Chromium tolerance mechanisms in Cosmos sulphureus: antioxidant defense and potential role of pectin chelation-based detoxification

BACKGROUND: Soil chromium (Cr) contamination is escalating, and phytoremediation is optimal for its abatement. Cosmos sulphureus, an annual Asteraceae herb widely used in landscape greening and ecological restoration, has strong tolerance to heavy metals, but its Cr accumulation traits, stress response and molecular mechanisms remain unclear, limiting its application. Based on physiological measurements under four Cr concentration gradients (100-400 µM), combined with transcriptomic and metabolomic analysis at the critical stress concentration of 100 µM, this study systematically elucidated the physiological stress responses, tolerance mechanisms and early molecular expression profiles of C. sulphureus under varying Cr stress. RESULTS: Roots are the main Cr accumulation organ. 100 µM is the physiological tolerance threshold with high tolerance, low translocation and activated antioxidant system; Cell wall (CW) pectin serves as the main Cr immobilization site, and its Cr enrichment correlates strongly with PME-mediated demethylation, implying that low-esterified pectin might contribute to Cr detoxification through increased chelation sites; Transcriptome identified 26,505 upregulated and 14,791 downregulated genes: defense genes related to Cr-induced pectin modification and enzyme activity (PME29, PME2, GALE2, etc.) were induced, while growth-related genes involved in CW development and photosynthetic regulation (PMEU1, WAK1, LHCA3, etc.) were repressed. Metabolomics showed elevated defensive metabolites including organic acids, amino acids, flavonoids and sugars (GSH, Pro, AsA, etc.) and also promoted the enrichment of metabolites associated with the synthesis and modification of CW pectin and cellulose (L-arabinose, raffinose, etc.). CONCLUSION: These findings clarify the Cr accumulation characteristics of C. sulphureus roots and its remediation potential under Cr concentration ≤ 100 µM, confirming that the antioxidant system and pectin chelation detoxification are core tolerance mechanisms. Overall, C. sulphureus adopts a molecular response pattern of growth-to-defense transition. This study provides a basis for Cr remediation landscape plant breeding and plant Cr tolerance mechanism exploration.