Ammonium nutrition modifies cellular calcium distribution influencing ammonium-induced growth inhibition

Proper plant growth requires balanced nutrient levels. In this study, we analyzed the relationship between ammonium (NH₄⁺) nutrition and calcium (Ca²⁺) homeostasis in the leaf tissues of wild-type and mutant Arabidopsis specimens provided with different nitrogen sources (NH₄⁺ and nitrate, NO₃^-). Providing plants with NH₄⁺ as the sole nitrogen source disrupts Ca²⁺ homeostasis, which is essential for activating signaling pathways and maintaining the cell wall structure. The results revealed that the lower Ca²⁺ content in Arabidopsis leaves under NH₄⁺ stress might result from reduced transpiration pull, which could impair root-to-shoot Ca²⁺ transport. Moreover, NH₄⁺ nutrition increased the expression of genes encoding proteins responsible for exporting Ca²⁺ from the cytosol of leaf cells. Furthermore, overexpression of the Ca²⁺/H⁺ antiporter 1 (CAX1) gene alleviates the effects of NH₄⁺ syndrome, including stunted growth. The oeCAX1 plants, characterized by a lower apoplastic Ca²⁺ level, grew better under NH₄⁺ stress than wild-type plants. Evaluation of the mechanical properties of the leaf blades, including stiffness, strength, toughness, and extensibility, showed that the wild-type and oeCAX1 plants responded differently to the nitrogen source, highlighting the role of cell wall metabolism in inhibiting the growth of NH₄⁺-stressed plants.