Subcellular Dynamics, Cell Morphogenesis, and Organ Formation in Plants

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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.