14-3-3 proteins GRF6 and GRF8 negatively regulate phosphate acquisition by interacting with and inhibiting PHR1 in Arabidopsis.

Phosphate starvation response1 (PHR1) is a central transcription factor necessary for low phosphate (LP) response in plants, thus its activity is tightly regulated in plants. Here, we report that two members of the 14-3-3 protein family, GRF6 and GRF8, regulate PHR1 activity and LP response independently of SPX1 in Arabidopsis thaliana. GRF6 and GRF8 physically interact with PHR1, while disruption of both genes causes increased expression of PHR1-targeted phosphate transporter genes and elevated phosphate acquisition. Genetically, PHR1 acts downstream of GRF6 and GRF8, as the phr1 grf6/8 triple mutant phenocopies the phr1 single mutant under phosphate-deficient conditions. Furthermore, both GRF6 and GRF8 suppress PHR1 transcriptional activation activity, at least partially through interfering with PHR1 binding to its target genes. In addition, neither GRF6 nor GRF8 interacts with SPX1, a well-known inhibitor of PHR1; nevertheless, mutation of both GRF6 and GRF8 could further enhance the phosphate starvation response in the spx1 mutant, suggesting that GRF6 and GRF8 act in parallel with SPX1 in the negative regulation of plant LP response. Collectively, our study uncovers a critical role of GRF6 and GRF8 in the negative regulation of phosphate acquisition through interacting with and inhibiting PHR1, possibly in an SPX1-independent manner in Arabidopsis.