A protodermal miR394 signal defines a region of stem cell competence in the Arabidopsis shoot meristem

点击翻译按钮获取中文摘要

Plants form new organs throughout their life, which in extreme cases can last more than a thousands years, by the activity of pluripotent stem cells in their growth centers, the meristems. Unraveling the mechanisms that maintain meristem activity is central to understand plant growth and development. To maintain shoot growth, the spatial organization of the shoot meristem, which gives rise to all aerial organs, must be stably preserved despite the fact that all meristem cells divide and differentiating cells leave the meristem. The stem cell niche is positioned at the outermost cell layers in the shoot meristem center, whereas a differentiation occurs in the surrounding areas. The shoot meristem thus represents a fundamental problem in biology, the maintenance of a functional organisation in a dividing cell population. In conventional genetic screens, factors fixing stem cell competence to the tip of the plant have not been discovered, presumably due to genetic redundancy. To circumvent this limitation, we performed several genetic enhancer and suppressor screens for novel stem cell regulators in the model plant Arabidopsis thaliana in our preparatory work for this proposal. We discovered through the analysis of one mutant that the protoderm (the outermost layer of the shoot meristem) functions as a hitherto unknown signaling center in stem cell regulation. It supplies the microRNA miR394 in a gradient to the underlying stem cells, where miR394 inhibits the F-Box protein LEAF CURLING RESPONSIVENESS (LCR). This repression is necessary to enable stem cell maintenance through the transcription factor WUSCHEL (WUS), which moves upward from underneath the stem cells. The implication of these findings is that the protoderm as the outermost cell layer of the shoot meristem can act as a stable source of patterning information, although its resident cells may change. The interaction of inward (miR394) and outward (WUS) signals produced from opposite poles of the stem cell population provides a novel concept for how the stem cell niche is stably anchored in the self organizing shoot meristem. The goal of this proposal is to investigate the molecular mechanisms of how miR394 regulates the region of stem cell competence in the shoot meristem. We will explore the spatial, temporal, and dosimetric parameters of miR394 function, integrate miR394 signaling into the regulatory network of the shoot meristem, and investigate the molecular mechanism of how LCR inhibits stem cell fate. The results obtained from this project will also be used in collaboration with Prof. Fleck (Wageningen) to develop mathematical models for shoot meristem maintenance. This project will not only address a fundamental question in stem cell biology, but also has the potential to contribute to our understanding of stem cell-based regeneration strategies in economically important plants.