Two MYB and Three bHLH Family Genes Participate in Anthocyanin Accumulation in the Flesh of Peach Fruit Treated with Glucose, Sucrose, Sorbitol, and Fructose In Vitro

Intracellular Mn mobility and differential response to H2O2 accumulation explain the susceptibility of litchi cultivars to dark pericarp disease

Dark pericarp disease (DPD) in litchi is a physiological disease caused by excess manganese (Mn) in pericarp, impacting fruit appearance and marketability and leading to substantial economic loss. The susceptibility of litchi varieties to DPD differs greatly, but the underlying mechanisms remain vague. In this study, we investigated the discrepancies in physiological and biochemical processes in pericarp of two varieties (Feizixiao and Heiye) resistant to DPD and a susceptible cultivar (Guiwei) during fruit development from the same orchard. Pericarp Mn in Guiwei was significantly lower than that in Feizixiao and slightly higher than that in Heiye through fruit growth. Under Mn stress, Feizixiao and Heiye maintained ROS homeostasis, whereas substantial H2O2 accumulated in Guiwei. Reduced anthocyanins and soluble sugars and increased lignin were observed in diseased Guiwei compared to Feizixiao and Heiye. The expression of genes encoding Mn transporters, light-harvesting antenna complex, ROS scavenging proteins and enzymes involved in anthocyanin synthesis was downregulated, whereas that of genes functioning in H2O2 production and lignin synthesis was upregulated in Guiwei, and that of genes involved in glucose metabolism was altered, suggesting that Mn was poorly transported and sequestrated within Guiwei pericarp cells, and excess Mn boosted H2O2 overproduction. The inhibited anthocyanin synthesis, enhanced lignin accumulation and tuned sugar metabolism conferred Guiwei adaptability to Mn stress. Conclusively, the poor Mn intracellular transferability and variations in response to H2O2 accumulation associated with disturbed photosynthetic energy deliver under excess Mn, are collaboratively responsible for the cultivar-dependent DPD vulnerability in litchi.

Integration of volatile and non-volatile metabolites and the transcriptome reveals the formation mechanisms of differential aroma compounds between Pyrus communis and Pyrus pyrifolia cultivars

Introduction

Aroma compounds are important flavor components in pear fruit. Among cultivated pears, fruits from Pyrus communis (hereafter referred to as P. communis) cultivars are famous for their abundant aroma, while the fruits of most Pyrus pyrifolia (hereafter referred to as P. pyrifolia) cultivars lack aroma compounds. A comparative study on the formation of differential aroma compounds between the two species could provide a theoretical foundation for improving the aroma quality of P. pyrifolia cultivars. However, there is a lack of systematic research on this subject.

Methods

An analysis of volatile and non-volatile metabolites was combined with transcriptome analysis to explore the formation mechanism of differential aroma compounds between three P. communis and three P. pyrifolia cultivars.

Results

In this study, a total of 510 volatile compounds were identified in the six cultivars. Of these, sixteen ester and alcohol compounds, including butyl acetate, hexyl acetate, ethyl-2-methylbutyrate, ethanol, butanol, propanol, and 2-methylbutanol, with higher contents in the P. communis cultivars than in the P. pyrifolia cultivars were identified as the primary differential aroma compounds. Among the possible synthesis pathways for these 16 aroma compounds, certain amino acid degradation processes, including isoleucine, valine, and alanine oxidation and threonine dehydration, were found to provide important intermediate substances for synthesis. Within the key enzyme genes in the synthesis pathway, several critical enzyme genes, including monoacylglycerol lipase (PcMAGL, pycom08g09340), threonine dehydrase (PcTD, pycom12g10020), and acyl CoA dehydrogenase (PcACD, pycom16g13880), might be important factors contributing to the disparity in aromatic compounds between P. communis and P. pyrifolia cultivars.

Discussion

The aforementioned results provide valuable information into the formation mechanisms of differential aroma compounds and offer novel target sites for enhancing pear aroma quality through gene editing.

The basic helix-loop-helix transcription factor PpeUNE12 regulates peach ripening by promoting polyamine catabolism and anthocyanin synthesis

The basic helix-loop-helix (bHLH) transcription factors (TFs) play important roles in various plant developmental and biological processes. However, the precise mechanisms by which bHLH TFs regulate fruit ripening warrant further investigation. Polyamine oxidase (PAO) is crucial for polyamine (PA) catabolism and plays crucial roles in fruit ripening. However, the regulatory mechanism of PAO gene expression during fruit ripening remains largely unexplored. In this study, we identified a peach bHLH TF, PpeUNE12, which directly binds to and activates the promoter of PpePAO1. Silencing PpeUNE12 substantially increased PA accumulation and delayed peach fruit ripening, while overexpressing PpeUNE12 decreased PA accumulation and accelerated peach fruit ripening. Additionally, anthocyanin content decreased in PpeUNE12-silenced fruits but increased in PpeUNE12-overexpressing peach fruits compared to the control. RNA-seq and RT-qPCR analyses revealed that the majority of genes involved in anthocyanin biosynthesis, including PpeF3H, PpeCHS, PpeDFR, PpeUFGT and PpeMYB10.1 exhibited down-regulation in fruits with silenced PpeUNE12, while these genes were up-regulated in fruits overexpressing PpeUNE12. Although PpeUNE12 exhibited no direct binding to the promoters of PpeUFGT and PpeMYB10.1, it substantially activated their activity. This investigation is the first to provide evidence that bHLH regulates fruit maturation via promoting both PA catabolism and anthocyanin synthesis. It reveals a novel mechanism of bHLH in regulating fruit ripening and enhances our comprehension of the regulatory mechanism of PA catabolism and anthocyanin synthesis during fruit maturation.

Integrated Transcriptome and Targeted Metabolome for Resolving Flavonoid Biosynthesis in Figs (Ficus carica Linn.)

Figs are an edible and medicinal plant rich in polyphenols and flavonoids with unique pharmacological effects. However, the mechanism of flavonoid synthesis in figs is not clear. In this study, fig fruits of six varieties were collected for RNA sequencing and UPLC-MS data collection. The results showed that a total of 39 differential metabolites were identified by targeted metabolomics, and their contents were determined by UPLC-MS. The clustered heat map analysis showed that most of the differential metabolites were highly accumulated in BRD and FY. A total of 62 flavonoid biosynthesis pathway genes were identified by transcriptome analysis, and FcCHS, FcCHI, FcFLS, FcCYP, and FcDFR were the key genes identified for the accumulation of flavonoids and flavonols in the dark-colored varieties. In addition, a total of 1671 transcription factor genes, mainly MYBs, bHLHs, and AP2/ERFs, were identified. This study will enrich the transcriptomic data of figs and provide some help in resolving the synthesis mechanism of fig flavonoids.

PsbHLH58 positively regulates sucrose accumulation by modulating Sucrose synthase 4 in 'Fengtang' plum (Prunus salicina Lindl.)

Sugar content is an important factor that largely determines fruit quality. 'Fengtang' plum (Prunus salicina Lindl.) is recognized for its high soluble sugar content, and the Sucrose synthase 4 (PsSUS4) functions as the controlling step in sucrose accumulation. Nevertheless, the transcriptional mechanism of PsSUS4 underlying sucrose production in this high-sugar plum remains unclear. In this work, a bHLH transcription factor PsbHLH58 was identified to be positively correlated with PsSUS4 expression and fruit sucrose content based on the transcriptome data and qRT-PCR validation. Further yeast one-hybrid and dual-luciferase assays confirmed that PsbHLH58 acts as a transcriptional activator of PsSUS4 via specifically binding to its promoter. Manipulation of the PsbHLH58 expression in fruits through transient overexpression and gene silencing resulted in significant corresponding changes in PsSUS4 expression and sucrose accumulation. In addition, it was also found that the expression levels of PsbHLH58 and PsSUS4 are positively regulated by ethylene, accompanied by an increased sugar accumulation in the fruits. Consequently, these results demonstrated a novel mechanism involving the PsbHLH58-PsSUS4 module that mediates the sugar accumulation in 'Fengtang' plum and provided a new insight into the future fruit quality improvement.

Transcriptomic Insights into Higher Anthocyanin Accumulation in 'Summer Black' Table Grapes in Winter Crop Under Double-Cropping Viticulture System

Anthocyanins are responsible for grape (Vitis vinifera L.) skin color. To obtain a more detailed understanding of the anthocyanin regulatory networks across' the summer and winter seasons in grapes under a double-cropping viticulture system, the transcriptomes of 'Summer Black' grapes were analyzed using RNA sequencing. The average daily temperature during the harvest stage in the summer crop, ranging from 26.18 °C to 32.98 °C, was higher than that in the winter crop, ranging from 11.03 °C to 23.90 °C. Grapes from the winter crop accumulated a greater content of anthocyanins than those from the summer crop, peaking in the harvest stage (E-L38) with 207.51 mg·100 g-1. Among them, malvidin-3-O-glucoside (Mv-3-G) had the highest monomer content, accounting for 32%. The content of Cy-3-G during winter increased by 55% compared to summer. KEGG analysis indicated that the flavonoid biosynthesis and circadian rhythm-plant pathways are involved in the regulation of anthocyanin biosynthesis during fruit development. Pearson's coefficient showed significant positive correlations between anthocyanin content and the VvDFR, VvUFGT, VvOMT, VvMYB, and VvbHLH genes in the winter crop; at full veraison stage, their expressions were 1.34, 1.98, 1.28, 1.17, and 1.34 times greater than in summer, respectively. The higher expression of VvUFGT and VvOMT led to higher contents of Cy-3-G and Mv-3-G in the winter berries, respectively.

Exogenous Melatonin Enhances Dihydrochalcone Accumulation in Lithocarpus litseifolius Leaves via Regulating Hormonal Crosstalk and Transcriptional Profiling

Dihydrochalcones (DHCs) constitute a specific class of flavonoids widely known for their various health-related advantages. Melatonin (MLT) has received attention worldwide as a master regulator in plants, but its roles in DHC accumulation remain unclear. Herein, the elicitation impacts of MLT on DHC biosynthesis were examined in Lithocarpus litseifolius, a valuable medicinal plant famous for its sweet flavor and anti-diabetes effect. Compared to the control, the foliar application of MLT significantly increased total flavonoid and DHC (phlorizin, trilobatin, and phloretin) levels in L. litseifolius leaves, especially when 100 μM MLT was utilized for 14 days. Moreover, antioxidant enzyme activities were boosted after MLT treatments, resulting in a decrease in the levels of intracellular reactive oxygen species. Remarkably, MLT triggered the biosynthesis of numerous phytohormones linked to secondary metabolism (salicylic acid, methyl jasmonic acid (MeJA), and ethylene), while reducing free JA contents in L. litseifolius. Additionally, the flavonoid biosynthetic enzyme activities were enhanced by the MLT in leaves. Multiple differentially expressed genes (DEGs) in RNA-seq might play a crucial role in MLT-elicited pathways, particularly those associated with the antioxidant system (SOD, CAT, and POD), transcription factor regulation (MYBs and bHLHs), and DHC metabolism (4CL, C4H, UGT71K1, and UGT88A1). As a result, MLT enhanced DHC accumulation in L. litseifolius leaves, primarily by modulating the antioxidant activity and co-regulating the physiological, hormonal, and transcriptional pathways of DHC metabolism.

The PpMYB75-PpDFR module reveals the difference between 'SR' and its bud variant 'RMHC' in peach red flesh

'Red Meat Honey Crisp (RMHC)' has been widely cultivated by growers in recent years due to its early maturity, and red meat type characteristics. As a bud variant of 'Super Red (SR)' peach, red flesh is the most distinctive characteristic of 'Red Meat Honey Crisp (RMHC)'. However, the mechanism of red flesh formation in 'RMHC' remains unclear. In this study, 79 differentially produced metabolites were identified by metabolomics analysis. The anthocyanin content in 'RMHC' was significantly higher than that in 'SR' during the same period, such as cyanidin O-syringic acid and cyanidin 3-O-glucoside. Other flavonoids also increased during the formation of red flesh, including flavonols (6-hydroxykaempferol-7-O-glucoside, hyperin), flavanols (protocatechuic acid, (+)-gallocatechin), and flavonoids (chrysoeriol 5-O-hexoside, tricetin). In addition, transcriptomic analysis and RT-qPCR showed that the expression levels of the flavonoid synthesis pathway transcription factor MYB75 and some structural genes, such as PpDFR, PpCHS, PpC4H, and PpLDOX increased significantly in 'RMHC'. Subcellular localization analysis revealed that MYB75 was localized to the nucleus. Yeast single hybridization assays showed that MYB75 bound to the cis-acting element CCGTTG of the PpDFR promoter region. The MYB75-PpDFR regulatory network was identified to be a key pathway in the reddening of 'RMHC' flesh. Moreover, this is the first study to describe the cause for red meat reddening in 'RMHC' compared to 'SR' peaches using transcriptomics, metabolomics and molecular methods. Our study identified a key transcription factor involved in the regulation of the flavonoid synthetic pathway and contributes to peach breeding-related efforts as well as the identification of genes involved in color formation in other species.

The Impact of Rootstock on "Big Top" Nectarine Postharvest Concerning Chilling Injury, Biochemical and Molecular Parameters

Peaches and nectarines have a short shelf life even when harvested at appropriate physiological maturity. Market life is increased by storage at low temperatures. However, chilling injury symptoms can appear, causing physiological disorders and limiting shipping potential. The rootstock effect on the post-harvest quality has hardly been explored. Thus, the principal aim of this work was to study the influence of seven different Prunus rootstocks on the "Big Top" nectarine cv, considering harvest and post-harvest quality parameters and their correlation with chilling injury disorders. Basic fruit quality traits, individual sugars and organic acids analyzed by HPLC and other biochemical compounds such as relative antioxidant capacity, total phenolics content, flavonoids, anthocyanins, vitamin C and related enzyme activities (PAL, POD, PPO) were considered. In addition, correlations with possible candidate genes for chilling injury (CI) tolerance were searched by qPCR. Although a low susceptibility to CI symptoms has been found in "Big Top", rootstocks "PADAC 9902-01", "PADAC 99-05" and "ReplantPAC" exhibited lower CI symptoms. A statistically significant influence of the evaluated rootstocks was found concerning the parameters of this study. Phenols and anthocyanins seem to be important parameters to be considered in the prevention of chilling injury disorders. Moreover, PAL1, PPO4, PG2 and LDOX genes relative expressions were positively associated with chilling injury susceptibility. This study opens new perspectives for understanding peach fruit adaptation and response to cold storage temperatures during the post-harvest period.

Insights into tissue-specific anthocyanin accumulation in Japanese plum (Prunus salicina L.) fruits: A comparative study of three cultivars

In the present study, three matured Japanese plum cultivars with different colored peel and flesh were selected to mine the key transcription factors regulating anthocyanin formation in tissues. Results showed that PsMYB10 was correlated with structural genes C4H, F3H, and ANS. PsMYB6 could positively regulate C4H (r = 0.732) and accumulated anthocyanins in Sanhua plum's flesh. Sanhua plum has the highest phenolic and anthocyanin contents (10.24 ± 0.37 gallic acid equivalent mg g-1 dry weight (DW) and 68.95 ± 1.03 μg g-1 DW), resulting itself superior biological activity as 367.1 ± 42.9 Trolox equivalent mg g-1 DW in oxygen radical absorbance capacity value and 72.79 ± 4.34 quercetin equivalent mg g-1 DW in cellular antioxidant activity value. The present work provides new insights into the regulatory mechanism of tissue-specific anthocyanin biosynthesis, confirming the pivotal role of anthocyanins in the biological activity of plums, providing essential support for the development of horticultural products enriched with anthocyanins.

Determination of the effects of pre-harvest bagging treatment on kiwifruit appearance and quality via transcriptome and metabolome analyses

Bagging is an effective cultivation strategy to produce attractive and pollution-free kiwifruit. However, the effect and metabolic regulatory mechanism of bagging treatment on kiwifruit quality remain unclear. In this study, transcriptome and metabolome analyses were conducted to determine the regulatory network of the differential metabolites and genes after bagging. Using outer and inner yellow single-layer fruit bags, we found that bagging treatment improved the appearance of kiwifruit, increased the soluble solid content (SSC) and carotenoid and anthocyanin levels, and decreased the chlorophyll levels. We also identified 41 differentially expressed metabolites and 897 differentially expressed genes (DEGs) between the bagged and control 'Hongyang' fruit. Transcriptome and metabolome analyses revealed that the increase in SSC after bagging treatment was mainly due to the increase in D-glucosamine metabolite levels and eight DEGs involved in amino sugar and nucleotide sugar metabolic pathways. A decrease in glutamyl-tRNA reductase may be the main reason for the decrease in chlorophyll. Downregulation of lycopene epsilon cyclase and 9-cis-epoxycarotenoid dioxygenase increased carotenoid levels. Additionally, an increase in the levels of the taxifolin-3'-O-glucoside metabolite, flavonoid 3'-monooxygenase, and some transcription factors led to the increase in anthocyanin levels. This study provides novel insights into the effects of bagging on the appearance and internal quality of kiwifruit and enriches our theoretical knowledge on the regulation of color pigment synthesis in kiwifruit.

Asymmetric distribution of mineral nutrients aggravates uneven fruit pigmentation driven by sunlight exposure in litchi

MAIN CONCLUSION

Sunlight boosts anthocyanin synthesis/accumulation in sunny pericarp of litchi fruit, directly leading to uneven pigmentation. Distribution discrepancy of mineral element aggravates uneven coloration by modulating synthesis/accumulation of anthocyanin and sugar. Uneven coloration, characterized by red pericarp on sunny side and green pericarp on shady side, impacts fruit quality of 'Feizixiao' (cv.) litchi. The mechanisms of this phenomenon were explored by investigating the distribution of chlorophyll, flavonoids, sugars, and mineral elements in both types of pericarp. Transcriptome analysis in pericarp was conducted as well. Sunny pericarp contained higher anthocyanins in an order of magnitude and higher fructose, glucose, co-pigments (flavanols, flavonols, ferulic acid), and mineral elements like Ca, Mg and Mn, along with lower N, P, K, S, Cu, Zn and B (P < 0.01), compared to shady pericarp. Sunlight regulated the expression of genes involved in synthesis/accumulation of flavonoids and sugars and genes functioning in nutrient uptake and transport, leading to asymmetric distribution of these substances. Anthocyanins conferred red color on sunny pericarp, sugars, Ca and Mg promoted synthesis/accumulation of anthocyanins, and co-pigments enhanced color display of anthocyanins. The insufficiencies of anthocyanins, sugars and co-pigments, and inhibition effect of excess K, S, N and P on synthesis/accumulation of anthocyanins and sugars, jointly contributed to green color of shady pericarp. These findings highlight the role of asymmetric distribution of substances, mineral elements in particular, on uneven pigmentation in litchi, and provide insights into coloration improvement via precise fertilization.

Involvement of a MYB Transcription Factor in Anthocyanin Biosynthesis during Chinese Bayberry (Morella rubra) Fruit Ripening

Anthocyanin is a class of water-soluble flavonoids found in Chinese bayberry (Morella rubra) that is not only responsible for the variety of colors visible in nature but also has numerous health-promoting benefits in humans. Through comparative transcriptomics, we isolated and identified a transcription factor (TF) of the R2R3-MYB type, MrMYB9, in order to explore the anthocyanin biosynthesis pathway in red and white Chinese bayberries. MrMYB9 transcript was positively correlated with anthocyanin level and anthocyanin biosynthetic gene expression during Chinese bayberry fruit maturation (R-values in the range 0.54-0.84, p < 0.05). Sequence analysis revealed that MrMYB9 shared a similar R2R3 domain with MYB activators of anthocyanin biosynthesis in other plants. MrMYB9 substantially transactivated promoters of anthocyanin biosynthesis-related EBGs (MrCHI, MrF3'H, and MrANS) and LBGs (MrUFGT) upon co-expression of the AtEGL3 gene. Our findings indicated that MrMYB9 may positively modulate anthocyanin accumulation in Chinese bayberry.