A novel purple acid phosphatase gene (MtPAP1) was isolated from the model legume Medicago truncatula Barrel Medic. The cDNA was 1 698 bp in length with an open reading frame (ORF) of 1 398 bp capable of encoding an N-terminal signal peptide of 23 amino acids. The transcripts of MtPAP1 were mainly detected in leaves under high-phosphate conditions, whereas under low-phosphate conditions the transcript level was reduced in leaves and increased in roots, with the strongest hybridization signal detected in roots. A chimeric gene construct fusing MtPAP1 and GFPwas made in which the fusion was driven by the CaMV35S promoter. Transgenlc Arabidopsis plants carrying the chimeric gene constructs showed that the fusion protein was mainly located at the apoplast based on confocal microscopic analysis, showing that MtPAP1 could be secreted to the outside of the cell directed by the signal peptide at the N-terminal. The coding region of MtPAP1 without signal peptide was inserted into the prokaryotic expression vector pET-30a (+) and overexpressed in Escherlchia coll BL21(DE3). The acid phosphatase (APase) proteins extracted from bacterial culture were found largely based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. An enzyme activity assay demonstrated that the APase activity in the transformed bacteria was 3.16-fold higher than that of control. The results imply that MtPAP1 functions to improve phosphorus acquisition in plants under conditions of phosphorus (P) stress.