Plants acquire phosphorus in the form of phosphate (Pi), the concentration of which is often limited for plant uptake. Plants have developed diverse responses to conserve and remobilize internal Pi and to enhance Pi acquisition to secure them against Pi deficiency. These responses are achieved by the coordination of an elaborate signaling network comprising local and systemic machineries. Recent advances have revealed several important components involved in this network. Pi functions as a signal to report its own availability. miR399 and sugars act as systemic signals to regulate responses occurring in roots. Hormones also play crucial roles in modulating gene expression and in altering root system architecture. Transcription factors function as a hub to perceive the signals and to elicit steady outputs. In this review, we outline the current knowledge on this subject and present hypotheses pertaining to other potential signals and to the organization and coordination of signaling.