Fulvic acid enhances lettuce stress tolerance: Molecular mechanisms and in situ changes of lignin

Soil salinization limits global agriculture, threatening food security and impairing crop productivity on irrigated lands worldwide. Using hydroponic lettuce (Lactuca sativa L.) to exclude soil confounders, this study investigated the mechanism of FA-mediated salt tolerance in lettuce under NaCl stress. It further examined FA-induced lignin remodeling across NaCl salinity of 7 mS cm^-1 with FA concentrations ranging from 0, 50, 100, 150, 200 mg L^-1. Transcriptomics, antioxidant profiling, and advanced structural analyses (2D-HSQC NMR, GPC, GC-MS, etc.) were combined to elucidate the interplay among lignin modification, redox homeostasis, and photosynthetic protection. The treatment cleaved β-O-4 linkages, oxidised S/G units, and reassembled them into 400-700 Da FA-lignin hetero-conjugates. Concurrently, FA decreased ROS accumulation, stabilized chloroplast ultrastructure, increased thylakoid density and plastoglobuli, and, compared with salt-stressed controls, improved photosynthetic electron transport efficiency by 11 %, biomass by 27 %, and soluble sugars 3.9-fold (p < 0.01). Transcriptomic analysis revealed that FA modulated the expression of genes associated with lignin synthesis, redox balance, and photosynthesis. FA significantly upregulated genes like CCT7 and CSLA9, enhancing cell cycle progression, cell wall synthesis, and photosynthetic efficiency, providing molecular evidence for FA-mediated physiological improvements. Excessive FA (200 mg L^-1) exerted inhibitory effects, confirming 150 mg L^-1 as the optimal dose. Based on these findings, we propose a "lignin remodelling-redox homeostasis-photosynthetic protection" model, wherein FA-mediated lignin modification reinforces xylem development and leaf cell wall stability, thereby enhancing salt tolerance and providing a precise dosage basis for saline vegetable production.