Background: Bud sport mutants of apple (Malus domestica Borkh.) trees with a highly blushed colouring pattern are mainly caused by the accumulation of anthocyanins in the fruit skin. Hormones are important factors modulating anthocyanin accumulation. However, a good understanding of the interplay between hormones and anthocyanin synthesis in apples, especially in mutants at the molecular level, remains elusive. Here, physiological and comparative transcriptome approaches were used to reveal the molecular basis of color pigmentation in the skin of 'Red Delicious' (G0) and its mutants, including 'Starking Red' (G1), 'Starkrimson' (G2), 'Campbell Redchief' (G3) and 'Vallee spur' (G4).
Results: Pigmentation in the skin gradually proliferated from G0 to G4. The anthocyanin content was higher in the mutants than in 'Red Delicious'. The activation of early phenylpropanoid biosynthesis genes, including ASP3, PAL, 4CL, PER, CHS, CYP98A and F3'H, was more responsible for anthocyanin accumulation in mutants at the color break stage. In addition, IAA and ABA had a positive regulatory effect on the synthesis of anthocyanins, while GA had the reverse effect. The down-regulation of AACT1, HMGS, HMGR, MVK, MVD2, IDI1 and FPPS2 involved in terpenoid biosynthesis influences anthocyanin accumulation by positively regulating transcripts of AUX1 and SAUR that contribute to the synthesis of IAA, GID2 to GA, PP2C and SnRK2 to ABA. Furthermore, MYB and bHLH members, which are highly correlated (r=0.882-0.980) with anthocyanin content, modulated anthocyanin accumulation by regulating the transcription of structural genes, including CHS and F3'H, involved in the flavonoid biosynthesis pathway.
Conclusions: The present comprehensive transcriptome analyses contribute to the understanding of the the relationship between hormones and anthocyanin synthesis as well as the molecular mechanism involved in apple skin pigmentation.
Keywords: Apple; Bud sport; Fruit skin; Hormone; Pigmentation; RNA-seq.