Nitric oxide confers cadmium tolerance in soybean via cell wall modification, transporter regulation, and activation of stress signaling pathways.

Cadmium (Cd) pollution in soils poses a serious threat to crop production and human health. Although nitric oxide (NO) is known to mitigate metal stress, its role in response to Cd stress remains controversial. This study investigated the underlying mechanisms by which NO mitigates Cd toxicity in soybean. Results showed that Cd-induced NO generation mainly arises from the nitrate reductase (NR) pathway. Application of an NO donor (50 μM) mitigated Cd-induced growth inhibition (+83.7 %) and oxidative damage by enhancing antioxidant defense and mineral nutrient uptake. Under Cd stress (20 μM), NO promoted Cd retention in roots (+55.2 % and +76.8 %) and reduced its translocation to shoots by increasing Cd binding to root cell walls (+36.2 % and +40.2 %), through elevated levels of pectin and hemicellulose, enhanced pectin methylesterase activity, and greater pectin demethylation. Additionally, NO stimulated lignin biosynthesis (+35.5 %) by upregulating the expression of lignin biosynthesis genes, resulting in thicker cell walls (+94.9 %) that restricted Cd entry. NO also modulated the expression of Cd transport genes, reducing intracellular Cd accumulation (-19.2 %) while promoting vacuolar sequestration. Furthermore, NO activated MAPK and CDPK signaling cascades, which contribute to Cd detoxification. Overall, these findings suggest that NR-mediated NO mitigates Cd toxicity in soybean by reinforcing cell wall Cd fixation, limiting Cd uptake and translocation, and activating stress signaling pathways.