From far away, the top of a leaf looks like one seamless surface; however, up close, that smooth exterior¹ is actually made up of a patchwork²（拼缀物） of cells in a variety of shapes and sizes. Interested in how these cells individually take on their own unique forms, Caltech biologist Elliot Meyerowitz, postdoctoral scholar Arun Sampathkumar, and colleagues sought to pinpoint³ the shape-controlling factors in pavement cells, which are puzzle-piece-shaped epithelial（上皮的） cells found on the leaves of flowering plants. They found that these unusual shapes were the cell's response to mechanical stress on the microtubule cytoskeleton（细胞骨架） -- protein tubes that act as a scaffolding inside the cells. These microtubules guide oriented deposition⁴ of cell-wall components⁵, thus providing structural⁶ support. The researchers studied this supportive microtubule arrangement in the tissue of pavement cells from the first leaves -- or cotyledons -- of a young Arabidopsis thaliana plant. By fluorescently marking the cells' microtubules (yellow, top surface of cell; purple, bottom surface of cell), the researchers could image the cell's structural arrangement -- and watch how this arrangement changed over time. They could also watch the microtubule modifications⁷ that occurred due to changes in the mechanical forces experienced by the cells.

 

Microtubules strengthen a cell's structure by lining⁸ up in the direction of stress or pressure experienced by the cell and guiding the deposition of new cell-wall material, providing a supportive scaffold for the cell's shape. However, Meyerowitz and colleagues found that this internal stress is also influenced by the cell's shape. The result is a feedback loop: the cell's shape influences the microtubule arrangement; this arrangement, in turn, affects the cell's shape, which modulates⁹ the microtubules, and so on. Therefore, the unusual shape of the pavement cell represents a state of balance -- an individual cell's tug-of-war to maintain structural integrity while also dynamically responding to the pushes and pulls of mechanical stress.

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