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.

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.

The timing of fruit bag removal is important for achieving optimum fruit quality, coloration, and visual appearance. Therefore, this study investigated the effects of fruit bag removal timing on fruit quality and color and the occurrence of physiological disorders in ‘Arisoo’ apples. Fruits were bagged in two-layer paper bags, which were removed 30, 20, and 10 days before harvest (DBH). Unbagged fruits served as the control. The incidence of fruit cracking, sunburn, and pathogen infection was highest in the unbagged group, followed by the group with bag removal at 30 DBH, and lowest in those with bag removal at 20 and 10 DBH. However, bag removal at 10 DBH significantly reduced fruit weight and soluble solids content but increased firmness. Additionally, bag removal at 10 DBH resulted in poorly colored fruits with higher chlorophyll and lower anthocyanin and carotenoid pigments and lower expression levels of pigment-related genes, including anthocyanin-, carotenoid-, and chlorophyll degradation-associated genes, compared with those in the other treatment groups. Bag removal at 30 and 20 DBH did not significantly affect fruit quality or coloration, but it did affect fruit size. Overall, this study serves as a reference for determining the optimal timing of fruit bag removal to enhance the quality and coloration of ‘Arisoo’ apples.

"‘Arisoo’, a Midseason Apple" published on 22 Jul 2021 by American Society for Horticultural Science.

FB_MR5 is a nucleotide-binding domain and leucine-rich repeat protein identified from wild apple species Malus × robusta 5 conferring disease resistance to bacterial fire blight. FB_MR5 (hereafter MrMR5) recognizes the cysteine protease effector EaAvrRpt2 secreted from the causal agent of bacterial fire blight, Erwinia amylovora. We previously reported that MrMR5 is activated by the C-terminal cleavage product (ACP3) of Malus domestica RIN4 (MdRIN4) produced by EaAvrRpt2-directed proteolysis. We show that MbMR5 from a wild apple species Malus baccata shares 99.4% amino acid sequence identity with MrMR5. Surprisingly, transient expression of MbMR5 in Nicotiana benthamiana showed autoactivity in contrast to MrMR5. Domain swap and mutational analyses revealed that 1 amino acid polymorphism in the MbMR5 CC domain is critical in enhancing autoactivity. We further demonstrated that MrMR5 carrying 7 amino acid polymorphisms present in MbMR5 is not activated by MdRIN4 ACP3 but recognizes AvrRpt2 without MdRIN4 in N. benthamiana. Our findings indicate that naturally occurring polymorphisms of MR5 natural variants can confer its cell death-inducing activity and the effector recognition mechanism likely due to altered compatibility with RIN4.

Fruit quality and skin color are key determinants of apple marketability. This study investigated the effect of fruit maturity and 1-methylcyclopropene (1-MCP) treatment on quality attributes and chlorophyll (Chl) degradation in ‘Golden Ball’ apples during long-term cold storage. Apples were harvested at two maturity stages—low maturity (LM, starch pattern index [SPI] 6) and high maturity (HM, SPI 8)—based on Cornell’s SPI method, and subsequently treated with 1-MCP (LM+1-MCP and HM+1-MCP) before storage. At harvest, HM fruits exhibited significantly higher soluble solid content (SSC), titratable acidity (TA), red coloration (a*), index of absorbance difference (I_AD), and respiration rate compared to LM fruits, although ethylene production, internal ethylene concentration (IEC), and firmness were not affected by maturity stage. During storage, HM fruits showed a more rapid decline in firmness, SSC, and TA, along with increased IEC, respiration rate, and a* value, and reduced I_AD and Chl contents. Consistently, expression levels of genes associated with ethylene biosynthesis and Chl degradation were also higher in HM fruits. However, application of 1-MCP effectively suppressed ethylene production and respiration, thereby slowing the deterioration of fruit quality and Chl degradation in both maturity groups. Notably, the LM+1-MCP group demonstrated stronger ethylene inhibition and retained higher Chl levels than the HM+1-MCP group. Overall, the findings demonstrate that applying 1-MCP at the LM stage is a superior strategy for maintaining the postharvest quality of the apples.