Proposed working model of the way in which the PtoWRKY40–PtoPHL3 module regulates PSR in poplar. Under Pi-sufficient conditions, PtoWRKY40 interacts with itself to bind to the W-box of the PtoPHT1s promoter and to inhibit gene expression, while PtoWRKY40 interacts with PtoPHL3 to prevent the binding of PtoPHL3 to P1BS and further to inhibit the expression of PtoPHT1s. Under Pi starvation conditions, PtoWRKY40 is inhibited at the transcriptional level, and the PtoWRKY40 protein can be degraded via the ubiquitination-mediated 26S proteasome degradation pathway. PtoPHL3 is therefore released to interact with itself and promotes expression of PtoPHT1s.