Ammonium mitigates Cd toxicity in rice (Oryza sativa) via putrescine-dependent alterations of cell wall composition

In plants, different forms of nitrogen (NO₃^- or NH₄⁺) affect nutrient uptake and environmental stress responses. In the present study, we tested whether NO₃^- and NH₄⁺ affect the ability of rice (Oryza sativa) to tolerate the toxic heavy metal cadmium (Cd). Compared with NO₃^-, NH₄⁺ treatment significantly increased chlorophyll contents and reduced Cd²⁺ levels in rice cultivars Nipponbare (japonica) and Kasalath (indica) grown in 0.2 mM Cd²⁺. NH₄⁺ significantly reduced the pectin and hemicellulose contents and inhibited the pectin methylesterase (PME) activity in rice roots, thereby reducing the negative charges in the cell wall and decreasing the accumulation of Cd²⁺ in roots. In addition, NH₄⁺ reduced the absorption and root-to-shoot translocation of Cd²⁺ by decreasing the expression of OsHMA2 and OsNramp5 in the root. Levels of the signaling molecule putrescine were significantly higher in the roots of both rice cultivars provided with NH₄⁺ compared with NO₃^-. The addition of putrescine reduced Cd²⁺ contents in both rice cultivars and increased the chlorophyll content in shoots by reducing root cell wall pectin and hemicellulose contents, inhibiting PME activity and suppressing the expression of OsHMA2 and OsNramp5 in the root. Taken together, these results indicate that NH₄⁺ treatment alleviated Cd toxicity, enabling rice to withstand the noxious effects of Cd by modifying the cell wall Cd-binding capacity due to alterations of pectin and hemicellulose contents and Cd transport, processes induced by increasing putrescine levels. Our findings suggest methods to decrease Cd accumulation in rice by applying NH₄⁺ fertilizers.