Elucidating the role of a GTPase-regulating protein family during tip growth initiation and regulation in roots

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The diversity of cellular shapes among eukaryotes depends on the ability of cells to define polar growth axes and regulate the rate of cell expansion. The interdependence of form and function is particularly evident in cells that use directed growth to explore and invade their environment and to penetrate dense substrates. To initiate a new growing tip, cells concentrate their resources and maximize mechanical forces at a confined spot at the plasma membrane. Such polarized growth is a common feature among eukaryotes and can be observed, for instance, in fungal hyphae, neuronal axons, and plant pollen tubes and root hairs. While numerous components and regulators of polarized growth in various cell types and organisms have been identified, fundamental questions remain to be answered, such as how the position of a polar domain is defined, how the growth machinery is assembled, or how tip growth is initiated, regulated and ultimately completed.In order to elucidate the mechanisms of cell morphogenesis in plants, we investigate the formation of root hairs, tip-growing structures of trichoblast cells, a cell type of the root epidermis. Root hairs help plants to extract water and mineral nutrients from soil, to anchor the root and to interact with microbes in the rhizosphere.In this project, we are focussing on the protein family of growth-regulating RopGEFs and their functions during cell polarization and tip growth regulation. RopGEFs are responsible to activate molecular switches, so-called ROP-GTPases, that integrate intra- and extracellular signals to modulate the growth machinery. We recently found that the polar association of RopGEFs with the plasma membrane is specifically timed accross different stages of root hair development: some members are involved in marking the site of outgrowth, while others have a regulatory function after growth has begun. Here we aim to elucidate the mechanisms that underlie this timed polar association and activity, as well as the different functions that are exerted by different RopGEFs. The findings of this study are expected to lead us to a better understanding of cell morphogenesis and the the emergence of tip-growing structures in plant cells.