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Fruit skin color is an important characteristic of fruit quality. The light-mediated regulation on fruit skin coloration in Actinidia arguta remains unclear. To better understand the role of light in fruit skin coloration, we performed bagging treatments in both on-tree and off-tree ‘Hongbaoshixing’, which is a kind of all-red-typed A. arguta cultivar. Non-bagging kiwifruits were used as control. For off-tree fruits, there was no difference between non-bagging and bagging treatments. For on-tree fruits, physiological and molecular changes were investigated during fruit development in non-bagging and bagging treatments. Phenotypic identification and the hue angle measurement showed that the stage of most significant color difference between non-bagging and bagging treatments was 130 days (after full bloom). Determination of five anthocyanin components suggested cyanidin-3-O-galactoside and cyanidin-3-O-xylose-galactoside made a main contribution to the fruit skin coloration. Gene expression profiles and cluster analysis showed AaLDOX and AaUFGT were highly expressed at 130 days and obviously clustered into the same class in non-bagging treatment, respectively. Correlation analysis suggested only AaLDOX expression was significantly correlated with anthocyanin content in non-bagging treatment while no correlation in bagging treatment. Similar results was observed for MYB1 transcription factor. The result of subcellular localization showed that AaLDOX was located in the cytoplasm, indicating AaLDOX is indeed structural gene that encodes leucoanthocyanidin dioxygenase participated in anthocyanin biosynthesis. All results were used to establish a possible working model, showing that light is indispensable for normal fruit skin coloration, and bagging treatment suppresses anthocyanin biosynthesis and accumulation mainly by inhibiting AaMYB1 and AaLDOX expression in A. arguta.

Rice is a model system for monocot but the molecular features of rice flavonoid biosynthesis have not been extensively characterized. Rice structural gene homologs encoding chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H), dihydroflavonol 4-reductase (DFR), and anthocyanidin synthase (ANS) were identified by homology searches. Unique differential expression of OsF3H, OsDFR, and OsANS1 controlled by the Pl w locus, which contains the R/B-type regulatory genes OSB1 and OSB2, was demonstrated during light-induced anthocyanin accumulation in T65-Plw seedlings. Previously, F3H genes were often considered as early genes co-regulated with CHS and CHI genes in other plants. In selected non-pigmented rice lines, OSB2 is not expressed following illumination while their expressed OSB1sequences all contain the same nucleotide change leading to the T⁶⁴ M substitution within the conserved N-terminal interacting domain. Furthermore, the biochemical roles of the expressed rice structural genes (OsCHS1, OsCHI, OsF3H, and OsF3'H) were established in planta for the first time by complementation in the appropriate Arabidopsis transparent testa mutants. Using yeast two-hybrid analysis, OsCHS1 was demonstrated to interact physically with OsF3H, OsF3'H, OsDFR, and OsANS1, suggesting the existence of a macromolecular complex for anthocyanin biosynthesis in rice. Finally, flavones were identified as the major flavonoid class in the non-pigmented T65 seedlings in which the single-copy OsF3H gene was not expressed. Competition between flavone and anthocyanin pathways was evidenced by the significant reduction of tricin accumulation in the T65-Plw seedlings.

BACKGROUND AND AIMS: When sunlight was excluded (opacity coefficient 100%) from fruitset until maturity, the red grape cultivar Jingxiu failed to colour while Jingyan (Jingxiu × white grape Xiangfei) developed red colour. The aim of this study was to gain insight into the genetic background that may be responsible for sunlight‐dependent versus sunlight‐independent anthocyanin biosynthesis in berry skin. METHODS AND RESULTS: The composition and concentration of anthocyanins via high‐performance liquid chromatography, and the transcriptional level of structural and regulatory genes via real‐time polymerase chain reaction were investigated. Light exclusion suppressed all the anthocyanins in Jingxiu berry skin, but did not change the proportion of the various anthocyanins in Jingyan. UDP‐glucose:flavonoid 3‐O‐glucosyltransferase (UFGT) and a v‐myb myeloblastosis viral oncogene homolog transcription factor (VvMYBA1) were expressed at a high level in sunlight‐exposed Jingxiu, sunlight‐exposed and excluded Jingyan, but not in sunlight‐excluded Jingxiu, which correlated with phenotypic colouration (red vs not red). There was no difference in the DNA sequence of the VvMYBA1b promoter region between Jingxiu and Jingyan. CONCLUSIONS: The regulatory gene VvMYBA1 is involved in the regulation of anthocyanin biosynthesis via the structural gene UFGT, while there must be some other regulatory factors or post‐transcriptional regulation mechanisms that differentially regulated VvMYBA1 in Jingxiu and Jingyan in response to sunlight. SIGNIFICANCE OF THE STUDY: The primary control mechanism of anthocyanin biosynthesis in the absence of sunlight would provide information for breeding sunlight‐independent red grape cultivars, which would be valuable for cultivation in areas with low sunlight, e.g. glasshouses or other covered cropping.

Key messageThis review contains functional roles of MYB transcription factors in the transcriptional regulation of anthocyanin biosynthesis in horticultural plants. This review describes potential uses of MYB TFs as tools for metabolic engineering for anthocyanin production.Anthocyanins (ranging from red to blue) are controlled by specific branches of the anthocyanin biosynthetic pathway and are mostly visible in ornamentals, fruits, and vegetables. In the present review, we describe which R2R3-MYB transcription factors (TFs) control the transcriptional regulation of anthocyanin structural genes involved in the specific branches of the anthocyanin biosynthetic pathway in various horticultural plants (e.g., ornamentals, fruits, and vegetables). In addition, some MYBs responsible for anthocyanin accumulation in specific tissues are described. Moreover, we highlight the phylogenetic relationships of the MYBs that suppress or promote anthocyanin synthesis in horticultural crops. Enhancement of anthocyanin synthesis via metabolic genetic engineering of anthocyanin MYBs, which is described in the review, is indicative of the potential use of the mentioned anthocyanin-related MYBs as tools for anthocyanin production. Therefore, the MYBs would be suitable for metabolic genetic engineering for improvement of flower colors, fruit quality, and vegetable nutrients.

Proanthocyanidins (PAs) are major defense phenolic compounds in the leaves of poplar (Populus spp.) in response to abiotic and biotic stresses. Transcriptional regulation of PA biosynthetic genes by the MYB-basic helix–loop–helix (bHLH)-WD40 complexes in poplar is not still fully understood. Here, an Arabidopsis TT2-like gene MYB115 was isolated from Populus tomentosa and characterized by various molecular, genetic and biochemical approaches. MYB115 restored PA productions in the seed coat of the Arabidopsis tt2 mutant. Overexpression of MYB115 in poplar activated expression of PA biosynthetic genes, resulting in a significant increase in PA concentrations. By contrast, the CRISPR/Cas9-generated myb115 mutant exhibited reduced PA content and decreased expression of PA biosynthetic genes. MYB115 directly activated the promoters of PA-specific structural genes. MYB115 interacted with poplar TT8. Coexpression of MYB115, TT8 and poplar TTG1 significantly enhanced the expression of ANR1 and LAR3. Additionally, transgenic plants overexpressing MYB115 had increased resistance to the fungal pathogen Dothiorella gregaria, whereas myb115 mutant exhibited greater sensitivity compared with wild-type plants. Our data provide insight into the regulatory mechanisms controlling PA biosynthesis by MYB115 in poplar, which could be effectively employed for metabolic engineering of PAs to improve resistance to fungal pathogens.

Kiwifruit (Actinidia spp.) is a climacteric fruit with high sensitivity to ethylene, influenced by multiple ethylene-responsive structural genes and transcription factors. However, the roles of other post-transcriptional regulators (e.g. miRNAs) necessary for ripening remain elusive. High-throughput sequencing sRNAome, degradome and transcriptome methods were used to identify further contributors to ripening control in the kiwifruit (A. deliciosa cv ‘Hayward’). Two NAM/ATAF/CUC domain transcription factors (AdNAC6 and AdNAC7), both predicted targets for miR164, showed significant upregulation by exogenous ethylene. Gene expression analysis and luciferase reporter assays indicated that Ade-miR164 and one of its precursor miRNAs (Ade-MIR164b) were repressed by ethylene treatment and negatively correlated with AdNAC6/7 expression. Subsequent analysis indicated that both AdNAC6 and AdNAC7 proteins are transcriptional activators and physically bind the promoters of AdACS1 (1-aminocyclopropane-1-carboxylate synthase), AdACO1 (1-aminocyclopropane-1-carboxylic acid oxidase), AdMAN1 (endo-β-mannanase) and AaTPS1 (terpene synthase). Moreover, subcellular analysis indicated that the location of the AdNAC6/7 proteins was influenced by Ade-miR164. Multiple omics-based approaches revealed a novel regulatory link for fruit ripening that involved ethylene-miR164-NAC. The regulatory pathway for miR164-NAC is present in various fruit (e.g. Rosaceae fruit, citrus, grape), with implications for fruit ripening regulation.

Iron (Fe) homeostasis is crucial for all living organisms. In mammals, an integrated posttranscriptional mechanism couples the regulation of both Fe deficiency and Fe excess responses. Whether in plants an integrated control mechanism involving common players regulates responses both to deficiency and to excess is still to be determined. In this study, molecular, genetic and biochemical approaches were used to investigate transcriptional responses to both Fe deficiency and excess. A transcriptional activator of responses to Fe shortage in Arabidopsis, called bHLH105/ILR3, was found to also negatively regulate the expression of ferritin genes, which are markers of the plant’s response to Fe excess. Further investigations revealed that ILR3 repressed the expression of several structural genes that function in the control of Fe homeostasis. ILR3 interacts directly with the promoter of its target genes, and repressive activity was conferred by its dimerisation with bHLH47/PYE. Last, this study highlighted that important facets of plant growth in response to Fe deficiency or excess rely on ILR3 activity. Altogether, the data presented herein support that ILR3 is at the centre of the transcriptional regulatory network that controls Fe homeostasis in Arabidopsis, in which it acts as both transcriptional activator and repressor.

Camellia japonica petals are colorful, rich in anthocyanins, and possess important ornamental, edible, and medicinal value. However, the regulatory mechanism of anthocyanin accumulation in C. japonica is still unclear. In this study, an integrative analysis of the metabolome and transcriptome was conducted in five C. japonica cultivars with different petal colors. Overall, a total of 187 flavonoids were identified (including 25 anthocyanins), and 11 anthocyanins were markedly differentially accumulated among these petals, contributing to the different petal colors in C. japonica . Moreover, cyanidin-3- O- (6 ″ - O- malonyl) glucoside was confirmed as the main contributor to the red petal phenotype, while cyanidin-3- O- rutinoside, peonidin-3- O- glucoside, cyanidin-3- O- glucoside, and pelargonidin-3- O- glucoside were responsible for the deep coloration of the C. japonica petals. Furthermore, a total of 12,531 differentially expressed genes (DEGs) and overlapping DEGs (634 DEGs) were identified by RNA sequencing, and the correlation between the expression level of the DEGs and the anthocyanin content was explored. The candidate genes regulating anthocyanin accumulation in the C. japonica petals were identified and included 37 structural genes (especially CjANS and Cj4CL ), 18 keys differentially expressed transcription factors (such as GATA , MYB , bHLH , WRKY , and NAC ), and 16 other regulators (mainly including transporter proteins, zinc-finger proteins, and others). Our results provide new insights for elucidating the function of anthocyanins in C. japonica petal color expression.

The MYB gene family is one of the largest groups of transcription factors (TFs) playing diverse roles in several biological processes. Hedychium coronarium (white ginger lily) is a renowned ornamental plant both in tropical and subtropical regions due to its flower shape and strong floral scent mainly composed of terpenes and benzenoids. However, there is no information available regarding the role of the MYB gene family in H. coronarium . In the current study, the MYB gene family was identified and extensively analyzed. The identified 253 HcMYB genes were unevenly mapped on 17 chromosomes at a different density. Promoter sequence analysis showed numerous phytohormones related to cis -regulatory elements. The majority of HcMYB genes contain two to three introns and motif composition analysis showed their functional conservation. Phylogenetic analysis revealed that HcMYBs could be classified into 15 distinct clades, and the segmental duplication events played an essential role in the expansion of the HcMYB gene family. Tissue-specific expression patterns of HcMYB genes displayed spatial and temporal expression. Furthermore, seven HcMYB ( HcMYB7/8/75/79/145/238/248 ) were selected for further investigation. Through RT-qPCR, the response of candidates HcMYB genes toward jasmonic acid methyl ester (MeJA), abscisic acid (ABA), ethylene, and auxin was examined. Yeast one-hybrid (Y1H) assays revealed that candidate genes directly bind to the promoter of bottom structural volatile synthesis genes ( HcTPS1 , HcTPS3 , HcTPS10 , and HcBSMT2 ). Moreover, yeast two-hybrid (Y2H) assay showed that HcMYB7/8/75/145/248 interact with HcJAZ1 protein. In HcMYB7/8/79/145/248 -silenced flowers, the floral volatile contents were decreased and downregulated the expression of key structural genes, suggesting that these genes might play crucial roles in floral scent formation in H. coronarium by regulating the expression of floral scent biosynthesis genes. Collectively, these findings indicate that HcMYB genes might be involved in the regulatory mechanism of terpenoids and benzenoid biosynthesis in H. coronarium .

FaMYB44.2 is a novel transcriptional repressor that modulates both ripening-related and jasmonic acid-related sucrose accumulation in strawberry receptacles. Abstract Sugar and acid metabolism are critical for fruit ripening and quality formation, but the underlying regulatory mechanisms are largely unknown. Here, we identified a transcriptional repressor, FaMYB44.2, that regulates sugar and acid accumulation in strawberry (Fragaria × ananassa 'Benihoppe') receptacles. We transiently expressed FaMYB44.2 in strawberry fruit and conducted metabolic and molecular analyses to explore the role of FaMYB44.2 in sugar and acid accumulation in strawberry. We found that FaMYB44.2 negatively regulates soluble sugar accumulation and malic acid content and represses the expression of numerous structural genes, including FaSPS3, a key gene in sucrose accumulation. From the white fruit stage onwards, the repressive effect of FaMYB44.2 on FaSPS3 is reversed by FaMYB10, which positively regulates anthocyanin accumulation. Our results indicate that FaMYB10 suppresses FaMYB44.2 expression; weakens the interaction between FaMYB44.2 and its co-repressor, FabHLH3; and cooperates with FabHLH3 to activate the expression of FaSPS3. The interplay between FaMYB10 and FaMYB44.2 results in sucrose accumulation in ripe strawberry fruits. In addition, the repressive effect of FaMYB44.2 on sucrose accumulation is enhanced by jasmonic acid. This study provides new insights into the regulatory mechanisms of sucrose accumulation and sheds light on the interplay between regulatory proteins during strawberry fruit ripening and quality formation.