Abstract
The anthocyanin content in apple skin determines its red coloration, as seen in a Fuji apple mutant. Comparative RNA-seq analysis was performed to determine differentially expressed genes at different fruit development stages between the wild-type and the skin color mutant. A novel R2R3-MYB transcription factor, MdMYB90-like, was uncovered as the key regulatory gene for enhanced coloration in the mutant. The expression of MdMYB90-like was 21.3 times higher in the mutant. MdMYB90-like regulates anthocyanin biosynthesis directly through the activation of anthocyanin biosynthesis genes and indirectly through the activation of other transcription factors that activate anthocyanin biosynthesis. MdMYB90-like bound to the promoters of both structural genes (MdCHS and MdUFGT) and other transcription factor genes (MdMYB1 and MdbHLH3) in the yeast one-hybrid system, electrophoretic mobility shift assay, and dual-luciferase assay. Transgenic analysis showed that MdMYB90-like was localized in the nucleus, and its overexpression induced the expression of other anthocyanin-related genes, including MdCHS, MdCHI, MdANS, MdUFGT, MdbHLH3, and MdMYB1. The mutant had reduced levels of DNA methylation in two regions (−1183 to −988 and −2018 to −1778) of the MdMYB90-like gene promoter, which might explain the enhanced expression of the gene and the increased anthocyanin content in the mutant apple skin.
Highlights
A bud sport is a naturally occurring mutation in a tree branch, and these have been widely used by breeders to select desirable characteristics for breeding[1,2]
Various genes in the anthocyanin biosynthesis pathway were detected as differentially expressed genes between Fuji apples and the mutant
UFGT catalyzes the glycosylation of anthocyanidins to anthocyanins and has been found to contribute to cyanidin 3-galactoside biosynthesis in apple skin[41]
Summary
A bud sport is a naturally occurring mutation in a tree branch, and these have been widely used by breeders to select desirable characteristics for breeding[1,2]. The phenotypes of bud sport mutants mainly include early development, fruiting spurs, and coloring. High-throughput sequencing technology has been widely used in studies to uncover the molecular mechanisms of these natural mutations. A large number of excellent varieties have been produced by utilizing bud mutants in fruit trees, such as apple, pear, cherry, orange, and grape[4,7,8,9,10,11]. Apple fruit coloration determines fruit appearance and economic characteristics and has beneficial value for
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