The growing need to increase agricultural production in today's world competes with the negative effects of nitrogen fertilizer application on climate change, as more than 50% of applied nitrogen is not utilized by plants (1). It is well known that nitrate (NO¿¿) is the primary source of nitrogen for higher plants, playing an essential role in their growth and development. However, optimizing its application in crops is crucial. Our research group reported that chloride (Cl¿) application at macronutrient levels improves nitrogen, carbon, and water use efficiency in higher plants (2). This finding aligns with studies on 670 plant species, where the most common Cl¿ concentration was found to be 5 mg g¿¹ DW (3), significantly exceeding (at least one order of magnitude) the levels required as a micronutrient. Thus, Cl¿ may serve as a valuable tool to address the emerging challenges of modern agriculture. Cl¿ has traditionally been considered an antagonist of NO¿¿ because it supposedly impairs NO¿¿ uptake and transport in the plant. For this reason and its potential toxicity in saline soils (4,5), Cl¿ has been considered a harmful element for agriculture, despite being an essential plant micronutrient (6) and beneficial macronutrient (2). We therefore consider it of great relevance to understand how plants regulate chloride uptake and transport. In this study, we investigate Cl¿ homeostasis when provided at macronutrient levels, demonstrating that the relationship between NO¿¿ and Cl¿ nutrition does not conform to the classical definition of antagonism but instead reveals synergistic interactions. Our results show that increasing NO¿¿ availability in the medium reduces Cl¿ uptake. However, the presence of Cl¿ in the medium at concentrations required for macronutrient-levels accumulation, does not impair NO¿¿ uptake. To fully understand the interaction between NO¿¿ and Cl¿ nutrition, it is essential to elucidate the molecular mechanisms regulating the homeostasis of both anions. Here, we propose a molecular model for Cl¿ uptake regulation based on the presence of both external and internal nitrate sensors. We propose that regulation of Cl¿ uptake according to NO¿¿ availability primarily contributes to optimizing NO¿¿-use efficiency in plants.

[1] Cakmak et al. (2022). Marschner's Mineral Nutrition of Plants 4th Edition, Chapter 7. [2] Colmenero-Flores et al. (2019) International Journal of Molecular Sciences, 20(19), 4686 [3] Watanabe et al. (2007) New Phytologist, 174(3), 516-523 [4] Cerezo et al. (1997) Plant Science, 126(1), 572 105-112 [5] Xu et al. (1999) Advances in agronomy (pp. 97-150) [6] Boyer et al. (1954) Plant Physiology, 29(6), 526-532

Resumen de la comunicación oral presentada en II ANDALUCÍA AGRO-HUB PhD students Meeting 11-12 marzo 2025 Estación Experimental del Zaidín, Granada

We acknowledge funding by MICINN FEDER Projects PID2021-125157OB-I00 and RTI 2018094460 B100