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Arabidopsis Plastid AMOS1/EGY1 Integrates Abscisic Acid Signaling to Regulate Global Gene Expression Response to Ammonium Stress
- Li, Baohai, Li, Qing, Xiong, Liming, Kronzucker, Herbert J., Krämer, Ute, Shi, Weiming
- Plant physiology 2012 v.160 no.4 pp. 2040-2051
- Arabidopsis thaliana, abscisic acid, ammonium compounds, chloroplasts, chlorosis, gene expression, genes, hypersensitivity, leaves, metalloproteinases, molecular cloning, mutants, mutation, nitrogen metabolism, promoter regions, toxicity, transcriptomics
- Ammonium (NH ₄⁺) is a ubiquitous intermediate of nitrogen metabolism but is notorious for its toxic effects on most organisms. Extensive studies of the underlying mechanisms of NH ₄⁺ toxicity have been reported in plants, but it is poorly understood how plants acclimate to high levels of NH ₄⁺. Here, we identified an Arabidopsis (Arabidopsis thaliana) mutant, ammonium overly sensitive1 (amos1), that displays severe chlorosis under NH ₄⁺ stress. Map-based cloning shows amos1 to carry a mutation in EGY1 (for ethylene-dependent, gravitropism-deficient, and yellow-green-like protein1), which encodes a plastid metalloprotease. Transcriptomic analysis reveals that among the genes activated in response to NH ₄⁺, 90% are regulated dependent on AMOS1 / EGY1 . Furthermore, 63% of AMOS1 / EGY1 -dependent NH ₄⁺-activated genes contain an ACGTG motif in their promoter region, a core motif of abscisic acid (ABA)-responsive elements. Consistent with this, our physiological, pharmacological, transcriptomic, and genetic data show that ABA signaling is a critical, but not the sole, downstream component of the AMOS1 / EGY1 -dependent pathway that regulates the expression of NH ₄⁺-responsive genes and maintains chloroplast functionality under NH ₄⁺ stress. Importantly, abi4 mutants defective in ABA-dependent and retrograde signaling, but not ABA-deficient mutants, mimic leaf NH ₄⁺ hypersensitivity of amos1 . In summary, our findings suggest that an NH ₄⁺-responsive plastid retrograde pathway, which depends on AMOS1 / EGY1 function and integrates with ABA signaling, is required for the regulation of expression of NH ₄⁺-responsive genes that maintain chloroplast integrity in the presence of high NH ₄⁺ levels.