Centering the Organizing Center in the Arabidopsis thaliana Shoot Apical Meristem by a Combination of Cytokinin Signaling and Self-Organization

Plants have the ability to continously generate new organs by maintaining populations of stem cells throught their lives. The shoot apical meristem (SAM) provides a stable environment for the maintenance of stem cells. All cells inside the SAM divide, yet boundaries and patterns are maintained. Experimental evidence indicates that patterning is independent of cell lineage, thus a dynamic self-regulatory mechanism is required. A pivotal role in the organization of the SAM is played by the WUSCHEL gene (WUS). An important question in this regard is that how WUS expression is positioned in the SAM via a cell-lineage independent signaling mechanism. In this study we demonstrate via mathematical modeling that a combination of an inhibitor of the Cytokinin (CK) receptor, Arabidopsis histidine kinase 4 (AHK4) and two morphogens originating from the top cell layer, can plausibly account for the cell lineage-independent centering of WUS expression within SAM. Furthermore, our laser ablation and microsurgical experiments support the hypothesis that patterning in SAM occurs at the level of CK reception and signaling. The model suggests that the interplay between CK signaling, WUS/CLV feedback loop and boundary signals can account for positioning of the WUS expression, and provides directions for further experimental investigation.

植物在整个生命周期中可通过维持干细胞种群持续生成新器官。茎尖分生组织（shoot apical meristem, SAM）为干细胞的维持提供稳定环境。SAM内部所有细胞均处于分裂状态，但其边界与模式建成却得以维持。实验证据表明，该组织的模式建成不依赖细胞谱系，因此必然存在动态的自我调控机制。WUSCHEL基因（WUSCHEL gene, WUS）在SAM的组织构建中发挥关键作用。该领域当前的一个核心问题是：WUS的表达如何通过不依赖细胞谱系的信号机制在SAM中实现精准定位。本研究通过数学建模证明，细胞分裂素（Cytokinin, CK）受体抑制剂拟南芥组氨酸激酶4（Arabidopsis histidine kinase 4, AHK4）与两种源自顶层细胞层的形态发生因子的协同作用，可合理解释SAM内WUS表达不依赖细胞谱系的居中定位现象。此外，本研究的激光消融与显微手术实验支持了“SAM的模式建成发生于CK信号接收与转导层面”这一假说。该模型表明，CK信号通路、WUS/CLV反馈环与边界信号之间的相互作用，可解释WUS表达的定位机制，并为后续实验研究指明方向。