Complex Regulation of Plant Phosphate Transporters and the Gap between Molecular Mechanisms and Practical Application: What Is Missing?  Mian Gu, Aiqun Chen, Shubin Sun, Guohua Xu  Molecular Plant  Volume 9, Issue 3, Pages 396-416 (March 2016) DOI: 10.1016/j.molp.2015.12.012 Copyright © 2016 The Authors Terms and Conditions

Figure 1 Regulation of Genes Encoding PHOSPHATE TRANSPORTER 1s (PHT1s) and SPX-Domain-Containing Proteins with Phosphate (Pi) Transport Activities Under Pi-sufficient (A) and Pi-deficient (B) Conditions in Arabidopsis thaliana and Rice (Oryza sativa). (A) Arabidopsis WRKY42 transcription factor (TF) and other unknown TFs are responsible for the basal transcription of PHT1 genes, AtPHO1, OsPHO1;2 (cis-natural transcript of OsPHO1;2 [cis-NATPHO1;2]), OsSPX-MFS1/2. Arabidopsis WRKY6/42 blocks PHO1 transcription by binding to its promoter. PHR TFs are sequestered by SPX1/2 in the nucleus, while OsPHR2 is sequestered by OsSPX4 in the cytosol. Rice PHT1s are phosphorylated by CK2α3/β3 holoenzyme in the endoplasmic reticulum (ER), leading to weakened interaction with PHF1 and thus trafficking to the plasma membrane (PM). Arabidopsis PHO1 and PHT1s are both degraded via the PHO2-involving ubiquitination pathway in post-ER compartments (e.g., the Golgi apparatus). AtPHO1 and AtPHT1s are finally degraded in the vacuole, and the former is mediated by mutivesicular bodies. Arabidopsis PHT1s undergo another ubiquitination pathway involving NLA, which are endocytosed and degraded in the vacuole. A basal amount of OsSPX-MFS1/2 proteins are localized to the tonoplast. ALIX associated with the ESCRT-III complex to facilitate the internalization of PHT1;1 at the MVB for degradation at the vacuole. (B) PHR, Arabidopsis WRKY45 and other unknown TFs are responsible for the transcriptional activation of PHT1 genes, cis-NATPHO1;2, and OsSPX-MFS2. AtPHO1 transcription is induced because of the release of AtWRKY6/42 binding to its promoter, and other unknown TFs might also be required for its expression. cis-NATPHO1;2 promotes OsPHO1;2 translation by its interaction with OsPHO1;2 mRNA. OsCK2β3 is degraded in the ER, thus PHF1 facilitates the exit of the non-phosphorylated PHT1s from the ER and then trafficking to the PM in a COPII-dependent manner. PHO2, which regulates PHT1s and AtPHO1 abundance, is post-transcriptionally regulated by microRNA399s (miR399s). AtNLA and OsSPX-MFS1/2 are negatively regulated by miR827s, yet OsSPX-MFS2 transcripts and probably its protein abundance are increased. Regulatory events in different tissues or cell types are integrated in one cell for simplicity. Gene names in blue refer to Arabidopsis genes; gene names in red refer to rice genes; gene names in black refer to genes of Arabidopsis and rice. Question marks, TFs responsible for the transcriptional activation or basal expression of the phosphate transporter genes are unknown; boxes with dotted outlines, degraded proteins; green circle, COPII-coated vesicle; orange circle, multivesicular body. Molecular Plant 2016 9, 396-416DOI: (10.1016/j.molp.2015.12.012) Copyright © 2016 The Authors Terms and Conditions