Foxtail millet [Setaria italica (L.) Beauv.] over-accumulates ammonium under low nitrogen supply

Nitrogen (N) deficiency is a primary limiting factor for crop production worldwide. Previously, we reported root system architectural modifications of hydroponically cultured foxtail millet [Setaria italica (L.) Beauv.] to facilitate N translocation under N limitation. Here, we investigated foxtail millet for its shoot adaptation to low N in terms of internal N regulation under hydroponic culture. The results of this study revealed that the shoot N and nitrate (NO₃^-) concentrations significantly declined as compared to control (CK); however, the shoot over-accumulated ammonium (NH₄⁺) under low N (LN). N shortage resulted in down-regulation of expressions of SiPetA, SiccsA, SipsbA, SirpoB, SipsaA, SiatpA, Sirps16, and SiPEPC which, undermined chloroplast functioning and CO₂ assimilation for the provision of carbon skeleton. Carbon deficiency and lower activities of GS decelerated ammonia assimilation and led to over-accumulation of NH₄⁺ in the LN-shoot, as indicated by lower concentrations of total amino acids. Thus, enhanced GOGAT activity was to assimilate NH₄⁺ while, those of catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) were to scavenge reactive oxygen species (ROS) of NH₄⁺ toxicity framework. The weakened chloroplast factory eventually minimized photosynthesis and reduced dry mass of the LN shoot. Such regulation of N by the shoot, perhaps, resurrected physiological functions which maintained internal mineral status under nitrogen limitation in foxtail millet.