Signalling Research Centres BIOSS and CIBSS, Faculty of Biology, University of Freiburg, Schänzlestrasse 1, 79104 Freiburg, Germany.
Institut Jean-Pierre Bourgin, INRA, AgroParisTech, CNRS, Université Paris-Saclay, 78000 Versailles, France.
Plant Commun. 2019 Sep 16;1(1):100002. doi: 10.1016/j.xplc.2019.100002. eCollection 2020 Jan 13.
Pluripotent stem cells localized in proliferating growth centers, the meristems, are the origin of life-long organ formation and growth in higher plants. In the shoot apical meristem of , the closely related ARGONAUTE proteins AGO1 and ZLL/AGO10 bind miR165/166 species to regulate mRNAs of HD-ZIP III transcription factors that are essential to maintaining stem cells. Several genetic studies showed that AGO1 and ZLL/AGO10 act redundantly to maintain stem cells. By contrast, the reported biochemical data suggested antagonistic functions: AGO1 utilizes miR165/166 to slice mRNAs, whereas ZLL/AGO10 promotes degradation of miR165/166 and thus stabilizes mRNAs. How these different functions are balanced in stem cell regulation has remained enigmatic. Here, we show that autorepression of AGO1 through miR168-mediated slicing of its own RNA is required to maintain the ability of AGO1 to suppress mRNAs. Increased AGO1 expression, either in the mutant or by transgenic expression, inhibits this ability despite the presence of high levels of miR165/166, effectively uncoupling and miR165/166 expression. AGO1 activity can be restored, however, by increasing the levels of chaperones SQN and HSP90, which promote assembly of RNA-induced silencing complex (RISC). This suggests that cellular abundance of SQN and HSP chaperones limits AGO1-mediated RNA interference in shoot meristem stem cell regulation. Localized misexpression of AGO1 indicates that the cells surrounding the shoot meristem primordium play a crucial role in stem cell development. Taken together, our study provides a framework that reconciles biochemical and genetic data, showing that restriction of AGO1 levels by miR168-mediated autorepression is key to RISC homeostasis and the function of AGO1 in stem cell regulation.
多能干细胞位于增殖生长中心的分生组织中，是高等植物终身器官形成和生长的起源。在拟南芥的茎尖分生组织中，密切相关的 ARGONAUTE 蛋白 AGO1 和 ZLL/AGO10 结合 miR165/166 物种来调节 HD-ZIP III 转录因子的 mRNA，这些转录因子对于维持干细胞至关重要。几项遗传研究表明，AGO1 和 ZLL/AGO10 冗余地作用以维持干细胞。相比之下，报道的生化数据表明它们具有拮抗作用：AGO1 利用 miR165/166 来切割 mRNAs，而 ZLL/AGO10 促进 miR165/166 的降解，从而稳定 mRNAs。这些不同的功能如何在干细胞调控中达到平衡仍然是个谜。在这里，我们表明通过 miR168 介导的自身 RNA 切割的 AGO1 自动抑制是维持 AGO1 抑制 mRNAs 的能力所必需的。增加 AGO1 的表达，无论是在 突变体中还是通过转基因表达，都会抑制这种能力，尽管存在高水平的 miR165/166，但有效地将 和 miR165/166 的表达解耦。然而，通过增加伴侣蛋白 SQN 和 HSP90 的水平，可以恢复 AGO1 的活性，这两种蛋白促进 RNA 诱导沉默复合物 (RISC) 的组装。这表明 SQN 和 HSP 伴侣蛋白的细胞丰度限制了 RISC 在茎尖分生组织干细胞调控中的 RNA 干扰作用。AGO1 的局部异位表达表明，围绕茎尖原基的细胞在干细胞发育中起着至关重要的作用。总之，我们的研究提供了一个框架，协调了生化和遗传数据，表明 miR168 介导的自动抑制对 AGO1 水平的限制是 RISC 平衡和 AGO1 在干细胞调控中的功能的关键。
