Interplay between HEAT SHOCK PROTEIN 90 and HY5 Controls PhANG Expression in Response to the GUN5 Plastid Signal

The presence of genes encoding organellar proteins in different cellular compartments necessitates a tight coordination of expression by the different genomes of the eukaryotic cell. This coordination of gene expression is achieved by organelle-to-nucleus or retrograde communication. Stress-induced perturbations of the tetrapyrrole pathway trigger large changes in nuclear gene expression in plants. Recently, we identified HSP90 proteins as ligands of the pu- tative plastid signal Mg-ProtolX. In order to investigate whether the interaction between HSP90 and Mg-ProtolX is bio- logically relevant, we produced transgenic lines with reduced levels of cytosolic HSP90 in wild-type and gun5 backgrounds. Our work reveals that HSP90 proteins respond to the tetrapyrrole-mediated plastid signal to control expres- sion of photosynthesis-associated nuclear genes （PhANG） during the response to oxidative stress. We also show that the hy5 mutant is insensitive to tetrapyrrole accumulation and that Mg-ProtolX, cytosolic HSP90, and HY5 are all part of the same signaling pathway. These findings suggest that a regulatory complex controlling gene expression that includes HSP90 proteins and a transcription factor that is modified by tetrapyrroles in response to changes in the environment is evolutionarily conserved between yeast and plants.