Background: Nitrogen is quantitatively the most important inorganic nutrient for plants. Roots mainly take nitrogen up in the form of nitrate (NO3–). Once inside the cells, NO3– can be assimilated into amino acids or stored in the vacuole, where it regulates cell water content to sustain plant growth. CLCa is the main transporter mediating the entry of NO3– into vacuoles. It works as an exchanger removing one proton (H+) from the vacuole to store two NO3– anions. CLCa belongs to a conserved family composed of both anion/proton exchangers and anion channels, with closely similar structures. Most of the exchangers share a conserved glutamate residue (E203 in CLCa).
Question: Is the NO3–/H+ exchange mechanism of CLCa required for plants to stabilize water and nitrate status? What are the physiological consequences of converting CLCa from an exchanger to a nitrate channel?
Findings: We generated Arabidopsis thaliana lines that express a form of CLCa in which the amino acid E203 is mutated, turning this protein into a NO3– channel. This modification decreases nitrate accumulation in the vacuole and increases amino acids and protein synthesis, thereby leading to high N content in the seeds. Although these plants have higher nitrogen use efficiency (NUE), their growth is reduced, in association with a diminution of water content. This finding reveals the importance of the CLCa exchange mechanism in allowing plant cells to maintain cell turgor irrespective of fluctuating nitrogen concentrations in the soil.
Next steps: We would like to investigate the possibility of selecting plants with higher NUE without affecting growth, through root-specific expression of the mutated version of CLCa. Decreasing NO3– accumulation in roots should stimulate NO3– translocation to the shoot, without disturbing the water content in aerial parts.
Julie Hodin, Christof Lind, Anne Marmagne, Christelle Espagne, Michele Wolfe Bianchi, Alexis De Angeli, Fadi Abou-Choucha, Mickaël Bourge, Fabien Chardon, Sebastien Thomine and Sophie Filleur (2023) Proton exchange by the vacuolar nitrate transporter CLCa is required for plant growth and nitrogen use efficiency. Plant Cell. https://doi.org/10.1093/plcell/koac325