Green flower coloration through AT-hook protein-mediated histone H3K27ac deacetylation in chrysanthemum

Abstract Chloroplast development and photosynthetic pigment synthesis in leaves are governed by a highly conserved Golden2-like (GLK) transcription factor network. However, the mechanisms sustaining green pigmentation in non-foliar organs such as flowers remain unclear. Here, we show that floral pigmentation is controlled by a GLK-independent pathway, identifying the AT-hook protein CmAHL8L as a central regulator of green coloration in chrysanthemum. Silencing CmAHL8L reduced green pigmentation in ray florets, accompanied by decreased chlorophyll and carotenoid levels. CmAHL8L promotes chlorophyll accumulation by repressing the transcriptional repressor CmAHL23, thereby derepressing GLUTAMYL-tRNA REDUCTASE 1 (CmHEMA1), while concurrently preventing carotenoid degradation through direct inhibition of CAROTENOID CLEAVAGE DIOXYGENASE (CmCCD4a). Mechanistically, CmAHL8L recruits the histone deacetylase CmHDT1 to remove H3K27ac marks at the CmAHL23 and CmCCD4a promoters, suppressing their transcription. We further demonstrate that this AHL8L–HDT1 complex is conserved across diverse species. Collectively, our work defines a predominant regulatory mechanism in photosynthetic pigments of the flower, distinct from leaf mechanisms, providing key genetic insights into the regulation and evolution of flower coloration.