Anthocyanins are key pigments responsible for apple fruit coloration, influencing not only its appearance and marketability but also contributing significantly to its nutritional and health benefits. In their natural state, anthocyanins are chemically unstable. However, glycosylation modifies them into anthocyanin derivatives known as anthocyanin glycosides, which exhibit markedly enhanced stability and improved water solubility. As a result, most naturally occurring anthocyanins exist in the form of anthocyanin glycosides. The biosynthesis of anthocyanins involves a series of structural genes within the phenylpropanoid and flavonoid pathways (including PAL, C4H, 4CL, CHS, CHI, F3H, DFR, ANS/LDOX, and UFGT). The MYB–bHLH–WD40 transcriptional complex serves as the core regulatory mechanism controlling anthocyanin synthesis, with additional transcription factors also playing important roles. This review systematically summarizes recent advances in the structural characteristics, biosynthetic pathways, molecular regulatory mechanisms, and environmental factors affecting anthocyanin accumulation in apples. These insights are important both for consumer health and for breeding apple cultivars with enhanced anthocyanin content.

Fruit size is a key economic trait in horticultural crops, determined by an integrated network of genetic, hormonal, epigenetic, and environmental factors. This review synthesizes recent advances in understanding this regulation. Genetically, key quantitative trait loci (QTLs) such as fw2.2, fw3.2, FAS, and LC control cell number and organ size. Hormones including auxin, gibberellin, cytokinin, and brassinosteroid regulate cell division and expansion. Transcription factors (e.g., YABBY, WOX families) and epigenetic mechanisms (DNA methylation, histone modifications, endoreduplication) provide additional regulatory layers. Environmental factors (light, temperature, water, nutrition) and cultivation practices (pruning, thinning) ultimately shape the final fruit phenotype. Emerging technologies like multi-omics, CRISPR-Cas9 gene editing, and artificial intelligence offer powerful tools for future research and molecular breeding, which aim to decode the complex regulatory network and enhance crop yield and quality.

Apple peel color is a key determinant of fruit quality and consumer preference. To elucidate the molecular basis underlying the peel color difference between ‘Jinhong’ (JH) apples and ‘Hongjinhong’ apples (HJH, a deep red bud sport of JH apples), we integrated physiological assays, widely targeted metabolomics, transcriptomics, and quantitative real-time PCR (qRT-PCR) to analyze peel pigment contents, metabolite profiles, and gene expression dynamics during fruit development. The deep red phenotype of HJH peel was primarily attributed to enhanced anthocyanin accumulation rather than alterations in chlorophyll content. Metabolomics identified 418 differentially accumulated metabolites. Among these, three cyanidin-type anthocyanins––cyanidin-3-O-(6″-O-malonyl) glucoside, cyanidin-3-O-(2″-O-xylosyl) galactoside, and cyanidin-3-O-glucoside––were significantly upregulated in HJH peel, serving as the key pigments responsible for its deep red coloration. Transcriptomics revealed 533 differentially expressed genes. Combined with qRT-PCR validation, the anthocyanin biosynthetic gene MdDFR (dihydroflavonol reductase) exhibited consistently high expression during critical developmental stages of HJH fruit, closely mirroring the anthocyanin accumulation pattern. Collectively, the upregulation of MdDFR drives the specific accumulation of cyanidin-type anthocyanins, representing the core molecular event underlying the deep red peel color of HJH apples. This study provides novel insights into the regulatory network of apple coloration and offers candidate genes for the genetic improvement of fruit quality.