Clock and riboswitch control of THIC in tandem are essential for appropriate gauging of TDP levels under light/dark cycles in Arabidopsis

SUMMARYMetabolic homeostasis is regulated by enzyme activities but the importance of regulating their corresponding coenzyme levels is unexplored. The organic coenzyme thiamine diphosphate (TDP) is supplied as needed and controlled by a riboswitch sensing mechanism in plants through the circadian regulated THIC gene. Riboswitch disruption leads to loss of time-of-day regulation of THIC expression, negatively impacting plant fitness. Pathway precursor balancing combined with enhancing the biosynthesis pathway demonstrate the importance of the riboswitch in gauging TDP levels and indicate that TDP impacts the clock in Arabidopsis. Altering the phase of THIC expression to be synchronous with TDP transporters disrupts the precision of the riboswitch suggesting that temporal separation of these processes is important. All defects are bypassed by growing plants under continuous light conditions highlighting the need to control levels of this coenzyme under diel cycles. Thus, consideration of coenzyme homeostasis within the well-studied domain of metabolic homeostasis is highlighted.