Comparative Secretome Profiling of Arabidopsis Seedlings Under Pi Starvation Unveils the Regulatory Role of PHR1.

Phosphate (Pi) starvation severely limits plant growth, and the transcription factor PHR1 is a central regulator of the phosphate starvation response (PSR). However, its role in modulating the secreted proteome remains poorly understood. Here, we performed comparative secretome profiling of wild-type (WT) and phr1 mutant Arabidopsis seedlings under Pi-sufficient (+P) and Pi-deficient (-P) conditions using LC-MS/MS. We quantified 2077 proteins from culture media across four biological replicates. Pi starvation induced 394 differentially abundant secreted proteins in WT, while the phr1 mutation severely attenuated this response, particularly down-regulating 213 proteins under -P conditions. PHR1 was essential for the starvation-induced secretion of phosphate-acquisition enzymes, including GDPD6, PAP1, and RNS1, and its role in the promotion of extracellular phosphatases was confirmed by acid phosphatase activity assays. Additionally, PHR1 modulated redox-related and defense-related secreted proteins, linking nutrient stress with oxidative and immune signaling. Integrative transcriptomic analysis revealed partial concordance between protein and mRNA levels, suggesting both transcriptional and post-transcriptional regulation. Collectively, our findings establish PHR1 as a vital regulator of the Pi-starvation-induced secretome and provide new insights into plant adaptation to phosphorus limitation through coordinated protein secretion.