Assembly of a polar domain - the role of electrostatic interactions at the plasma membrane during the initiation of tip growth in Arabidopsis roots

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Cell polarity is a unifying feature of virtually all living cells. It is characterized by localized specialization of cellular functions that provide, among others, directionality for growth or motility. At the plasma membrane, symmetry breaking and, hence, functional polarization, occurs microscale- and mesoscale domains that can be persistent or short-lived. How local membrane specialization is achieved is under debate for decades. Since most cells are hardly ever unpolarized it is challenging to investigate the initiation of polarity and the formation of polar domains.Plant roots feature a cell type, called hair cells or trichoblasts, that transition from elongation to tip growth and, thus, need to establish a new polar axis and a polar domain at the plasma membrane. How the protein assembly at this polar domain is coordinated and which signals or receptors trigger the initiation of hair formation is not well understood. We take advantage of the high predictability of this process to study the recruitment of specific proteins and lipids to the emerging root hair initiation domain. Our first goal is to analyze the step-wise assembly of proteins and determine specific protein-protein interactions along the time axis of hair development. Our second goal is to unveil mechanisms of the membrane protein recruitment by investigating the role of clustered phosphatidylinositide 4,5-bisphosphate (PIP2), a class of negatively charged phospholipids at the plasma membrane. PIP2 clustering can drive protein recruitment via electrostatic interactions, which can be modulated rapidly by the cell through changes in membrane potential or cytosolic ion concentration.Our third goal is to unveil the function of a novel receptor-like kinase in the initiation of hair emergence. Its specific gene-expression, its early appearance at the root hair initiation domain and an increased hair formation in gain-of-function mutants point toward an involvement of this RLK in triggering or coordinating the formation a new root hair. Taken together, by dissecting the dynamic interaction networks during the assembly of the polar hair initiation domain, by studying a mechanism for dynamic protein recruitment and by investigating a potential coordinator of this process we aim to provide new insights in the establishment of mesoscale domains at the plasma membrane and the initiation of polar growth.