The Phytochrome-Interacting Factor PIF7 Negatively RegulatesDREB1Expression under Circadian Control in Arabidopsis

Abstract

Transcription factors of the DRE-Binding1 (DREB1)/C-repeat binding factor family specifically interact with a cis-acting dehydration-responsive element/C-repeat involved in low-temperature stress-responsive gene expression in Arabidopsis (Arabidopsis thaliana). Expression of DREB1s is induced by low temperatures and is regulated by the circadian clock under unstressed conditions. Promoter sequences of DREB1s contain six conserved motifs, boxes I to VI. We analyzed the promoter region of DREB1C using transgenic plants and found that box V with the G-box sequence negatively regulates DREB1C expression under circadian control. The region around box VI contains positive regulatory elements for low-temperatureinduced expression of DREB1C. Using yeast one-hybrid screens, we isolated cDNA encoding the transcriptional factor Phytochrome-Interacting Factor7 (PIF7), which specifically binds to the G-box of the DREB1C promoter. The PIF7 gene was expressed in rosette leaves, and the PIF7 protein was localized in the nuclei of the cells. Transactivation experiments using Arabidopsis protoplasts indicated that PIF7 functions as a transcriptional repressor for DREB1C expression and that its activity is regulated by PIF7-interacting factors TIMING OF CAB EXPRESSION1 and Phytochrome B, which are components of the circadian oscillator and the red light photoreceptor, respectively. Moreover, in the pif7 mutant, expression of DREB1B and DREB1C was not repressed under light conditions, indicating that PIF7 functions as a transcriptional repressor for the expression of DREB1B and DREB1C under circadian control. This negative regulation of DREB1 expression may be important for avoiding plant growth retardation by the accumulation of DREB1 proteins under unstressed conditions.

PIFs: Systems Integrators in Plant Development

“…TOC1, PRR5, and PRR7 also repress PIF4 and PIF5 expression (Yamashino et al, 2003;Niwa et al, 2009). Moreover, PIF7 oscillates in SD, long-day (LD), and free-running conditions, suggesting that expression of PIF7 is probably under clock regulation as well (Kidokoro et al, 2009;Lee and Thomashow, 2012).…”

“…In the case of PIF7, the interaction with phyB-Pfr does not lead to a significant rapid reduction of PIF7 protein during dark-light transitions (Leivar et al, 2008a;Kidokoro et al, 2009). Instead, the phy induces the accumulation of a relatively stable phosphorylated form of PIF7 under constant light (high R/FR), and this form is rapidly dephosphorylated in response to phy inactivation under simulated shade (low R/FR) conditions .…”

“…The other subfamily members either do not bind phyB or this interaction has not been tested ; therefore, they are not the focus of this review. Some of these phyB-PIF interactions (e.g., for PIF1, PIF3, and PIF7) have been validated in vivo using dual fluorescent imaging or coimmunoprecipitation assays (Bauer et al, 2004;Leivar et al, 2008a;Shen et al, 2008;Clack et al, 2009;Kidokoro et al, 2009;Park et al, 2012;Ni et al, 2013). In addition to binding phyB, two members (PIF1 and PIF3) have also been shown to bind phyA in a Pfr conformerspecific manner, through a less conserved domain called the APA (for active phyA binding) ( Figure 1C) Oh et al, 2004;Al-Sady et al, 2006;Shen et al, 2008).…”

“…The mechanism involves direct CBF induction in the morning by CCA1 and LHY and evening repression by PRR5, 7, and 9 (Dong et al, 2011;Nakamichi et al, 2012). PIF7 and PIF4 also regulate the circadian and diurnal expression of CBFs at warm temperatures, thus potentially participating in the photoperiodic control of the CBF cold acclimation pathway (Kidokoro et al, 2009;Lee and Thomashow, 2012). The proposed model involves direct transcriptional repression of CBFs by PIF7 and presumably PIF4 during daytime in LDs, which is relieved in SDs through an unknown mechanism that might involve lower accumulation of PIF4 and PIF7 proteins during the day hours compared with LDs (Lee and Thomashow, 2012).…”

“…First, although they appear to represent a minority, a significant number of PIFq-bound genes are upregulated in pif mutants especially during the deetiolation response ( Figure 2C; Oh et al, 2009Oh et al, , 2012Zhang et al, 2013), suggesting that PIFs act as transcriptional repressors of these genes. Second, evidence for PIFs acting as intrinsic transcriptional repressors of specific genes has been provided in a number of cases, such as PIF1 repression of PSY expression in the dark (Toledo-Ortiz et al, 2010), PIF7 repression of CBF2 expression under diurnal conditions (Kidokoro et al, 2009;Lee and Thomashow, 2012), and PIF1 and/or PIF3 repression of genes associated with chlorophyll biosynthesis, photosynthesis, and reactive oxygen species (ROS) responses during deetiolation (Chen et al, 2013;Liu et al, 2013b). Collectively, the data are consistent with a model whereby in environments with low or absent phy activation, PIFs accumulate and either activate the expression of PIF-induced genes or inhibit the expression of PIF-repressed genes, and this activity is rapidly reversed by light through phy-induced degradation of the PIFs (Figures 3A and 3B).…”