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 (NH) nutrition and calcium (Ca) homeostasis in the leaf tissues of wild-type and mutant Arabidopsis specimens provided with different nitrogen sources (NH and nitrate, NO). Providing plants with NH as the sole nitrogen source disrupts Ca homeostasis, which is essential for activating signaling pathways and maintaining the cell wall structure. The results revealed that the lower Ca content in Arabidopsis leaves under NH stress might result from reduced transpiration pull, which could impair root-to-shoot Ca transport. Moreover, NH nutrition increased the expression of genes encoding proteins responsible for exporting Ca from the cytosol of leaf cells. Furthermore, overexpression of the Ca/H antiporter 1 (CAX1) gene alleviates the effects of NH syndrome, including stunted growth. The oeCAX1 plants, characterized by a lower apoplastic Ca level, grew better under NH 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 NH-stressed plants.