The ppk-expressing transgenic rice floating bed improves P removal in slightly polluted water

With significant economic advantages, the plant floating bed has been widely utilized in the ecological remediation of eutrophic water because of the excessive phosphorus (P) and nitrogen discharge in China. Previous research has demonstrated that polyphosphate kinase (ppk)-expressing transgenic rice (Oryza sativa L. ssp. japonica) (ETR) can increase the P absorption capacity to support rice growth and boost rice yield. In this study, the floating beds of ETR with single copy line (ETRS) and double copy line (ETRD) are built to investigate their capacity to remove aqueous P in slightly polluted water. Compared with the wild type Nipponbare (WT) floating bed, the ETR floating beds greatly reduce the total P concentration in slightly polluted water though the ETR floating beds have the same removal rates of chlorophyll-a, NO₃^--N, and total nitrogen in slightly polluted water. The P uptake rate of ETRD on the floating bed is 72.37% in slightly polluted water, which is higher than that of ETRS and WT on the floating beds. Polyphosphate (polyP) synthesis is a critical factor for the excessive phosphate uptake of ETR on the floating beds. The synthesis of polyP decreases the level of free intracellular phosphate (Pi) in ETR on the floating beds, simulating the phosphate starvation signaling. The OsPHR2 expression in the shoot and root of ETR on the floating bed increased, and the corresponding P metabolism gene expression in ETR was changed, which promoted Pi uptake by ETR in slightly polluted water. The Pi accumulation further promoted the growth of ETR on the floating beds. These findings highlight that the ETR floating beds, especially ETRD floating bed, have significant potential for P removal and can be exploited as a novel method for phytoremediation in slightly polluted water.