Abstract
The core of the plant circadian clock involves multiple interlocking gene expression loops and post-translational controls along with inputs from light and metabolism. The complexity of the interactions is such that few specific functions can be ascribed to single components. In previous work, we reported differences in the operation of the clocks in Arabidopsis shoots and roots, including the effects of mutations of key clock components. Here, we have used luciferase imaging to study prr7 mutants expressing CCA1::LUC and GI::LUC markers. In mature shoots expressing CCA1::LUC, loss of PRR7 radically altered behaviour in light:dark cycles and caused loss of rhythmicity in constant light but had little effect on roots. In contrast, in mature plants expressing GI::LUC, loss of PRR7 had little effect in light:dark cycles but in constant light increased the circadian period in shoots and reduced it in roots. We conclude that most or all of the circadian input to the CCA1 promoter in shoots is mediated by PRR7 and that loss of PRR7 has organ-specific effects. The results emphasise the differences in operation of the shoot and root clocks, and the importance of studying clock mutants in both light:dark cycles and constant light.
Highlights
Circadian clocks have evolved in many organisms in response to the daily rotation of the earth and the resulting light:dark (LD) cycle
Given the importance of PRR7 in sugar signalling, we have studied the effects of its loss on mature shoots and roots in both light:dark cycles (LD) and constant light (LL) using two different reporters
All seeds were surface sterilised, stratified for 4 days at 4°C and sown on 1.2% agar in 0.5 strength Murashige and Skoog (MS) medium adjusted to pH 5.7 in 120 mm square vertical plates which were exposed to LD cycles (12 h white light provided by fluorescent tubes, 110–130 μmol . m−2 . s−1, 12 h dark) at 20°C. 10 days after sowing, seedlings were transferred to fresh plates in which the top 3 cm of agar had been replaced with 1.8% agar and 2% charcoal in 0.5 strength MS medium, readjusted to pH 5.7 after addition of charcoal
Summary
Circadian clocks have evolved in many organisms in response to the daily rotation of the earth and the resulting light:dark (LD) cycle They drive rhythms at the molecular and cellular levels, regulate the timing of many aspects of physiology and behaviour and provide a fitness benefit (Green et al, 2002; Dodd et al, 2005). The numerous components of the clock are arranged in a complex set of gene expression loops with multiple interactions, with additional inputs from factors, such as light and metabolism. It is negatively regulated by CCA1 itself, by LHY, by PRR5, 7 and 9, and by a combination of TOC1 and CHE (Sanchez et al, 2020) It is activated by light; this involves the phytochrome signalling proteins FAR-RED ELONGATED HYPOCOTYL 3 (FHY3) and FAR-RED IMPAIRED RESPONSE 1 (FAR1) as activators, and TOC1 and PHYTOCHROME INTERACTING FACTOR5 (PIF5) as inhibitors (Liu et al, 2020). The data show that loss of PRR7 has opposite effects on the periods of the shoot and root clocks, and that PRR7 provides the main circadian input to the CCA1 promoter in shoots
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