New findings on the regulation of phosphate homeostasis.
As part of her research at the University of Geneva, IPB junior scientist Martina Ried, together with partners from Germany, France and Switzerland, has gained new insights into the regulation of plant phosphate homeostasis. Phosphorus, as an essential nutrient, is taken up by all organisms in the form of inorganic phosphate (Pi) and kept in balance by fine-tuned Pi sensing and signaling cascades. Different derivatives of the signaling molecule inositol pyrophosphate (PP-InsP) play a key role in Pi homeostasis. In plants, Pi starvation response transcription factors (PHRs) regulate intracellular phosphate concentration. At sufficiently high Pi concentrations, PHRs are bound to a repressor (SPX) and consequently inactivated. The binding between PHR and its SPX repressor is facilitated by inositol pyrophosphate, which accumulates in the cell under sufficient phosphate conditions. However, the exact mechanism of this PP-InsP-mediated coupling was previously unknown.
Ried et al. have now figured out that a PP-InsP-SPX complex recognizes the plant unique coiled-coil (CC) domain of PHRs in plants. Crystal structures of this CC domain showed an unusual four-stranded anti-parallel arrangement of the distinct helices. Interface mutations in the CC domain yield monomeric PHR, which is no longer able to bind DNA with high affinity. Under Pi starvation, when PP-InsP levels are reduced, SPX–PHR complexes dissociate, enabling the transcription factors to activate the corresponding Pi starvation response genes. The high-affinity binding of PHR to its promoters is thereby only possible in the oligomeric state of the transcription factor. A better understanding of this central process of Pi homeostasis may, in the long term, contribute to the development of Pi starvation resilient crops. Given the alarming scarcity of rock phosphate, this is one of the major challenges facing civilization today.
