Specific members of the Rho GTPase of plants (ROP) family of proteins play a key role in shaping these cells. The processes underlying the formation of these cells are presently unclear, and it has been proposed that they emerge from either the localized outgrowth of lobes (also called protrusions) ( Fu, 2002 Mathur, 2006 Xu et al., 2010 Zhang et al., 2011), localized restriction of indentations ( Fu et al., 2009 Sampathkumar et al., 2014 Lin et al., 2013), or a combination of both ( Fu et al., 2005 Abley et al., 2013 Armour et al., 2015 Higaki et al., 2016 Majda et al., 2017). This results in large elongated cells, such as those in roots and stems, or much more intricate forms, such as the jigsaw puzzle-shaped epidermal cells of Arabidopsis thaliana leaves and cotyledons ( Figure 1A), which we call puzzle cells. Starting from small isodiametric cells in proliferative tissues, cells stop dividing and can expand to over 100 times their original size. have highlighted that plant cell shapes also adapt to mechanical forces, further research is needed to uncover how these forces are sensed.ĭuring growth and morphogenesis, plant cells undergo dramatic changes in size and shape. For example, animal cells can develop into different cell types depending on the stiffness of the surface they are placed on. Mechanical forces are known to have important effects on the shape and behavior of cells from other species too. Instead, mechanical forces and the shape the puzzle cells themselves may transmit this information. does not require such chemical signaling. However, the model developed by Sapala, Runions et al. It had previously been proposed that mobile chemical signals passed between cells coordinate the process by which a lobe in one puzzle cell matches an indentation in its neighbor. This confirmed that puzzle cell shape is related to both organ shape and how isotropically the plant grows. tested this idea by analyzing the shape of organs and cells in many plant species and by genetically modifying growth directions in Arabidopsis thaliana plants. If a plant organ grows mostly in one direction, like in a root or stem, long thin cells are sufficient to reduce the stress on the epidermal cell wall. The simulations show that ‘paving’ the leaf surface with puzzle shaped cells instead of more regularly shaped cells reduces the stress in the epidermal cell walls.Ĭounterintuitively, the simulations also show that complex puzzle shapes develop in parts of the plant that grow isotropically (uniformly in all directions), such as leaves. developed a computer simulation that models how a plant grows and re-creates a variety of realistic puzzle cell shapes. To investigate this possibility, Sapala, Runions et al. This could mean that the shape of puzzle cells is an adaptation used by plants to reduce the stress on their surface. The extent of the stress depends on the shape and size of the cells for example, large cells bulge out and experience more stress than small cells. The pressure inside a cell creates a lot of mechanical stress on the epidermal cell walls – those that make up the surface of the plant. Take away the pressure, and the plant wilts. It is this pressure that gives non-woody plant tissue its shape. Their internal pressure can be higher than the pressure in a car tire. Plant cells are like small balloons surrounded by a strong cell wall. Why do these curious shapes form, and what benefit do they provide to the plant? Cells with complex interlocking shapes, similar to pieces of a jigsaw puzzle, cover the surface of many leaves.
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