Unraveling the Mechanism of Cork Spot-like Physiological Disorders in 'Kurenainoyume' Apples Based on Occurrence Location

Cork spot-like physiological disorder (CSPD) is a newly identified issue in 'Kurenainoyume' apples, yet its mechanism remains unclear. To investigate CSPD, we conducted morphological observations on 'Kurenainoyume' apples with and without pre-harvest fruit-bagging treatment using light-impermeable paper bags. Non-bagged fruit developed CSPD in mid-August, while no CSPD symptoms were observed in bagged fruit. The bagging treatment significantly reduced the proportion of opened lenticels, with only 17.9% in bagged fruit compared to 52.0% in non-bagged fruits. In non-bagged fruit, CSPD spots tended to increase from the lenticels, growing in size during fruit development. The cuticular thickness and cross-sectional area of fresh cells in CSPD spots were approximately 16 µm and 1600 µm², respectively. Healthy non-bagged fruit reached these values around 100 to 115 days after full bloom from mid- to late August. Microscopic and computerized tomography scanning observations revealed that many CSPD spots developed at the tips of vascular bundles. Therefore, CSPD initiation between opened lenticels and vascular bundle tips may be influenced by water stress, which is potentially caused by water loss, leading to cell death and the formation of CSPD spots.

Red-Fleshed Apple Flavonoids Extract Alleviates Male Reproductive Injury Caused by Busulfan in Mice

In this research, we analyzed the protective effects of red-fleshed apple flavonoid extracts (RAFEs) on male reproductive injury induced by busulfan, using both in vitro and in vivo models. In the cell-based experiments, RAFEs significantly improved cell viability and proliferation rates compared to control groups. Similarly, in vivo testing with male mice showed that RAFEs and whole apple flavonoid extracts (WAFEs) enhanced various biochemical and liver function-related indicators in the testes; however, RAFEs demonstrated superior efficacy in mitigating testicular damage. Through immunohistochemistry, qRT-PCR, and Western blotting, we found that RAFEs notably enhanced the expression of spermatogenesis-related genes. Moreover, RAFEs increased the expression of oxidative stress- and apoptosis-related genes, thereby effectively reducing oxidative damage in the testes. These findings highlight the potential of RAFEs as natural agents for the prevention and treatment of male reproductive injury, paving the way for future research and potential therapeutic applications.

MdMYB305-MdbHLH33-MdMYB10 regulates sugar and anthocyanin balance in red-fleshed apple fruits

Sugar and anthocyanin are important indicators of fruit quality, and understanding the mechanism underlying their accumulation is essential for breeding high-quality fruit. We identified an R2R3-MYB transcription factor MdMYB305 in the red-fleshed apple progeny, which was positively correlated with fruit sugar content but negatively correlated with anthocyanin content. Transient injection, stable expression [overexpressing and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)], and heterologous transformation of tomato confirmed that MdMYB305 promotes the accumulation of sugar and inhibits the synthesis of anthocyanin. A series of molecular experiments (such as electrophoretic mobility shift and luciferase assays) confirmed that MdMYB305 combines with sugar-related genes (MdCWI1/MdVGT3/MdTMT2) and anthocyanin-related genes (MdF3H/MdDFR/MdUFGT), promoting and inhibiting their activities, and finally regulating the sugar and anthocyanin content of fruits. In addition, the study also found that MdMYB305 competes with MdMYB10 for the MdbHLH33 binding site to balance sugar and anthocyanin accumulation in the fruits, which provides a reference value for exploring more functions of the MYB-bHLH-MYB complex and the balance relationship between sugar and anthocyanin in the future.

Mdm-miR858 targets MdMYB9 and MdMYBPA1 to participate anthocyanin biosynthesis in red-fleshed apple

Anthocyanins are important secondary metabolites in plants. They are important for human health because of their antioxidant activities and because their dietary intake reduces the incidence of cardiovascular and cerebrovascular diseases and tumors. The biosynthesis of anthocyanins and its regulation in fruits and vegetables is a global research hotspot. Compared with cultivated apples, the red-fleshed apple is a relatively new and popular commodity in the market. Previous studies on red-fleshed apples have focused on the basis for the high anthocyanin content and the transcriptional regulation of anthocyanin synthesis. In the present study, we focused on the mechanism of microRNA-mediated post-transcriptional regulation of anthocyanin synthesis in red-fleshed apples. We identified a microRNA (miRNA), designated mdm-miR858, that is specifically expressed in the flesh of apple fruit. The expression level of miR858 was significantly lower in red-fleshed apples than in white-fleshed apples. The overexpression of mdm-miR858 significantly inhibited anthocyanin accumulation, whereas the silencing of mdm-miR858 promoted anthocyanin synthesis in STTM858 transgenic apple calli. Further analyses showed that mdm-miR858 targets the transcription factor genes MdMYB9 and MdMYBPA1 to participate anthocyanin accumulation in apple. Our results also show that MdHY5, a transcription factor in the light signaling pathway, can bind to the promoter of mdm-miR858 to inhibit its transcription, thereby regulating anthocyanin synthesis. Based on our results, we describe a novel HY5-miR858-MYB loop involved in the modulation of anthocyanin biosynthesis. These findings provide new information about how plant miRNAs regulate anthocyanin anabolism and provide a basis for breeding new anthocyanin-rich, red-fleshed apple varieties.

Red-fleshed apple flavonoid extract alleviates CCl4-induced liver injury in mice

In recent years, the global incidence of liver damage has increased. Despite the many known health benefits of red-fleshed apple flavonoids, their potential liver-protective effects have not yet been investigated. In this study, we analyzed the composition of red-fleshed apple flavonoid extract (RAFE) by high-performance liquid chromatography (HPLC). We then induced liver damage in mice with carbon tetrachloride (CCl4) and performed interventions with RAFE to analyze its effect on liver damage, using bifendate as a positive control. The results showed that catechin was the most abundant flavonoid in 'XJ4' RAFE (49.346 mg/100 g). In liver-injured mice, the liver coefficients converged to normal levels following RAFE intervention. Moreover, RAFE significantly reduced the enzymatic activity levels of glutamic oxaloacetic transaminase (ALT), glutamic alanine transaminase (AST), and alkaline phosphatase (ALP) in mouse serum. Furthermore, RAFE significantly increased the content or enzyme activity level of total glutathione, total antioxidant capacity, and superoxide dismutase, and significantly decreased the content of malondialdehyde in the liver of mice. In parallel, we performed histopathological observations of mouse livers for each group. The results showed that RAFE restored the pathological changes caused by CCl4 around the central hepatic vein in mice and resulted in tightly bound hepatocytes. The recovery effect of RAFE was dose-dependent in the liver tissue. Regarding intestinal microorganisms, we found that RAFE restored the microbial diversity in liver-injured mice, with a similar microbial composition in the RAFE intervention group and normal group. RAFE reduced the ratio of Firmicutes to Bacteroidetes, increased the levels of probiotic bacteria, such as Lactobacillus acidophilus, and Clostridium, and reduced the levels of harmful bacteria, such as Erysipelothrix Rosenbach. Therefore, RAFE ameliorated CCl4-induced liver damage by modulating the abundance and composition of intestinal microorganisms in mice. In conclusion, RAFE alleviated CCl4-induced liver damage in mice, with H-RAFE (5 mg kg-1) significantly improving liver damage in mice but M-RAFE (1 mg kg-1) significantly improving the imbalance of intestinal microorganisms in mice. Our research suggests that RAFE could be employed for the adjuvant treatment and prevention of liver damage, and may have important applications in food and medicine.

MdJa2 Participates in the Brassinosteroid Signaling Pathway to Regulate the Synthesis of Anthocyanin and Proanthocyanidin in Red-Fleshed Apple

Anthocyanin and proanthocyanidin play important roles in plant secondary metabolism. Although previous studies identified many transcription factors involved in anthocyanin and proanthocyanidin synthesis, the effects of MADS-box transcription factors are unclear in apple. Brassinosteroids (BRs) are steroid hormones that affect plant flavonoid biosynthesis, but the underlying regulatory mechanism is not yet well established. In this study, we identified a MADS-box transcription factor, MdJa2, which contained a highly conserved MADS-box domain and belonged to the STMADS11 subfamily. Additionally, MdJa2 was responsive to BR signal, and the overexpression of MdJa2 inhibited the synthesis of anthocyanin and proanthocyanidin. The silencing of MdJa2 in "Orin" calli promoted anthocyanin and proanthocyanidin accumulations. Moreover, MdJa2 interacted with MdBZR1. MdJa2 was revealed to independently regulate anthocyanin and proanthocyanidin synthesis pathways. The MdJa2-MdBZR1 complex enhanced the binding of MdJa2 to the promoters of downstream target genes. Our research provides new insights into how MADS-box transcription factors in the BR signaling pathway control the accumulations of anthocyanin and proanthocyanidin in red-fleshed apple.

Combined Analysis of Gut Microbiota and Plasma Metabolites Reveals the Effect of Red-Fleshed Apple Anthocyanin Extract on Dysfunction of Mice Reproductive System Induced by Busulfan

Busulfan is currently an indispensable anti-cancer drug, but the side effects on male reproductive system are so serious. Meanwhile, red-fleshed apples are natural products with high anthocyanin content. In this research, we analyzed the effect of red-fleshed apple anthocyanin extract (RAAE) on busulfan-treated mice. Compared with the busulfan group, main plasma biochemical indicators were significantly improved after RAAE treatment. Compared with BA0 (busulfan without RAAE) group, total antioxidant capacity(T-AOC) and the activity of superoxide dismutase (SOD) and glutathione catalase (GSH-Px) in RAAE treatment groups were obviously increased, while the activity of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were significantly decreased. Malondialdehyde (MDA) was significantly decreased in the RAAE groups. In addition, we found RAAE alleviated busulfan-disrupted spermatogenesis through improving genes expression which are important for spermatogenesis, such as DDX4, PGK2, and TP1. Furthermore, we found that RAAE increased beneficial bacteria Akkermansia and Lactobacillaceae, and significantly depleted harmful bacteria Erysipelotrichia. The correlation studies indicated that RAAE ameliorated busulfan-induced rise in LysoPC levels through regulating gut microbial community and their associated metabolites. In conclusion, this study extends our understanding of the alleviated effect of RAAE on busulfan-induced male reproductive dysfunction through regulating the relationships between gut microbiota and metabolites.

Red-Fleshed Apple Anthocyanin Extract Reduces Furan Content in Ground Coffee, Maillard Model System, and Not-from-Concentrate Apple Juice

Furan is a volatile and carcinogenic heterocyclic chemical compound that occurs in a wide range of thermally processed food. It can be induced during food-preparing processes by high temperatures and UV-C light. In the present study, the degradation of furan content in ground coffee, Maillard model system, and not-from-concentrate (NFC) apple juice by red-fleshed apple anthocyanin extract (RAAE) was studied. The results demonstrated that RAAEs had different degrees of degradation of furan content in coffee powder, and the RAAE from ‘XJ3’ had the most significant effect, with a reduction rate of up to 20%. Moreover, by adding RAAE to the Maillard model system, we found the amounts of furan were significantly reduced. At the same time, RAAE from ‘XJ3’ could observably reduce the content of furan in pasteurized NFC juice, with ‘Fuli’ NFC juice furan content decreasing the most, which was 68%. Taken together, our study demonstrated that the use of RAAE could be a feasible way to reduce furan content in ground coffee, Maillard model system, and NFC apple juice.

Differential effects of phenolic extracts from red-fleshed apple peels and flesh induced G1 cell cycle arrest and apoptosis in human breast cancer MDA-MB-231 cells

Red-fleshedapples are preferredbecause of their high content of phenolics and antioxidants in peel and pulp. Herein, we evaluated the mechanisms of apple peel polyphenolic extracts (APP) and apple flesh polyphenolic extracts (AFP) from the new red-fleshed apple in inhibiting cell proliferation and inducing apoptosis on human breast cancer MDA-MB-231 cells. The antiproliferative activities were determined by the CCK8 assay. The expression of proteins was determined using Western blot. We found that the content of polyphenols and flavonoids in APP was significantly higher than that in AFP, and 14 main phenolic compounds in APP and AFP were quantified using UPLC-MS/MS techniques. Besides, the significant inhibition effects of APP and AFP were achieved through Akt pathway by inducing apoptosis (significantly upregulating reactive oxygen species [ROS] levels, and downregulating expression of pAkt, pBad, Bcl-2, promoting Cytochrome c release, activating Cle-Caspase 9, and inducing expressions of Cle-Caspase 3 and Cle-PARP), and inducing G0/G1 cell cycle arrest (increased expressions of p-p53 and p21 and decreased expressions of PCNA and Cyclin D1). And the inhibition effect of APP was stronger than that of AFP. These results suggest that AFP and APP may be excellent sources of natural chemicals for treating triple-negative breast cancer MDA-MB-231 cells. PRACTICAL APPLICATION: The effects of antiproliferation of phenolic extracts from red-fleshed apple peels and flesh on human breast cancer MDA-MB-231 cells were evaluated. The data may clarify the functional parts of red-fleshed apple and provide some basis for scientific researchers and consumers to recognize and exploit red-fleshed apple.

Effects of Different Drying Methods on the Retention of Bioactive Compounds, On-Line Antioxidant Capacity and Color of the Novel Snack from Red-Fleshed Apples

The aim of this study was to determine the effect of different drying methods: convective (at 50, 60, 70 °C), vacuum-microwave (at 120, 240, 360, 480 W and 360 W with reduction to 120 W) and hybrid (convective pre-drying at 50, 60, 70 °C followed by vacuum-microwave drying at 120 W) on the quality parameters of novel red-fleshed apple fruit snacks (RFAs), such as phenolics, on-line antioxidant capacity, water activity and color. Drying kinetics, including a temperature profile of dried material, and modified Page model were determined. Freeze-drying was used as a control method. The highest content of bioactive compounds in the samples was retained following freeze-drying, then hybrid, vacuum-microwave and finally convection drying. The antioxidant capacity measured by on-line 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), identified anthocyanins, flavan-3-ols and phenolic acid as the main compounds responsible for this activity. Unfavorable changes in color, formation of hydroxymethylfurfural (HMF) and degradation of polyphenolics were noted along with increasing drying temperature and magnetron power. The red-fleshed apple snacks are a promising high-quality dehydrated food product belonging to functional foods category.

Antioxidant and hepatoprotective effects against acute CCl4 -induced liver damage in mice from red-fleshed apple flesh flavonoid extract

Apple (Malus domestica Borkh.) is an important fruit tree species worldwide. Apple fruits are favored by consumers because of their antioxidative, anti-inflammatory, and antitumor effects as well as their protective effects against cardiovascular diseases and other health benefits. There is considerable interest in red-fleshed apple fruits among breeders because of their high flavonoid and anthocyanin contents. However, the flavonoids extracted from red-fleshed apple fruits must still be functionally characterized, especially regarding their protective effects against certain pathologies. In this study, the flavonoid components and contents in the extracts prepared from red-fleshed apple cultivar "Meihong" were determined. Additionally, the in vitro antioxidant activities and protective effects of the extracts against CCl₄ -induced acute liver injury were investigated. The red-fleshed apple flesh flavonoid extract (RAFF) exhibited strong in vitro antioxidant activities. Compared with the model control mice treated with CCl₄ , the mice pretreated with high (800 mg/kg·bw), middle (400 mg/kg·bw), and low (200 mg/kg·bw) RAFF doses had significantly lower CCl₄ -induced serum aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase activities. Moreover, the RAFF pretreatment also significantly decreased the liver malondialdehyde activity and prevented the CCl₄ -induced decrease in liver superoxide dismutase, glutathione peroxidase, catalase, and reduced glutathione levels. Furthermore, a histopathological examination revealed that RAFF inhibited the inflammatory cell infiltration and cell boundary loss caused by CCl₄ in the liver. Thus, RAFF is a natural antioxidant with significant antioxidative activities and liver protective effects. The results of this study may be relevant for enhancing the application of the red-fleshed apple fruit extract as a food additive. PRACTICAL APPLICATION: We took the self-selected red-fleshed apple cultivar "Meihong" as the unique research material, and the active ingredients of its flavonoid extract, in vitro antioxidant activity and hepatoprotective effect were analyzed. It is of great significance to promote the development of the red-fleshed apple industry, and also provides an important reference for the development of natural antioxidants.

Analysis of Antioxidant Activity and Flavonoids Metabolites in Peel and Flesh of Red-Fleshed Apple Varieties

In this research, we compared the phenotypical characters, total anthocyanins content, total phenols content, and antioxidant activity of red-fleshed apple cultivars ‘XJ4’, ‘QN-5’, ‘DH’ and ‘HX1’ at three fruit developmental stages. A further flavonoids metabolites study was conducted in ‘XJ4’ and ‘DH’. We found broader variation of total anthocyanins content in the peel of the four cultivars, which might result in larger differences of free radicals scavenging rate. The most significant difference in fruit phenotype, anthocyanins content, and DPPH scavenging rate was observed between ‘XJ4’ and ‘DH’ at mature stage. Therefore, the flavonoids metabolites of ‘XJ4’ and ‘DH’ at mature stage were compared to unveil the details of anthocyanins compounds. The unique compounds pelargonidin 3-O-β-d-glucoside and cyanidin-3-O-malonylhexoside were detected only in peel and flesh of ‘XJ4’ but not in ‘DH’, which might contribute to the purple peel and dark-red flesh color of ‘XJ4’. Significantly decreased upstream metabolites in the early biosynthetic genes regulated domain were found only in ‘XJ4’ peel but not in the flesh. This might explain why the anthocyanins content in ‘XJ4’ peel was decreased largely at the mature stage. Taken together, our findings will give some insight into the metabolites study in flavonoid biosynthetic pathway of red-fleshed apple.

A novel NAC transcription factor, MdNAC42, regulates anthocyanin accumulation in red-fleshed apple by interacting with MdMYB10

Anthocyanin pigmentation is an important consumption trait of apple (Malus domestica Borkh.). In this study, we focused on the identification of NAC (NAM, ATAF1/2 and CUC2) proteins involved in the regulation of anthocyanin accumulation in apple flesh. A group of MdNACs was selected for comparison of expression patterns between the white-fleshed cultivar 'Granny Smith' and red-fleshed 'Redlove'. Among them, MdNAC42 was screened, which exhibited a higher expression level in red-fleshed than in white-fleshed fruit, and has a positive correlation with anthocyanin content as fruits ripened. Moreover, overexpression of MdNAC42 in apple calli resulted in the up-regulation of flavonoid pathway genes, including MdCHS, MdCHI, MdF3H, MdDFR, MdANS and MdUFGT, thereby increasing the accumulation of anthocyanins, which confirmed the roles of MdNAC42 in anthocyanin biosynthesis. Notably, MdNAC42 was demonstrated to have an obvious interaction with MdMYB10 either in vitro or in vivo by yeast two-hybrid combined with bimolecular fluorescence complementation, further suggesting that MdNAC42 is an important part of the regulatory network controlling the anthocyanin pigmentation of red-fleshed apples. To the best of our knowledge, this is the first report identifying the MdNAC gene as related to anthocyanin accumulation in red-fleshed apples. This study provides valuable information for improving the regulatory model of anthocyanin biosynthesis in apple fruit.

MdWRKY11 Participates in Anthocyanin Accumulation in Red-Fleshed Apples by Affecting MYB Transcription Factors and the Photoresponse Factor MdHY5

Red-fleshed apples are popular as a result of their high anthocyanin content. MdMYB10 and its homologues are known to be important regulators of anthocyanin synthesis in apple, but the roles of other transcription factors are not well-understood. Here, we explored the role of MdWRKY11 in regulating anthocyanin synthesis in apple flesh. Overexpression of MdWRKY11 in apple callus could significantly promote anthocyanin accumulation, and the expression of some MYB transcription factors and structural genes increased significantly. In binding analyses, MdWRKY11 bound to W-box cis-elements in the promoters of MdMYB10, MdMYB11, and MdUFGT. However, MdWRKY11 did not interact with MdMYB10, MdbHLH3, or MdWD40 proteins, the members of the MBW complex. Sequence analyses revealed that another W-box cis-element was present in the promoter of MdHY5 (encoding a photoresponse factor), and MdWRKY11 was able to bind to the promoter of MdHY5 and promote its activity. Our findings clarify the role of MdWRKY11 in anthocyanin synthesis in red-fleshed apple and imply that other novel genes may be involved in anthocyanin synthesis.

Enhanced stability of red-fleshed apple anthocyanins by copigmentation and encapsulation

BACKGROUND

Red-fleshed apples are a great source of natural colorants and functional food ingredients because of their high anthocyanin content. Generally, anthocyanins are highly unstable after extraction, which limits their wide applications in the food and pharmaceutical industries. This study was aimed at investigating the effects of combining copigmentation with encapsulation on the stability of anthocyanins from red-fleshed apples. In this study, red-fleshed apple anthocyanins were copigmented with caffeic acid, and then the copigmented complexes were encapsulated using gum arabic and maltodextrin using spray drying and freeze drying.

RESULTS

All anthocyanin microcapsules had high encapsulation efficiencies ranging from 93.84 to 96.85% with mean hydrodynamic diameter smaller than 350 nm. After heating at 80 °C for 2 h, the dispersions of microencapsulated anthocyanins with copigments exhibited the highest absorbance values at λ_max (515 nm) (P < 0.05). Light stability experiments demonstrated that the half-life of the red-fleshed apple anthocyanins increased from 5 to 12 days after being treated with copigmentation and encapsulation. The drying methods (spray/freeze drying) did not significantly influence the stability of the microencapsulated anthocyanins.

CONCLUSIONS

Applying copigmentation and spray-drying encapsulation in tandem has great potential for enhancing the stability of red-fleshed apple anthocyanins. Thus, such anthocyanins with enhanced stability may be increasingly used in the food and pharmaceutical industries as value-added natural food pigments. © 2018 Society of Chemical Industry.

Nanocrystallization of Anthocyanin Extract from Red-Fleshed Apple 'QN-5' Improved Its Antioxidant Effect through Enhanced Stability and Activity under Stressful Conditions

Red-flesh apples are known as functional fruits because of their rich anthocyanin. The anthocyanin content of the red flesh apple cultivar ′QN-5′ we bred can reach 361 mg·kg⁻¹ (FW), and showed higher scavenging capacity to DPPH radicals, hydroxyl radicals, and superoxide anion radicals, with scavenging rates of 80.0%, 54.0%, and 43.3%, respectively. We used this particular anthocyanin-rich ′QN-5′ apple as material to examine how nanocrystallization affects the antixodiant effect of anthocyanin. The anthocyanin extract was encapsulated with biocompatible zein to form zein-anthocyanin nanoparticles (ZANPs). Transmission electron microscopy (TEM) scanning showed that ZANPs had a regular spherical shape with an average diameter size of 50–60nm. When the ratio of the zein and the anthocyanin was 1:0.5, the results suggested that the encapsulation efficiency (EE) of the ZANPs could reach as high as 92.8%, and that scavenging rate for DPPH radicals was increased from 87.1% to 97.2% compared to the non-nanocrystallized anthocyanin extract. Interestingly, treatment under alkaline conditions (pH 9.0), high temperature (90 °C), and a storage time of 7 days could decrease the scavenging capacity of the ZANPs for DPPH radicals, but this scavenging capacity loss for ZANPs was significantly lower than that observed in the non-nanocrystallized anthocyanin, suggesting the higher stability of ZANPs is caused by encapsulation. These results would provide a theoretical basis for the application of the anthocyanin in scavenging free radicals under stress conditions.

Transcriptomic Analysis of Red-Fleshed Apples Reveals the Novel Role of MdWRKY11 in Flavonoid and Anthocyanin Biosynthesis

In plants, flavonoids are important secondary metabolites that contribute to the nutritional quality of many foods. Apple is a popular and frequently consumed food because of its high flavonoid content. In this study, flavonoid composition and content were detected and compared between red- and white-fleshed apples in a BC₁ hybrid population using ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry. Transcriptomic analysis of the red- and white-fleshed apples was then performed using RNA-seq technology. By screening differentially expressed genes encoding transcription factors, we unearthed a WRKY-family transcription factor designated MdWRKY11. Overexpression of MdWRKY11 promoted the expression of F3H, FLS, DFR, ANS, and UFGT and increased the accumulation of flavonoids and anthocyanin in apple calli. Our findings explored the novel role of MdWRKY11 in flavonoid biosynthesis and suggest several other genes that may also be potentially involved. This provides valuable information on flavonoid synthesis for the breeding of elite red-fleshed apples.

MdHB1 down-regulation activates anthocyanin biosynthesis in the white-fleshed apple cultivar 'Granny Smith'

Coloration in apple (Malus×domestica) flesh is mainly caused by the accumulation of anthocyanin. Anthocyanin is biosynthesized through the flavonoid pathway and regulated by MYB, bHLH, and WD40 transcription factors (TFs). Here, we report that the HD-Zip I TF MdHB1 was also involved in the regulation of anthocyanin accumulation. MdHB1 silencing caused the accumulation of anthocyanin in 'Granny Smith' flesh, whereas its overexpression reduced the flesh content of anthocyanin in 'Ballerina' (red-fleshed apple). Moreover, flowers of transgenic tobacco (Nicotiana tabacum 'NC89') overexpressing MdHB1 showed a remarkable reduction in pigmentation. Transient promoter activation assays and yeast one-hybrid results indicated that MdHB1 indirectly inhibited expression of the anthocyanin biosynthetic genes encoding dihydroflavonol-4-reductase (DFR) and UDP-glucose:flavonoid 3-O-glycosyltransferase (UFGT). Yeast two-hybrid and bimolecular fluorescence complementation determined that MdHB1 acted as a homodimer and could interact with MYB, bHLH, and WD40 in the cytoplasm, consistent with its cytoplasmic localization by green fluorescent protein fluorescence observations. Together, these results suggest that MdHB1 constrains MdMYB10, MdbHLH3, and MdTTG1 to the cytoplasm, and then represses the transcription of MdDFR and MdUFGT indirectly. When MdHB1 is silenced, these TFs are released to activate the expression of MdDFR and MdUFGT and also anthocyanin biosynthesis, resulting in red flesh in 'Granny Smith'.