Molecular Genetic Analysis of Plant Circadian Rhythms

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Rhythmic biological phenomena which recur at approximately 24 hour intervals in the absence of environmental time cues are termed circadian rhythms, and their timing is regulated by an endogenous biological clock. Circadian rhythms are widespread both within any given organism and among diverse taxa. Genetic and molecular biological studies, primarily in a subset of model organisms, have begun to identify the components of circadian systems. Although plants have provided many examples of rhythmic outputs and our understanding of photoreceptors of circadian input pathways is well-advanced, plants have lagged in the identification of components of the central circadian oscillator. Increased understanding of plant responses to environmental input and to endogenous temporal cues has agricultural importance, in particular because both environmental cues and the circadian clock contribute to the photoperiodic decision to flower. In addition, there is temporal variation in the response to biotic and abiotic stresses, which may prove particularly relevant to attempts to increase agricultural productivity. Arabidopsis mutants affected in clock function, termed circadian timing defective (ctd) and out of phase (oop), have been isolated using the circadian rhythm in resistance/sensitivity to SO2 exposure that results from circadian rhythms in stomatal aperture and gas exchange. For example, oop1 and oop2 represent novel alleles of the photoreceptors PHYTOCHROME B and CRYPTOCHROME 1, respectively. The determination of the molecular mechanisms by which phyBoop1 and cry1oop2 affect circadian phase will afford insight into this integral but poorly understood aspect of circadian rhythmicity. phyBoop1 and cry1oop2 affect both blue and red light signaling in a semi-dominant manner, suggesting that the mutant proteins titrate components common to red and blue light signaling pathways that provide photic input to the clock. The identification of proteins interacting with phyBoop1 and cry1oop2 represents an important goal in the elucidation of this pathway. A second goal is the cloning and the genetic and molecular characterization of a long period mutant, ctd2. Although it has not yet been established that the CTD2 gene encodes a component of a central clock oscillator, the characterization of the ctd2 mutant and the isolation of the CTD2 gene will contribute to the elucidation of the mechanisms of clock function. Potential functions for the CTD2 gene and its product include roles as a component of the central oscillator, as a component of input pathways that provide environmental information to the clock, as well as a component of output pathways that transmit temporal information from the clock to the overtly rhythmic phenomena.