PRR3 Is a Vascular Regulator of TOC1 Stability in theArabidopsisCircadian Clock

Abstract

The pseudoresponse regulators (PRRs) participate in the progression of the circadian clock in Arabidopsis thaliana. The founding member of the family, TIMING OF CAB EXPRESSION1 (TOC1), is an essential component of the transcriptional network that constitutes the core mechanism of the circadian oscillator. Recent data suggest a role in circadian regulation for all five members of the PRR family; however, the molecular function of TOC1 or any other PRRs remains unknown. In this work, we present evidence for the involvement of PRR3 in the regulation of TOC1 protein stability. PRR3 was temporally coexpressed with TOC1 under different photoperiods, yet its tissue expression was only partially overlapping with that of TOC1, as PRR3 appeared restricted to the vasculature. Decreased expression of PRR3 resulted in reduced levels of TOC1 protein, while overexpression of PRR3 caused an increase in the levels of TOC1, all without affecting the amount of TOC1 transcript. PRR3 was able to bind to TOC1 in yeast and in plants and to perturb TOC1 interaction with ZEITLUPE (ZTL), which targets TOC1 for proteasome-dependent degradation. Together, our results indicate that PRR3 might function to modulate TOC1 stability by hindering ZTL-dependent TOC1 degradation, suggesting the existence of local regulators of clock activity and adding to the growing importance of posttranslational regulation in the design of circadian timing mechanisms in plants.

PSEUDO RESPONSE REGULATORs stabilize CONSTANS protein to promote flowering in response to day length

“…PRRs were originally identified as components of the circadian clock in A. thaliana and generally proposed to contribute to day‐length measurement through control of the time‐keeping mechanism associated with CO transcription (Strayer et al , 2000; Yanovsky & Kay, 2002; Ito et al , 2003, 2008; Yamamoto et al , 2003; Murakami et al , 2004; Nakamichi et al , 2007; Para et al , 2007). By contrast, we defined a previously unidentified role of these circadian‐clock components in regulating CO activity, where they interact with and stabilize CO protein during the day, ensuring its accumulation under LDs.…”

“…According to this model, in our study the effect of toc1 prr5 prr7 prr9 mutations on HY5 levels in the vasculature would not be detected because HY5 is broadly expressed (Oyama et al , 1997). However, in this case, since PRRs are also broadly expressed in A. thaliana to control circadian‐clock functions (Para et al , 2007; Fujiwara et al , 2008), the activity of PRRs to directly control COP1 activity might be expected to occur in other tissues as well.…”

Organ specificity in the plant circadian system is explained by different light inputs to the shoot and root clocks

“…The cca1‐11,lhy‐21 double mutant expressing CCR2:LUC+ was obtained from the Nottingham Arabidopsis Stock Centre (N9809). PRR7:LUC+ expressing lines and the toc1‐4 mutant expressing CCR2:LUC+ are in the Columbia (Col‐0) background (Salome & McClung, 2005; Para et al ., 2007). …”

“…Recent work has focussed on the potential for hierarchical coupling of tissue‐specific clocks. The proposed vascular‐specific expression of PRR3 (Para et al ., 2007) might be part of a distinct vascular clock that can influence the mesophyll clock (Endo et al ., 2014). Takahashi et al .…”

Abiotic stress and the plant circadian clock

“…28 In contrast to the action of ZTL on TOC1 instability, PRR3 protects TOC1 from degradation. 29 Targeted protein degradation also takes place in the morning phase of the oscillator. As an example, LHY proteolysis was shown to be regulated by DEETIOLATED 1 (DET1).…”