![]() Yet, even when cells are fully adhering to one another, they can still display wavy cell walls. Similarly, when cell-cell adhesion is less prominent naturally, cells can also round up or exhibit irregular shapes, as in the leaf mesophyll and spongy parenchyma for instance. When cell-cell adhesion is artificially affected, cells can round up. This explains why most plant cells in fully adhesive tissues have a brick shape (e.g., hypocotyl cells). In tissues, cell shape is also constrained by the presence of adjacent cells, through packing and adhesion at the middle lamella. Typically, when they are still growing, larger cells are more susceptible to wall failure than smaller cells. Beyond the wall properties, the mechanical balance operating in plant cells also depends on cell shape. In fact, when cellulose deposition is impaired, cells also tend to become spherical, as in protoplasts. Cellulose microfibrils are classically thought to play a load-bearing role here, and their alignment supports the mechanical anisotropy of the wall. Typically, wall-less protoplasts are spherical. Because turgor pressure is in essence isotropic, any deviation from a spherical shape is determined by the mechanical anisotropy of the cell wall. An isolated plant cell is shaped by the balance between turgor pressure and cell wall resistance to turgor. Plant cell shapes depend on internal and external factors. Whether in kinematic analyses (e.g., ), in functional genetics (e.g., ), in cell biology (e.g., ), and in computational modeling (e.g., ), quantifying cell contours during growth is thus crucial to understand plant development as a whole. From a geometric perspective, this means that plant morphogenesis mainly depends on the cell growth rate and growth anisotropy. Because plant cells do not migrate, and usually do not go through apoptosis in young tissues, plant morphogenesis primarily relies on cell elongation and cell division. Cell shape is a primary variable in morphogenesis in all kingdoms, either as a building block for multicellular shape or because cell shape in turn biases the behavior of structural elements (e.g., cytoskeleton) or morphogens.
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