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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.

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