The amino acid sequence of the trans-membrane protein, PM19, unique to the plant kingdom has been highly conserved over 450 million years of evolution in all plant genera including ferns and mosses, but is not found in aqueous plants such as algae. Thus, the function of this protein is possibly linked to the plant ability to grow on land. We have investigated the PM19 protein using a number of molecular biological tools in Arabidopsis thaliana. Gene expression studies using bioinformatics, northern blotting and promoter-GUS fusions show that the AtPM19 gene is highly expressed in seeds and seedlings, in addition to expression observed in leaves under drought stress. A translational fusion with GFP reveals that the protein is located in the plasma membrane. T-DNA insertion mutants have a germination phenotype; the mutant is more sensitive to high levels of salts in the medium, and in addition, the mutant has a lower stomatal conductance indicative of reduced guard cell turgor. The predicted secondary structure of the protein and the mutant phenotype suggest that PM19 may be a cation transporter and this is being tested by functional complementation of yeast mutants.