Arabidopsis Plastid AMOS1/EGY1 Integrates Abscisic Acid Signaling to Regulate Global Gene Expression Response to Ammonium Stress

Abstract

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.

Molecular components of stress-responsive plastid retrograde signaling networks and their involvement in ammonium stress

“…45 The H 2 O 2 response is defective in guard-cell chloroplasts of the amos1 mutant, independent of external ammonium treatment and the absence or presence of ABA. 31 The expression of H 2 O 2 -responsive genes, such as small heat shock proteins (HSPs), 27,46 is induced greatly in wild-type Arabidopsis, but not in the amos1 mutant, upon exposure to excess ammonium. 31 These data indicate that a chloroplastic ROS signal is a highly plausible candidate for the AMOS1/EGY1-dependent ammonium-induced plastid retrograde signal.…”

“…35,36 A mutant of EGY1 was found to reduce H 2 O 2 accumulation in guard cell chloroplasts, and interact with ABA signaling to regulate the expression of nuclear genes in response to excess ammonium. 31 Thus, the function of EGY1 in retrograde signaling at least incorporates, but may not be limited to, the fifth type. Therefore, the diversity of molecular components and signal compounds suggests a fundamental regulatory function of plastid retrograde signaling in plant acclimation to a variety of environmental stresses.…”

“…37,38 However, ABA is not regarded as a direct candidate for the plastid signal, 39 because ABA biosynthesis occurs in the cytosol, 40 ABA can repress the expression of mutant revealed that plastid metalloprotease EGY1 is required for plastid retrograde signaling in response to ammonium stress. 31 Further, the role chloroplastic H 2 O 2 plays in triggering retrograde signaling to regulate the expression of nuclear genes induced by abiotic and biotic stresses was established by the construction of silencing lines of thylakoid membrane-bound ascorbate peroxidase (tAPX), using RNAi. 27,28 The regulatory role of these molecular components in stressresponsive plastid retrograde signaling can be preliminarily divided into five functional types, based on their targets: regulation of the signal compound level, signal transfer, gene transcription in the nucleus, chloroplast homeostasis, and interaction with other signaling pathways.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] More recently, several plastid retrograde signaling pathways have been identified to regulate the expression of stress-related nuclear genes during several stresses such as high light, drought, wounding, heat, and excess ammonium. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] Based on the new stress-responsive plastid retrograde signaling pathways just reported in 2012 alone, this review will focus on properties of currently proposed molecular components in the plastid retrograde signaling network under stress. We also specifically discuss possible plastid retrograde signals involved in plant response to ammonium stress.…”

“…However, the involvement of these signals remains to be established for heat and ammonium stress. 30,31 It is typically assumed that the plasma membrane serves as the first site of perception of environmental changes in a cell, 32,33 and chloroplasts, as the metabolic center of photosynthetic activity, may act as a secondary, but vital, downstream modulator in the sensing of environmental signals originally perceived by the plasma membrane. As a result of effective signal transduction, modulation of nuclear gene expression ensues, impacting chloroplast function and, more generally, cell growth.…”