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.
Highlights
It is generally accepted that a temperature upshift, usually 10– 15uC above an optimum temperature for growth, is considered as heat stress for leaf photosynthesis in higher plants [1,2]
We have identified chloroplast ribosomal protein S1 (RPS1) as a heat-responsive protein through proteomic screening of heat-responsive proteins
We have established a previously unrecognized molecular connection between the downregulation of RPS1 expression in chloroplast and the activation of HsfA2-dependent heat-responsive genes in nucleus, which is required for heat tolerance in higher plants
Summary
It is generally accepted that a temperature upshift, usually 10– 15uC above an optimum temperature for growth, is considered as heat stress for leaf photosynthesis in higher plants [1,2]. Oxygen evolving complex (OEC) in PSII is highly thermolabile and heat stress may cause the dissociation of OEC, resulting in an imbalance in the electron flow from OEC toward the acceptor side of PSII in the direction of PSI reaction center [1,2,3,8,9]. It is known that under heat stress, the decline in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity is mainly due to inactivation of Rubisco activase that is extremely sensitive to elevated temperatures because the enzyme Rubisco of higher plants is heat stable [5,11]. In addition to the early effects on photochemical reactions and carbon assimilation, heat stress usually leads to alterations in the microscopic ultrastructures of chloroplast and the integrity of thylakoid membranes, including membrane destacking and reorganization [2,12,13,14,15]
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