The Basic Helix-Loop-Helix Transcription Factor SmbHLH1 Represses Anthocyanin Biosynthesis in Eggplant

Genetic and Biotechnological Approaches to Improve Fruit Bioactive Content: A Focus on Eggplant and Tomato Anthocyanins

Anthocyanins are a large group of water-soluble flavonoid pigments. These specialized metabolites are ubiquitous in the plant kingdom and play an essential role not only in plant reproduction and dispersal but also in responses to biotic and abiotic stresses. Anthocyanins are recognized as important health-promoting and chronic-disease-preventing components in the human diet. Therefore, interest in developing food crops with improved levels and compositions of these important nutraceuticals is growing. This review focuses on work conducted to elucidate the genetic control of the anthocyanin pathway and modulate anthocyanin content in eggplant (Solanum melongena L.) and tomato (Solanum lycopersicum L.), two solanaceous fruit vegetables of worldwide relevance. While anthocyanin levels in eggplant fruit have always been an important quality trait, anthocyanin-based, purple-fruited tomato cultivars are currently a novelty. As detailed in this review, this difference in the anthocyanin content of the cultivated germplasm has largely influenced genetic studies as well as breeding and transgenic approaches to improve the anthocyanin content/profile of these two important solanaceous crops. The information provided should be of help to researchers and breeders in devising strategies to address the increasing consumer demand for nutraceutical foods.

Bulk Segregant Analysis Sequencing and RNA-Seq Analyses Reveal Candidate Genes Associated with Sepal Color Phenotype of Eggplant (Solanum melongena L.)

Eggplant is a highly significant vegetable crop and extensively cultivated worldwide. Sepal color is considered one of the major commercial traits of eggplant. Eggplant sepals develop from petals, and sepals have the ability to change color by accumulating anthocyanins, but whether the eggplants in sepal and their biosynthetic pathways are the same as those in petals is not known. To date, little is known about the underlying mechanisms of sepal color formation. In this study, we performed bulked segregant analysis and transcriptome sequencing using eggplant sepals and obtained 1,452,898 SNPs and 182,543 InDel markers, respectively, as well as 123.65 Gb of clean data using transcriptome sequencing. Through marker screening, the genes regulating eggplant sepals were localized to an interval of 2.6 cM on chromosome 10 by bulked segregant analysis sequencing and transcriptome sequencing and co-analysis, combined with screening of molecular markers by capillary electrophoresis. Eight possible candidate genes were then screened to further interpret the regulatory incentives for the eggplant sepal color.

Fine Mapping of Candidate Gene Controlling Anthocyanin Biosynthesis for Purple Peel in Solanum melongena L

Fruit color is an intuitive quality of horticultural crops that can be used as an evaluation criterion for fruit ripening and is an important factor affecting consumers' purchase choices. In this study, a genetic population from the cross of green peel 'Qidong' and purple peel '8 guo' revealed that the purple to green color of eggplant peel is dominant and controlled by a pair of alleles. Bulked segregant analysis (BSA), SNP haplotyping, and fine genetic mapping delimited candidate genes to a 350 kb region of eggplant chromosome 10 flanked by markers KA2381 and CA8828. One ANS gene (EGP22363) was predicted to be a candidate gene based on gene annotation and sequence alignment of the 350-kb region. Sequence analysis revealed that a single base mutation of 'T' to 'C' on the exon green peel, which caused hydrophobicity to become hydrophilic serine, led to a change in the three-level spatial structure. Additionally, EGP22363 was more highly expressed in purple peels than in green peels. Collectively, EGP22363 is a strong candidate gene for anthocyanin biosynthesis in purple eggplant peels. These results provide important information for molecular marker-assisted selection in eggplants, and a basis for analyzing the regulatory pathways responsible for anthocyanin biosynthesis in eggplants.

MdbHLH162 connects the gibberellin and jasmonic acid signals to regulate anthocyanin biosynthesis in apple

Anthocyanins are secondary metabolites induced by environmental stimuli and developmental signals. The positive regulators of anthocyanin biosynthesis have been reported, whereas the anthocyanin repressors have been neglected. Although the signal transduction pathways of gibberellin (GA) and jasmonic acid (JA) and their regulation of anthocyanin biosynthesis have been investigated, the cross-talk between GA and JA and the antagonistic mechanism of regulating anthocyanin biosynthesis remain to be investigated. In this study, we identified the anthocyanin repressor MdbHLH162 in apple and revealed its molecular mechanism of regulating anthocyanin biosynthesis by integrating the GA and JA signals. MdbHLH162 exerted passive repression by interacting with MdbHLH3 and MdbHLH33, which are two recognized positive regulators of anthocyanin biosynthesis. MdbHLH162 negatively regulated anthocyanin biosynthesis by disrupting the formation of the anthocyanin-activated MdMYB1-MdbHLH3/33 complexes and weakening transcriptional activation of the anthocyanin biosynthetic genes MdDFR and MdUF3GT by MdbHLH3 and MdbHLH33. The GA repressor MdRGL2a antagonized MdbHLH162-mediated inhibition of anthocyanins by sequestering MdbHLH162 from the MdbHLH162-MdbHLH3/33 complex. The JA repressors MdJAZ1 and MdJAZ2 interfered with the antagonistic regulation of MdbHLH162 by MdRGL2a by titrating the formation of the MdRGL2a-MdbHLH162 complex. Our findings reveal that MdbHLH162 integrates the GA and JA signals to negatively regulate anthocyanin biosynthesis. This study provides new information for discovering more anthocyanin biosynthesis repressors and explores the cross-talk between hormone signals.

SmCIP7, a COP1 interactive protein, positively regulates anthocyanin accumulation and fruit size in eggplant

In higher plants, COP1 (Constitutively Photomorphogenic 1) acts as a central regulator of light-signaling networks and globally conditions the target proteins via the ubiquitin-proteasome pathway. However, the function of COP1-interacting proteins in light-regulated fruit coloration and development remains unknown in Solanaceous plants. Here, a COP1-interacting protein-encoding gene, SmCIP7, expressed specifically in the eggplant (Solanum melongena L.) fruit, was isolated. Gene-specific silencing of SmCIP7 using RNA interference (RNAi) significantly altered fruit coloration, fruit size, flesh browning, and seed yield. SmCIP7-RNAi fruits showed evident repression of the accumulation of anthocyanins and chlorophyll, indicating functional similarities between SmCIP7 and AtCIP7. However, the reduced fruit size and seed yield indicated SmCIP7 had evolved a distinctly new function. With the comprehensive application of HPLC-MS, RNA-seq, qRT-PCR, Y2H, BiFC, LCI, and dual-luciferase reporter system (DLR™), it was found that SmCIP7, a COP1 interactive protein in light signaling promoted anthocyanin accumulation, probably by regulating the transcription of SmTT8. Additionally, the drastic up-regulation of SmYABBY1, a homologous gene of SlFAS, might account for the strongly retarded fruit growth in SmCIP7-RNAi eggplant. Altogether, this study proved that SmCIP7 is an essential regulatory gene to modulate fruit coloration and development, serving as a key gene locus in eggplant molecular breeding.

Integrated Metabolome and Transcriptome Analysis Reveals a Regulatory Network of Fruit Peel Pigmentation in Eggplant (Solanum melongena L.)

The color of fruit peel is an economically important character of eggplant, and black-purple eggplant has received much attention for being rich in anthocyanin. However, the reason why different fruit peel colors form in eggplant is not well understood. In the present study, an integrative analysis of the metabolome and transcriptome profiles was performed in five eggplant varieties with different fruit colors. A total of 260 flavonoids were identified, and most of them showed significantly higher abundance in black-purple varieties than in other varieties. The transcriptome analysis indicated the activation of early phenylpropanoid biosynthesis genes (SmPAL, SmC4H, and Sm4CL) was more responsible for anthocyanin accumulation, while SmF3′5′H was the key factor for the formation of a purple color. Furthermore, two transcription factors, SmGL2 and SmGATA26, were identified as new hub genes associated with anthocyanin accumulation. The silencing of SmGL2 and SmGATA26 reduced anthocyanin accumulation in eggplant fruit peels, suggesting the possible involvement of SmGL2 and SmGATA26 in regulating anthocyanin biosynthesis. In addition, the pathway of plant hormone signal transduction was significantly enriched, indicating that phytohormones may cooperatively interact to modulate flavonoid biosynthesis. This study provides comprehensive information of flavonoid metabolites and new insights into the regulatory network of fruit coloration, which might be useful for the molecular breeding of eggplant.

Genome-wide identification and expression analysis of the bHLH gene family in cauliflower (Brassica oleracea L.)

Basic helix-loop-helix (bHLH) transcription factors (TFs) are one of the largest TF families in plant species, and they play important roles in plant growth, development and stress responses. The present study systematically identified members of the cauliflower (Brassica oleracea L.) bHLH gene family based on genomic data. Analysis of bHLH family gene numbers, evolution, collinearity, gene structures and motifs indicated that cauliflower contained 256 bHLH family genes distributed on 10 chromosomes. Most of these genes have been localized in the nucleus, and they were divided into 18 subgroups which have been relatively conserved during evolution. Promoter analysis showed that most cis-acting elements were related to MeJA and ABA. Expression analysis suggested that 14 bHLH genes may be involved in the transformation of cauliflower curd from white to purple. An expression analysis of these 14 genes in FQ136 material was performed using qRT-PCR, and 9 bHLH genes (BobHLH1, 14, 58, 61, 63, 84, 231, 239 and 243) showed significantly increased or decreased expression in cauliflower from white to purple, which suggests that these 9 genes play important roles in the accumulation of anthocyanins in cauliflower. The coexpression network of these 9 genes and anthocyanin synthesis-related key genes was analyzed using weighted gene coexpression network analysis (WGCNA). In conclusion, our observations suggested that the bHLH gene family plays an important role in the accumulation of anthocyanins in cauliflower and provide an important theoretical basis for further research on the functions of the bHLH gene family and the molecular mechanism of cauliflower coloration.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-022-01238-9.

Integrating Transcriptomic and Metabolomic Analyses to Explore the Effect of Color Under Fruit Calyx on That of Fruit Apex in Eggplant (Solanum melongena L.)

Fruit color is an important commercial characteristic of eggplant (Solanum melongena L.), which affects both the profits of growers and consumer choice. Two eggplant inbred lines were discovered: “Z,” which is a light purple color under the fruit calyx, with purple on the fruit apex; and “L,” fruits of which are green under the calyx and at the apex. To determine the molecular mechanisms underlying the effect of fruit peel color under the calyx on that at the fruit apex, we conducted a combined transcriptomic and metabolomic analyses of the Z and L inbred eggplant lines. Transcriptome analysis of peel samples from three fruit regions (under the calyx, the apex, and the middle surface) of each line was conducted by RNA sequencing, and generated a total of 791,512,404 clean reads from 18 samples (three biological replicates). Differentially expressed genes (DEGs; n = 424) were identified in comparisons of peel samples from the three sites of L line fruits. Gene ontology analysis showed that “catalytic activity” was extremely significantly enriched. Further, DEGs (n = 8) were enriched in the Kyoto Encyclopedia of Genes and Genomes pathway “flavonoid biosynthesis.” Levels of CHI, LDOX, F3′5′H, and dihydroflavonol reductase were higher in the Z line than the L line. In addition, metabolome analysis showed that, 10 differentially accumulated metabolites were detected between peel samples from the apex of L and Z line fruit. The most significant DAM was delphinidin-3-O-rutinoside (Z line content, 34.89 μg/g vs. L line content 0.01 μg/g). Combined transcriptomic and metabolomic analyses indicated that DFR and F3′5′H were closely related to content of the metabolites, cyanidin and delphinidin, and that some downstream metabolites differed significantly between the L and Z lines. Content levels of delphinidin-3-O-rutinoside, delphinidin-3-O-glucoside, cyanidin-3-O-glucoside, and cyanidin-3-O-rutinoside were markedly down-regulated in the L line. Altogether, increased CHI levels could up-regulate the downstream genes, LDOX, F3′5′H, and DFR, which further lead to increasing the content of delphindin. Thus, the uniform purple color was presented at the apex of fruits in Z plants. These findings not only identify key candidate genes, but will also improve understanding of the genetics and the efficiency of breeding for eggplant fruit color.

Genome–Wide Identification and Functional Characterization of Auxin Response Factor (ARF) Genes in Eggplant

Auxin response factors (ARFs) are important plant transcription factors that are differentially expressed in response to auxin and various abiotic stresses. ARFs play important roles in mediating plant growth and stress responses; however, these factors have not been studied in eggplants. In this study, genome–wide identification and the functional analysis of the ARF gene family in eggplants (Solanum melongena L.) were performed. A total of 20 ARF (SmARF) genes were identified and phylogenetically classified into three groups. Our analysis revealed four functional domains and 10 motifs in these proteins. Subcellular localization showed that the SmARFs localized in the nucleus. To investigate the biological functions of the SmARFs under 2,4–D and salt stress treatments, quantitative real–time RT–PCR (qRT–PCR) was conducted. Most SmARF genes exhibited changes in expression in response to 2,4–D treatments in the flowers, especially SmARF4 and 7B. All SmARF genes quickly responded to salt stress, except SmARF17 and 19 in leaves, SmARF1A and 7B in roots, and SmARF2A, SmARF7B, and SmARF16B in stems. These results helped to elucidate the role of ARFs in auxin signaling under 2,4–D and salt stress in eggplants.

SmMYB113 Is a Key Transcription Factor Responsible for Compositional Variation of Anthocyanin and Color Diversity Among Eggplant Peels

To understand the color formation mechanism in eggplant (Solanum melongena L.) peel, a metabolomic analysis was performed in six cultivars with different peel colors. A total of 167 flavonoids, including 16 anthocyanins, were identified based on a UPLC-MS/MS approach. Further analysis revealed that the delphinidins/flavonoids ratio was consistent with the purple coloration of eggplant peels, and SmF3'5'H expression level was consistent with the delphinidin 3-O-glucoside and delphinidin 3-O-rutinoside contents, the main anthocyanins in the purple-peels eggplant cultivars identified in this study. SmMYB113 overexpression promoted anthocyanins accumulation in eggplant peels and pulps. Metabolomic analysis revealed that delphinidins were still the main anthocyanins class in the peels and pulps of SmMYB113-OE4, but most anthocyanins were glycosylated at the 5-position of the B-ring. Our results provide new insights into the anthocyanin composition of eggplant peels and demonstrate the importance of SmMYB113 in stimulating anthocyanin biosynthesis in eggplant fruits.