An Arabidopsis Cell Culture With Weak Circadian Rhythms Under Constant Light Compared With Constant Dark Can Be Rescued by ELF3.

Callus and cell suspension culture techniques are valuable tools in plant biotechnology and are widely used in fundamental and applied research. For studies in callus and cell suspension cultures to be relevant, it is essential to know if the underlying biochemistry is similar to intact plants. This study examined the expression of core circadian genes in Arabidopsis callus from the cell suspension named AT2 and found that the circadian rhythms were impaired. The circadian waveforms were like intact plants in the light/dark cycles, but the circadian expression in the AT2 callus became weaker in the free-running, constant light conditions. Temperature cycles could drive the rhythmic expression in constant conditions, but there were novel peaks at the point of temperature transitions unique to each clock gene. We found that callus freshly induced from seedlings had normal oscillations, like intact plants, suggesting that the loss of the circadian oscillation in the AT2 callus was specific to this callus. We determined that neither the media composition nor the source of the AT2 callus caused this disruption. We observed that ELF3 expression was not differentially expressed between dawn and dusk in both entrained, light-dark cycles and constant light conditions. Overexpression of AtELF3 in the AT2 callus partially recovers the circadian oscillation in the AT2 callus. This work shows that while callus and cell suspension cultures can be valuable tools for investigating plant responses, careful evaluation of their phenotype is important. Moreover, the altered circadian rhythms under constant light and temperature cycles in the AT2 callus could be useful backgrounds to understand the connections driving circadian oscillators and light and temperature sensing at the cellular level.