Comparative Transcriptome Analysis of the Skin-Specific Accumulation of Anthocyanins in Black Peanut ( Arachis hypogaea L.)

Overexpression of a BR inactivating enzyme gene GhPAG1 impacts eggplant fruit development and anthocyanin accumulation mainly by altering hormone homeostasis

Brassinosteroids (BRs) function importantly in plant growth and development, but the roles in regulating fruit development and anthocyanin pigmentation remain unclear. Eggplant (Solanum melongena L.) is an important Solanaceae vegetable crop rich in anthocyanins. The fruit size and coloration are important agronomic traits for eggplant breeding. In this study, transgenic eggplant exhibiting endogenous BRs deficiency was created by overexpressing a heterologous BRs-inactivating enzyme gene GhPAG1 driven by CaMV 35 S promoter. 35 S::GhPAG1 eggplant exhibited severe dwarfism, reduced fruit size, and less anthocyanin accumulation. Microscopic observation showed that the cell size of 35 S::GhPAG1 eggplant was significantly reduced compared to WT. Furthermore, the levels of IAA, ME-IAA, and active JAs (JA, JA-ILE, and H2JA) all decreased in 35 S::GhPAG1 eggplant fruit. RNA-Seq analyses showed a decrease in the expression of genes involved in cell elongation, auxin signaling, and JA signaling. Besides, overexpression of GhPAG1 significantly downregulated anthocyanin biosynthetic genes and associated transcription regulators. Altogether, these results strongly suggest that endogenous brassinosteroid deficiency arising from GhPAG1 overexpression impacts eggplant fruit development and anthocyanin coloration mainly by altering hormone homeostasis.

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.

Transcriptional networks orchestrating red and pink testa color in peanut

BACKGROUND

Testa color is an important trait of peanut (Arachis hypogaea L.) which is closely related with the nutritional and commercial value. Pink and red are main color of peanut testa. However, the genetic mechanism of testa color regulation in peanut is not fully understood. To elucidate a clear picture of peanut testa regulatory model, samples of pink cultivar (Y9102), red cultivar (ZH12), and two RNA pools (bulk red and bulk pink) constructed from F₄ lines of Y9102 x ZH12 were compared through a bulk RNA-seq approach.

RESULTS

A total of 2992 differential expressed genes (DEGs) were identified among which 317 and 1334 were up-regulated and 225 and 1116 were down-regulated in the bulk red-vs-bulk pink RNA pools and Y9102-vs-ZH12, respectively. KEGG analysis indicates that these genes were divided into significantly enriched metabolic pathways including phenylpropanoid, flavonoid/anthocyanin, isoflavonoid and lignin biosynthetic pathways. Notably, the expression of the anthocyanin upstream regulatory genes PAL, CHS, and CHI was upregulated in pink and red testa peanuts, indicating that their regulation may occur before to the advent of testa pigmentation. However, the differential expression of down-stream regulatory genes including F3H, DFR, and ANS revealed that deepening of testa color not only depends on their gene expression bias, but also linked with FLS inhibition. In addition, the down-regulation of HCT, IFS, HID, 7-IOMT, and I2'H genes provided an alternative mechanism for promoting anthocyanin accumulation via perturbation of lignin and isoflavone pathways. Furthermore, the co-expression module of MYB, bHLH, and WRKY transcription factors also suggested a fascinating transcriptional activation complex, where MYB-bHLH could utilize WRKY as a co-option during the testa color regulation by augmenting anthocyanin biosynthesis in peanut.

CONCLUSIONS

These findings reveal candidate functional genes and potential strategies for the manipulation of anthocyanin biosynthesis to improve peanut varieties with desirable testa color.

Metabolomics combined with transcriptomics analyses of mechanism regulating testa pigmentation in peanut

Peanut testa (seed coat) contains large amounts of flavonoids that significantly influence seed color, taste, and nutritional qualities. There are various colors of peanut testa, however, their precise flavonoid components and regulatory mechanism of pigmentation remain unclear. In this study, a total of 133 flavonoids were identified and absolutely quantified in the seed coat of four peanut cultivars with different testa color using a widely targeted metabolomic approach. Black peanut skin had more types and substantial higher levels of cyanidin-based anthocyanins, which possibly contribute to its testa coloration. Procyanidins and flavan-3-ols were the major co-pigmented flavonoids in the red, spot and black peanuts, while flavanols were the most abundant constitutes in white cultivar. Although the concentrations as well as composition characteristics varied, the content ratios of procyanidins to flavan-3-ols were similar in all samples except for white peanut. Furthermore, MYB-like transcription factors, anthocyanidin reductases (ANR), and UDP-glycosyltransferases (UGT) were found to be candidate genes involved in testa pigmentation via RNA-seq and weighted gene co-expression network analysis. It is proposed that UGTs and ANR compete for the substrate cyanidin and the prevalence of UGTs activities over ANR one will determine the color pattern of peanut testa. Our results provide a comprehensive report examining the absolute abundance of flavonoid profiles in peanut seed coat, and the finding are expected to be useful for further understanding of regulation mechanisms of seed coat pigmentation in peanut and other crops.

Integrated Transcriptomic and Metabolomic Analyses Reveal the Mechanisms Underlying Anthocyanin Coloration and Aroma Formation in Purple Fennel

The color and aroma are the significant traits of vegetables and fruits, but the metabolic and molecular mechanisms underlying anthocyanin accumulation and aroma formation remain almost unknown in fennel (Anethum foeniculum L.), which is a crucial vegetable crop and grown widely for aromatic leaves and bulbs. Here, ten major anthocyanins identified and quantified by ultra-high performance liquid chromatography coupled with quadrupole Orbitrap high-resolution mass spectrometry (UHPLC-Q-Orbitrap HRMS) were mainly responsible for the coloration of purple fennel leaf. With the application of GC-MS, it was found that the reduced volatile phenylpropanoids including isoeugenol, trans-isoeugenol, and apiol chiefly account for the characteristic aroma changes of the purple fennel. Moreover, the characteristic anthocyanin coloration and aroma formation in purple fennel were systematically studied with the integrated transcriptomics and metabolomics. The critical genes associated with the biosynthesis and regulation of anthocyanins and volatile phenylpropanoids were isolated and studied carefully in transiently transfected tobacco cells and transgenic tomato plants. Together with the results of UHPLC-Q-Orbitrap HRMS, RT-qPCR, and yeast two hybrid (Y2H), it is proved that the metabolic flux redirection of phenylpropanoid pathway primarily regulated by a functional MYB-bHLH-WD40 complex consisting of AfTT8, AfMYB7, and AfTTG1 accounts for the characteristic anthocyanin coloration and aroma formation in purple fennel leaf. The systematic understanding of the anthocyanin accumulation and aroma formation will assist in the improvement of fennel resource utilization and breeding.

Comparative Transcriptome Analysis Reveals Differential Regulation of Flavonoids Biosynthesis Between Kernels of Two Pecan Cultivars

Flavonoids influence the flavor and nutritional value of pecan nuts. However, limited information is available regarding the molecular mechanisms underlying pecan flavonoid biosynthesis. Here, we used a high ("YLC28") and a low ("Oconee") flavonoid content cultivar as the research objects. The changes in flavonoid content and the gene transcription patterns during kernel development were identified. Different accumulation patterns of total flavonoids (TF) and condensed tannins (CT) were observed between the two cultivars. The contents of TF and CT in "YLC28" were 1.76- and 2.67-fold higher levels than that of "Oconee" on 150 days after full bloom of female flowers, respectively. In total, 30 RNA-Seq libraries were constructed and sequenced. The upregulated genes in "YLC28" were highly enriched in flavonoid-related pathways. Thirty-three structural genes were identified, and the expression of two phenylalanine ammonia lyases, one chalcone synthase, one flavonoid 3',5'-hydroxylase, and one flavonol synthase exhibited high correlation (r ≥ 0.7, p < 0.01) with the condensed tannin content in "YLC28." A putative MYB transcription factor, CIL1093S0100, might act as a flavonoid biosynthesis repressor during kernel development. Altogether, these results will be useful for uncovering the molecular mechanisms of flavonoid biosynthesis and subsequently accelerating quality pecan breeding.

Multi-Omics and miRNA Interaction Joint Analysis Highlight New Insights Into Anthocyanin Biosynthesis in Peanuts (Arachis hypogaea L.)

Peanut (Arachis hypogaea L.) is one of the most important economic and oil crops in the world. At present, peanut varieties with rich anthocyanin in testa are rare in the market, but the selection and breeding of varieties with the related traits has always attracted the attention of breeders. In this study, two peanut varieties with the pink and purple testa, G110 (G) and Z18-40 (Z) were used to conduct interaction joint analysis of multi-omics and miRNA-target gene. The anthocyanin content of Z18-40 was 7.49-8.62-folds higher than G110 on 30 DAF (days after flowering) and 45 DAF via Ultraviolet-visible Spectrophotometer (UV-5800, Shanghai, China). And then, a total of 14 candidate genes related with the anthocyanin biosynthesis were identified for correlation in different comparison groups (R ² ≥ 0.80), among of a novel gene Ah21440 related with hydroxycinnamoyl transferase (HCT) biosynthesis was identified. In addition, Cyanidin 3-O-glucoside (Kuromanin, pmb0550) was the only common differentially accumulated metabolite (DAM) identified using multi-omics joint analysis in G1_vs_G2, Z1_vs_Z2, G1_vs_Z1, and G2_vs_Z2, respectively. Correlation analysis of miRNA-target genes and DEGs in the transcriptome shows that, AhmiR2950, AhmiR398, AhmiR50, and AhmiR51 regulated to HCT and chalcone biosynthesis related candidate genes (Ah21440, AhCHS, AhCHI). Lastly, all of 14 candidate genes and 4 differentially expressed miRNAs were validated using quantitative real-time PCR (qRT-PCR), which trends were consistent with that of the former transcriptome data. The results provide important reference for in-depth research on the anthocyanin metabolism mechanism in peanut testa.

An overview on extraction, composition, bioactivity and food applications of peanut phenolics

Peanuts contain a diverse and vast array of phenolic compounds having important biological properties. They are allocated mostly in the seed coat (skin), an industrial waste with minor and undervalued applications. In the last few years, a considerable amount of scientific knowledge about extraction, composition, bioactivities and health benefits of peanut skin phenolics has been generated. The present review was focused on four main aspects: a) extraction methods and technologies for obtaining peanut skin phenolics with an emphasis on green-solvent extraction processes; b) variations in chemical profiles including those due to genetic variability, extraction methodologies and process-related issues; c) bioactive properties, especially antioxidant activities in food and biological systems; d) update of promising food applications. The revision was also aimed at identifying areas where knowledge is insufficient and to set priorities for further research.

Peanut Seed Coat Acts as a Physical and Biochemical Barrier against Aspergillus flavus Infection

Aflatoxin contamination is a global menace that adversely affects food crops and human health. Peanut seed coat is the outer layer protecting the cotyledon both at pre- and post-harvest stages from biotic and abiotic stresses. The aim of the present study is to investigate the role of seed coat against A. flavus infection. In-vitro seed colonization (IVSC) with and without seed coat showed that the seed coat acts as a physical barrier, and the developmental series of peanut seed coat showed the formation of a robust multilayered protective seed coat. Radial growth bioassay revealed that both insoluble and soluble seed coat extracts from 55-437 line (resistant) showed higher A. flavus inhibition compared to TMV-2 line (susceptible). Further analysis of seed coat biochemicals showed that hydroxycinnamic and hydroxybenzoic acid derivatives are the predominant phenolic compounds, and addition of these compounds to the media inhibited A. flavus growth. Gene expression analysis showed that genes involved in lignin monomer, proanthocyanidin, and flavonoid biosynthesis are highly abundant in 55-437 compared to TMV-2 seed coats. Overall, the present study showed that the seed coat acts as a physical and biochemical barrier against A. flavus infection and its potential use in mitigating the aflatoxin contamination.

Comparative Transcriptome Analysis of the Accumulation of Anthocyanins Revealed the Underlying Metabolic and Molecular Mechanisms of Purple Pod Coloration in Okra (Abelmoschus esculentus L.)

Color is an essential agronomic trait and the consumption of high anthocyanin containing vegetables in daily diet does provide benefits to human health, but the mechanisms on anthocyanin accumulation in tender pods of okra (Abelmoschus esculentus L.) were totally unknown. In this study, a wide characterization and quantitation of anthocyanins and flavonols in tender pods of 15 okra varieties were performed by UHPLC-Q-Orbitrap HRMS for the first time. Two major anthocyanins (delphinidin 3-O-sambubioside and cyanidin 3-O-sambubioside) and six kinds of flavonol glycosides (most are quercetin-based) were identified and quantified. The coloration of the purple okra pod mainly arises from the accumulation of both delphinidin 3-O-sambubioside and cyanidin 3-O-sambubioside in most of purple varieties (Hong Yu, Bowling Red and Burgundy), except Jing Orange. The significant differences in the compositions and contents of anthocyanins are responsible for the pod color ranging from brick-red to purplish-red among the various okra cultivars. Furthermore, four representative okra cultivars exhibiting obvious differences in anthocyanin accumulation were further analyzed with transcriptome and more than 4000 conserved differentially expressed genes were identified across the three compared groups (B vs. BR, B vs. HY and B vs. JO). Based on the comprehensive analysis of transcriptomic data, it was indicated that MBW complex consisting of AeMYB114, AeTT8, and AeTTG1 and other transcriptional factors coordinately regulate the accumulation of anthocyanins via the transcriptional regulation of structural genes. Moreover, four independent working models explaining the diversities of anthocyanin pigmentation in okra pods were also proposed. Altogether, these results improved our understanding on anthocyanin accumulation in okra pods, and provided strong supports for the development of okra pod as a functional food in the future.

Extracts of Peanut Skins as a Source of Bioactive Compounds: Methodology and Applications

Peanut skins are a waste product of the peanut processing industry with little commercial value. They are also significant sources of the polyphenolic compounds that are noted for their bioactivity. The extraction procedures for these compounds range from simple single solvent extracts to sophisticated separation schemes to isolate and identify the large range of compounds present. To take advantage of the bioactivities attributed to the polyphenols present, a range of products both edible and nonedible containing peanut skin extracts have been developed. This review presents the range of studies to date that are dedicated to extracting these compounds from peanut skins and their various applications.

Integrated Metabolomics and Transcriptomics Analyses Reveal the Molecular Mechanisms Underlying the Accumulation of Anthocyanins and Other Flavonoids in Cowpea Pod (Vigna unguiculata L.)

As an important vegetable crop of the legume family, cowpea (Vigna unguiculata L.) is grown widely for its tender pod with good taste and nutrition. The purple cowpea pods attract more attention mainly for the eye-catching color and health-promoting ingredients. Initially, large quantities of two major anthocyanins (delphinidin 3-O-glucoside and cyanidin 3-O-glucoside) and nine kinds of flavonoids (most are quercetin-based flavonol glycosides) were separated and identified from purple cowpea pod by ultra-high performance liquid chromatography coupled with quadrupole Orbitrap high-resolution mass spectrometry. To study them systematically, two representative cowpea cultivars with a drastic difference in anthocyanin accumulation were further analyzed by the integration of metabolomics and transcriptomics. A total of 56 differentially accumulated metabolites and 4142 differentially expressed genes were identified, respectively. On the basis of the comprehensive analysis of multiomic data, it was shown that VuMYB90-1, VuMYB90-2, VuMYB90-3, VuCPC, VuMYB4, and endogenous bHLH and WD40 proteins coordinately control anthocyanin and flavonoid accumulation via transcriptional regulation of structural genes in purple cowpea pod.

Comparative transcriptome analysis of anthocyanin synthesis in black and pink peanut

In recent years, black testa peanut (Arachis hypogaea L.) has been favored because of its nutritional value and health function. To explore the genetic basis of peanut testa color, high-throughput sequencing technology was used to sequence the transcriptome of black testa peanut 'ZH9' and pink testa peanut 'ZH8.' Over 18 million high-quality reads were assembled into 49,404-52,578 genes for these two cultivars using a combined assembly strategy. Totally, 4,122 differentially expressed genes (DEGs) were identified between ZH8 and ZH9, among which 1317 (32%) were up-regulated and 2805 (68%) were down-regulated. KEGG analysis showed that the pathways of anthocyanin biosynthesis, isoflavonoid biosynthesis, flavone and flavonol biosynthesis, and phenylpropanoid biosynthesis were in the top 20 differentially expressed genes enriched pathways. Further analysis showed that the formation of the black color of ZH9 testa was mainly due to the reduction of lignin biosynthesis and isoflavonoid biosynthesis, and as a result, more substrate flow to anthocyanin biosynthesis. The up-regulation of all genes associated with DFR, a key enzyme determining flavonoid synthesis or anthocyanin synthesis in the flavonoid metabolic pathway, is also a strategy for increasing dihydroflavonol, a substrate for anthocyanin and flavonol biosynthesis. In addition, we identified three up-regulated R2R3MYB transcription factors associated with anthocyanin biosynthesis in ZH9. Finally, we verified the expressions of 15 genes that encode key enzymes and transcription factors using quantitative real-time PCR (qRT-PCR).

Transcriptome and Flavonoids Metabolomic Analysis Identifies Regulatory Networks and Hub Genes in Black and White Fruits of Lycium ruthenicum Murray

Lycium ruthenicum Murry. is a highly nutritional cash crop due to its fruit abundant anthocyanins. To understand the complex metabolic networks underlying the color formation in black and white fruits of L. ruthenicum, we conducted transcriptome and flavonoid metabolic profiling to identify the candidate genes possibly involved in flavonoid biosynthesis. As a result, 147 flavonoids were identified and there was almost no anthocyanin in white fruits, while luteolin, kaempferol, and quercetin derivatives showed markedly higher abundance. Furthermore, applying weighted gene co-expression network analyses, 3 MYB, 2 bHLH, 1WRKY and 1 NAC transcription factor, associated with anthocyanin biosynthesis were identified. A bHLH transcription factor, LrAN1b showed the greatest correlations with anthocyanin accumulation with no expression in white fruits. In addition, gene function analysis and qRT-PCR experiments identified a new activated anthocyanin MYB transcription factor designed as LrAN2-like. Yeast two-hybrid and transient tobacco overexpression experiments showed that LrAN1b could interact with LrAN2-like and LrAN11 to form MBW complex to activate the anthocyanin pathway. The yeast one-hybrid experiment indicated that LrAN2-like bonded anthocyanin structural gene LrDFR and LrANS promoters. Heterologous expression of LrAN1b in tobacco can significantly increase the anthocyanin content of tobacco florals and capsules, and activate anthocyanin synthesis related genes. Taken together, an anthocyanin regulatory network model in L. ruthenicum fruit was proposed firstly and we speculate that the white fruit phenotype was due to abnormal expression of LrAN1b. The findings provide new insight into the underlying mechanism of flavonoids, laying the foundation for future functional and molecular biological research in L. ruthenicum.

Physiological and transcriptome analyses of Opisthopappus taihangensis in response to drought stress

Background

Water scarcity is considered to be a severe environmental constraint to plant survival and productivity. Studies on drought-tolerant plants would definitely promote a better understanding of the regulatory mechanism lying behind the adaptive response of plants to drought. Opisthopappus taihangensis (ling) shih is a typical drought-tolerant perennial plant species endemically distributed across the Taihang Mountains in China, but the underlying mechanism for drought tolerance of this particular species remains elusive.

Results

To mimic natural drought stress, O. taihangensis plants were treated with two different concentrations (25% and 5%) of polyethylene glycol (PEG6000), which represent the H group (high salinity) and the L group (low salinity), respectively. The physiological characteristics of these two groups of plants, including relative water content maintenance (RWC), proline content and chlorophyll content were assessed and compared with plants in the control group (CK), which had normal irrigation. There was not a significant difference in RWC when comparing plants in the L group with the control group. Proline was accumulated to a higher level, and chlorophyll content was decreased slightly in plants under low drought stress. In plants from the H group, a lower RWC was observed. Proline was accumulated to an even higher level when compared with plants from the L group, and chlorophyll content was further reduced in plants under high drought stress. Transcriptomic analysis was carried out to look for genes that are differentially expressed (DEGs) in O. taihangensis plants coping adaptively with the two levels of drought stress. A total of 23,056 genes are differentially expressed between CK and L, among which 12,180 genes are up-regulated and 10,876 genes are down-regulated. Between H and L, 6182 genes are up-regulated and 1850 genes are down-regulated, which gives a total of 8032 genes. The highest number of genes, that are differentially expressed, was obtained when a comparison was made between CK and H. A total of 43,074 genes were found to be differentially expressed with 26,977 genes up-regulated and 16,097 genes down-regulated. Further analysis of these genes suggests that many of the up-regulated genes are enriched in pathways involved in amino acid metabolism. Besides, 39 transcription factors (TFs) were found to be continuously up-regulated with the increase of drought stress level.

Conclusion

Taken together, the results indicate that O. taihangensis plants are able to live adaptively under drought stress by responding physiologically and regulating the expression of a substantial number of drought-responsive genes and TFs to avoid adverse effects.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0318-7) contains supplementary material, which is available to authorized users.

Metabolic and molecular analysis of nonuniform anthocyanin pigmentation in tomato fruit under high light

Pigment intensity and patterns are important factors that determine the nutritional and market values of tomato fruits. The acropetal manner of light-dependent anthocyanin accumulation with the highest levels at the stem end of the fruit makes Pro35S:BrTT8 tomato plants an ideal system for investigating the effects of light intensity on anthocyanin biosynthesis. Extensive transcript analyses indicate that anthocyanin pigmentation in Pro35S:BrTT8 plants under high light might be coordinately regulated by the exogenous protein BrTT8 and endogenous proteins SlAN2 and SlMYBL2. Furthermore, yeast two-hybrid assays showed that BrTT8 could interact efficiently with SlAN2, SlMYBL2, and SlAN11. Moreover, the physical interaction between BrTT8 and SlAN2 was validated by FRET. Simultaneous overexpression of SlAN2 and BrTT8 activated significant anthocyanin biosynthesis in infiltrated tobacco leaves. In addition, the ability of SlMYBL2 to suppress anthocyanin accumulation was also demonstrated in infiltrated tobacco leaves. Altogether, these results prove that tissue-specific assemblage of the heterogeneous MYB-bHLH-WD40 complex consisting of SlAN2, BrTT8 and SlAN11 triggers nonuniform anthocyanin accumulation in tomato fruit under high light. Additionally, it is proposed that a negative-feedback loop fulfilled by SlMYBL2 also participates in the regulation of anthocyanin production.