How Mechanical Forces Shape Plant Organs.

Plants produce organs of various shapes and sizes. While much has been learned about genetic regulation of organogenesis, the integration of mechanics in the process is also gaining attention. Here, we consider the role of forces as instructive signals in organ morphogenesis. Turgor pressure is the primary cause of mechanical signals in developing organs. Because plant cells are glued to each other, mechanical signals act, in essence, at multiple scales, through cell wall contiguity and water flux. In turn, cells use such signals to resist mechanical stress, for instance, by reinforcing their cell walls. We show that the three elemental shapes behind plant organs - spheres, cylinders and lamina - can be actively maintained by such a mechanical feedback. Combinations of this 3-letter alphabet can generate more complex shapes. Furthermore, mechanical conflicts emerge at the boundary between domains exhibiting different growth rates or directions. These secondary mechanical signals contribute to three other organ shape features - folds, shape reproducibility and growth arrest. The further integration of mechanical signals with the molecular network offers many fruitful prospects for the scientific community, including the role of proprioception in organ shape robustness or the definition of cell and organ identities as a result of an interplay between biochemical and mechanical signals.