The photorespiratory pathway is comprised of enzymes localized within three distinct cellular compartments: chloroplasts, peroxisomes, and mitochondria. Photorespiratory enzymes are encoded by nuclear genes, translated in the cytosol, and targeted into these distinct subcellular compartments. One likely means by which to regulate the expression of the genes encoding photorespiratory enzymes is coordinated temporal control. We have previously shown in Arabidopsis that a circadian clock regulates the expression of the nuclear genes encoding both chloroplastic (Rubisco small subunit and Rubisco activase) and peroxisomal (catalase) components of the photorespiratory pathway. To determine whether a circadian clock also regulates the expression of genes encoding mitochondrial components of the photorespiratory pathway, we characterized a family of Arabidopsis serine hydroxymethyltransferase (SHM) genes. We examined mRNA accumulation for two of these family members, including one probable photorespiratory gene (SHM1) and a second gene expressed maximally in roots (SHM4), and show that both exhibit circadian oscillations in mRNA abundance that are in phase with those described for other photorespiratory genes. In addition, we show that SHM1 mRNA accumulates in light-grown seedlings, although this response is probably an indirect consequence of the induction of photosynthesis and photorespiration by illumination.
Circadian rhythms in the abundance of the CAT2 catalase mRNA were not seen in etiolated seedlings but developed upon illumination. These circadian oscillations were preceded by a rapid and transient induction of CAT2 mRNA abundance that varied strikingly according to the timing (circadian phase) of the onset of illumination. This variation oscillated with a circadian periodicity of approximately 28 hr, indicating that the circadian oscillator is running in etiolated seedlings and regulates (gates) the induction of CAT2 by light. Moreover, because we assayed populations of seedlings, we infer that the individual clocks among populations of etiolated seedlings were synchronized before the onset of illumination. What developmental or environmental signals synchronized the clocks among seedlings? Varying the phase of the onset of illumination relative to release from stratification failed to affect the acute induction of CAT2, indicating that the temperature step from 4 to 22 degrees C associated with release from stratification did not reset the circadian clock. However, the acute induction of CAT2 mRNA varied with time after imbibition, demonstrating that imbibition provides a signal capable of resetting the circadian clock and of synchronizing the clocks among populations of seedlings.
Circadian rhythms in the abundance of the CAT2 catalase mRNA were not seen in etiolated seedlings but developed upon illumination. These circadian oscillations were preceded by a rapid and transient induction of CAT2 mRNA abundance that varied strikingly according to the timing (circadian phase) of the onset of illumination. This variation oscillated with a circadian periodicity of ف 28 hr, indicating that the circadian oscillator is running in etiolated seedlings and regulates (gates) the induction of CAT2 by light. Moreover, because we assayed populations of seedlings, we infer that the individual clocks among populations of etiolated seedlings were synchronized before the onset of illumination. What developmental or environmental signals synchronized the clocks among seedlings? Varying the phase of the onset of illumination relative to release from stratification failed to affect the acute induction of CAT2 , indicating that the temperature step from 4 to 22 ؇ C associated with release from stratification did not reset the circadian clock. However, the acute induction of CAT2 mRNA varied with time after imbibition, demonstrating that imbibition provides a signal capable of resetting the circadian clock and of synchronizing the clocks among populations of seedlings. INTRODUCTIONThe biological circadian clock generates circadian rhythms with periods of ف 24 hr. Plants are richly rhythmic, and the clock provides endogenous timing information that is used to coordinate numerous aspects of plant development and physiology (Sweeney, 1987;Kreps and Kay, 1997;Lumsden and Millar, 1998). The circadian clock regulates the expression of many plant genes that serve as molecular markers of the status of the clock. These circadian rhythms are not unique to plants but are widespread, if not ubiquitous, among eukaryotes and prokaryotes (Edmunds, 1988; Dunlap, 1996; Golden et al., 1997).The catalase ( CAT ) gene family of Arabidopsis provides a particularly useful set of molecular markers for the circadian clock because the individual members of this small family of three genes each respond differently to temporal information supplied by the circadian clock (McClung, 1997). Although CAT1 mRNA abundance does not exhibit circadian oscillation (Frugoli et al., 1996), there are circadian oscillations in the mRNA abundance of both CAT2 and CAT3. It is intriguing that the peaks in mRNA abundance of CAT2 and CAT3 are gated to distinct phases (times of day) by the circadian clock, with CAT2 mRNA being most abundant at dawn and CAT3 mRNA being most abundant at dusk (Zhong et al., 1994;Zhong and McClung, 1996). The physiological significance of the circadian regulation of CAT mRNA abundance is not yet apparent. Catalase (H 2 O 2 :H 2 O 2 oxidoreductase; EC 1.11.1.6) belongs to a group of enzymes involved in regulating the cellular levels of active oxygen species. Catalase is present in all aerobic organisms and protects cells from the damaging effects of H 2 O 2 by converting H 2 O 2 to O 2 and H 2 O. However, it remains to be dete...
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