Downregulation of Chloroplast RPS1 Negatively Modulates Nuclear Heat-Responsive Expression of HsfA2 and Its Target Genes in Arabidopsis

Abstract

Heat stress commonly leads to inhibition of photosynthesis in higher plants. The transcriptional induction of heat stress-responsive genes represents the first line of inducible defense against imbalances in cellular homeostasis. Although heat stress transcription factor HsfA2 and its downstream target genes are well studied, the regulatory mechanisms by which HsfA2 is activated in response to heat stress remain elusive. Here, we show that chloroplast ribosomal protein S1 (RPS1) is a heat-responsive protein and functions in protein biosynthesis in chloroplast. Knockdown of RPS1 expression in the rps1 mutant nearly eliminates the heat stress-activated expression of HsfA2 and its target genes, leading to a considerable loss of heat tolerance. We further confirm the relationship existed between the downregulation of RPS1 expression and the loss of heat tolerance by generating RNA interference-transgenic lines of RPS1. Consistent with the notion that the inhibited activation of HsfA2 in response to heat stress in the rps1 mutant causes heat-susceptibility, we further demonstrate that overexpression of HsfA2 with a viral promoter leads to constitutive expressions of its target genes in the rps1 mutant, which is sufficient to reestablish lost heat tolerance and recovers heat-susceptible thylakoid stability to wild-type levels. Our findings reveal a heat-responsive retrograde pathway in which chloroplast translation capacity is a critical factor in heat-responsive activation of HsfA2 and its target genes required for cellular homeostasis under heat stress. Thus, RPS1 is an essential yet previously unknown determinant involved in retrograde activation of heat stress responses in higher plants.

Systems-Wide Analysis of Acclimation Responses to Long-Term Heat Stress and Recovery in the Photosynthetic Model OrganismChlamydomonas reinhardtii

“…In eukaryotes, these are known to be sensed in the cytosol, the endoplasmic reticulum (ER), and mitochondria and signaled to the nucleus to initiate appropriate transcriptional responses (Voellmy and Boellmann, 2007;Walter and Ron, 2011;Haynes et al, 2013). Recent work suggested that an unfolded protein response also exists in the chloroplast (Yu et al, 2012;Schmollinger et al, 2013;Ramundo et al, 2014). Plasma membrane fluidity has been identified as an important temperature sensor in land plants that appears to lie upstream of the unfolded protein response (Sangwan et al, 2002;Suri and Dhindsa, 2008;Saidi et al, 2009;Wu et al, 2012).…”

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

“…26,29 Analysis of the rps1 mutant demonstrated that the chloroplast ribosomal protein S1 (RPS1) mediates retrograde signaling to modulate the expression of the heat-responsive nuclear transcription factor HsfA2 and its target genes during heat stress, although the specific plastid retrograde signal is unknown. 30 Recently, gene cloning and transcriptional analysis of the Arabidopsis ammonium-overly-sensitive 1 (amos1) that are needed for maintaining the stability of the thylakoid membrane system under normal growth conditions. 30 As an example of the fifth type, PLEIOTROPIC RESPONSE LOCUS 1 (PRL1) protein, located in the nucleus, appears to play a major role in modulating plant responses to environmental changes by interconnecting 1 O 2 -mediated retrograde signaling with energy signaling pathways.…”

“…30 Recently, gene cloning and transcriptional analysis of the Arabidopsis ammonium-overly-sensitive 1 (amos1) that are needed for maintaining the stability of the thylakoid membrane system under normal growth conditions. 30 As an example of the fifth type, PLEIOTROPIC RESPONSE LOCUS 1 (PRL1) protein, located in the nucleus, appears to play a major role in modulating plant responses to environmental changes by interconnecting 1 O 2 -mediated retrograde signaling with energy signaling pathways. 23 The most recently characterized molecular component, EGY1, the first metalloprotease site-2 protease (S2P) homolog discovered in plants and located in plastids, is required for normal chloroplast development such as thylakoid grana stacking, the development of the lamellae system, and the accumulation of chlorophyll and chlorophyll a/b binding proteins.…”

“…[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.…”