Abstract
Phosphite (H2PO3 -, Phi) is an analog of phosphate (H3PO4 -, Pi) which is not metabolized by plants and therefore accumulates in the tissue. Because of their close steric resemblance Phi is able to mimic Pi, thus impeding Pi sensing and signalling mechanisms. In Pi limited plants Phi inhibits phosphate starvation responses, i.e. adaptations aimed at coping with limited Pi supply and increasing Pi uptake capacity. Thus, Phi has constrictive effects on plant growth under low P supply. Phi is also commercially marketed as a biostat against oomycete pathogens (e.g. Phytophthora spp.) for agricultural application and even used on an ecosystems scale. Phi directly inhibits pathogen growth through interference with phosphate-dependent metabolism and/or phosphate signaling which is paralleled in plants grown on high phosphite concentrations. In addition, Phi activates plant defense responses, e.g. by SA-dependent induction of defense gene expression or increased callose deposition, by a yet unknown mode of action. We have started to characterize the impact of prolonged phosphite treatment by analyses of growth responses, root development, gene expression and metabolic adjustments. We show that at low levels Phi has indirect impact on plant growth by inhibiting phosphate uptake whereas at elevated tissue contents Phi directly impairs growth. Phi is not able to suppress the induction of lateral roots under P limiting conditions and the up-regulation of a set of phosphate starvation induced genes. Furthermore, phosphite induces specific changes in the abundance of several metabolites, especially amino acids, which may have relevance for the understanding of phosphite induced resistance.