Phosphate starvation response 1 (PHR1): a versatile master regulator shaping plant resilience beyond phosphate deprivation

In the last two decades, AtPHR1 and its homologs in Arabidopsis and other plant species have emerged as undisputed master regulators of phosphorus starvation response (PSR). The role of SPX proteins as the negative regulators of PHR1 activity and binding of this transcription factor to the P1BS element of the target Pi starvation inducible genes during the regulation of PSR is well established. Given the centrality of phosphate in cell structure, metabolism, and functioning, and modulation in the expression of hundreds of genes upon Pi starvation, the roles of PHR1 proteins are anticipated beyond atypical PSR. The newly emerged evidence implicates PHR1 in the direct regulation of processes such as hypocotyl-root-cotyledon growth during early seedling establishment, nitrogen-phosphorus balance, anthocyanin and proline biosynthesis, jasmonic acid responses, mycorrhizal symbiosis, and abiotic stress adaptation. These diverse functions of PHR1 seemingly arise from the well-distributed roles among PHR1 homologs within a species and their dynamic interactions with other regulatory proteins. In this review, we explore recent advances revealing PHR1's involvement in a wide array of plant processes, including hormonal cross-talk, abiotic and biotic stress responses, and developmental regulation. We take cues from emerging research across multiple crop species to provide a timely synthesis of PHR1's multifaceted functions and its potential as a target for crop improvement under nutrient and environmental constraints.