From micronutrient to beneficial macronutrient: the impact of chloride on nitrogen metabolism

Poster presentado en el BP2021: XXIV Reunión de la Sociedad Española de Biología de Plantas y XVII Congreso Hispano-Luso de Biología de Plantas, 7 y 8 de julio de 2021. online

Chloride (Cl-) is an essential micronutrient recently defined as a beneficial macronutrient for plants, with new specific functions that improve the efficiency in the use of water (WUE), nitrogen (NUE) and CO2. Nitrate (NO3-) and Cl- are the most abundant inorganic anions in plants, sharing physical and osmoregulatory properties, transport mechanisms and strong dynamic interactions (Colmenero-Flores et al., 2019). Cl- has been traditionally considered harmful for agriculture due to the belief of its competitive interaction with NO3- and the toxicity generated by excessive Cl- accumulation in sensitive crops during salt-stress conditions (Wege et al., 2017; Geilfus, 2018). However, when Cl- is accumulated in leaves at beneficial macronutrient levels (1-5 mM), NO3- content is reduced while plant growth and nitrogen use efficiency (NUE) increased (Rosales et al., 2020). Therefore, in this work we determined how Cl- nutrition affects nitrate accumulation by studying different nitrogen metabolism pathways. To this aim, tobacco plants were growth under experimental greenhouse conditions and watered with a basal nutrient solution supplemented with 6 mM Cl- and a control treatment containing a mix of sulphate+phosphate to compensate for cations added with Cl- salts. After 60 days of nutritional treatments, mature tobacco leaves were collected and, afterwards, different nitrogen forms, key enzyme activities involved in nitrogen metabolism and protein content were spectrophotometrically analysed. In addition, a free amino acids profile was determined by HPLC. Our results showed that the Cl- treatment stimulated plant growth and increased the content of total nitrogen and ammonium in leaves, whereas the nitrate content was reduced. The analysis of key enzymes involved in nitrogen metabolism showed increased activities of nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT) and aspartate aminotransferase (AAT) in Cl--treated plants, supporting the Cl--dependant stimulation of nitrate assimilation. Interestingly, the analysis of free amino acids content showed a reduction of aspartate, glutamate, glutamine, glycine and threonine, whereas serine and asparagine significantly increased. These results also showed possible implications of Cl- nutrition in other pathways such as photorespiration or urea pathways. Taken together, in this work we demonstrate that Cl- nutrition at macronutrient levels improves plant growth and NUE as a result of an induced nitrogen metabolism, a process that is commonly a bottleneck for crop yield.

Financial support of the Spanish Ministry of science Innovaction and Universities-FEDER grant RTI2018-094460-B-100, European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement Nº 895613 and Spaish National Research Council grants CSIC-202040E266 and CSIC-202140E023; ii)

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