Synergistic regulation by the stay-green gene family maintains green flower coloration via chlorophyll degradation in chrysanthemum

Chrysanthemum is an economically important species with substantial ornamental, edible, and medicinal value. Green-flowered chrysanthemums represent a rare and highly valued phenotype among cultivated varieties; however, the molecular mechanisms underlying green flower coloration remain poorly understood. Here, we integrated pigment quantification, transcriptome profiling, and functional validation to systematically elucidate the regulatory network governing green pigmentation. Chlorophyll content increased progressively during flower development. Transcriptomic analyses identified 11 key genes associated with chlorophyll metabolism, encompassing biosynthetic genes (GSA, HEMB, HEMY, CLH) and degradation-related genes (SGR, PPH, PAO). Notably, SGR expression was consistently downregulated throughout development. Three SGR homologs were identified in chrysanthemum-CmSGR1a, CmSGR1b, and CmSGRL-all of which localized to chloroplasts. Heterologous expression of each CmSGR in tobacco promoted chlorophyll degradation and induced leaf etiolation. In chrysanthemum, individual silencing of CmSGRL resulted in compensatory upregulation of CmSGR1a/b and accelerated chlorophyll degradation, producing lighter-colored flowers. Similarly, silencing either CmSGR1a or CmSGR1b induced upregulation of the remaining homologs and reduced chlorophyll accumulation. By contrast, simultaneous silencing of all three CmSGR genes markedly inhibited chlorophyll degradation, leading to significantly darker flowers with elevated chlorophyll levels. Collectively, these results demonstrate that CmSGR family members function redundantly and synergistically to regulate chlorophyll degradation, thereby maintaining the green flower phenotype. This study clarifies the molecular basis of green coloration in chrysanthemum and provides valuable genetic resources and theoretical support for flower color improvement.