Summary: The gaseous plant hormone ethylene induces the ripening of climacteric fruit, including apple (Malus domestica). Another phytohormone, auxin, is known to promote ethylene production in many horticultural crops, but the regulatory mechanism remains unclear. Here, we found that auxin application induces ethylene production in apple fruit before the stage of commercial harvest, when they are not otherwise capable of ripening naturally. The expression of MdARF5, a member of the auxin response factor transcription factor (TF) family involved in the auxin signaling pathway, was enhanced by treatment with the synthetic auxin naphthaleneacetic acid (NAA). Further studies revealed that MdARF5 binds to the promoter of MdERF2, encoding a TF in the ethylene signaling pathway, as well as the promoters of two 1‐aminocyclopropane‐1‐carboxylic acid synthase (ACS) genes (MdACS3a and MdACS1) and an ACC oxidase (ACO) gene, MdACO1, all of which encode key steps in ethylene biosynthesis, thereby inducing their expression. We also observed that auxin‐induced ethylene production was dependent on the methylation of the MdACS3a promoter. Our findings reveal that auxin induces ethylene biosynthesis in apple fruit through activation of MdARF5 expression.

High acetylation in the promoter of a transcription factor gene is associated with sucrose accumulation in pear fruit by activating the expression of a sucrose

Auxin plays an important role in regulating plant development, and Auxin/indole acetic acid (Aux/IAA) is a type of auxin-responsive gene and plays an important role in auxin signaling; to date, although 29 Aux/IAA proteins have been reported in Abrabidopsis thaliana, only parts of the Aux/IAA family gene functions have been identified. We previously reported that a bud sport of ‘Longfeng’ (LF) apple (Malus domestica), named ‘Grand longfeng’ (GLF), which showed a larger fruit size than LF, has lower expression of MdAux/IAA2. In this study, we identified the function of the MdAux/IAA2 gene in apple fruit size difference using Agrobacterium-mediated genetic transformation. Overexpression of MdAux/IAA2 decreased the apple flesh callus increment and caused a smaller globular cell size. In addition, overexpression of MdAux/IAA2 in GLF fruit resulted in the reduction of apple fruit size, weight, and cell size, while silencing MdAux/IAA2 in LF apple fruit resulted in an increase in apple fruit weight and cell size. We suggest that the high auxin content depressed the expression of MdAux/IAA2, and that the downregulated expression of MdAux/IAA2 led to the formation of GLF. Our study suggests a mechanism for fruit size regulation in plants and we will explore the transcription factors functioning in this process in the future.

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.