Chloride uptake is regulated by nitrate availability in plants: identification of regulatory components

Resumen del póster presentado en XVIII Spanish-Portuguese Congress on Plant Biology y XXV Meeting of the Spanish Society of Plant Biology (IPB 2023). Braga (Portugal). 9 al 12 de julio de 2023

Chloride (Cl−) and nitrate (NO3−) are the most abundant inorganic anions in plants, showing strong dynamic interactions, in part because both anions compete for common transport mechanisms. Since NO3− is an essential nitrogen source, it is currently considered that plants strongly prioritize the uptake of NO3− over that of Cl−. However, plants also accumulate Cl− to content levels which are typical of a macronutrient, inducing beneficial roles that improve the efficiency in the use of water, Nitrogen (NUE) and Carbon/energy (Colmenero‐Flores et al, 2019; Cakmak et al, 2023). The NO3− transceptor (transporter + receptor) AtNPF6.3, and its orthologs in other species, is the only root Cl− uptake mechanism identified in plants so far (Xiao et al, 2021). We hypothesize that reciprocal regulation of NO3− vs Cl− homeostasis in plants is more complex than currently believed. We aim to assess whether components that regulate NO3− uptake, including plasma membrane transporters, also regulate Cl− uptake, and to what extent AtNPF6.3 is the only mechanism involved in Cl− uptake in higher plants. Chloride uptake has been quantified in mutant lines of Arabidopsis thaliana affected in: i) high‐ and lowaffinity NO3− uptake transporters; ii) regulatory components that signal NO3− availability in plants. Quantifications were performed under high, mid and low NO3− availability.The dual‐affinity AtNPF6.3, but not the high‐affinity NO3− transporters tested in this work, mediated Cl− uptake, but only when NO3− availability was low. This strategy allows plants to improve NUE under NO3− limitation. Mutant lines dealing to a reduction in plant NO3− content gave rise to increased Cl− content, indicating the occurrence of an intracellular sensor that down‐regulates Cl− uptake when NO3− nutrition is adequate. A new regulatory component was also identified that participates in the induction of Cl− uptake under low NO3− availability. In A. thaliana, Cl− uptake mediated by AtNPF6 represented 20% of total Cl− accumulated, so other transporters with relevant roles in Cl− nutrition have yet to be identified in plants. References Cakmak et al (2022) Micronutrients. In Marschner’s Mineral Nutrition of Higher Plants (4th Ed) Colmenero‐Flores et al (2019) IJMS 20, 4686. Xiao et al (2021) EMBO J 40: e106847.

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