Metabolism in Sync: The Circadian Clock, a Central Hub for Light-Driven Chloroplastic and Mitochondrial Entrainment.

Plants align their physiology with daily environmental cycles through the circadian clock, which integrates light and metabolic signals to optimize growth and stress responses. While light entrainment has been extensively studied, emerging evidence highlights the central role of metabolism-particularly from chloroplasts and mitochondria-in tuning circadian rhythms. In this review, we explore the bidirectional relationship between organelle metabolism and the circadian clock, focusing on how metabolic signals such as sugars, ROS, and organic acids function as entrainment cues.

Retrograde signals control dynamic changes to the chromatin state at photosynthesis-associated loci.

Retrograde signalling networks originating in the organelles dictate nuclear gene expression and are essential for control and regulation of cellular energy metabolism. We investigate whether such plastid retrograde signals control nuclear gene expression by altering the chromatin state during the establishment of photosynthetic function in response to light. An Arabidopsis thaliana cell culture provides the required temporal resolution to map four histone modifications during the greening process.

CDK8 of the mediator kinase module connects leaf development to the establishment of correct stomata patterning by regulating the levels of the transcription factor SPEECHLESS (SPCH).

The components of the mediator kinase module are highly conserved across all eukaryotic lineages, and cyclin-dependent kinase 8 (CDK8) is essential for correct cell proliferation and differentiation in diverse eukaryotic systems. We show that CDK8 couples leaf development with the establishment of correct stomata patterning for prevailing CO conditions. In Arabidopsis, the basic helix-loop-helix (bHLH) transcription factor SPEECHLESS (SPCH) controls cellular entry into the stomatal cell lineage, and CDK8 interacts with and phosphorylates SPCH, controlling SPCH protein levels and thereby also expression of the SPCH target genes encoding key regulators of cell fate and asymmetric cell divisions.

The Arabidopsis Circadian Clock and Metabolic Energy: A Question of Time.

A fundamental principle shared by all organisms is the metabolic conversion of nutrients into energy for cellular processes and structural building blocks. A highly precise spatiotemporal programming is required to couple metabolic capacity with energy allocation. Cellular metabolism is also able to adapt to the external time, and the mechanisms governing such an adaptation rely on the circadian clock.

Circadian Control of Metabolism by the Clock Component TOC1.

Photosynthesis in chloroplasts during the day and mitochondrial respiration during the night execute nearly opposing reactions that are coordinated with the internal cellular status and the external conditions. Here, we describe a mechanism by which the Arabidopsis clock component TIMING OF CAB EXPRESSION1 (TOC1) contributes to the diurnal regulation of metabolism. Proper expression of TOC1 is important for sustaining cellular energy and for the diel and circadian oscillations of sugars, amino acids and tricarboxylic acid (TCA) cycle intermediates.