NTRC mediates the coupling of chloroplast redox rhythm with nuclear circadian clock in plant cells.

The intricate interplay between cellular circadian rhythms, primarily manifested in the chloroplast redox oscillations-characterized by diel hyperoxidation/reduction cycles of 2-Cys Peroxiredoxins-and the nuclear transcription/translation feedback loop (TTFL) machinery within plant cells, demonstrates a remarkable temporal coherence. However, the molecular mechanisms underlying the integration of these circadian rhythms remain elusive. Here, we elucidate that the chloroplast redox protein, NADPH-dependent thioredoxin reductase type-C (NTRC), modulates the integration of the chloroplast redox rhythms and nuclear circadian clocks by regulating intracellular levels of reactive oxygen species and sucrose. In NTRC-deficient ntrc mutants, the perturbed temporal dynamics of cytosolic metabolite pools substantially attenuated the amplitude of CIRCADIAN CLOCK ASSOCIATED-1 (CCA1) mRNA oscillation, while maintaining its inherent periodicity. In contrast, these fluctuations extended the period and ameliorated the amplitude of GIGANTEA (GI). In alignment with its regulatory role, the chloroplast redox rhythm and TTFL-driven nuclear oscillators are severely disrupted in ntrc plants. The impairments are rescued by NTRC expression, but not by the catalytically inactive NTRC(C/S) mutant, indicating that NTRC's redox activity is essential for synchronizing intracellular circadian rhythms. In return, the canonical nuclear clock component, TIMING OF CAB EXPRESSION-1 (TOC1), regulates the diel chloroplast redox rhythm by controlling NTRC expression, as evidenced by the redox cycle of chloroplast 2-Cys Peroxiredoxins. This reciprocal regulation suggests a tight coupling between chloroplast redox rhythms and nuclear oscillators. Consequently, our research has successfully identified NTRC as a key circadian modulator, elucidating the intricate connection between the metabolite-dependent chloroplast redox rhythm and the temporal dynamics of nuclear canonical clocks.