Identification of the Na⁺ transporter gene <i>MdNCL4</i> in response to high salinity stress in apple rootstock (<i>Malus domestica</i> (Suckow) Borkh.)

Sodium ion (Na⁺) transport in plants directly affects salt tolerance. Therefore, identifying key Na⁺ transport genes is important for breeding salt-tolerant plants. In this study, a total of 97 genes were identified in six transporter families in apple (<i>Malus domestica</i> (Suckow) Borkh.) through a genome-wide screening for Na⁺/K⁺ transport-related genes. Under high-salt stress, 12 genes were up-regulated in the roots. Notably, Na⁺/Ca²⁺ exchanger-like protein 4 (<i>MdNCL4</i>) showed the strongest positive response to salt stress and the highest root-tissue specific expression. We determined that MdNCL4 localised to the plasma membrane in <i>Arabidopsis</i> protoplasts; heterologous expression showed function complementation in yeast Na⁺ efflux mutants. This supported its function in transporting Na⁺ from the cytoplasm to the extracellular space. Association analysis showed that <i>MdNCL4</i> co-expressed with known salt-tolerance genes, including NAC transcription factor (<i>NAC69</i>) and dehydrin 1 (<i>DHN1</i>). These findings enrich current understanding of Na⁺ transport and may be useful for salt-tolerant breeding of apple rootstocks.