Anthocyanin accumulation is responsible for the coloration of apple fruit, and their accumulation depends on the expression of anthocyanin biosynthesis-related genes. Light is an environmental stimulus that induces fruit color by regulating genes involved in the anthocyanin biosynthesis pathway. In this study, the roles of light and genetic factors on fruit coloration and anthocyanin accumulation in apple fruit were investigated. Three genes in the anthocyanin biosynthesis pathway, MdCHS, MdANS, and MdUFGT1, were synthesized and cloned into a viral-based expression vector system for transient expression in ‘Ruby S’ apple fruits. Apple fruits were agroinfiltrated with expression vectors harboring MdCHS, MdANS, and MdUFGT1. Agroinfiltrated apple fruits were then either kept in the dark (bagged fruits) or exposed to light (exposed fruits). The agroinfiltrated fruits showed significantly different coloration patterns, transcript expression levels, and anthocyanin accumulation compared to the control fruits. Moreover, these parameters were higher in exposed fruits than in bagged fruits. For stable expression, MdCHS was introduced into a binary vector under the control of the rice α-amylase 3D (RAmy3D) promoter. The ectopic overexpression of MdCHS in transgenic rice calli showed a high accumulation of anthocyanin content. Taken together, our findings suggest that light, together with the overexpression of anthocyanin biosynthesis genes, induced the coloration and accumulation of anthocyanin content in apple fruits by upregulating the expression of the genes involved in the anthocyanin biosynthesis pathway.

Increasing fruit crop production sustainability under climate change, particularly increasing temperatures, is a major challenge in modern agriculture. High temperatures affect apple fruit quality and decrease its color. Herein, we constructed an experimental field under temperature simulation to evaluate climate change mitigation strategies for apples. ‘Hongro’ apples were subjected to three treatments: (1) cultivation inside a vinyl house for heat treatment (heat induction), (2) cultivation under water irrigation (heat reduction), and (3) cultivation under normal atmospheric temperature (control). At harvest, the fruits of the heat treatment group exhibited poor coloration, with a lower gene expression and pigment accumulation than those of the water irrigation and control groups. Furthermore, the fruit quality of the heat treatment group decreased, with a lower soluble solid content (SSC) and titratable acidity (TA), and smaller fruits. Additionally, a higher fruit disorder (cracking and spots) ratio was observed in the heat treatment group than in the water irrigation and control groups. However, the fruits of the water irrigation group exhibited higher quality indexes (flesh firmness, SSC, and TA) and less cracking than those of the heat treatment and control groups. Heat reduction, including water irrigation, may be used for orchard management to prevent climate change-induced increasing temperatures.

The coloration of the apple fruit (Malus × domestica Borkh.) depends on pigment content. Light stimulus activates a broad range of photosynthesis-related genes, including carotenoids. The effect of light on two red commercial apple cultivars, ‘Summer Prince’ and ‘Arisoo’ at the juvenile stage were examined. Apple fruits were either bagged to reduce light irradiation or were exposed to direct, enhanced sunlight (reflected). The pigment content and the expression of carotenoid metabolism genes in the peel and flesh of apple fruits were significantly different between the shaded and the reflected parts. These parameters were also different in the two cultivars, highlighting the contribution of the genetic background. Further, a combination of light and transient overexpression of carotenogenic genes increased fruit coloration and pigment content in the variety ‘RubyS’. Western blot analysis showed the expression of small heat shock proteins (smHSP) in lysates extracted from the reflected part of the fruits but not in the bagged fruits, indicating the activation of smHSP in response to heat generated by the reflected light. Therefore, the synergy between the genes and the environment dictates the color of apple fruits.

Apples (Malus × domestica Borkh.) require up to several years for flowering and bearing fruits. The transition from vegetative to reproductive phase is controlled by floral regulators such as TERMINAL FLOWER 1 (TFL1) and FLOWERING LOCUS T (FT). TFL1 mediates the maintenance of vegetative phase, unlike the antagonistic function of FT to promote the transition into reproductive phase. In this study, we isolated apple TFL1-like gene (MdTFL1) to elucidate various phenotypic traits triggered by the antisense expression of MdTFL1 in tobacco apart from its floral induction function. Early flowering was observed in the tobacco line with MdTFL1 knockout, indicating the reduced time for transition to vegetative phases. Quantitative reverse-transcription PCR showed upregulation of genes involved in the regulation of floral induction, including NtAP1, NtSOC1, NFL1, and NtFTs, and downregulation of carotenoid cleavage dioxygenases (CCDs) and CEN-like genes in transgenic lines. Interestingly, transgenic tobacco expressing antisense MdTFL1 exhibited distinct morphological changes in lateral shoot outgrowth, internode length, and the development of leaves, flowers, and fruits. The results suggested that using the antisense expression of MdTFL1 gene is one of the approaches to shorten the vegetable phase and proposed improvement of plant architecture in horticultural crops.

Coloration, a major factor contributing to apple quality, attracts consumer attraction, thereby increasing the sale of fruits in the market. Among the fruit pigments, anthocyanins and carotenoids are the most important pigments that impart coloration to apples. Pigment accumulation, which is regulated by transcriptional factors in the anthocyanin and carotenoid biosynthesis pathways, varies according to genetics, transcription, and developmental stage. To understand the regulation of color development in apples, we evaluated the expression of the genes in the anthocyanin and carotenoid biosynthetic pathways and analyzed the accumulation of pigment, including anthocyanin and carotenoid in the skin and flesh of the red-fleshed apple ‘Enbu’ cultivar at different fruit development stages (30, 60, 90, 120, and 150 days after full bloom, and ripe). Color development increased according to the developmental stage, with considerable variation in both the transcript levels and pigment concentrations observed in tissues, skin, and flesh. Moreover, we identified key transcription factors among the anthocyanin and carotenoid biosynthesis genes that regulated pigment accumulation. Pearson’s correlation analysis showed a strong correlation between the coloration patterns and the expression levels of anthocyanin biosynthesis-related genes (MdPAL, MdCHI, and MdF3H) and carotenoid biosynthesis-related genes (MdGGPPS, MdPSY, MdZDS, MdCRTISO, MdCRHβ, and MdZEP). This study provides insight into the molecular mechanisms underlying pigment biosynthesis for breeding high-quality red-fleshed apple varieties to cater to consumer attention and preference.