Unveiling the PHR-centered regulatory network orchestrating the phosphate starvation signaling in Chinese fir (Cunninghamia lanceolata)

Phosphorus (P) is an essential mineral element for plant growth and is absorbed and utilized in the form of inorganic phosphate (Pi). However, Pi deficiency largely restricts plant growth in forest ecosystems, while the molecular mechanism of Pi deficiency in woody plants remains unclear. Here, we show that PHOSPHATE STARVATION RESPONSE (PHRs) were central regulators of Pi starvation signaling in Chinese fir, a gymnosperm woody plant. Pi deficiency repressed the shoot growth by decreasing the net photosynthesis rate, reducing the size and number of needle leaves, suppressing the plant height, and reducing the biomass accumulation of shoots in Chinese fir seedlings. Thirteen Chinese fir PHRs (ClPHRs) were characterized, which evolved differently from model and angiosperm woody plants. ClPHRs did not respond to Pi deficiency at the transcriptional level, whereas three ClPHRs responded to Pi deficiency by increasing the nuclear/cytoplasmic protein abundance ratio. Four ClPHRs can restore Pi starvation signaling by activating transcription of AtPHT1;1 and AtPHT1;4 in the atphr1 mutant. Notably, ClPHR7, which is evolutionarily distinct from AtPHR1, was the only ClPHR that could respond to Pi deficiency and restore Pi starvation signals. ClPHR7 could also interact with SPX through protein-protein interaction analysis. Thus, the SPX-PHR regulatory module was also present in gymnosperm woody plants, but the exactly responsible proteins were evolutionarily different from those of model plants. In summary, our results revealed the function of the SPX-PHR regulatory module in Pi starvation signaling and provided genetic information for engineered woody plants with high Pi use efficiency.