Alternative polyadenylation (APA) is a pervasive mechanism for gene regulation in eukaryotes. Stachyose is the main assimilate translocated in the cucumber phloem. Stachyose synthase (CsSTS) catalyzes the last step of stachyose biosynthesis in cucumber leaves and plays a key role in the regulation of assimilate partitioning between source and sink. In this study, three CsSTS mRNAs with the same open reading frame and the 5`untranslated region (UTR), but differing in their 3`UTRs, named CsSTS1 (short), CsSTS2 (medium), and CsSTS3 (long), were identified. Southern blot and sequence analysis of the cucumber genome confirmed that these transcripts are regulated through APA from a single gene. No significant difference of in vitro translation efficiency was found among three mRNAs. However, the relative stabilities of three transcripts varied among different tissues and different leaf development stages of cucumber. CsSTS1 expression in cucumber calli was up-regulated by the raffinose (substrate of CsSTS) and down-regulated by stachyose (product of CsSTS), respectively. In cucumber plants, all three isoforms have considerable expression in non-fruit node leaves. However, in fruit-carrying node leaves, the expression of CsSTS2 and CsSTS3 was severely inhibited and only CsSTS1 was highly expressed, indicating fruit setting has a remarkable effect on the relative expression level of three transcripts. This "fruit setting" effect could be observed until at least 36 h after the fruit was removed from the node. Our results suggest that abundant expression of CsSTS1 is beneficial for stachyose loading in source leaves, and APA is a delicate mechanism for CsSTS to regulate cucumber source-sink balance.

中文翻译：

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水苏糖合酶基因的替代多聚腺苷酸化介导黄瓜的源库调节

替代性聚腺苷酸化 (APA) 是真核生物基因调控的普遍机制。水苏糖是黄瓜韧皮部中转移的主要同化物。水苏糖合酶 (CsSTS) 催化黄瓜叶片中水苏糖生物合成的最后一步，并在调节源和汇之间的同化物分配中起关键作用。在本研究中，鉴定了具有相同开放阅读框和 5`非翻译区 (UTR) 的三个 CsSTS mRNA，但它们的 3`UTR 不同，命名为 CsSTS1（短）、CsSTS2（中）和 CsSTS3（长） . 黄瓜基因组的 Southern 印迹和序列分析证实，这些转录本是通过 APA 从单个基因进行调节的。三种mRNA的体外翻译效率无显着差异。然而，三种转录本的相对稳定性在黄瓜不同组织和不同叶片发育阶段之间存在差异。CsSTS1 在黄瓜愈伤组织中的表达分别被棉子糖（CsSTS 的底物）上调和被水苏糖（CsSTS 的产物）下调。在黄瓜植物中，所有三种同工型在非果实节叶中都有相当多的表达。然而，在结果节叶中，CsSTS2和CsSTS3的表达受到严重抑制，只有CsSTS1高表达，表明坐果对三种转录本的相对表达水平有显着影响。这种“坐果”效应可以观察到，直到果实从节点上移开后至少 36 小时。我们的结果表明，CsSTS1 的大量表达有利于源叶中水苏糖的加载，