Epidermal root cells as a suitable cell biological model system to unravel PIN2 auxin efflux carrier turnover and differential growth regulation in plants

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The polar distribution of the phytohormone auxin within the plant is crucial for growth, reaction to the environment and for the development of plant organs such as leaves and flowers. Key players in the auxin distribution are auxin efflux carriers of the conserved PIN (Pin-formed) protein family. Developmental and environmental signals (such as gravity and light) are processed by dynamic adjustments in directional vesicle trafficking of the PIN proteins. Among these adjustments are conditional endocytosis and subsequent degradation of PINs in the vacuole. Starting with observations on gravistimulated as well as dark-incubated seedlings, it was demonstrated that a portion of PIN proteins is internalized and sorted into the lytic vacuolar compartment. Further experiments, using pharmacological approaches, PIN2 trafficking to the vacuole was abolished, presumably by interference with pre-vacuolar compartment (PVC) function. Hence, PVC-dependent degradation and therefore down regulation of PIN2 seems crucial for adaptive growth responses, such as gravitropism. However, mechanisms that signal endocytosis and degradation of plasma membrane proteins in plant cells are still poorly understood. Since PIN2 is expressed in roots, a suitable system in this organ has to be established in order to analyze how PIN2 stability affects differential growth regulations. In both primary and secondary roots, the epidermis differentiates in regularly spaced tricho- and atrichoblasts. Trichoblasts display polar outgrowth leading to the formation of root hairs, contributing largely to surface creation for plant-soil interface, while atrichoblasts do not form root hairs.We intend to investigate differential growth regulation by utilizing this easily accessible system of tricho- and atrichoblasts. I propose that growth regulation in the root epidermal cell file is a suitable model to study these fundamental processes on a cellular level. We will particularly assess PIN2-dependent regulation of cellular auxin homeostasis and its contribution to epidermal growth control.