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 (NO) concentrations significantly declined as compared to control (CK); however, the shoot over-accumulated ammonium (NH) 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 CO assimilation for the provision of carbon skeleton. Carbon deficiency and lower activities of GS decelerated ammonia assimilation and led to over-accumulation of NH in the LN-shoot, as indicated by lower concentrations of total amino acids. Thus, enhanced GOGAT activity was to assimilate NH while, those of catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) were to scavenge reactive oxygen species (ROS) of NH 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.