A major question in biology is how external and internal cues are perceived by the organism and translated into an adequate response at the cellular level. For instance, organ growth in plants involves a phase of cell expansion, which requires extensive rearrangement of cell wall structure and is triggered by systemic cues, such as phytohormones. The complex topic how such cues are interpreted is of major interest in the field of cell biology, which has succeeded in deciphering various intra-cellular signaling pathways. Nevertheless, additional, conceptually different pathways very likely remain to be discovered. A recent example for a conceptually novel pathway is the perception of the phytohormone auxin in the nucleus by direct binding to and regulation of a certain class of E3 ubiquitin ligases, the auxin receptors . Auxin is of pivotal importance in various processes of plant development throughout the life cycle, mainly through its dose-dependent influence on cell proliferation and elongation. In this proposal, we aim to investigate a novel, parallel auxin signaling pathway involving the plant-specific BRX protein that has been recently discovered in the host lab. Unlike the nuclear auxin receptor pathway, this pathway involves plasma membrane to nucleus signaling and responds to alterations in trans-cellular auxin transport, a key phenomenon in plant organogenesis. The aim of this proposal is to solidify the evidence for this pathway and characterize it in more detail to answer two key questions: i) how is BRX, a transcriptional co-activator, targeted to the plasma membrane in a polar fashion? and ii) what is the mechanistic basis for endocytosis-rate and hormone-triggered nuclear import of BRX? We expect that the anticipated results will not only shed light on a novel pathway of pivotal importance in cellular growth of plants, but also conceptually add to our understanding of intra-cellular signaling pathways in general.
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