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Abstract

Plants are characterized by an unparalleled ability to adapt to their environment by forming organs post-embryonically. In contrast to animal cells, plant cells are encaged in a rigid extracellular matrix, the cell wall, which glues cells tightly to their neighbours. The presence of the cell wall precludes cell migration and determines that plant morphogenesis relies on the ability of cells to control the direction of growth and the orientation of cell division. The microtubule and actin cytoskeletons play an essential role in the control of these processes. Cortical microtubules guide the deposition of cellulose fibrils in the cell wall and in consequence dictate the shape and growth direction of the cell. Actin is essential for the establishment and maintenance of cell polarity and together with microtubules orchestrates cell division. The root system is formed by the continuous growth of root tips and branching of lateral roots. This enables anchoring and efficient absorption of water and nutrients. The branching of lateral root starts with the coordinated division of two pericycle founder cells located in the outer most layer of the main root vasculature. During this phase, called lateral root initiation (LRI), founder cells swell synchronously and their nuclei migrate towards the common cell wall Once nuclei are asymmetrically localized, founder cells divide asymmetrically originating two small daughter cells in the centre and two long daughter cells at the flanks. In this work, we combined cytoskeleton tissue-specific markers and confocal live imaging to determine the role of the cytoskeleton in the coordination of the swelling and nuclear migration of founder cells during LRI. We observed that founder cells expand more in the central domain than in the periphery, a process amplified by the asymmetric positioning of the nucleus and pursued after the first asymmetric cell division (ACD). We observed that after ACD, microtubules reorganise in a parallel array at the periphery and remain more isotropic in the central domain. Genetic and pharmacological perturbation of microtubule organization, support a model in which the graded organization of microtubules constraints radial expansion at the periphery and allow fast expansion of the central cells. We detected that before cell division actin bundles reorganize in a polarized mesh around the nucleus. Pharmacological disruption of actin network revealed that actin is essential for the migration of the nucleus, the asymmetry of cell division and cell expansion. Auxin is an essential regulator of LRI that controls in particular the expression of LBD16, a transcription factor involved in the polar migration of nuclei during LRI. Expression of the repressor LBD16-SRDX abolishes nuclear migration and asymmetric cell division. We observed that cytoskeleton polarity and organization are disrupted in the LBD16-SRDX background, where asymmetric cell expansion is also impaired. After initiation, founder cells divide and expand to form a dome shaped primordium (LRP) in response to the spatial accommodation of the overlaying endodermis. In a second part of this work, we observed that disturbing microtubule dynamics hinders the spatial remodelling of the endodermis and delays the growth of the LRP. We found that microtubule organization in the endodermis is dependent on SHY2-mediated auxin signalling and shows a cell face-determined pattern. In conclusion, our results evidence the importance of cytoskeleton dynamics during LRI and the spatial accommodation of the endodermis and reveal a role for auxin signalling in these processes.