Abstract
Heat stress is a major environmental constraint for crop production worldwide. To respond to and cope with heat stress, plants synthesize heat shock proteins (HSPs), which are often molecular chaperones and are under the control of heat stress transcription factors (HSFs). Very little is known about the upstream regulators of HSFs. In a forward genetic screen for regulators of C-REPEAT BINDING FACTOR (CBF) gene expression (RCFs), we identified RCF2 and found that it is allelic to CPL1/FIERY2, which encodes a homolog of C-terminal domain phosphatase. Our results also showed that, in addition to being critical for cold stress tolerance, RCF2 is required for heat stress-responsive gene regulation and thermotolerance, because, compared with the wild type, the rcf2-1 mutant is hypersensitive to heat stress and because the reduced thermotolerance is correlated with lower expression of most of the 21 HSFs and some of the HSPs in the mutant plants. We found that RCF2 interacts with the NAC transcription factor NAC019 and that RCF2 dephosphorylates NAC019 in vivo. The nac019 mutant is more sensitive to heat stress than the wild type, and chromatin immunoprecipitation followed by quantitative PCR analysis revealed that NAC019 binds to the promoters of HSFA1b, HSFA6b, HSFA7a, and HSFC1. Overexpression of RCF2 or NAC019 in Arabidopsis thaliana increases thermotolerance. Together, our results suggest that, through dephosphorylation of NAC019, RCF2 is an integrator of high-temperature signal transduction and a mechanism for HSF and HSP activation.