"Phosphate (Pi)-based fertilizers are essential to agriculture but are a non-renewable resource and a major cause of water pollution. Understanding how plants sense and adapt to Pi deficiency is key to attain sustainable agriculture. Contrary to Pi import, mechanisms regulating Pi export are unknown despite being essential for Pi homeostasis in most eukaryotes (e.g. human kidney or mychorizzal roots). The only protein known to mediate Pi efflux is the Arabidopsis thaliana PHO1. PHO1 mediates Pi efflux when expressed in Xenopus oocytes and is responsible for loading Pi into the root xylem. PHO1 is also involved in Pi deficiency signaling, linking low Pi to reduced growth and gene activation. PHO1 has thus a dual function in transport and signaling, a feature typical of transceptors. The protein contains two distinct domains, called SPX and EXS, and is localized to endosomes. Plant PHO1 homologues have functions other than Pi homeostasis; PHO1;H4 (SHB1) is associated with the nucleus and modulates hypocotyl growth under blue light. This project aims on a detailed structure-function analysis of PHO1 to determine; 1- its topology in endosomes; 2- the domains responsible for Pi export, Pi signaling, and the distinct sub-cellular localization and function of PHO1 and PHO1;H4, 3- the role of phosphorylation on PHO1 localization and activity. Membrane topology will be determined by bimolecular fluorescence and redox-sensitive GFP. Mutagenized and truncated versions of PHO1, as well as chimeras between PHO1 and PHO1;H4 will be expressed, their localization determined by confocal and two-photon microscopy, and their Pi export activity performed in Xenopus oocytes and plants. Signal transduction for Pi deficiency and blue light will be determined by growth assays and gene expression profiling. The information will be transferred to other PHO1-family members with poorly determined function, such as the human XRP1 and other plant proteins containing SPX or EXS domains."