Pseudo-Response Regulators: Molecular Mediators Transducing NTRC-Dependent Metabolic Oscillatory Signals to the Nuclear Clocks in Plants

Abstract

A recent study has demonstrated that NADPH-thioredoxin reductase type C (NTRC) plays a crucial role in coordinating the diel redox rhythms of the chloroplast with nuclear circadian oscillators, particularly by regulating the expression of CCA1, which is significantly reduced in ntrc mutants compared to wild-type (WT) Col-0 plants. However, the specific molecules responsible for transmitting NTRC-mediated alterations of metabolite signals to nuclear clock components remained elusive. To address this intriguing question, we investigated the changes in intracellular reactive oxygen species (ROS) and sucrose levels in NTRC-deficient, ntrc, mutant, and compared them with WT plants. Our study revealed a notable increase in ROS levels and a decrease in sucrose concentration in ntrc mutant plants compared to Col-0 controls, highlighting the distinct influence of NTRC deficiency on these metabolites. Through extensive bioinformatic analyses that elucidated the mechanistic role of pseudo-response regulators (PRRs) linking fluctuations in cytoplasmic metabolites to the nuclear clock components CCA1/LHY, we performed comparative transcriptional profiling of PRR expression patterns between ntrc mutant and the WT controls. The results indicate a substantial upregulation of PRR genes in ntrc mutants compared to Col-0 plants. Furthermore, in prr975 triple mutants, the expression of CCA1 could not be restored by treatment with DMTU and sucrose. These results support that PRRs act as transducers, conveying NTRC-dependent metabolic signals to the nuclear TTFL clock component, CCA1.