NITROGEN ASSIMILATION IN MONOCOT CROPS UNDER BIOTIC AND ABIOTIC STRESS: A COMPARATIVE PERSPECTIVE WITH ARABIDOPSIS THALIANA

Abstract

Nitrogen (N) is the most critical macronutrient governing plant growth, development, and yield, forming the elemental backbone of amino acids, proteins, nucleic acids, chlorophyll, and hormones. The primary assimilatory pathway sequential reduction of nitrate (NO₃⁻) by nitrate reductase (NR) and nitrite reductase (NiR), followed by ammonium (NH₄⁺) incorporation via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle is conserved across higher plants but shows remarkable diversity in gene family size, regulation, and tissue specificity between the dicot model Arabidopsis thaliana and economically dominant monocot crops such as rice (Oryza sativa), maize (Zea mays), wheat (Triticum aestivum), sorghum (Sorghum bicolor), and barley (Hordeum vulgare). These cereals feed over half the world's population yet use only 30-50% of applied N fertilizer, motivating intensive study of nitrogen use efficiency (NUE). Biotic stresses (pathogen infection, herbivory, nematodes) and abiotic stresses (salinity, drought, heat, cold, nitrogen starvation) each disrupt distinct nodes of N metabolism through enzyme inactivation, transporter downregulation, hormonal reprogramming, and metabolic flux redirection. This review systematically examines: (i) the core N assimilation enzymes across Arabidopsis and major monocots; (ii) the nitrate transporter (NPF/NRT1, NRT2) and ammonium transporter (AMT) gene families; (iii) the impact of individual and combined abiotic stresses on N metabolism in cereals; (iv) the crosstalk between biotic stress and N nutrition; (v) regulatory networks including NIN-like protein (NLP) and Dof transcription factors, calcium signaling, reactive oxygen species (ROS), nitric oxide (NO), polyamines, phytohormones, and TOR kinase; and (vi) biotechnological and breeding strategies to improve NUE under stress. The ureide pathway, NH₄⁺ toxicity, CLE peptide N signaling, and the NLP-NIGT1 feedforward loop receive special attention as monocot-relevant topics underexplored by the Arabidopsis model.