Integrated metabolome and transcriptome analysis of fulvic acid relieves nitrate stress-induced damage in spinach (Spinacia oleracea L.) by regulating multiple defense pathways

In the current environment of agricultural production, abiotic stress caused by high nitrate is becoming more and more serious. Fulvic acid (FA), a plant growth regulator, acts a vital role in improving plant stress tolerance. However, specific impacts of FA on vegetables under nitrate stress have been less studied. This study focused on spinach as the research object, changes in growth physiology, gene expression, and metabolites of spinach under nitrate stress by 0.15% FA application were investigated by analyzing physiology, transcriptomics, and metabolomics. The results indicated that FA could alleviate the adverse impacts of nitrate stress on spinach growth and chlorophyll synthesis, inhibit the accumulation of ROS in leaves caused by nitrate stress, the increase in malondialdehyde (MDA) content and relative conductivity (REC). Omics analysis indicated that there were 5097 differentially expressed genes (DEGs) and 100 differentially expressed metabolites (DEMs) in spinach leaves treated with nitrate stress and the control, 735 DEGs and 71 DEMs in spinach leaves treated with nitrate stress combined with FA and nitrate stress alone, which jointly participated in biological processes such as ascorbate-glutathione (AsA-GSH) cycle, plant hormone signaling, biosynthesis of phenylpropanoids, phenylalanine, and flavonoids. Changes of leaves in AsA and GSH contents, related enzyme activities, and hormone levels such as ABA further verified that FA relieves nitrate stress-induced damage in spinach by adjusting the above biological processes, thereby improving nitrate tolerance.