HSP90 Contributes to Entrainment of the Arabidopsis Circadian Clock via the Morning Loop.

The plant circadian clock allows the synchronization of internal physiological responses to match the predicted environment. HSP90.2 is a molecular chaperone that has been previously described as required for the proper functioning of the Arabidopsis oscillator under both ambient and warm temperatures. Here, we have characterized the circadian phenotype of the hsp90.2-3 mutant. As previously reported using pharmacological or RNA interference inhibitors of HSP90 function, we found that hsp90.2-3 lengthens the circadian period and that the observed period lengthening was more exaggerated in warm-cold-entrained seedlings. However, we observed no role for the previously identified interactors of HSP90.2, GIGANTEA and ZEITLUPPE, in HSP90-mediated period lengthening. We constructed phase-response curves (PRCs) in response to warmth pulses to identify the entry point of HSP90.2 to the oscillator. These PRCs revealed that hsp90.2-3 has a circadian defect within the morning. Analysis of the cca1, lhy, prr9, and prr7 mutants revealed a role for CCA1, LHY, and PRR7, but not PRR9, in HSP90.2 action to the circadian oscillator. Overall, we define a potential pathway for how HSP90.2 can entrain the Arabidopsis circadian oscillator.