Nitrogen assimilation and photorespiration become more efficient under chloride nutrition as a beneficial macronutrient

17 páginas.- 7 figuras.- 98 referencias.- The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpls.2022.1058774/full#supplementary-material .- This article is part of the Research Topic XVII Spanish Portuguese Congress on Plant Biology (BP2021) - Plant Biochemistry and Metabolism

Chloride (Cl−) and nitrate (NO−3) are closely related anions involved in plant growth. Their similar physical and chemical properties make them to interact in cellular processes like electrical balance and osmoregulation. Since both anions share transport mechanisms, Cl− has been considered to antagonize NO−3 uptake and accumulation in plants. However, we have recently demonstrated that Cl− provided at beneficial macronutrient levels improves nitrogen (N) use efficiency (NUE). Biochemical mechanisms by which beneficial Cl− nutrition improves NUE in plants are poorly understood. First, we determined that Cl− nutrition at beneficial macronutrient levels did not impair the NO−3 uptake efficiency, maintaining similar NO−3 content in the root and in the xylem sap. Second, leaf NO−3 content was significantly reduced by the treatment of 6 mM Cl− in parallel with an increase in NO−3 utilization and NUE. To verify whether Cl− nutrition reduces leaf NO−3 accumulation by inducing its assimilation, we analysed the content of N forms and the activity of different enzymes and genes involved in N metabolism. Chloride supply increased transcript accumulation and activity of most enzymes involved in NO−3 assimilation into amino acids, along with a greater accumulation of organic N (mostly proteins). A reduced glycine/serine ratio and a greater ammonium accumulation pointed to a higher activity of the photorespiration pathway in leaves of Cl−-treated plants. Chloride, in turn, promoted higher transcript levels of genes encoding enzymes of the photorespiration pathway. Accordingly, microscopy observations suggested strong interactions between different cellular organelles involved in photorespiration. Therefore, in this work we demonstrate for the first time that the greater NO−3 utilization and NUE induced by beneficial Cl− nutrition is mainly due to the stimulation of NO−3 assimilation and photorespiration, possibly favouring the production of ammonia, reductants and intermediates that optimize C-N reutilization and plant growth. This work demonstrates new Cl− functions and remarks on its relevance as a potential tool to manipulate NUE in plants.

This work was supported by the Spanish Ministry of Science Innovation and Universities-FEDER grants, RTI2018-094460-B-I00 and PID2021-125157OB-I00, Spanish National Research Council grants CSIC-201840E132 and CSIC-202040E266, and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 895613.

Peer reviewed