Circadian Studies Show Plant, Animal Similarities

Plant and animal circadian rhythms, 24-hour cycles that regulate many physiological and metabolic functions, are ultimately influenced by the same thing: light. Scientists are now beginning to realize that plants and animals may also share some of the mechanisms for receiving and processing that light so that the day/night cycles in both organisms are optimized. Two reports in the Nov. 20 issue of Science --one by researchers from The Scripps Research Institute in La Jolla, Calif. (D.E. Somers et al., Science, 282:1488-90), and one by University of North Carolina, Chapel Hill, and Northwestern University researchers (R.J. Thresher et al., Science, 282:1491-4)--identify light-sensitive molecules called photoreceptors that act as intermediaries between environmental cues and the machinery of the circadian clock. The first paper outlines the roles of plant photoreceptors in the Arabidopsis thaliana that detect blue and red light, the two wavelengths plants primarily absorb; the second investigates the role of one particular photoreceptor in mice, cryptochrome 2, one of a type of blue light photoreceptors identified first in Arabidopsis (M. Ahmad, A.R. Cashmore, Nature, 366:162-6, 1993). These studies, in conjunction with past studies in Arabidopsis and as-yet-unpublished studies in Drosophila, which scientists know also has cryptochromes, suggest that circadian input pathways have, at least in part, been conserved throughout evolution among several species. The Scripps researchers examined the roles of plants' red light photoreceptors, phytochromes A (phyA) and B (phyB), and blue light photoreceptors, cryptochromes 1 (cry1) and 2 (cry2), by using mutant plants missing the genes for each. To measure the period length of the photoreceptor mutants' biological clocks, the investigators genetically engineered each to contain the clock-regulating genes fused with the firefly luciferase gene (luc) so that the plants glowed on and off rhythmically in the course of their 24-hour cycles. Special cameras and video equipment measured the bioluminescence, and computers traced the bioluminescent pattern. They found that phyA was important for signaling to the clock in dim light, an observation that made biological sense since scientists know that phyA builds up to high levels in young, light-deprived seedlings just after germination. PhyB transmitted the high-intensity red light, which is plentiful in full sunlight, to the clock. "What this result says is that there's plasticity here," explains senior author Steven Kay, an associate professor of cell biology at Scripps. "That is, the plants recruit different photoreceptors to cope with different kinds of light conditions." Kay called the results from the blue light photoreceptors "a bit more interesting." While they couldn't find much of an effect from cry2, cry1 appeared to be responsible for detecting both dim and bright blue light. Somewhat surprisingly, they found that phyA was also necessary for dim blue light signaling. Apparently, in order for plants to send light signals to the clock under dim blue light, these two photoreceptors must act together. Interestingly, this result matches that of a paper published recently by Anthony Cashmore, the University of Pennsylvania plant biologist who discovered cryptochromes. This past June, Cashmore demonstrated (M. Ahmad et al., Molecular Cell, 1:939-48, June 1998) that phyA and cry1 in Arabidopsis actually engage in direct protein-protein interactions; the former phosphorylates the latter. "Finally in plants now we're beginning to get the picture," says Kay. "Prior to [our Nov. 20 Science ] study we had no idea how any of these photoreceptors were hooked up to the clock in any organism." More evidence of conservation between species: cry2 also plays a role in the regulation of animals' circadian rhythms. According to the Science paper by UNC and Northwestern researchers, cry2-deficient mice ran on their treadmills out of sync with their daily cycle, and they had reduced light-mediated induction of period (per), a gene thought to be important for clock "resetting" and one of several genes known to be involved in the molecular cascade that controls animals' circadian rhythms. Similar results were first reported in May of this year by the UNC group (Y. Miyamoto, A. Sancar , Proceedings of the National Academy of Sciences, 95:6097-102, May 26, 1998); less conclusive data--based on in situ hybridization--led the researchers to back up their results with the more recent knockout experiment in mice. "This is really the first time anyone has identified a pigment [in animals] unambiguously ... that's involved in regulating the circadian clock," says senior author Aziz Sancar, a professor of biochemistry and biophysics at UNC. Sancar and his colleagues now hope to clarify the process of phototransduction, a kind of signal transduction in which photoreceptors such as cryptochromes transmit light signals to activate transcription factors that start protein production. "One is struck by the fact that indeed there are cryptochrome sequences ... in both animals and plants," remarks Cashmore. "It remains to be seen mechanistically how similar these two sequences are." Kay, Cashmore, and others continue to investigate the extent to which plants have molecular homologues to the known animal circadian machinery that includes, for example, the PERIOD (PER) protein (E. Russo, The Scientist, 12[16]:13, Aug. 17, 1998), a protein that is somehow produced as a result of light-photoreceptor interactions that set phototransduction in motion. Either such a homology in plants has not yet been identified at all, has not been recognized because plants have diverged from animals too much, or is simply nonexistent because plants have a different set of regulatory components. A recent published study (M. Ishiura et al., Science, 281:1519-23, Sept. 4, 1998) suggests that the latter option may be the most likely. Using a bioluminescent trick similar to the one employed by Kay, researchers from Vanderbilt and Texas A & M Universities discovered a cluster of three genes, dubbed kaiABC, which makes the proteins (KaiA, B, and C) that regulate the circadian clock in cyanobacteria. Those genes and proteins are, in all likelihood, not homologous to anything found in animals. "The photoreceptors [in plants and animals] are clearly similar," says Cashmore. "It remains to be seen how similar the signaling systems are."

Circadian Studies Show Plant, Animal Similarities

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