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Abstract

Multicellular development requires coordination of polarity, growth, and division both within individual cells and between neighboring cells. In plants, where cells are immobile and interconnected by cell walls, this cross-scale coordination is critical. Yet, the mechanisms that link cellular processes to multicellular organization remain poorly understood. The recently discovered plant-specific SOSEKI proteins provide robust ‘global’ polarity markers and are a promising new tool to investigate how global polarity is formed. In most contexts, their localization reflects a higher-order ‘global’ polarity field, whereas in the post-embryonic lateral root they instead reflect an organ-autonomous polarity field. Using fluorescent SOSEKI reporters, I investigated how this global polarity field transforms into an organ-autonomous field during lateral root organogenesis. I found that SOSEKI polarity is not globally fixed, but instead exhibits features of a local, mechano-sensitive polarity field. Indeed, mechanical perturbations and analysis of root mechanics showed that SOSEKI localization is responsive to mechanical cues, dissipating in mechanically stressed environments and accumulating in the mechanically heterogeneous environments. Complementary analyses in mutants and chemically treated plants further revealed that intercellular interfaces are critical for stable SOSEKI polarity, implicating the cell wall as a conduit for polarity signals. Furthermore, I show that SOSEKI5, one of the essential family members, has dynamic cell-cycle-linked expression, peaking before division and declining just prior to cytokinesis. To circumvent the lethality of complete SOSEKI loss-of-function, collaborators generated a higher-order mutant combining loss-offunction with knockdown. I found that this mutant exhibited reduced regenerative divisions during wound healing. Together with collaborative data, these results implicate SOSEKI5 in the punctual execution of cell division. These findings establish SOSEKIs as mechano-sensitive proteins that may link multicellular mechanics to the cell cycle. In addition to these functional and conceptual insights, this work takes steps towards plant optogenetics by piloting a strategy for cell-level control of gene expression, and it introduces GreenBraid, a GreenGatebased molecular cloning tool for efficient transgene stacking.