Improved phosphorus acquisition and biomass production in Arabidopsis by transgenic expression of a purple acid phosphatase gene from M. truncatula

Phosphorus (P) is one of the least-available nutrients in soils; improvement in plant P acquisition will significantly impact agriculture and the environment. A purple acid phosphatase (MtPAP1) cDNA was isolated from the model legume Medicago truncatula. During growth in high-phosphate conditions, MtPAP1 transcripts were highest in leaves, while under low-phosphate conditions, MtPAP1 transcript levels were reduced in leaves and increased significantly in roots. Three chimeric gene constructs were made in which the MtPAP1 was either driven by the root-specific MtPT1 promoter or by the constitutive CaMV35S promoter. The constructs also included either the native signal sequence of MtPAP1 or a patatin signal sequence from potato. Transgenic Arabidopsis plants carrying the chimeric MtPAP1 gene constructs showed 4.6- to 9.9-fold higher acid phosphatase activities in the root apoplast relative to the control plants. The increase in enzyme activity correlated closely with the expression levels of the transgenes. Highly expressed transgenic lines were chosen for each construct and used for further analyses. The expressed acid phosphatase was secreted into the rhizosphere as shown by enzyme activity staining and HPLC analysis of organic P degradation in liquid culture. Compared with empty vector control, all the tested transgenic lines showed a minimum of a two-fold increase in biomass production and total phosphorus content after about 1 month growth when 2 mM phytate was supplied as the sole source of phosphorus. The results clearly demonstrate that overexpression of plant acid phosphatase gene is an effective approach to improve organic phosphorus acquisition.