Abstract
Environmentally-responsive genes can affect fruit red colour via the activation of MYB transcription factors. The apple B-box (BBX) gene, BBX33/CONSTANS-like 11 (COL11) has been reported to influence apple red-skin colour in a light- and temperature-dependent manner. To further understand the role of apple BBX genes, other members of the BBX family were examined for effects on colour regulation. Expression of 23 BBX genes in apple skin was analysed during fruit development. We investigated the diurnal rhythm of expression of the BBX genes, the anthocyanin biosynthetic genes and a MYB activator, MYB10. Transactivation assays on the MYB10 promoter, showed that BBX proteins 1, 17, 15, 35, 51, and 54 were able to directly function as activators. Using truncated versions of the MYB10 promoter, a key region was identified for activation by BBX1. BBX1 enhanced the activation of MYB10 and MdbHLH3 on the promoter of the anthocyanin biosynthetic gene DFR. In transformed apple lines, over-expression of BBX1 reduced internal ethylene content and altered both cyanidin concentration and associated gene expression. We propose that, along with environmental signals, the control of MYB10 expression by BBXs in ‘Royal Gala’ fruit involves the integration of the expression of multiple BBXs to regulate fruit colour.
Highlights
Environmentally-responsive genes can affect fruit red colour via the activation of MYB transcription factors
Using dual luciferase assays it was shown that BBX33 upregulates the MYB10 promoter. These findings suggest that BBX33 functions downstream of HYPOCOTYL 5 (HY5) and upstream of MYB10 as a component in the environmental sensing pathway, which leads to anthocyanin production in apple
Cyanidin-3-O-galactoside content was detectable at 110 days after full bloom (DAFB), increasing up to 140 DAFB (Fig. 1B)
Summary
Environmentally-responsive genes can affect fruit red colour via the activation of MYB transcription factors. Research on the regulation of anthocyanin production has identified both the biosynthetic pathways genes[14,15,16] and the major regulating transcription factors (TFs), comprising the MYB-bHLH-WD40 (MBW) complex[17]. This complex binds promoter regions of anthocyanin biosynthetic genes and MYB10 and increases transcription, as shown for Arabidopsis (Arabidopsis thaliana)[17], nectarine[18], petunia (Petunia hybrida)[15], strawberry[19], legumes[20], Norway spruce[21], and apple[22], and reviewed by Li23. A family of MYB repressors of anthocyanin were identified after heat treatment and were characterised by the EAR motif[32]
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