Plant PHR Transcription Factors: Put on A Map

The phosphate starvation response (PHR) protein family exhibits the MYB and coiled-coil domains. In plants, within the either 5' untranslated regions (UTRs) or promoter regions of phosphate starvation-induced (PSI) genes are characteristic cis-regulatory elements, namely PHR1 binding sequence (P1BS). The most widely studied PHR protein family members, such as AtPHR1 in Arabidopsis thaliana (L.) and OsPHR2 in Oryza sativa (L.), may activate the gene expression of a broad range of PSI genes by binding to such elements in a phosphate (Pi) dependent manner. In Pi signaling, PHR transcription factors (TFs) can be selectively activated or deactivated by other proteins to execute the final step of signal transduction. Several new proteins have been associated with the AtPHR1/OsPHR2 signaling cascade in the last few years. While the PHR TF transcriptional role has been studied intensively, here we highlight the recent findings of upstream molecular components and other signaling pathways that may interfere with the PHR final mode of action in plants. Detailed information about transcriptional regulation of the AtPHR1 gene itself and its upstream molecular events has been reviewed.