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Abstract: Ammonium (NH₄⁺) is an important form of nitrogen nutrient for most plants, yet is also a stressor for many of them. However, the primary events of NH₄⁺ toxicity at the cellular level are still unclear. Here, we showed that NH₄⁺ toxicity can induce the root cell death in a temporal pattern which primarily occurs in the cells of root maturation and elongation zones, and then spreads to the cells in the meristem and root cap. The results from the NH₄⁺-hypersensitive mutant hsn1 further confirmed our findings. Taken together, NH₄⁺ toxicity inhibits primary root growth by inhibiting cell elongation and division and inducing root cell death.

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