Rational utilization of exogenous additives is the key to enhance phytoremediation efficiency for removal of heavy metal contamination in soil. However, the physiological, biochemical, and molecular mechanisms of organic acids promoting the removal of Cd from soil by hyperaccumulator Solanum nigrum are still not well understood. EDTA and five organic acids, oxalic acid (OA), citric acid (CA), malic acid (MA), tartaric acid (TA), and succinic acid (SA), were added as exogenous additives to S. nigrum seedlings treated with Cd. The effect of EDTA and organic acids on physiological processes, Cd accumulation, and the expression of Cd stress related-genes in S. nigrum under Cd stress were analyzed. The growth of S. nigrum seedlings was significantly inhibited by Cd, but the addition of CA resulted in a 3.01-fold increase in seedling biomass compared to those treated with Cd alone. The content of the photosynthetic pigments in seedlings decreased significantly due to Cd toxicity, while CA could effectively restore the photosynthetic pigments to the level of CK. The levels of MDA, proline, and H2O2 induced by Cd toxicity decreased after CA application, which revealed that CA effectively alleviated Cd toxicity to S. nigrum. CA alleviates the Cd toxicity in the following aspects: it maintains the non-enzymatic antioxidant content, mediates the expression of enzymatic antioxidant genes such as SnCAT1, and alters the chemical form of Cd from high toxicity to low toxicity. Notably, CA effectively stimulates the expression of Cd transport and compartment-related genes, such as SnNRAMP3 and SnPCS1, thus facilitating Cd transport from the roots to the shoots. Through comprehensive evaluation, CA is considered to be the most effective chelating agent to enhance the efficiency of S. nigrum in removing Cd contamination. CA effectively alleviates photosynthetic pigment degradation and ROS homeostasis imbalance caused by Cd toxicity by regulating gene expression, thereby maintaining the growth of S. nigrum under Cd stress. Additionally, CA facilitates the accumulation of Cd in the shoots of S. nigrum. These findings suggest that CA can replace EDTA as a chelating agent for enhancing the ability of S. nigrum to remove Cd contamination.
