Magnesium supports nitrogen uptake through regulating NRT2.1/2.2 in soybean

Abstract

Nitrogen (N) and magnesium (Mg) are two of the essential elements for plant growth. Application of Mg may increase N use efficiency in plants, but the mechanisms underlying Mg and N interaction are not well understood. The objectives of this study were to assess how Mg supply affects N uptake in soybean, and to further investigate underlying molecular mechanisms. Soybean (Glycine max L.) seedlings were provided with various concentrations of Mg under different N conditions in hydroponic cultures. Biomass, nutrient and sugar concentrations, and gene expression were determined. Tissue specificity of gene expression was performed in hairy-root transformants. Increasing Mg supply enhanced N concentration and total N uptake in soybean. The promotive effect is due to the enhanced uptake of nitrate but not ammonium. Moreover, elevating nitrate supply can partially restore growth retardation and leaf chlorosis caused by low Mg availability. A comparative transcriptomic study revealed that two Nitrate Transporter 2 (NRT2) family genes NRT2.1 and NRT2.2 were positively regulated by Mg in roots. Both NRT2.1 and 2.2 are mainly expressed in the exodermis and epidermis of soybean roots, suggesting roles in nitrate uptake. In addition, under low-Mg conditions, expression of NRT2.1 and 2.2 and subsequent N uptake were restored upon exogenous addition of sucrose, implying the possibility that Mg may regulate NRT2.1/2.2 expression through facilitation of sucrose allocation. The results of this study indicate that Mg promotes N uptake through positive regulation of NRT2.1/2.2 expression in soybean roots. This Mg-induced gene expression appears to act through-facilitation of sucrose allocation from shoots to roots.