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

In the last two decades, the transcription factor phosphate starvation response 1 (PHR1) and its homologues in Arabidopsis and other plant species have emerged as undisputed master regulators of phosphorus starvation response (PSR). The role of Suppressor of Yeast Gpa1 (SYG1)/PHOsphate metabolism 81 (PHO81)/Xenotropic and Polytropic Retrovirus receptor 1 (XPR1) (SPX) proteins as negative regulators of PHR1 activity, and binding of this transcription factor to the P1BS element of the target phosphate starvation-inducible genes during the regulation of PSR, is well established. Given the centrality of phosphate in cell structure, metabolism and function, and modulation of the expression of hundreds of genes upon Pi starvation, the roles of PHR1 proteins are anticipated beyond the atypical PSR. 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 homologues within a species, and their dynamic interactions with other regulatory proteins. In this review, we explore recent advances revealing the involvement of PHR1 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 the multifaceted functions of PHR1, and its potential as a target for crop improvement under nutrient and environmental constraints.