Silicon and iron co-application modulates cadmium accumulation and cell wall composition in tomato seedlings

Abstract Background Cadmium (Cd) in farmland soils has severely hindered agricultural production. Our previous studies have found that the combined application of silicon (Si) and iron (Fe) can effectively alleviate cadmium toxicity in tomatoes. But the underlying mechanisms remain unclear. In this study, authors investigated the effects of individual or combined application of silicon and iron by physiological, biochemical (mainly on cell wall components) and transcriptomic analysis under cadmium stress. Results The results showed that the application of silicon and iron under cadmium stress effectively reduced cadmium accumulation in the roots of tomato seedlings, with the combined application of silicon and iron showing the best effect. It also significantly reduced cadmium accumulation in the stems and leaves of the aboveground parts. The cadmium content in the cell wall was significantly higher than that in the soluble fraction and organelles. Compared to the cadmium treatment alone, the application of silicon and iron increased the content of pectin and hemicellulose in the root cell wall, decreased cellulose content, downregulated the expression of ABCA1, and upregulated the expression of HMA5. However, the application of silicon alone resulted in higher cadmium accumulation in pectin and hemicellulose in the root cell wall, while the combined application of Silicon and Iron reduced cadmium content in the cell wall components and resulted in the highest expression of HMA5, which reduced the accumulation of cadmium in roots and its translocation to the aboveground parts. Based on transcriptome KEGG differential enrichment analysis, CSF vs Cd phenylpropane synthesis pathway and pectin metabolism gene expression were up-regulated and down-regulated, respectively, suggesting that the application of Si and Fe may remodel the cell wall and enhance Cd fixation by alleviating excessive cellular lignification as well as sustaining pectin methyl esterification. This indicated that the combined application of silicon and iron could reduce cadmium uptake and accumulation in tomato seedlings by regulating cell wall components and cadmium transport-related gene expression. Conclusions The synergistic application of silicon and iron effectively reduces cadmium uptaking and accumulation in tomato seedlings by modifying cell wall composition and regulating key metal transport genes. This strategy presents a promising approach for mitigating cadmium toxicity in crops. Graphical Abstract