By James CW Locke, California Institute of Technology
A new Science paper from the lab of Alex Webb (Dodd et al, Science, 2007) represents an important step forward in plant circadian research (read also commentary by Imaizumi et al, Science, 2007). The circadian (24 h) clock controls processes throughout the day and night in most organisms, and in plants is involved in multiple pathways including photosynthesis, leaf movement and floral opening. The circadian clock has evolved to consist of multiple interlocking transcriptional feedback loops (at least in eukaryotes), which generate the 24 h rhythm even under constant environmental conditions.
Using a series of elegant experiments Dodd et al uncover a new level of complexity to the plant clock. They first show that cytosolic signaling molecule cyclic adenosine diphosphate ribose (cADPR) is regulated by the clock and is responsible for the previously reported circadian rhythm in intracellular calcium. They go on to show that disruption of cADPR signaling by addition of nicotinamide causes a strong period lengthening of the clock. Thus they have discovered an additional feedback loop (see drawing), and revealed a new class of circadian clock components: cytosolic signaling molecules.
It is also of note that Dodd et al use an existing mathematical model of the Arabidopsis clock (Locke et al, Mol Syst Biol, 2005) to frame the expected effects of their predicted feedback loop. A large amount of systems modeling work has recently been carried out on the clock (summarized in Ueda, Mol Syst Biol, 2006). It is exciting to see how the Arabidopsis clock community has responded to this work. In some cases the models have been used to simulate experiments and to test putative mechanisms (Dodd et al, Science, 2007, Martin-Tryon et al, Plant Physiol, 2007). I believe for systems biology approaches to succeed it is crucial that models must be made easily accessible to experimentalists, which is the case in this field (eg Circadian Modelling). I look forward to seeing the next iteration of the plant circadian clock model, perhaps from the Webb lab including the cADPR feedback loop.