Metabolism depends on inorganic phosphate (P(i)) as reactant, allosteric effector and regulatory moiety in covalent protein modification. To cope with P(i) shortage (a common situation in many ecosystems), plants activate a set of adaptive responses to enhance P(i) recycling and acquisition by reprogramming metabolism and restructuring root system architecture. The physiology of P(i) starvation responses has become well understood, and so current research focuses on the initial molecular events that sense, transmit and integrate information about external and internal P(i) status. Recent studies have provided evidence for P(i) as a signaling molecule and initial insight into the coordination of P(i) deficiency responses at the cellular and molecular level.