SPX4 acts on PHR1-dependent and -independent regulation of shoot phosphorus status (Plant Physiol)

Plant phosphorus homeostasis is important to understand as humans seek to increase food production while also reducing the amount of agricultural inputs and environmental pollution. In their recent paper, Osorio and colleagues expand our current understanding of phosphorus homeostasis in Arabidopsis thaliana through their focus on SPX4 regulation of the phosphorus starvation response. SPX4 is one of a four-member family of negative regulators of phosphorus uptake in Arabidopsis. The authors identified that SPX4 abundance is controlled by the rate of protein turnover based on internal phosphorus concentration. However, unlike rice OsSPX4, this turnover isn’t based on common proteasomal degradation, nor is it responsive to external phosphite (Phi) supply. SPX4 also showed phosphorus-dependent sequestration of PHR1, an important phosphorus starvation response transcription factor, in the cytosol. Furthermore, the use of spx4 mutants demonstrated that misregulation of many the phosphorus starvation response genes was dependent on SPX4, PHR1, and PHO2. (Summary by Nathan Scinto-Madonich)  Plant Physiol.10.1104/pp.18.00594