In plants, metabolic homeostasis-the driving force of growth and development-is achieved through the dynamic behavior of a network of enzymes, many of which depend on coenzymes for activity. The circadian clock is established to influence coordination of supply and demand of metabolites. Metabolic oscillations independent of the circadian clock, particularly at the subcellular level is unexplored. Here, we reveal a metabolic rhythm of the essential coenzyme thiamine diphosphate (TDP) in the Arabidopsis nucleus. We show there is temporal separation of the clock control of cellular biosynthesis and transport of TDP at the transcriptional level. Taking advantage of the sole reported riboswitch metabolite sensor in plants, we show that TDP oscillates in the nucleus. This oscillation is a function of a light-dark cycle and is independent of circadian clock control. The findings are important to understand plant fitness in terms of metabolite rhythms.

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BackgroundPDX1.2 has recently been shown to be a regulator of vitamin B6 biosynthesis in plants and is implicated in biotic and abiotic stress resistance. PDX1.2 expression is strongly and rapidly induced by heat stress. Interestingly, PDX1.2 is restricted to eudicota, wherein it behaves as a non-catalytic pseudoenzyme and is suggested to provide an adaptive advantage to this clade. A first report on an Arabidopsis insertion mutant claims that PDX1.2 is indispensable for viability, being essential for embryogenesis. However, a later study using an independent insertion allele suggests that knockout mutants of pdx1.2 are viable. Therefore, the essentiality of PDX1.2 for Arabidopsis viability is a matter of debate. Given the important implications of PDX1.2 in stress responses, it is imperative to clarify if it is essential for plant viability.ResultsWe have studied the previously reported insertion alleles of PDX1.2, one of which is claimed to be essential for embryogenesis (pdx1.2–1), whereas the other is viable (pdx1.2–2). Our study shows that pdx1.2–1 carries multiple T-DNA insertions, but the T-DNA insertion in PDX1.2 is not responsible for the loss of embryogenesis. By contrast, the pdx1.2–2 allele is an overexpressor of PDX1.2 under standard growth conditions and not a null allele as previously reported. Nonetheless, upregulation of PDX1.2 expression under heat stress is impaired in this mutant line. In wild type Arabidopsis, studies of PDX1.2-YFP fusion proteins show that the protein is enhanced under heat stress conditions. To clarify if PDX1.2 is essential for Arabidopsis viability, we generated several independent mutant lines using the CRISPR-Cas9 gene editing technology. All of these lines are viable and behave similar to wild type under standard growth conditions. Reciprocal crosses of a subset of the CRISPR lines with pdx1.2–1 recovers viability of the latter line and demonstrates that knocking out the functionality of PDX1.2 does not impair embryogenesis.ConclusionsGene editing reveals that PDX1.2 is dispensable for Arabidopsis viability and resolves conflicting reports in the literature on its function.

Metabolic 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.