Satoshi Kidokoro, Kentaro Hayashi, Hiroki Haraguchi, Tomona Ishikawa, Fumiyuki Soma, Izumi Konoura, Satomi Toda, Junya Mizoi, Takamasa Suzuki, Kazuo Shinozaki, and Kazuko Yamaguchi-Shinozaki

PNAS March 9, 2021 118 (10) e2021048118

Significance

The DREB1/CBF transcription factors function as master switches for cold stress adaptation in Arabidopsis. Cold stress strongly induced the DREB1 genes, and elucidation of the mechanisms of DREB1 induction is important to clarify the cold stress responses in plants. We revealed that the clock-related MYB proteins RVE4 and RVE8 were quickly transferred from the cytoplasm to the nucleus in response to cold stress and functioned as direct transcriptional activators of DREB1 expression. The other clock-related MYB proteins, CCA1 and LHY, suppressed DREB1 expression under unstressed conditions and were rapidly degraded under cold stress, which suggests that these factors indirectly regulate cold-inducible DREB1 expression. We concluded that posttranslational regulation of the clock-related transcription factors triggered cold-inducible gene expression.

Abstract

Cold stress is an adverse environmental condition that affects plant growth, development, and crop productivity. Under cold stress conditions, the expression of numerous genes that function in the stress response and tolerance is induced in various plant species, and the dehydration-responsive element (DRE) binding protein 1/C-repeat binding factor (DREB1/CBF) transcription factors function as master switches for cold-inducible gene expression. Cold stress strongly induces these DREB1 genes. Therefore, it is important to elucidate the mechanisms of DREB1 expression in response to cold stress to clarify the perception and response of cold stress in plants. Previous studies indicated that the central oscillator components of the circadian clock, CIRCADIAN CLOCK-ASSOCIATED 1 (CCA1) and LATE ELONGATED HYPOCOTYL (LHY), are involved in cold-inducible DREB1 expression, but the underlying mechanisms are not clear. We revealed that the clock-related MYB proteins REVEILLE4/LHY-CCA1-Like1 (RVE4/LCL1) and RVE8/LCL5 are quickly and reversibly transferred from the cytoplasm to the nucleus under cold stress conditions and function as direct transcriptional activators of DREB1 expression. We found that CCA1 and LHY suppressed the expression of DREB1s under unstressed conditions and were rapidly degraded specifically in response to cold stress, which suggests that they act as transcriptional repressors and indirectly regulate the cold-inducible expression of DREB1s. We concluded that posttranslational regulation of multiple clock-related transcription factors triggers cold-inducible gene expression. Our findings clarify the complex relationship between the plant circadian clock and the regulatory mechanisms of cold-inducible gene expression.

See https://www.pnas.org/content/118/10/e2021048118

Figure 1: Mutation analysis of the 1AR element in the DREB1A promoter and identification of transcription factors that bind to the Evening Element (EE) of 1AR. (A) Schematic diagram of the 1AR:ELUC reporter construct. The emerald luciferase (ELUC) reporter gene driven by four tandem repeats of a DREB1A promoter fragment (−143 to −55; 1AR) and its minimal promoter (TATA) is expressed in transgenic Arabidopsis. (B) Expression of the ELUC reporter gene in response to cold stress. Two representative lines of each construct are shown. The bars refer to the mean ± SD of three biological replicates. (C) Phylogenetic tree of the CCA1/LHY/RVE transcription factors in Brassicaceae species. The alignment is shown in SI Appendix, Fig. S3. (D) Yeast one-hybrid assays between the 1AR element and CCA1/LHY/RVE transcription factors. For the reporter, four tandemly repeated 1AR fragments were cloned in a pHisi-1 vector. (E) Electrophoretic mobility shift assays of CCA1, RVE2, and RVE8.