LDH-Melatonin Nanocomposite Alleviates Saline-Alkali Stress in Soybean by Modulating Iron Homeostasis and Carbon-Nitrogen Metabolism.

Abstract

Soil salinization has become a major abiotic stress affecting ecosystems and agriculture globally. Although melatonin (MT) has been proven to alleviate these abiotic stresses, its efficacy is limited by poor photostability and brief duration of action. Here, we constructed a layered double hydroxide (LDH)-based nano-delivery system for MT, designated LMT, and investigated its effects on two soybean cultivars (HF50 and HN95) with contrasting tolerance to saline-alkali stress. The synthesized Mg-Al LDH exhibits uniform quasi-spherical morphology (200-300nm), positive zeta potential (20-30mV), and efficient MT loading (encapsulation efficiency=34.86%, loading capacity = 52.33%). LDH nanoparticles readily adhere to leaf surfaces, protect MT from photodegradation, and enable sustained release, leading to earlier peak and prolonged duration of MT in leaves. LMT treatment significantly alleviated saline-alkali-induced growth inhibition, outperforming either LDH or MT alone. Transcriptome analysis across four treatments identified 1,528 differentially expressed genes (DEGs), of which 223 core co-regulated DEGs were significantly enriched in conserved pathways governing iron homeostasis, redox balance, nitrogen metabolism, and photosynthetic carbon assimilation under saline-alkali stress. Physiological and molecular validation demonstrated that LMT upregulated iron transporters and chelators, restored chloroplastic iron content and K+/Na+ homeostasis, enhanced antioxidant enzyme activities (SOD, POD, CAT, APX) and glutathione redox status, elevated nitrogen assimilation enzyme activities (NR, GS, GOGAT), and recovered photosynthetic pigment content and electron transport efficiency. Collectively, LMT alleviates saline-alkali stress via synergistic regulation of iron homeostasis, photosynthesis, nitrogen metabolism, and antioxidant defense, offering a promising nano-strategy for soybean production on saline-alkali soils.