Fig. 7
The model of anthocyanin accumulation in RG and anr mutant fruits. A The pattern of procyanidin accumulation in early development fruits of wild-type RG. B The pattern of anthocyanidin accumulation in early development fruits of the anr mutant. C The pattern of anthocyanidin accumulation in ripe fruits of wild-type RG. D The pattern of anthocyanidin accumulation in ripe fruits of the anr mutant. During the early stage of fruit development, MYB9/11/105 binds to the promoter of ANR and structural genes shared by the flavonoid metabolic pathway, promoting its expression. Meanwhile, the expression level of UFGT is low at this stage, leading to a predominant flow of flavonoid metabolism towards procyanidin synthesis. Consequently, a significant amount of procyanidins accumulates in the fruit (A). However, in the anr mutant, despite the expression level of ANR being unaffected, the mutated ANR (mANR) fails to function as an enzyme protein. This disruption results in the blockage of procyanidin synthesis. Simultaneously, the upregulated expression of MYB105 during this stage enhances the expression of structural genes. This shift in flavonoid metabolism directs the pathway towards anthocyanidin synthesis, leading to the accumulation of abundant anthocyanins in the early development fruits, characterized by a red coloration (B). As the fruits progress towards maturity, both RG and anr mutant fruits exhibit an increased expression level of MYB10. This elevated expression promotes the expression of structural genes, facilitating the accumulation of anthocyanins in the fruits. Notably, RG fruits do not accumulate anthocyanins during the early stage of fruit development. Conversely, anr mutant fruits accumulate anthocyanins at this early stage and store them in vesicles until they are superimposed on newly synthesized anthocyanins during the ripening stage. Consequently, the anr mutant fruits exhibit a much higher anthocyanin content during the ripening stage compared to wild-type RG fruits, resulting in a deep red coloration (D) as opposed to the red coloration observed in wild-type RG fruits (C)