Metabolomics Analysis of Root Exudates of Cotton Seedling Under Cadmium Stress and the Alleviating Effect of Citric Acid

In the past few decades, Cadmium-contaminated soil in agricultural fields has been a major global issue. The wide attention followed in agricultural production and the remediation of cadmium pollution in soil by cotton plants, due to the characteristics of wide planting area, the large biomass, strong capacity of cadmium accumulation, and non-edible properties of fiber. But the root secretion mechanism of cotton plants in response to cadmium threat is still unclear. In this study, four CdCl2 concentrations (0, 150, 300,450 mu mol/L) were applied to the soil at seedling stage, and physiological indicators of cotton seedling were detected and root exudates were collected after 10 days of cadmium exposure. The results showed that the cadmium tolerance of cotton seedlings was activated to the greatest extent under 300 mu mol/L cadmium, and inhibited when the concentration reached 400 mu mol/L. A total of 407 metabolites were detected based on UPLC-MS/MS. The composition and content of root exudates of cotton seedlings were significantly changed by cadmium stress, and there were 7 common differential accumulated metabolites, including isomaltulose, quinic acid, citric acid, gamma-aminobutyric acid, isomaltulose, galactinol and gluconic acid. KEGG analysis showed that there were 7 metabolic pathways highly related to cadmium stress, including pyruvate metabolism, glyoxylate and dicarboxylate metabolism, citrate cycle, galactose metabolism, starch and sucrose metabolism, ABC transporters and carbon metabolism. These metabolic pathways were involved in osmoregulation, energy supply and resilience in plants. In addition, exogenous addition of citric acid can enhance the antioxidant capacity of cotton leaves, and promote the absorption and accumulation of cadmium in cotton. This study provides a theoretical basis for further research on elucidating the response mechanism of root exudates in cotton plants to cadmium stress and for utilizing root exudates such as citric acid to alleviate cadmium stress.