Iwanaga, H.; Arai, Y.; Nezuo, M.; Doi, A.; Takei, T.; Fujiwara, M.; Hachiya, T.; Hamada, T.

Nitrogen is essential for plant growth and is sourced primarily from nitrate and ammonium in the soil. Even though plants can take ammonium up for nutrition, it often results in toxic effects such as growth suppression and chlorosis. To elucidate the mechanism of ammonium toxicity, a time-course analysis of the transcriptome was performed on A. thaliana leaves treated with high concentrations of ammonium sulfate in the presence of sufficient nitrate. The expression of nitrate-inducible genes tended to be downregulated by the treatment. The expression of genes relating to abscisic acid, jasmonic acid (JA), sali-cylic acid (SA), and membrane trafficking was upregulated, whereas that of photosynthesis-, auxin-, and cytokinin-related genes involved in growth and development was down-regulated. The induction of many osmotic stress-responsive genes suggests the involvement of osmotic stress in ammonium toxicity. Furthermore, the upregulation of nitric oxide (NO)-inducible genes and the simultaneous upregulation of genes involved in JA biosynthesis, glutathione metabolism, and SA response suggested the involvement of endogenous NO and protein S-nitrosylation in response to high concentrations of ammonium sulfate. This study provides a novel and comprehensive overview of transcriptional changes occurring in response to high ammonium sulfate concentrations and proposes possible mechanisms of ammonium toxicity that can be explored in future research.