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 NH4+-responsive genes that maintain chloroplast integrity in the presence of high NH4+ levels.