Significance When P levels are low, plants activate an array of adaptive responses to increase efficient acquisition and use of phosphate (Pi), the form in which P is preferentially absorbed, and to protect themselves from Pi starvation stress. Considerable progress has been made recently in dissecting the plant Pi starvation signaling pathway. Nonetheless, little is known as to how Pi levels are perceived by plants. Here, we identify the nuclear protein SPX1 as a Pi-dependent inhibitor of DNA binding by PHOSPHATE STARVATION RESPONSE 1 (PHR1), a master regulator of Pi starvation responses. We show that the Pi dependence of SPX1 inhibition of PHR1 activity can be recreated in vitro using purified proteins, which indicates that the SPX1/PHR1 module links Pi sensing and signaling.

SPX1 is a phosphate-dependent inhibitor of PHOSPHATE STARVATION RESPONSE 1 in Arabidopsis