Conserved mechanical hallmark guides four-way junction avoidance during plant cytokinesis

When building an organ, adjacent cells coordinate to form topologically stable junctions by integrating mechanosensitive signaling pathways. Positioning the newly formed tricellular junction in walled multicellular organisms is critical, as cells cannot migrate. The cell division site must avoid existing cellular junctions to prevent unstable four-way junctions. The microtubule preprophase band (PPB) is a guideline for the plant cell's future cell division site. Yet, mutants impaired in PPB formation hardly display defects, suggesting the presence of an alternative mechanism. Here, we report the existence of a process that guides the cell division site close to an adjacent tricellular junction. This PPB-independent mechanism depends on the phosphoinositide phosphatase SUPPRESSION OF ACTIN 9 (SAC9): in the Arabidopsis mutant sac9-3, the cell plate abnormally attaches at two equidistant positions from an adjacent tricellular junction. Numerical simulations suggest that elastic energy accumulation at the subcellular level within walls, due to their rheological response to mechanical stresses, could act as positional cues revealing the positions of tricellular junctions. Moreover, perturbation of the root mechanical homeostasis results in four-way junction formation. This provides a scenario in which cells avoid four-way junction formation during cytokinesis through discrete subcellular mechanical hallmarks.