Model-based analysis on the generation of the circadian rhythm of the bioluminescence reporter activities in duckweed

Bioluminescence monitoring techniques are widely used for the study of gene expression dynamics in living plants. Monitoring bioluminescence from a luciferase gene under a circadian promoter is indispensable to examine the plant circadian system. The bioluminescence monitoring technique has been successfully applied to spaciotemporal analysis on circadian rhythms in duckweed plants. It has been reported that the luciferase gene under a constitutive promoter also shows a bioluminescence circadian rhythm in duckweed. The mechanism to generate the rhythm is unknown. Here we performed a model-based analysis to evaluate the generation machinery of the bioluminescence rhythm. Regarding the bioluminescence intensities of luciferase in the cells, we hypothesized the rhythmic factor at three aspects: luminescence efficiency, production rate, degradation rate. Theoretically, if the latter two are involved in the rhythm generation, the difference in luciferase stability affects the amplitude and the phase relation of the bioluminescence rhythm. The luciferase stability is irrelevant to these rhythm properties if the luminescence efficiency is only involved. We first simulated bioluminescence rhythms of two luciferases with different stabilities in association with each of the three rhythmic factors. Luciferase stabilities were set on the basis of the reported values of Emerald-luciferase and Emerald-luciferase-PEST. We then experimentally examined bioluminescence rhythms of those reporters driven under the CaMV35S promoter in a duckweed, Lemna japonica . Their circadian properties matched those obtained in the simulation for the luminescence efficiency. This supports the idea that cells in duckweed show a circadian rhythm in physiological conditions that are related with the luciferase enzyme reaction.