The Role of Chloride Channels in Plant Responses to NaCl

Abstract

Chloride (Cl^-) is considered a crucial nutrient for plant growth, but it can be a challenge under saline conditions. Excessive accumulation of Cl^- in leaves can cause toxicity. Chloride channels (CLCs) are expressed in the inner membranes of plant cells and function as essential Cl^- exchangers or channels. In response to salt stress in plants, CLCs play a crucial role, and CLC proteins assist in maintaining the intracellular Cl^- homeostasis by sequestering Cl^- into vacuoles. Sodium chloride (NaCl) is the primary substance responsible for causing salt-induced phytotoxicity. However, research on plant responses to Cl^- stress is comparatively rare, in contrast to that emphasizing Na⁺. This review provides a comprehensive overview of the plant response and tolerance to Cl^- stress, specifically focusing on comparative analysis of CLC protein structures in different species. Additionally, to further gain insights into the underlying mechanisms, the study summarizes the identified CLC genes that respond to salt stress. This review provides a comprehensive overview of the response of CLCs in terrestrial plants to salt stress and their biological functions, aiming to gain further insights into the mechanisms underlying the response of CLCs in plants to salt stress.

Keywords: Cl− stress; NaCl; biological functions; chloride channel; plants.

Figure 1

Cryo-EM structure of the CLCs. (a) Human CLC-1 channel hCLC-1 (PDB: 6COY and 6COZ), the transmembrane domain (TMD, blue), and the cytosolic domain (CTD, light blue). (b) Chicken CLC chloride–proton exchanger ggCLC-7 (PDB: 7JM6); TMD, purple; CTD, light purple. (c) Arabidopsis thaliana CLC transporter AtCLCa (PDB: 8IAB); TMD, green; CTD, light green. (d) Escherichia coli CLC channel EcCLC (PDB: 1OTS); the two subunits are yellow. The selectivity filter residues of CLCs are shown with red. (e) The anion selectivity filter of hCLC-1 (blue), AtCLCa (green), ggCLC-7 (purple), and EcCLC (yellow) were compared between structures.

Figure 2

Phylogenetic analysis and conserved motif analysis of CLC proteins. The full-length amino acid sequences of CLC proteins were aligned by Clustal W, and the phylogenetic tree was built using the neighbor-joining (NJ) method in MEGA (v11.0). The grouping of the CLC proteins is indicated by different colors. AtCLC-x (green box); CsCLC-c (yellow box); CsCLC-x (purple box); EcCLC (brown box); ggCLC-7 (olive box); GmCLC-x (khaki box); GsCLC-x (khaki box); GhCLC-x (black star); hCLC-1 (indigo box); MhCLC-x (cyan box); NtCLC-x (pink box); OsCLC-x (gray box); PgCLC-x (blue box); SaCLC-x (red box); ZmCLC-x (orange box). The red star represents the positions of residues.

Figure 3

Model showing known Cl⁻ transporters affecting Cl⁻ transport and Cl⁻ tolerance under salt stress. Transporters upregulated in response to salt stress are indicated in red; those that are downregulated are indicated in blue. Among the seven AtCLCs (Arabidopsis thaliana CLC proteins), four are related to Cl⁻ transport. AtCLC-c and AtCLC-g are localized in the tonoplast. AtCLC-c and AtCLC-g have been suggested to play a role in Cl⁻ transport within the vacuole. AtCLC-d is localized in the Golgi apparatus, and AtCLC-d is known to regulate Golgi pH through Cl⁻ transport. AtCLC-e is found in the thylakoid membrane and has been reported to facilitate Cl⁻ transport, which is essential for efficient photosynthesis. NPFs, SLAHs, and ALMTs relate to Cl⁻ transport to xylem. The figure was generated using BioRender.com.