Color is an important trait for horticultural crops. Carotenoids are one of the main pigments for coloration and have important implications for photosynthesis in plants and benefits for human health. Here, we identified an APETALA2 (AP2)/ETHYLENE RESPONSE FACTOR (ERF) transcription factor named MdAP2-34 in apple (Malus domestica Borkh.). MdAP2-34 expression exhibited a close correlation with carotenoid content in ‘Benin Shogun’ and ‘Yanfu 3’ fruit flesh. MdAP2-34 promotes carotenoid accumulation in MdAP2-34-OVX transgenic apple calli and fruits by participating in the carotenoid biosynthesis pathway. The major carotenoid contents of phytoene and β-carotene were much higher in overexpressing MdAP2-34 transgenic calli and fruit skin, yet the predominant compound of lutein showed no obvious difference, indicating that MdAP2-34 regulates phytoene and β-carotene accumulation but not lutein. MdPSY2-1 (phytoene synthase 2) is a major gene in the carotenoid biosynthesis pathway in apple fruit, and the MdPSY2-1 gene is directly bound and transcriptionally activated by MdAP2-34. In addition, overexpressing MdPSY2-1 in apple calli mainly increases phytoene and total carotenoid contents. Our findings will advance and extend our understanding of the complex molecular mechanisms of carotenoid biosynthesis in apple, and this research is valuable for accelerating the apple breeding process.

The color of apple skin, particularly anthocyanin-based coloration, is a key factor determining market acceptance. The mechanisms of anthocyanin accumulation in apples with different skin color patterns (i.e., striped and blushed) were analyzed. In total, 14 anthocyanins and 5 procyanidins were simultaneously assayed in red blushed-skin mutants (CF-B1 and CF-B2) and red striped-skin parents (CF-S1 and CF-S2), and 13 significant differences were revealed. Anthocyanin accumulation was significantly higher in the red blushed-skin apples than it was in the parents. The transcript levels of anthocyanin biosynthesis genes and regulatory factors (MdMYB10, MdbHLH3, and MdWD40) were associated with different skin color patterns during the coloring period at 4, 6, and 8 days after the fruits were debagged. The methylation levels of the MdMYB10 promoter regions −1203 to −779 bp, −1667 to −1180 bp, and −2295 to −1929 bp were associated with different skin color patterns, and there was more methylation in red striped-skin apples. These results improve our understanding of anthocyanin accumulation and its underlying molecular mechanism in apples with different skin color patterns, thereby providing valuable information for apple breeding.

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