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.