背景回顾：Cell-type-specific gene expression is critical for the specialized functions within multicellular organisms. In Arabidopsis, SWEET11 and SWEET12 sugar transporters are specifically expressed in phloem parenchyma (PP) cells and are responsible for sucrose efflux from the PP, the first step of a two-step apoplasmic phloem loading strategy that initiates the long-distance transport of sugar from leaves to non-photosynthetic sink tissues. 

提出问题：However, we know nothing about what determines the PP cell-specific expression of these SWEETs. 

主要研究：Sequence deletions, histochemical β-glucuronidase (GUS) analysis, cross-sectioning, live-cell imaging, and evolutionary analysis were used to elucidate domains responsible for PP specificity, while a FRET sensor-based transport assay was used to determine whether substrate specificity co-evolved with PP specificity.

结果：We identified two domains in the Arabidopsis SWEET11 coding sequence that, along with its promoter, regulate PP-specific expression at the post-transcriptional level, likely involving RNA-binding proteins. This mechanism is conserved among vascular plants but independent of transport substrate specificity. 

结论：We conclude that two evolutionarily duplicated coding sequence domains are essential and individually sufficient for PP-specific expression of SWEET11. We also provide a crucial experimental tool to study PP physiology and development.

 摘 要 

细胞类型特异性基因表达对于多细胞生物中的功能特化十分重要。在拟南芥中，SWEET11和SWEET12糖转运体特异性地在韧皮部薄壁组织细胞中表达，作用于韧皮部薄壁组织的糖外流，这是质外体韧皮部装载策略两步法的第一步，启动糖从叶片到非光合库组织的长距离运输。但是，我们对于是何机制决定了这些SWEETs基因在韧皮部薄壁组织中的特异性表达还不清楚。本文中，作者通过序列删除、组织化学GUS分析、切片、活细胞成像以及演化分析，用于研究韧皮部薄壁组织特异性相关的结构域，并且利用一种基于荧光共振能量转移（FRET）传感器的转运分析，研究底物特异性是否与韧皮部薄壁组织特异性存在共演化。作者鉴定到了SWEET11编码区存在两个结构域，这两个结构域与SWEET11的启动子一起，从转录后水平调控SWEET11在韧皮部薄壁组织的特异性表达，并且该过程很有可能涉及到RNA结合蛋白。该机制在维管植物中保守，但是独立于转运底物特异性。作者总结道，两个在演化上复制的编码序列结构域对于SWEET11基因的韧皮部薄壁组织特异表达是必须的，并且其中任意一个结构域均足以保证这种特异性。另外，本文还提供了研究韧皮部薄壁组织生理和发育的重要实验工具。