Chemical and Biological Mobilization of Soil Phosphorus by ppk-Expressing Transgenic Rice (Oryza sativa L.)

Phosphorus (P) availability in alkaline soils is constrained by the immobilization of legacy P, particularly through calcium phosphate precipitation. We used a confined soil–plant microcosm to examine how expression of the polyphosphate kinase gene (ppk) is associated with legacy P utilization in rice (Oryza sativa L.). We compared ppk-expressing transgenic rice (ETR) carrying single-copy (ETRS) and double-copy (ETRD) insertions with their isogenic wild type (WT), without P fertilization. Both transgenic lines had higher root organic acid release and acid phosphatase activity on day 30 and greater plant P acquisition by day 60 than WT; soil pH declined to a greater extent. On day 30, transgenic roots released more oxalate and malate, and their exudates mobilized more soil P than WT exudates in a separate incubation assay. These traits corresponded to greater depletion of the operationally defined HCl-P and NaOH-P pools. Bacterial profiles and PICRUSt2 predictions were descriptive because one pooled library per genotype precluded tests of genotype effects. Diffusive gradients in thin films (DGT) measurements revealed greater rhizosphere labile P in the ETRD early in plant growth, followed by a decline as plant P concentrations increased. This pattern was consistent with an early increase in P supply followed by plant-driven depletion. Net P acquisition in ETRD was 2.55 times that in WT, and ETRD exhibited 53% greater root biomass, while ETRS showed intermediate responses. Collectively, these results link enhanced plant P acquisition to greater P release in exudate incubation assays and higher acid phosphatase activity within this confined system.