Arabidopsis transcription factor TCP4 represses chlorophyll biosynthesis to prevent petal greening

Green petals pose a challenge for pollinators to distinguish flowers from leaves, but are valuable as a specialty flower trait. However, little is understood about the molecular mechanisms underlying the development of green petals. Here, we report that CINCINNATA (CIN)-like TEOSINTE BRANCHED 1/ CYCLOIDEA/ PCF (TCP) proteins play key roles in control of petal color. The septuple tcp2/3/4/5/10/13/17 mutant produced flowers with green petals due to chlorophyll accumulation. Expression of TCP4 complemented the petal phenotype of tcp2/3/4/5/10/13/17. We found that chloroplasts were converted into leucoplasts in the distal parts of wild-type petals, but not in the proximal parts, during flower development, while plastid conversion was compromised in the distal parts of tcp2/3/4/5/10/13/17 petals. TCP4 and most CIN-like TCPs were predominantly expressed in distal petal regions, consistent with the green-white pattern in wild-type petals and the petal greening observed in the distal parts of tcp2/3/4/5/10/13/17 petals. RNA-sequencing (RNA-seq) data revealed that most chlorophyll biosynthesis genes were down-regulated in the white distal parts of wild-type petals, but these genes had elevated expression in the distal green parts of tcp2/3/4/5/10/13/17 petals and the green proximal parts of wild-type petals. We revealed that TCP4 repressed chlorophyll biosynthesis by directly binding to the promoters of PROTOCHLOROPHYLLIDE REDUCTASE (PORB), DIVINYL REDUCTASE (DVR) and SUPPRESSOR OF OVEREXPRESSION OF CO 1 (SOC1), which are known to promote petal greening. We found that the conversion of chloroplasts to leucoplasts and green coloration in the proximal parts of petals appeared to be conserved among plant species. Our findings uncover a main molecular mechanism underpinning the formation of petal color patterns and provide a foundation for breeding of plants with green flowers.