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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.