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. We discuss how the clock regulates organelle function at multiple levels, including transcriptional, translational, and post-translational mechanisms, and how organelle-derived signals feedback to modulate core clock components through retrograde pathways. Special attention is given to the integration of chloroplast and mitochondrial signals, emphasizing their synergistic roles in maintaining cellular homeostasis. Drawing on the "three-body problem" analogy, we illustrate the dynamic and reciprocal interactions among light, clock, and metabolism. This perspective underscores the need to reframe the circadian system, not merely as light-driven but also as a central integrator of energy status and environmental cues. Understanding this integrated network is essential to improve plant performance and resilience under fluctuating environmental conditions.