Arabidopsis plastid AMOS1/EGY1 integrates abscisic acid signaling to regulate global gene expression response to ammonium stress

Abstract
Ammonium (NH4 +) is a ubiquitous intermediate of nitrogen metabolism but is notorious for its toxic effects on most organisms. Extensive studies of the underlying mechanisms of NH4 + toxicity have been reported in plants, but it is poorly understood how plants acclimate to high levels of NH4 +. Here, we identified an Arabidopsis (Arabidopsis thaliana) mutant, ammonium overly sensitive1 (amos1), that displays severe chlorosis under NH4 + 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 NH4 +, 90% are regulated dependent on AMOS1/ EGY1. Furthermore, 63% of AMOS1/EGY1-dependent NH4 +-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 NH4 +-responsive genes and maintains chloroplast functionality under NH4 + stress. Importantly, abi4 mutants defective in ABA-dependent and retrograde signaling, but not ABA-deficient mutants, mimic leaf NH4 + hypersensitivity of amos1. In summary, our findings suggest that an NH4 +-responsive plastid retrograde pathway, which depends on AMOS1/EGY1 function and integrates with ABA signaling, is required for the regulation of expression of the presence of high NH4 + levels. © 2012 American Society of Plant Biologists. All Rights Reserved.

Citation
Li, B., Li, Q., Xiong, L., Kronzucker, H. J., Krämer, U., & Shi, W. (2012). Arabidopsis Plastid AMOS1/EGY1 Integrates Abscisic Acid Signaling to Regulate Global Gene Expression Response to Ammonium Stress. Plant Physiology, 160(4), 2040–2051. doi:10.1104/pp.112.206508

Acknowledgements
This work was supported by the National Science Foundation of China (grant nos. 31200189 and 30771285), the Chinese Academy Sciences Innovation Program (grant no. ISSASIP1103), the National Sciences and Engineering Research Council of Canada (grant no. 217277-2009), and the National Nature Science Foundation (grant nos. 91017013 and 31070327).

Publisher
American Society of Plant Biologists (ASPB)

Journal
PLANT PHYSIOLOGY

DOI
10.1104/pp.112.206508

PubMed ID
23064408

PubMed Central ID
PMC3510130

Additional Links
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3510130

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