Rice SPX6 negatively regulates the phosphate starvation response through suppression of the transcription factor PHR2

Phosphorus (P) is an essential macronutrient for plant growth and development, but the molecular mechanism determining how plants sense external inorganic phosphate (Pi) levels and reprogram transcriptional and adaptive responses is incompletely understood. In this study, we investigated the function of OsSPX6 (hereafter SPX6), an uncharacterized member of SPX domain (SYG1, Pho81 and XPR1)-containing proteins in rice, using reverse genetics and biochemical approaches. Transgenic plants overexpressing SPX6 exhibited decreased Pi concentrations and suppression of phosphate starvation-induced (PSI) genes. By contrast, transgenic lines with decreased SPX6 transcript levels or spx6 mutant showed significant Pi accumulation in the leaf and upregulation of PSI genes. Overexpression of SPX6 genetically suppressed the overexpression of PHOSPHATE STARVATION RESPONSE REGULATOR 2 (PHR2) in terms of the accumulation of high Pi content. Moreover, direct interaction of SPX6 with PHR2 impeded PHR2 translocation into the nucleus, and inhibited PHR2 binding to the P1BS (PHR1 binding sequence) element. SPX6 protein was degraded in leaves under Pi-deficient conditions, whereas it accumulated in roots. We conclude that rice SPX6 is another important negative regulator in Pi starvation signaling through the interaction with PHR2. SPX6 shows different responses to Pi starvation in shoot and root, which differ from those of other SPX proteins.