The mitigation of citric acid on cadmium toxicity in Iris tectorum and its effects on the composition of cell walls

Cadmium (Cd), a widely distributed and highly toxic heavy metal, poses a severe threat to soil fertility and plant growth. Citric acid (CA), a small organic acid molecule, plays a crucial role in alleviating heavy metal toxicity in plants. However, the specific mechanism underlying how CA organizes and mitigates the damage caused by heavy metals to plant cells remains unclear. Therefore, we studied the impact of exogenous CA on Cd-induced stress in Iris tectorum. The results showed that the addition of exogenous CA significantly increased the activity of antioxidant enzymes and altered the content of mineral elements including Fe, Zn, Ca, and Mn. Notably, compared to the Cd-only treatment, the proportion of Cd in the root cell walls increased by 14% in the presence of CA, and this increase was due to the ability of CA to regulate the amount of polysaccharide components in the root cell walls. CA affected the activity of pectinesterase (PME), changed the degree of pectinesterification (PMD), and enhanced the root cell walls’ ability to bind Cd, thereby reducing the Cd content in the above-ground tissues and alleviating heavy metal toxicity in plants. In summary, this study provides robust evidence that supports the use of CA to improve the efficiency of urban soil remediation.