Dissecting Circadian Rhythm in Kalanchoë fedtschenkoi, a Model Species for Crassulacean Acid Metabolism Research

The circadian clock plays a major role in the temporal regulation of an organism’s metabolism and physiology. This role is even more important in plants due to their sessile lifestyle as it provides a fitness advantage by synchronizing internal metabolic and physiological processes with periodic environmental stimuli. A prime example of the importance of the circadian clock in plants can be seen in plants with crassulacean acid metabolism (CAM) photosynthesis. In comparison with C3 [C three] or C4 [C four] photosynthesis, CAM enhances water-use efficiency via inverted day/night stomatal movement pattern (i.e., stomata close during day and open at night). Also, CAM improves photosynthetic efficiency through a carbon concentration mechanism centered on a temporal separation of carbon dioxide fixation relative to C3 photosynthesis. These advantages of CAM have garnered increased interest in understanding the molecular mechanism of CAM pathway. However, little is known about the regulatory networks controlling the temporal phases of CAM. The goal of this research is to better understand how the circadian clock regulates CAM-related processes and how it possibly influenced the evolution of CAM.Through the use of various omics resources and computational methods and tools, the rhythmic transcriptomes of the model obligate CAM species, Kalanchoë fedtschenkoi and the model C3 species, Arabidopsis thaliana were examined and compared. Similarities and differences in rhythmicity of gene expression were observed between these two species, leading to testable hypotheses on the circadian clock’s role in CAM and its evolution. Furthermore, several candidate clock genes in K. fedtschenkoi were predicted as new members of the core clock network and the circadian output network. Evidence in support of the core clock network impacting the inversion of stomatal movement in K. fedtschenkoi was generated through use of gene regulation inference and an expanded list of stomata-related genes. Together, the results from this research provide a reduction in the hypothesis space for investigating the circadian regulation of CAM.