The proper functioning of many cytosolic proteins involved in signal transduction depends on protein folding steps carried out cooperatively by a multichaperone complex containing the Hsp90 and Hsp70 machineries. We have recently found that also in the chloroplast the Hsp90 and Hsp70 machineries form a multichaperone complex, although chloroplast Hsp90 and Hsp70 are from eukaryotic and prokaryotic origin, respectively. In earlier work by others it was shown that plants expressing a mutated form of a chloroplast-targeted Hsp90 were impaired in the light induction of several nuclear genes. These data suggest that, like in the cytosol, the folding of chloroplast proteins involved in chloroplast-to-nucleus signalling might depend on the cooperative action of the chloroplast Hsp70-Hsp90 machineries.