Precise timing of flowering in plants is critical for their growth and reproductive processes. One factor controlling flowering time is the cycle of light and darkness within a day, known as the photoperiod. Plants are classified into long-day, short-day, and day-neutral plants based on light requirements for floral initiation. Although the molecular mechanisms that govern this differentiation remain incompletely understood, studies have consistently shown that the circadian clock plays a central role in regulating photoperiod response across diverse plant species. However, there is a scarcity of reviews describing the regulatory network linking the circadian clock with photoperiodic flowering. This review summarizes that regulatory network, focusing on the distinct roles of clock genes in long-day and short-day plants. We also discuss the strategies of clock gene mutations contributing to geographic variation in long-day and short-day crops.

Plants are capable of regulating their shoot architecture in response to diverse internal and external environments. The circadian clock is an adaptive mechanism that integrates information from internal and ambient conditions to help plants cope with recurring environmental fluctuations. Despite the current understanding of plant circadian clock and genetic framework underlying plant shoot architecture, the intricate connection between these two adaptive mechanisms remains largely unclear. In this study, we elucidated how the core clock gene LUX ARRHYTHMO (LUX) regulates shoot architecture in the model legume plant Medicago truncatula. We show that mtlux mutant displays increased main stem height, reduced lateral shoot length, and decreased the number of lateral branches and biomass yield. Gene expression analysis revealed that MtLUX regulated shoot architecture by repressing the expression of strigolactone receptor MtD14 and MtTB1/MtTCP1A, a TCP gene that functions centrally in modulating shoot architecture. In vivo and in vitro experiments showed that MtLUX directly binds to a cis-element in the promoter of MtTB1/MtTCP1A, suggesting that MtLUX regulates branching by rhythmically suppressing MtTB1/MtTCP1A. This work demonstrates the regulatory effect of the circadian clock on shoot architecture, offering a new understanding underlying the genetic basis towards the flexibility of plant shoot architecture.