HsfB2b-mediated repression of PRR7 directs abiotic stress responses of the circadian clock

Significance The daily environmental changes in light and dark set the pace of the circadian clock, which at the molecular level is composed of transcription-based feedback loops. It is well known that temperature fluctuations affect the circadian clock, but the molecules involved remain largely undefined. We found that the heat shock transcription factor HsfB2b works in the repression of the morning clock gene PRR7 . We tested the properties of the circadian clock, and growth behavior, in the hsfB2b mutant and in transgenics with altered HsfB2b expression. Our results reveal that HsfB2b expression is important for accurate circadian rhythms following elevated temperature and/or salt treatment. Our work provides evidence of a molecular entry point for temperature signaling to the plant circadian clock. The circadian clock perceives environmental signals to reset to local time, but the underlying molecular mechanisms are not well understood. Here we present data revealing that a member of the heat shock factor (Hsf) family is involved in the input pathway to the plant circadian clock. Using the yeast one-hybrid approach, we isolated several Hsfs, including HEAT SHOCK FACTOR B2b (HsfB2b), a transcriptional repressor that binds the promoter of PSEUDO RESPONSE REGULATOR 7 ( PRR7 ) at a conserved binding site. The constitutive expression of HsfB2b leads to severely reduced levels of the PRR7 transcript and late flowering and elongated hypocotyls. HsfB2b function is important during heat and salt stress because HsfB2b overexpression sustains circadian rhythms, and the hsfB2b mutant has a short circadian period under these conditions. HsfB2b is also involved in the regulation of hypocotyl growth under warm, short days. Our findings highlight the role of the circadian clock as an integrator of ambient abiotic stress signals important for the growth and fitness of plants.