Control of EIR1 expression and its implications for root growth

The phytohormone auxin plays a central role in plant growth and development. In recent years, some elements required for auxin signal transduction have been isolated in the model organism Arabidopsis thaliana. Among those, proteins involved in the control of protein stability, gene expression and auxin transport have been characterized. One of the central questions remaining is, how all these different components interact in auxin signaling events. The regulation of auxin distribution within a plant is thought to play a critical role in both the control of plant morphogenesis and a variety of growth responses. With the characterization of proteins that appear to catalyze the active transport of auxin, the molecular tools for studying the regulation of auxin transport are now available. Specifically, by using the auxin efflux carrier ETHYLENE INSENSITIVE ROOT 1 (EIR1), it could be shown that EIR1 expression depends upon the activities of some genes, which have previously been described to participate in different auxin signaling events. Thus, so far unrelated, aspects of auxin signaling appear to merge at the level of EIR1 regulation, indicating a central role of EIR1 in auxin signaling pathways. The goal of this application is a deeper, more fundamental characterization of the role of EIR1 in these pathways that should lead to a better understanding of how auxin controls the life cycle of a higher plant.

Plants as sessile organism have evolved an amazing plasticity with respect to overall growth and development. Such adaptive growth response depend on a variety of signal perception and transduction mechanisms involved in sensing and responding to environmental changes, in general. A key component involved in the transduction of such external signal is represented by the plant growth regulator auxin. This tiny molecule is distributed throughout the entire plant body, thereby regulating a multitude of developmental processes. A tight regulation of auxin transport thus plays a central role in the regulation of plant growth. PIN proteins, a class of membrane proteins, were demonstrated to act as a rate-limiting determinant in the control of auxin transport. Regulation of these PINs involves control of gene expression, intra-cellular localization as well as degradation in a highly tissue-specific manner. In the course of this project determinants that control PIN levels via diverse regulatory mechanisms have been analyzed. Specifically, auxin itself could be demonstrated to affect PIN expression via control of gene transcription, localization as well as degradation. Auxin, thus, modulates the activity of its transport machinery, via a set of sophisticated mechanisms, which orchestrate the distribution of auxin in response to environmental as well as intrinsic developmental cues.