Proline-functionalized zein nano delivery system enables sustained brassinolide release for enhanced salt stress tolerance in cucumber cultivation

Soil salinization poses a global agricultural challenge with significant economic implications for salt-sensitive cucumber ( Cucumis sativus L. ) cultivation. To address this issue, we developed a proline-functionalized zein nano delivery system (BR@AM-Zein-Pro) through amide coupling of proline to aminated zein (AM-Zein), followed by brassinolide (BR) encapsulation to form self-assembled architectures. BR@AM-Zein-Pro exhibits sustained release for BR, thereby extending the phytohormone's duration efficacy. Furthermore, proline functionalization enhances BR@AM-Zein-Pro penetration through cucumber seed coats, thereby improving osmotic regulation capacity. Crucially, cucumber seeds treated with BR@AM-Zein-Pro exhibited a 90–116.67 % higher germination potential than untreated salt-stressed seeds. Following seven days of growth, cucumber seedlings from seeds treated with BR@AM-Zein-Pro exhibited significant increases in root and stem length of 39.91 % and 43.81 %, respectively, compared to untreated salt-stressed seedlings. At the mature plant stage, foliar application of BR@AM-Zein-Pro to salt-stressed plants significantly enhanced superoxide dismutase (SOD) and peroxidase (POD) activities by 98.64 % and 56.42 %, respectively, relative to the untreated control group. Additionally, malondialdehyde (MDA) content decreased by 53.14 %, proline content reduced by 72.95 %, electrolyte leakage rate dropped by 22.48 %, and chlorophyll content increased by 50.33 %. Mechanistically, BR@AM-Zein-Pro enhances cucumber salt tolerance through synergistic coordination of osmotic adjustment, antioxidant defense activation, and photosynthetic efficiency maintenance. It is important that BR@AM-Zein-Pro significantly alleviated the phytotoxicity from high BR concentrations (>0.25 mg/mL) by maintaining optimal therapeutic levels, resolving the trade-off between efficacy and safety. This nano delivery system provides an innovative strategy for enhancing crop salt tolerance, offering transformative potential for agricultural sustainability and global food security.