Networks in plant nitrate foraging: From model plants to crop improvement

Land plants must cope with the spatiotemporal heterogeneity of soil nutrients. During evolution, plants have developed sophisticated systems to perceive nutrient distribution and adaptively remodel their root system architecture (RSA) to maximize nutrient acquisition while minimizing energy expenditure. This process, referred to as nutrient foraging, involves local nutrient perception at the root, signal integration in the shoot, and subsequent RSA adjustment. Over the past decade, significant progress has been made in understanding how plants sense spatially heterogeneous nutrient availability and how systemic signals coordinate root development. Nitrogen (N) is a key limiting nutrient, often unevenly distributed in soils. Nitrate (NO₃ ^-), a predominant N source, displays pronounced heterogeneity in soils. To adapt to this heterogeneity, plants use nitrate transporters, small peptides, phytohormones, receptor-like kinases (RLKs), microRNAs (miRNAs), mobile transcription factors, and amino acid signals as central regulators to mediate long-distance bidirectional signaling between the root and the shoot, ultimately guiding RSA modulation to match whole-plant nutritional demands. In legumes, systemic signaling pathways also regulate symbiotic nitrogen fixation. The product of symbiotic nitrogen fixation, ammonia (NH₃)/ammonium (NH₄ ⁺), represents a nitrogen form fundamentally distinct from nitrate. This review summarizes current understandings of nitrate foraging, from local sensing to systemic signaling, and discusses how these insights can be harnessed to optimize RSA and improve nitrogen use efficiency (NUE) in crops. By integrating these concepts, we provide a framework for designing nitrogen-efficient crops adapted to heterogeneous soil environments.