Abstract:
Morphogenesis in multicellular organisms relies on mechanical forces, cell geometry, and biochemical cues such as hormones, and these elements must act in concert. Using *Arabidopsis thaliana* as a model, I investigated these aspects of morphogenesis and uncovered how they contribute to the *denovo* formation of a shoot meristem and the regeneration of a tapered root tip. Through my investigations of the fundamental processes governing tissue culture-mediated shoot regeneration in Arabidopsis, I discovered the critical role of mechanical forces in the making of a shoot meristem *denovo*. The *de novo* shoot meristem is made from a select few cells, named shoot progenitors, from the large pool of undifferentiated mass of cells, called callus. A cell-wall-loosening enzyme, XTH9, is expressed exclusively in a shell of cells surrounding the progenitors. This creates mechanical heterogeneity, with progenitor cells experiencing compression and their neighbors undergoing stretching. This conflict in mechanical forces activates intrinsic cell polarity proteins like SOSEKI and establishes the biochemical environment necessary for shoot meristem formation. Based on these findings, I propose a "stretch-compress" model to illustrate how these mechano-biochemical effects instruct formation of proper progenitors, and their progression into complete shoot systems. I further investigated the acquisition of specific cell geometry and its role in wound repair using root tip resection as an experimental system. The plant roots are tapering to support soil penetration and this tapering portion of the root tip is excised out during root tip regeneration. I investigated how damaged plant roots regain their tapered shape and suggest a two-step process driving this restoration. I discovered that the first step features the generation of a specific cell geometry, which are rhomboid-shaped, through differential between neighboring cells. The second step uses this specific cell geometry as template to establish arrangements, such as a unique diagonal division plane and redirecting the vertical longitudinal cell files into inclined trajectories. This redirection narrows the root’s diameter, re-establishing the tapering shape. I found that a gradient-expressed transcription factor (TF) conditioned the cells for their deformation into rhomboid shapes. This previously unknown shape-forming mechanism highlights how local cell geometries trigger tapering, offering new insights into wound repair in plants. In summary, my study highlights the fundamental role of cell geometry and mechanochemical feedback as key regulators of tissue morphogenesis, offering new insights into the forces shaping developmental processes in plant regeneration.
