Chloride as a Beneficial Macronutrient in Higher Plants: New Roles and Regulation

Abstract

Chloride (Cl−) has traditionally been considered a micronutrient largely excluded by plants due to its ubiquity and abundance in nature, its antagonism with nitrate (NO3−), and its toxicity when accumulated at high concentrations. In recent years, there has been a paradigm shift in this regard since Cl− has gone from being considered a harmful ion, accidentally absorbed through NO3− transporters, to being considered a beneficial macronutrient whose transport is finely regulated by plants. As a beneficial macronutrient, Cl− determines increased fresh and dry biomass, greater leaf expansion, increased elongation of leaf and root cells, improved water relations, higher mesophyll diffusion to CO2, and better water- and nitrogen-use efficiency. While optimal growth of plants requires the synchronic supply of both Cl− and NO3− molecules, the NO3−/Cl− plant selectivity varies between species and varieties, and in the same plant it can be modified by environmental cues such as water deficit or salinity. Recently, new genes encoding transporters mediating Cl− influx (ZmNPF6.4 and ZmNPF6.6), Cl− efflux (AtSLAH3 and AtSLAH1), and Cl− compartmentalization (AtDTX33, AtDTX35, AtALMT4, and GsCLC2) have been identified and characterized. These transporters have proven to be highly relevant for nutrition, long-distance transport and compartmentalization of Cl−, as well as for cell turgor regulation and stress tolerance in plants.

Coping With Water Shortage: An Update on the Role of K+, Cl-, and Water Membrane Transport Mechanisms on Drought Resistance

“…Plants accumulate Clto macronutrient levels, around 15-50 mg g -1 dry weight (DW), when the external concentration is 1 to 5 mM Cl - (Colmenero-Flores et al, 2019). This range of FIGURe 1 | Overview of the main processes associated to water deficit (WD) resistance.…”

“…Thus, Clrepresents the dominant inorganic anion in plant cells, with leaf contents that can be similar to those of the macronutrient K + (Franco-Navarro et al, 2016). This accumulation led to better plant performance, the reason why Clhas been recently defined as a beneficial macronutrient (Franco-Navarro et al, 2016;Raven, 2017;Wege et al, 2017;Colmenero-Flores et al, 2019). It is worth highlighting that field assays using durum wheat have shown that soil Cldeficiency caused a number of physiological disorders impairing the growth and yield (Schwenke et al, 2015).…”

“…Recently, it has also been observed that K + regulates the processes of photo-assimilation and translocation of carbohydrates in cotton plants, favoring their acclimation to drought stress (Zahoor et al, 2017). Regarding Clfertilization, substantial responses to Clcontaining fertilizers have been reported for different crops in many parts of the world (Xu et al, 1999;Chen et al, 2010;Colmenero-Flores et al, 2019). However, these studies do not clarify to what extent plant yield enhancement was due to the accompanying cations, or whether other anions could replace Clin such growth-promoting effect.…”

Editorial: Ion Homeostasis in Plant Stress and Development

“…In fact, it has been postulated that proteins regulating potassium uptake channels in the guard cells may be targets for biotechnological strategies against drought (Locascio et al, 2019). The study of chloride homeostasis has often been neglected due to the importance of potassium, which is the major ion in the cytoplasm, but Cl − has a specific beneficial effect in leaf cells by giving rise to larger cells (with higher water storage capacity), lower stomatal conductance and higher mesophyll conductance to CO 2 (Dreyer and Uozumi, 2011;Colmenero-Flores et al, 2019). Therefore, water use efficiency is increased under proper Cl − nutrition.…”

Chloride involvement in the synthesis, functioning and repair of the photosynthetic apparatus in vivo

“…Although the occurrence, and mechanism, of inhibitory effects of high, salinizing, Cl − concentrations on glycophytic (crop) plants has received some attention, the main focus in salinity studies has been on the effects of Na + (Raven, 2017; Wege et al , 2017; Colmenero‐Flores et al , 2019; Geilfus, 2019). Recently there has been some emphasis on Cl − at external concentrations between those satisfying the micronutrient requirement in PSII and those inhibiting the growth of glycophytes acting as a ‘beneficial nutrient’ that can increase growth rate (Raven, 2017; Wege et al , 2017; Colmenero‐Flores et al , 2019; Geilfus, 2019). The present paper analyses the quantitative role of Cl − in O 2 production by PSII, as well as other photosynthetic roles such as synthesis and repair of the PSII complex, some of which require high intracellular Cl − concentrations.…”