Subcellular Dynamics, Cell Morphogenesis, and Organ Formation in Plants

植物通过细胞分裂和细胞扩张的协调模式生长并对周围环境做出反应。例如，在根中，细胞分裂限于顶端，形成顶端分生组织，产生多层细胞。顶端后面是伸长区，细胞停止分裂，开始纵向扩张，形成成熟的根。由于植物细胞被细胞壁包围和支撑，所有的细胞分裂和扩张都取决于细胞壁的结构和组成。被称为纤维素微纤维(CMF)的纤维素链形成了一个网络，决定了细胞扩张的方向。例如，在根伸长区，CMF以平行的环排列，限制了细胞的横向扩张，有利于伸长。相反，分裂的分生组织细胞具有防止伸长的混合CMF取向。CMF的取向由微管(MT)的取向控制，微管形成排列在细胞膜上的线缆网络。MTS引导纤维素酶在膜中的线性运动，将CMF挤出到相邻的细胞壁。与CMF一样，一旦进入细胞扩展，MT就从混合模式切换到有组织模式。 我的研究计划的一个关键领域是如何控制MT和CMF模式，以确定细胞是重新进入分裂还是切换到扩张。由于分生组织细胞小而四四方方，它们有尖锐的边缘，MT必须穿过这些边缘才能产生混合的MT模式。我已经确定了三种蛋白质(CLASP、GCP2和GCP3)，它们聚集在这些尖锐的细胞边缘，使MTS能够交叉。然后，这些MT横跨细胞的多个侧面，这有助于形成阻止细胞扩张的混合MT模式。另一组专门的MT称为内质MT(EMTs)，也有助于混合MT模式。EMT从细胞中心发出，并在其尖端附着在细胞膜上。我发现卡环位于EMT-膜附着点，它帮助急诊医生急转弯进入膜并排列成线。我的研究计划的三个目标是：(1)确定其他细胞边缘蛋白，并确定它们如何影响CMT模式；(2)确定其他EMT-皮质附着蛋白，并确定它们如何影响EMT-皮质附着和CMT模式；以及(3)为了了解分叶和分枝等复杂细胞扩展模式是如何发展的，我们将分析叶肉细胞随着时间的变化而改变形状。 为了实现这些目标，我开发了表达荧光蛋白标签的拟南芥植物，以可视化活的根和叶中的MTS、CMF、细胞边缘、EMT膜锚和其他亚细胞结构。我和我的学生将使用荧光显微镜，随着时间的推移以三维方式追踪这些元素。我的研究项目的发现将促进我们对植物如何生长和对环境做出反应的了解。

Plants grow and respond to their surroundings through coordinated patterns of cell division and cell expansion. For example, in a root, cell division is restricted to the tip, forming an apical meristem that produces layers of cells. Behind the tip is the elongation zone, where cells cease dividing and begin expanding lengthwise to form the mature root. Since plant cells are surrounded and supported by cell walls, all cell division and expansion depends on cell wall structure and composition. Strands of cellulose called cellulose microfibrils (CMFs) form a meshwork that determines the direction of cell expansion. For example, in the root elongation zone, CMFs line up in parallel hoops that restrict lateral cell expansion and favor elongation. In contrast, dividing meristematic cells have mixed CMF orientations that prevent elongation. CMF orientation is controlled by the orientation of microtubules (MTs), which form a network of cables that line the cell membrane. MTs guide the linear movement of cellulose-synthesizing enzymes in the membrane that extrude CMFs into the adjacent cell wall. As with CMFs, MTs switch from mixed to organized patterns upon entry into cell expansion. A key area of my research program focuses on how MT and CMF patterns are controlled to determine whether a cell re-enters division or makes the switch to expansion. Being small and boxy, meristem cells have sharp edges that MTs must cross in order to generate mixed MT patterns. I have identified three proteins (CLASP, GCP2 and GCP3) that accumulate at these sharp cell edges to enable MTs to cross over. These MTs then span across multiple sides of the cell, which contributes to the mixed MT patterns that prevent cell expansion. Another specialized set of MTs called Endoplasmic MTs (EMTs) also contributes to mixed MT patterns. EMTs emanate from the cell center and attach at their tips to the cell membrane. I found that CLASP is located at the EMT-membrane attachment point, where it helps EMTs make the sharp turn to enter and line the membrane. Three objectives of my research program are: (1) to identify additional cell edge proteins and determine how they influence CMT patterns; (2) to identify additional EMT-cortex attachment proteins and determine how they influence EMT-cortex attachment and CMT patterns; and (3) to understand how complex cell expansion patterns such as lobing and branching develop, we will analyze leaf mesophyll cells as they change shape over time. To achieve these goals, I have developed Arabidopsis thaliana plants expressing fluorescent protein tags to visualize MTs, CMFs, cell edges, EMT-membrane anchors and other subcellular structures in living roots and leaves. My students and I will use fluorescence microscopy to track these elements in three-dimensions over time. The discoveries made by my research program will advance our knowledge of how plants grow and respond to their environments.