StWRKY31 Positively Regulates Salt Tolerance in Potato (Solanum tuberosum L.) by Modulating Ion Homeostasis, Photosynthetic Capacity, and Antioxidant Defense

Soil salinization is a major environmental constraint limiting global potato ( Solanum tuberosum L.) yield. WRKY transcription factors (TFs) modulate plant abiotic stress tolerance, although their role in salt stress tolerance in potato remains unexplored. In this study, a WRKY gene was isolated from potato and designated as StWRKY31 . Phylogenetic analysis indicated that StWRKY31 belongs to the Group II subfamily of WRKY proteins, shows high evolutionary conservation with its tomato ortholog SlWRKY31, and is localized in the nucleus. Different salt concentrations (50, 100, and 150 mM NaCl) upregulated StWRKY31 expression level in leaves of the potato cultivars Longshu 5 and Qingshu 9 . After introducing StWRKY31 into potato plants, overexpression (OE) lines of both cultivars ( Qingshu 9 : OE-4/5/6; Longshu 5 : OE-2/5/6) exhibited enhanced salt tolerance, reflected by improved growth and biomass (plant height, fresh and dry weight of shoots and roots), increased K⁺/Na⁺ ratio, and markedly enhanced net photosynthetic rate (Pn), transpiration rate (Tr), and stomatal conductance (Gs). Conversely, under the same stress conditions, RNA interference (RNAi) lines of both cultivars ( Qingshu 9 : RNAi-3/4/6; Longshu 5 : RNAi-3/4/5) showed opposite phenotypes compared with non-transgenic (NT) plants. Moreover, under salt stress, compared to NT plants, the OE lines accumulated higher contents of proline, soluble sugar, and soluble protein, showed increased activities of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), and exhibited reduced levels of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂). In contrast, the RNAi lines showed opposite trends in these physiological indices. Additionally, salt stress induced elevated expression of stress-responsive genes involved in oxidative stress response ( StCAT , StPOD , and StSOD ) and proline biosynthesis ( StP5C S) in StWRKY31 -overexpressing plants, whereas these genes were down-regulated in RNAi lines compared to NT plants under salt stress. Thus, StWRKY31 acts as a positive regulator in the salt stress response of potato. • StWRKY31 salt-inducible (qRT-PCR), localizes to the nucleus. • StWRKY31 elevates proline/antioxidants, mitigates ROS/MDA, and boosts stress resilience. • StWRKY31 regulates photosynthesis and K⁺/Na⁺ ratios in potato under salt stress. • StWRKY31 drives stress-resilient growth (height/biomass) under salt.