Phosphate (H2PO4-), as a source of phosphorous is one of the most critical macronutrients for plants. However, it is frequently not readily available to plants as it is often found in low concentrations and bound to other components of the soil. Therefore plants have developed strategies to adjust to phosphate starvation with a range of adaptations such as alterations of primary root length, number of lateral roots or secretion of organic acids. Additionally, phosphate-depleted plants increase the expression of genes involved in phosphate acquisition, e.g. phosphate transporters and purple acid phosphatases. Phosphite (H2PO3-) is a more reduced form of phosphorous than phosphate. Although physiologically inert and not metabolised, phosphite is able to suppress the phosphate starvation response which results in an exacerbated phosphate depletion leading to a inhibition of plant growth. Similarly, phosphite can lead to toxicity at elevated levels, likely through interference with phosphate-dependent reactions or remobilisation. These observations indicate a recognition and uptake capacity of the plant for phosphite similar to that of phosphate. In addition, phosphite is used as a fungicide against plant pathogens, especially oomycetes such as Phytophthora species, which have devastating effects in agriculture and ecosystems (e.g. Phytophthora Dieback). Phytophthora species are a major threat for Australia’s biodiversity with more than 3000 species under threat in the Western Australia alone. So far the only reliable measure to control these pathogens has proven to be phosphite for which the mode of action is yet to be elucidated. To gain a better understanding on the effects of phosphite on plant growth and defence we have started to investigate the natural genetic variation of phosphite-sensitivity in Arabidopsis thaliana. Findings from this research will improve our knowledge of the mode of action of phosphite on plant defence responses, and might also have implications for the understanding of phosphate signalling or metabolism. Investigations into the genetic background of phosphite-dependent adaptation have been made by observing the phenotypic responses of 19 different Arabidopsis accessions. We will present first results obtained by screening for root phenotypes on media containing a range of phosphite / phosphate ratios.