Nitrate enhances salt tolerance of wild jujube by regulating nutrient homeostasis and nitrate transporters

Wild jujube (Ziziphus jujuba var. spinosa), the wild relative of cultivated jujube, widely used as rootstock in jujube cultivation, demonstrates adaptive advantages under abiotic stress. Despite the critical role of nitrogen in plant growth and development, as well as its association with salt tolerance, the mechanisms underlying nitrate-mediated salinity adaptation in perennial woody plants remain poorly understood. In this study, we systematically investigated the effects of nitrate (0.5, 2, and 10 mM NaNO₃) on the growth and physiological responses of wild jujube seedlings under 100 mM NaCl stress. Our results revealed interactive effects between NaCl and NaNO₃ on growth performance and physiological responses. Elevated nitrate levels alleviated NaCl-induced toxicity by reducing MDA accumulation, enhancing antioxidant enzyme activities (superoxide dismutase, catalase, and peroxidase), and promoting the accumulation of potassium, calcium, magnesium, proline, and soluble proteins in leaves. Transcriptional profiling identified six nitrate transporters (ZjNPF5.4, ZjNPF7.2, ZjNPF4.6, ZjNPF3.4, ZjNPF5.13, and ZjNPF2.10) that were specifically upregulated in roots under combined salt-nitrogen stress. Functional validation using Agrobacterium tumefaciens-mediated hairy root transformation demonstrated that overexpression of ZjNPF5.4 and ZjNPF7.2 enhanced nitrogen assimilation, reduced oxidative damage, and improved the K⁺/Na⁺ ratio, thereby conferring salt tolerance. This study elucidates the molecular mechanisms underlying nitrate-mediated salt tolerance in wild jujube and provides insights that could be harnessed to develop salt-tolerant jujube cultivars.