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The ability to detect and respond to environmental changes is critical for all living organisms. One research project in my laboratory is using the genetic model Arabidopsis thaliana to study environmental stress perception and signal transduction. A major focus of our work is the dissection of the regulatory pathways that control intracellular sodium ion (Na+) homeostasis in organisms exposed to excessive salt. Plant cells are especially sensitive to NaCl in the environment because Na+ disrupts K+ uptake by root cells and, when Na+ enters into cells, it becomes toxic to enzymes. In order to prevent growth inhibition or cell death, excessive Na+ must be extruded from the cell or compartmentalized in the vacuole. Therefore, proper regulation of the proteins that mediate ion flux is necessary for cells to keep concentrations of toxic Na+ ions low and to accumulate essential ions. Recently, through the identification of Arabidopsis mutants that are salt overly sensitive (sos) and the cloning and characterization of the SOS genes, a novel signaling pathway that mediates ion homeostasis and salt tolerance in Arabidopsis has been discovered. In this pathway, a myristoylated calcium (Ca2+)-binding protein, SOS3, senses cytosolic Ca2+ changes elicited by salt stress. SOS3 physically interacts with and activates the protein kinase, SOS2. The SOS3-SOS2 kinase complex phosphorylates and activates the transport activity of the plasma membrane Na+/H+ exchanger encoded by the SOS1 gene. To fully understand how intracellular Na+ levels are regulated during salt stress, it will be necessary to determine how the individual proteins that are involved in the transport of Na+ (H+-pumps and Na+ transporters) are regulated and how they function together. The focus of this project is to determine the structure and function relationships of identified SOS pathway components, and to identify additional pathway targets and the mechanisms that underlie their regulation. Environmental stresses such as salinity limit crop productivity worldwide. Understanding how plants respond to high levels of salt is essential for designing strategies to combat the yield-reducing effects of this stress. While studies with Arabidopsis thaliana have identified salt tolerance determinants that are conserved in organisms ranging from cyanobacteria to fungi, algae to higher plants, what is missing from these studies is a functional understanding of if and how these elements alleviate the consequences of high levels of salt in plants that are naturally tolerant (halophytes). Our goal is to provide this information. In a second project in my laboratory, we are determining the function of the SOS pathway components Thellungiella halophila, a genetically tractable halophytic relative of Arabidopsis. Results from these studies will provide a critical starting point for the identification of traits enabling successful plant habitation of extreme environments. In addition, comparisons of SOS pathway function and regulation in two closely related species that differ in their ability to grow in salt will enable us to explore the evolutionary basis of plant salt tolerance. Ultimately, these studies may enable us to determine the feasibility of transferring important traits to crop plants and allow better manipulation of character evolution to meet our agricultural needs.