Circadian clock regulation of low R/FR light signaling and photosynthetic cross-talk in crop development

The environmental light condition serves as a pivotal external cue governing plant growth and development. In particular, a low ratio of red to far-red light (low R:FR), resulting from vegetative shading, significantly limits crop yield under high-density planting—as often seen in cereal and horticultural crops. Plants perceive alterations in R:FR ratio via phytochromes, transduce this signal into multifaceted intracellular responses and initiate adaptive processes such as shade avoidance syndrome (SAS). This review comprehensively examines the molecular mechanisms of low R:FR-controlled photosynthesis mediated by the circadian clock. First of all, we elaborate on phytochromes initiation of downstream signaling cascades through nucleocytoplasmic partitioning and interactions with key components such as PIFs. We then systematically outline the cross-talk between low R:FR signaling and the core circadian oscillator. The central clock components, including LHY/CCA1 and TOC1, act as critical regulatory nodes within the phytochrome signaling network. By integrating photoperiodic and circadian information, these components finely modulate photosynthesis-related gene expression, chloroplast development, stomatal movement, and photosynthetic carbon assimilation, thereby optimizing photosynthetic performance in dynamically changing light environments. This review aims to provide integrative insights into how plants synchronize light signaling with the circadian clock to maximize photosynthetic efficiency, and to suggest potential molecular targets for breeding crops with improved shade tolerance and higher productivity in dense cultivation systems.