OsAMT1.1 Expression by Nitrate-Inducible Promoter of OsNAR2.1 Increases Nitrogen Use Efficiency and Rice Yield

Nitrate (NO₃^–) and ammonium (NH₄⁺) are two main inorganic nitrogen (N) sources during crop growth. Here, we enhanced the expression of OsAMT1.1, which encodes a NH₄⁺ transporter, using the NO₃^–-inducible promoter of OsNAR2.1 and an ubiquitin promoter in transgenic rice plants. Under field condition of 120 kg /hm² N, agronomic N use efficiency, N recovery efficiency and N transport efficiency, and grain yield of the pOsNAR2.1:OsAMT1.1 transgenic lines were increased compared with those of the wild type (WT) and the pUbi:OsAMT1.1 transgenic plants. Under 2.0 mmol/L NO₃^– + 0.5 mmol/L NH₄⁺ and 0.5 mmol/L NO₃^– + 2.0 mmol/L NH₄⁺ conditions of hydroponic culture, compared with the WT, both biomass and total N content were increased in the pOsNAR2.1:OsAMT1.1 transgenic lines. However, biomass was significantly reduced in pUbi:OsAMT1.1 transgenic plants under 0.5 mmol/L NO₃^– + 2.0 mmol/L NH₄⁺ condition. The lines expressing pOsNAR2.1:OsAMT1.1 exhibited increased OsAMT1.1 expression and ¹⁵NH₄⁺ influx in roots under both 2.0 mmol/L NO₃^– + 0.5 mmol/L NH₄⁺ and 0.5 mmol/L NO₃^– + 2.0 mmol/L NH₄⁺ conditions. Our study showed that expression of OsAMT1.1 can be promoted when driven by the OsNAR2.1 promoter, especially under high-level nitrate condition, leading to enhancement of NH₄⁺ uptake, N use efficiency and grain yield.