Functional characterization of foxtail millet purple acid phosphatase SiPAP10 in enhancing phosphorus deficiency tolerance

Soil phosphorus (P) deficiency is a major limitation to crop production globally. Purple acid phosphatases (PAPs) play essential roles in plant P uptake and metabolism. However, the functional characterization of the PAP gene family in foxtail millet ( Setaria italica ) remains poorly understood. In this study, P deficiency significantly promoted root development and enhanced root-associated acid phosphatase (APase) activity in foxtail millet, with a stronger response in the P-tolerant genotype B248. We identified a total of 25 SiPAP genes containing at least one MYB-related cis-element in their promoter regions. Association analysis using 188 foxtail millet accessions showed that 63 single nucleotide polymorphisms (SNPs) from seven SiPAP genes were significantly associated with leaf APase activity, plant height, and root length under P deficiency ( p < 0.001). P deficiency significantly induced the expression of 13 SiPAP genes in the roots, with induction patterns varying with stress duration and genotype. Notably, the plasma membrane-localized SiPAP10 exhibited earlier and stronger induction in B248. Functional validation showed that heterologous overexpression of SiPAP10 in Arabidopsis and rice significantly promoted root growth and enhanced tolerance to P deficiency by increasing APase activity, P concentration, and antioxidant enzyme activities (SOD and POD). Yeast one-hybrid and dual-luciferase assays showed that SiMYB1a, a Zuotin-related transcription factor containing DnaJ and MYB-like domains, specifically binds to the SiPAP10 promoter, and its expression was downregulated in B248. Collectively, these findings indicate that SiPAP10 enhances P efficiency by regulating root architecture and phosphatase activity, and that differential regulation by the repressor SiMYB1a contributes to genotypic variation in P-deficiency tolerance in foxtail millet.