To cope with low phosphate (Pi) availability, plants have to adjust its gene expression profile to facilitate Pi acquisition and remobilization. Sensing the levels of Pi is essential for reprogramming the gene expression profile to adapt to the fluctuating Pi environment. AtPHR1 in Arabidopsis and OsPHR2 in rice are central regulators of Pi signaling, which regulates the expression of phosphate starvation-induced (PSI) genes by binding to the P1BS elements in the promoter of PSI genes. However, how the Pi level affects the central regulator to regulate the PSI genes have puzzled us for a decade. Recent progress in SPX proteins indicated that the SPX proteins play important role in regulating the activity of central regulator AtPHR1/OsPHR2 in a Pi dependent manner at different subcellular levels.