Objectives For several decades, PHR and SPX proteins are critical for the phosphate (Pi) signaling pathway. More research discovered that they were required for the (Pi) signaling pathway and played an essential role in the absorption of other nutrients. To comprehensively understand the function of the SPX-PHR module, we summarized the function of PHR and SPX in the development of plant root system, nutrient absorption, transport and redistribution.

Major advances Phosphate starvation response transcription factors (PHR TFs) regulate the transcription of downstream genes by binding to P1BS element of target genes and then participate in plant root development, nutrient uptake, transport, distribution and immune response. On the other hand, the transcription level of PHR TFs is up-regulated by different signals in a plant, such as an auxin signaling pathway ARF7/19, ethylene signaling pathway EIN3, light signaling pathway FHY3, and FAR1. However, the expression of HR or PHL TFs is inhibited by HY5 and PIF4/5. Studies have found that SPX proteins interact with PHR TFs and inhibit the transcription activation of PHR TFs. The transcription level of SPXs and their protein turnover is also regulated by nitrogen and phosphorus signaling pathway. Both NRT1.1(B)-NBIP1 in the nitrogen signaling pathway and SDELs in the phosphorus signaling pathway can mediate the degradation of SPX protein by the 26S protease complex pathway. Subsequently, NLP/PHR is released into the nucleus to activate nitrate and phosphate response genes. At the same time, NLP/PHR can activate the expression of NIGT1 to regulate the expression of nitrate and phosphate response genes.

Expectations  To understand the function of the SPX-PHR module in the nutrients signaling pathway, we need to identify the upstream genes of PHR TFs and identify the role of proteins interacting with SPXs in future.