Multiple-layered fine-tuning of flowering locus C --- Flowering control and beyond

FLOWERING LOCUS C (FLC) is a MADS-box transcription factor that integrates diverse internal and environmental signals to precisely regulate the growth and development of plants. While historically characterized as a key repressor of flowering in the vernalization, autonomous, and temperature pathways, recent research has revealed that FLC's functions extend far beyond flowering control. This review synthesizes current understanding of FLC's pleiotropic roles in various developmental processes, from seed germination, juvenile-to-adult phase transition and biomass determination, to the establishment of annual/perennial habits. Furthermore, it explores emerging links between FLC and broader environmental adaptation, including plant responses to drought stress, nitrogen availability, and pathogen tolerance, where its role may be both direct and indirect. The molecular mechanisms underlying FLC's expression are explored, encompassing complex multi-layered regulation at transcriptional, post-transcriptional, including alternative splicing (AS) and m⁶A RNA methylation, and epigenetic levels (notably PRC2-mediated H3K27me3 deposition). The discussion also covers how natural sequence variation and transposable elements in the FLC locus contribute to adaptive evolution. By contextualizing recent findings, this review aims not only to summarize FLC's functions as a developmental-stress integrator but also to critically evaluate the strength of evidence, identify persistent knowledge gaps, and propose key questions for future research to move from descriptive association to mechanistic understanding.