫。尽管花蜜的功能和成分已经被描述过了，但花蜜分泌的机制仍然不清楚。在这里，我们在三个真双子叶植物物种中鉴定出 SWEET9 是蜜腺特异性的糖转运蛋白：拟南芥、油菜（副蜜腺）和黄花烟草（雌蕊蜜腺）。我们表明 SWEET9 对花蜜的产生是必需的，并且可以作为外排转运蛋白发挥作用。我们还表明，蔗糖磷酸合酶基因，编码蔗糖生物合成的关键酶，在蜜腺中高度表达，其表达对花蜜分泌也是必需的。这些数据共同支持了这样一种模型，即蔗糖在蜜腺的薄壁组织中合成，然后通过 SWEET9 分泌到细胞外空间，在那里蔗糖被质外体中的蔗糖转化酶水解，产生蔗糖、葡萄糖和果糖的混合物。蔗糖外排的 SWEET9 的招募可能是一个关键的创新，它可能与核心真双子叶植物的进化同时发生，并有助于花蜜分泌以奖励传粉者。
