HOP family plays a major role in long term acquired thermotolerance in Arabidopsis affecting the HSR and protein QC

HOP is a family of cytosolic cochaperones whose molecular role in thermotolerance was quite unknown in eukaryotes and unexplored in plants. In this article, we describe that the three members of the HOP family display a different induction pattern under heat, being HOP3 highly regulated during the challenge and the attenuation period. Despite this tight HOP3 regulation, the analysis of the hop1 hop2 hop3 triple mutant demonstrates that the three AtHOP proteins act redundantly to promote long acquired thermotolerance in Arabidopsis. AtHOPs interact very strongly with HSP90 and part of the bulk of HOP shuttles from the cytoplasm to cytoplasmic foci and to the nucleus in response to heat. Consistent with this latter location, and with the formation of a HOP complex with HsfA1, RNAseq analyses demonstrate that HSR is altered in the hop1 hop2 hop3 triple mutant during the heat stress.In addition, this mutant displays an unusual high accumulation of insoluble and ubiquitinated proteins under heat. These data reveal that HOPs play a main role in two different aspects of the response to heat: the maintenance of protein homeostasis and the proper establishment of the HSR, affecting the plant capacity to acclimate to heat stress for long periods. 7 day old Arabidopsis seedlings from wt and hop1 hop2 hop3 triple mutant, grown under control conditions or challenged at 38ºC