Regulation of auxin transport

Research project P 13948 Regulation of auxin transport Christian LUSCHNIG 11.10.1999 The plant hormone auxin is an important effector in the regulation of cell division, cell elongation and differentiation processes. Within this context, distribution of the growth regulator to target sites appears to be crucial for the hormonal responses of the entire plant. In an effort to isolate genes involved in auxin transport the potential auxin transporter EIR1/AGR1/AtPIN2 was isolated from the model organism Arabidopsis thaliana. Moreover, experiments performed in both Arabidopsis as well as in the heterologous host Saccharomyces cerevisiae provided strong evidence that this gene encodes the catalytic subunit of a root-specific auxin efflux carrier complex. Several lines of evidence suggest that auxin efflux is the target of a number of different plant signal transduction pathways responding to a variety of parameters such as light, ethylene as well as temperature. The aim of this project is a characterisation of some of these regulatory factors which influence the activity of EIR1 at a. post- translational level. The strong 5-fluoro-indole resistance phenotype, mediated by overexpression of EIR1 in yeast offers the opportunity for a rather simply in vivo interaction screen in the heterologous; host. Any alterations in this phenotype obtained in yeast strains, overexpressing EIR1 and potentially interacting Arabidopsis proteins, would indicate a finctional interaction among these two proteins. After demonstrating protein-protein interaction by using standard techniques it is planned to introduce the newly isolated regulators of auxin transport into plants. Consequences of ectopic expression of these regulators as well as expression of several mutant alleles of EIR1 in Arabidopsis will be tested. Characterisation of interactions among factors involved in auxin transport in Arabidopsis should provide a major contribution towards the understanding of auxin involvement in plant

Plant growth can be considered as a highly dynamic process that is controlled by intrinsic developmental blueprints and by a variety of mechanisms, which allow the plant to respond to variations in environmental parameters. Among the internal factors that are known to play a crucial role for the regulation of plant growth, the plant hormone auxin turned out to be essential for a variety of growth responses, including the control of cell division, but also of subsequent cell differentiation processes. A prerequisite for the impact of auxin on plant development is the control of hormone concentrations in any given tissue. Besides metabolic control, which affects auxin homeostasis via its biosynthesis or catabolic inactivation, the control of auxin transport is essential for the regulation of auxin responses. In particular, the regulation of auxin efflux from the interior of the cell appears to represent a potential target for a variety of regulatory signaling cascades. Based on the genetic and molecular characterization of the putative auxin efflux carrier EIR1 (for ETHYLENE INSENSITIVE ROOT1, also known as AGR, WAV6 and AtPIN2) we intended to characterize some of these regulatory mechanisms that might affect auxin transport in planta. Experiments performed as a part of this project led to the characterization of a substrate- dependent control of EIR1 gene expression. More specifically, auxin the likely substrate for EIR1 turned out to control the expression of the likely auxin carrier at the transcriptional and the post-transcriptional level. Regulatory circuits, characterized in these studies, include feedback but also feed-forward control that seemingly depend on the overall concentration of auxin. Our current model suggests that such regulatory switches might be essential for the fine-tuning of EIR1 expression in response to variations in hormone concentrations. Future work in the project leader`s laboratory is essentially based on the findings made throughout this project and should eventually lead to a concise characterization of cis-and trans-acting elements that affect EIR1 expression, thereby playing an important role in the overall control of plant growth and development.