Nitrogen Deficiency Identifies Nitrogen use Efficiency Management Pathways in Maize

Nitrogen (N) is an essential macronutrient required for plant growth, development, and metabolic processes. However, the regulatory mechanisms modulating N assimilate allocation and utilization under different N forms remain unclear. This study examines N metabolism, utilization and spatial distribution in maize, examined at 20 and 40 days after seedling transfer (DAT) and subjected to three N treatments: 1 mM NO₃⁻ (low N, LN), 2 mM NO₃⁻ (medium N) and 10 mM NO₃⁻ (high N). LN treatment induced significant physiological and molecular adaptations, such as enhanced root biomass and root/shoot (R/S) ratio, which prioritized N acquisition efficiency under a N deficiency condition. LN-treated plants exhibited significantly ( P ≤ 0.05) reduced shoot biomass, protein synthesis, NO₃⁻ and NO₂⁻ accumulation but increased NH₄⁺ levels. The activities of key N metabolism enzymes, such as nitrate reductase (NR) and nitrite reductase (NiR) were decreased, while glutamine synthetase (GS) and glutamate synthase (GOGAT) activities were upregulated, facilitating efficient NH₄⁺ assimilation. Molecular analysis revealed transcriptional reprogramming under LN condition, with downregulation of NO₃⁻ metabolism-related genes, ZmNR and ZmNiR , and upregulation of NO₃⁻ and NH₄⁺ transporter genes, ZmNPF6.2 , ZmNRT2 , ZmNRT3 , ZmAMT1 and ZmAMT2 , as well as GS-GOGAT pathway genes ZmGS and ZmGOGAT . Spatial and diurnal analysis highlighted dynamic N partitioning and adaptive regulation, with LN-treated plants exhibiting consistently higher NH₄⁺ levels and reduced NO₃⁻ concentrations in leaves, roots, and sheath tissues. These findings underscore the robust plasticity of maize N metabolism under different N conditions, providing useful insights into optimizing nitrogen use efficiency (NUE) for sustainable agriculture.