The Levels of Sulfur-containing Metabolites in Brassica napus are Not Influenced by the Circadian Clock but Diurnally

Adapting biological processes to an endogenous rhythm enables plants to cope with the daily changes in light and temperature in a more predictable way enhancing growth and fitness. A number of biological processes such as metabolic pathways as well as the immunity in plants are under diurnal or circadian control. In this study a possible circadian regulation of key enzymes in the sulfur assimilation and the corresponding metabolites was investigated in the agriculturally important crop plant oilseed rape (Brassica napus). Leaves of a commercially available cultivar were harvested in the course of a day under diurnal and under free-running conditions with constant light. Analyses in this study were focused on sulfur-containing metabolites and expression analysis of enzymes involved in sulfur assimilation. Expression analysis showed that the transcript levels of the sulfate transporters Sultr3;1 and Sultr4;2 as well as APR2 and APR3 oscillated diurnally. Results revealed a periodic rhythm of sulfur-containing metabolites such as glutathione, sulfate and certain glucosinolates in the course of a day which were only partly maintained under constant light. Therefore, we conclude that a diurnal rhythm and not the circadian clock regulates sulfur metabolism in plants.