A bHLH transcription factor, DvIVS, is involved in regulation of anthocyanin synthesis in dahlia (Dahlia variabilis)

Anthocyanin biosynthesis in goji berry is inactivated by deletion in a bHLH transcription factor LrLAN1b promoter

Black goji berry (Lycium ruthenicum Murray) contains a rich source of health-promoting anthocyanins which are used in herbal medicine and nutraceutical foods in China. A natural variant producing white berries allowed us to identify two key genes involved in the regulation of anthocyanin biosynthesis in goji berries: one encoding a MYB transcription factor (LrAN2-like) and one encoding a basic helix-loop-helix (bHLH) transcription factor (LrAN1b). We previously found that LrAN1b expression was lost in the white berry variant, but the molecular basis for this phenotype was unknown. Here, we identified the molecular mechanism for loss of anthocyanins in white goji berries. In white goji, the LrAN1b promoter region has a 229 bp deletion that removes three MYB-binding elements and one bHLH-binding element, which are key to its expression. Complementation of the white goji berry LrAN1b allele with the LrAN1b promoter restored pigmentation. Virus-induced gene silencing of LrAN1b in black goji berry reduced fruit anthocyanin biosynthesis. Molecular analyses showed that LrAN2-like and another bHLH transcription factor LrJAF13 can activate LrAN1b by binding directly to the MYB-recognizing element and bHLH-recognizing element of its promoter-deletion region. LrAN1b expression is enhanced by the interaction of LrAN2-like with LrJAF13 and the WD40 protein LrAN11. LrAN2-like and LrAN11 interact with either LrJAF13 or LrAN1b to form two MYB-bHLH-WD40 complexes, which hierarchically regulate anthocyanin biosynthesis in black goji berry. This study on a natural variant builds a comprehensive anthocyanin regulatory network that may be manipulated to tailor goji berry traits.

Multi-omics analysis reveals key regulatory defense pathways and genes involved in salt tolerance of rose plants

Salinity stress causes serious damage to crops worldwide, limiting plant production. However, the metabolic and molecular mechanisms underlying the response to salt stress in rose (Rosa spp.) remain poorly studied. We therefore performed a multi-omics investigation of Rosa hybrida cv. Jardin de Granville (JDG) and Rosa damascena Mill. (DMS) under salt stress to determine the mechanisms underlying rose adaptability to salinity stress. Salt treatment of both JDG and DMS led to the buildup of reactive oxygen species (H2O2). Palisade tissue was more severely damaged in DMS than in JDG, while the relative electrolyte permeability was lower and the soluble protein content was higher in JDG than in DMS. Metabolome profiling revealed significant alterations in phenolic acid, lipids, and flavonoid metabolite levels in JDG and DMS under salt stress. Proteome analysis identified enrichment of flavone and flavonol pathways in JDG under salt stress. RNA sequencing showed that salt stress influenced primary metabolism in DMS, whereas it substantially affected secondary metabolism in JDG. Integrating these datasets revealed that the phenylpropane pathway, especially the flavonoid pathway, is strongly enhanced in rose under salt stress. Consistent with this, weighted gene coexpression network analysis (WGCNA) identified the key regulatory gene chalcone synthase 1 (CHS1), which is important in the phenylpropane pathway. Moreover, luciferase assays indicated that the bHLH74 transcription factor binds to the CHS1 promoter to block its transcription. These results clarify the role of the phenylpropane pathway, especially flavonoid and flavonol metabolism, in the response to salt stress in rose.

Identification of two 6'-deoxychalcone 4'-glucosyltransferase genes in dahlia (Dahlia variabilis)

MAIN CONCLUSION

Two glycosyltransferase genes belonging to UGT88 family were identified to have 6'-deoxychalcone 4'-glucosyltransferase activity in dahlia. 6'-Deoxychalcones (isoliquiritigenin and butein) are important pigments for yellow and orange to red flower color. 6'-Deoxychalcones are glucosylated at the 4'-position in vivo, but the genes encoding 6'-deoxychalcone 4'-glucosyltransferase have not yet been identified. In our previous study, it was indicated that snapdragon (Antirrhinum majus) chalcone 4'-O-glucosyltransferase (Am4'CGT) has isoliquiritigenin 4'-glucosylation activity. Therefore, to identify genes encoding 6'-deoxychalcone 4'-glucosyltransferase in dahlia (Dahlia variabilis), genes expressed in ray florets that shared high homology with Am4'CGT were explored. As a result, c34671_g1_i1 and c35662_g1_i1 were selected as candidate genes for 6'-deoxychalcone 4'-glucosyltransferases in dahlia. We conducted transient co-overexpression of three genes (c34671_g1_i1 or c35662_g1_i1, dahlia aldo-keto reductase1 (DvAKR1) or soybean (Glycine max) chalcone reductase5 (GmCHR5), and chili pepper (Capsicum annuum) MYB transcription factor (CaMYBA)) in Nicotiana benthamiana by agroinfiltration. Transient overexpression of c34671_g1_i1, DvAKR1, and CaMYBA resulted in increase in the accumulation of isoliquiritigenin 4'-glucosides, isoliquiritigenin 4'-O-glucoside, and isoliquiritigenin 4'-O-[6-O-(malonyl)-glucoside]. However, transient overexpression of c35662_g1_i1, DvAKR1, and CaMYBA did not increase accumulation of isoliquiritigenin 4'-glucosides. Using GmCHR5 instead of DvAKR1 showed similar results suggesting that c34671_g1_i1 has isoliquiritigenin 4'-glucosyltransferase activity. In addition, we conducted co-overexpression of four genes (c34671_g1_i1, c35662_g1_i1 or Am4'CGT, DvAKR1 or GmCHR5, CaMYBA, and chalcone 3-hydroxylase from dahlia). Accumulation of butein 4'-O-glucoside and butein 4'-O-[6-O-(malonyl)-glucoside] was detected for c35662_g1_i1, suggesting that c35662_g1_i1 has butein 4'-glucosyltransferase activity. Recombinant enzyme analysis also supported butein 4'-glucosyltransferases activity of c35662_g1_i1. Therefore, our results suggested that both c34671_g1_i1 and c35662_g1_i1 are 6'-deoxychalcone 4'-glucosyltransferases but with different substrate preference.

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.

Seed color in lettuce is determined by the LsTT2, LsCHS, and Ls2OGD genes from the flavonoid biosynthesis pathway

KEY MESSAGE

The mutated LsTT2 and Ls2OGD genes are responsible for white seeds and yellow seeds in lettuce, respectively. Three LsCHS genes are involved in the biosynthesis of flavonoid in seed coats. Lettuce seeds have several different colors, including black, yellow, and white. The genetic mechanisms underlying color variations of lettuce seeds remain unknown. We used genome-wide association studies (GWAS) and map-based cloning approaches to clone genes controlling the color of lettuce seeds. LsTT2, which encodes an R2R3-MYB transcription factor and is homologous to the TT2 gene in Arabidopsis, was shown to be the causal gene for the variation of black and white seeds in lettuce. A point mutation leads to the lack of stop codon in the LsTT2 transcript, resulting in white seeds. Knockout of the LsTT2 gene converted black seeds to white seeds. The locus controlling yellow seeds was mapped to Chromosome 2. Knockout of two 2-oxoglutarate-dependent dioxygenases (2OGD) genes from the candidate region converted black seeds to yellow seeds, suggesting that these two 2OGD proteins catalyze the conversion of yellow metabolites to black metabolites. We also showed that three LsCHS genes from the candidate region are associated with flavonoid biosynthesis in seeds. Knockout mutants of the three LsCHS genes decreased color intensity. This study provides new insights into the regulation of flavonoid biosynthesis in plants.

Comparative transcriptome analysis and flavonoid profiling of floral mutants reveals CmMYB11 regulating flavonoid biosynthesis in chrysanthemum

Flavonoids, of which the major groups are flavones, flavonols, and anthocyanins, confer a variety of colors on plants. Bud sports with variation of floral colors occur occasionally during chrysanthemum cultivation. Although it has been reported that methylation at the promoter of CmMYB6 was related to anthocyanin contents, the regulatory networks of flavonoid biosynthesis still remain largely unknown in mutation of chrysanthemum. We compared phenotypes, pigment composition and transcriptomes in two chrysanthemum cultivars, 'Anastasia Dark Green' and 'Anastasia Pink', and regenerated bud sports of these cultivars with altered floral colors. Increased anthocyanins turned the 'Anastasia Dark Green' mutant red, while decreased anthocyanins turned the 'Anastasia Pink' mutant white. Moreover, total flavonoids were reduced in both mutants. Multiple flavonoid biosynthetic genes and regulatory genes encoding MYBs and bHLHs transcription factors were differentially expressed in pairwise comparisons of transcriptomes in 'Anastasia Dark Green' or 'Anastasia Pink' and their mutants at different flowering stages. Among these regulatory genes, the expression patterns of CmMYB6 and CmbHLH2 correlated to changes of anthocyanin contents, and down-regulation of CmMYB11 correlated to decreased total flavonoid contents in two mutants. CmMYB11 was shown to directly activate the promoter activities of CmCHS2, CmCHI, CmDFR, CmANS, CmFNS, and CmFLS. Furthermore, overexpression of CmMYB11 increased both flavonols and anthocyanins in tobacco petals. Our work provides new insights into regulatory networks involved in flavonoid biosynthesis and coloration in chrysanthemum.

Multilayered regulation of secondary metabolism in medicinal plants

Medicinal plants represent a huge reservoir of secondary metabolites (SMs), substances with significant pharmaceutical and industrial potential. However, obtaining secondary metabolites remains a challenge due to their low-yield accumulation in medicinal plants; moreover, these secondary metabolites are produced through tightly coordinated pathways involving many spatiotemporally and environmentally regulated steps. The first regulatory layer involves a complex network of transcription factors; a second, more recently discovered layer of complexity in the regulation of SMs is epigenetic modification, such as DNA methylation, histone modification and small RNA-based mechanisms, which can jointly or separately influence secondary metabolites by regulating gene expression. Here, we summarize the findings in the fields of genetic and epigenetic regulation with a special emphasis on SMs in medicinal plants, providing a new perspective on the multiple layers of regulation of gene expression.

Pre and postharvest characteristics of Dahlia pinnata var. pinnata, cav. As affected by SiO2 and CaCO3 nanoparticles under two different planting dates

Agriculture faces many challenges because of climate changes. The nutrients present in nano-sized form improve plant productivity, especially when used at the appropriate planting time. Field experiments were conducted as a factorial experiment for evaluating two planting dates (20th September and 20th October), foliar application with nanoparticles (NPs) including silica nanoparticles (SiO₂-NPs) at 1.5 and 3 mM, calcium carbonate nanoparticles (CaCO₃-NPs) at 5 and 10 mM and distilled water (control) on pre- and post-harvest characteristics of Dahlia pinnata var. pinnata Cav. The results indicate that the interactions during the late planting time (20th October) and exogenous applications of SiO₂-NPs at 1.5 mM or CaCO₃-NPs at 10 mM have improved plant growth including plant height, stem diameter, fresh and dry weights of plant, leaf area, inflorescence diameter, inflorescence stalk length, branches number, tuber numbers, inflorescences number on the plant, and the vase life. At the same time, insignificant differences appeared in the interaction during the planting dates and SiO₂ or CaCO₃ -NPs concentrations on inflorescence stalk diameter, total soluble solids, membrane stability index, maximum increase in fresh weight (FW), and Si and Ca contents. In addition, all exogenous applications of NPs at the late planting time promoted the plant growth characteristics like lignin %, cellulose %, inflorescence water content, change in FW, and total water uptake. Moreover, the controls through the two planting dates recorded the maximum change in water uptake and water loss values. In short, it can be recommended to use SiO₂-NPs at 1.5 mM or CaCO₃-NPs at 10 mM as a foliar application at the late planting time (20th October) for obtaining the optimum quantitative and qualitative parameters of D. pinnata.

Genome-wide identification, characterization, and validation of the bHLH transcription factors in grass pea

Background: The basic helix-loop-helix (bHLH) transcription factor is a vital component in plant biology, with a significant impact on various aspects of plant growth, cell development, and physiological processes. Grass pea is a vital agricultural crop that plays a crucial role in food security. However, the lack of genomic information presents a major challenge to its improvement and development. This highlights the urgency for deeper investigation into the function of bHLH genes in grass pea to improve our understanding of this important crop.

Results: The identification of bHLH genes in grass pea was performed on a genome-wide scale using genomic and transcriptomic screening. A total of 122 genes were identified as having conserved bHLH domains and were functionally and fully annotated. The LsbHLH proteins could be classified into 18 subfamilies. There were variations in intron-exon distribution, with some genes lacking introns. The cis-element and gene enrichment analyses showed that the LsbHLHs were involved in various plant functions, including response to phytohormones, flower and fruit development, and anthocyanin synthesis. A total of 28 LsbHLHs were found to have cis-elements associated with light response and endosperm expression biosynthesis. Ten conserved motifs were identified across the LsbHLH proteins. The protein-protein interaction analysis showed that all LsbHLH proteins interacted with each other, and nine of them displayed high levels of interaction. RNA-seq analysis of four Sequence Read Archive (SRA) experiments showed high expression levels of LsbHLHs across a range of environmental conditions. Seven highly expressed genes were selected for qPCR validation, and their expression patterns in response to salt stress showed that LsbHLHD4, LsbHLHD5, LsbHLHR6, LsbHLHD8, LsbHLHR14, LsbHLHR68, and LsbHLHR86 were all expressed in response to salt stress.

Conclusion: The study provides an overview of the bHLH family in the grass pea genome and sheds light on the molecular mechanisms underlying the growth and evolution of this crop. The report covers the diversity in gene structure, expression patterns, and potential roles in regulating plant growth and response to environmental stress factors in grass pea. The identified candidate LsbHLHs could be utilized as a tool to enhance the resilience and adaptation of grass pea to environmental stress.

PlgMYBR1, an R2R3-MYB transcription factor, plays as a negative regulator of anthocyanin biosynthesis in Platycodon grandiflorus

Floral color plays a major role in pollinator specificity, and changes in color may result in pollinator shifts and pollinator-mediated speciation. In the purple flowers of Platycodon grandiflorus, anthocyanins are the major pigment metabolites, whereas white flowers result due to the absence of anthocyanins. The lack of anthocyanins may be due to the inhibition of the anthocyanin biosynthesis pathway. However, the molecular mechanism of anthocyanin biosynthesis in P. grandiflorus is not fully understood. Hence, we identified R2R3-MYB transcription factor, PlgMYBR1, as a negative regulator for anthocyanin biosynthesis using sequence homology and tissue-specific expression pattern analyses. A heterologous co-expression assay suggested that PlgMYBR1 inhibited the function of AtPAP1 (Arabidopsis thaliana production of anthocyanin pigment 1), indicating that PlgMYBR1 plays as a repressor of anthocyanin biosynthesis in P. grandiflorus. Our results provide a foundation for future efforts to understand the anthocyanin biosynthesis in P. grandiflorus and, thereby, to improve flower color through genetic engineering.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03490-6.

Basic Helix-Loop-Helix Transcription Factors: Regulators for Plant Growth Development and Abiotic Stress Responses

Plant basic helix-loop-helix (bHLH) transcription factors are involved in many physiological processes, and they play important roles in the abiotic stress responses. The literature related to genome sequences has increased, with genome-wide studies on the bHLH transcription factors in plants. Researchers have detailed the functionally characterized bHLH transcription factors from different aspects in the model plant Arabidopsis thaliana, such as iron homeostasis and abiotic stresses; however, other important economic crops, such as rice, have not been summarized and highlighted. The bHLH members in the same subfamily have similar functions; therefore, unraveling their regulatory mechanisms will help us to identify and understand the roles of some of the unknown bHLH transcription factors in the same subfamily. In this review, we summarize the available knowledge on functionally characterized bHLH transcription factors according to four categories: plant growth and development; metabolism synthesis; plant signaling, and abiotic stress responses. We also highlight the roles of the bHLH transcription factors in some economic crops, especially in rice, and discuss future research directions for possible genetic applications in crop breeding.

Transcriptomic and metabolomic analysis of autumn leaf color change in Fraxinus angustifolia

Fraxinus angustifolia is a type of street tree and shade tree with ornamental value. It has a beautiful shape and yellow or reddish purple autumn leaves, but its leaf color formation mechanism and molecular regulation network need to be studied. In this study, we integrated the metabolomes and transcriptomes of stage 1 (green leaf) and stage 2 (red-purple leaf) leaves at two different developmental stages to screen differential candidate genes and metabolites related to leaf color variation. The results of stage 1 and stage 2 transcriptome analysis showed that a total of 5,827 genes were differentially expressed, including 2,249 upregulated genes and 3,578 downregulated genes. Through functional enrichment analysis of differentially expressed genes, we found that they were involved in flavonoid biosynthesis, phenylpropanoid biosynthesis, pigment metabolism, carotene metabolism, terpenoid biosynthesis, secondary metabolite biosynthesis, pigment accumulation, and other biological processes. By measuring the metabolites of Fraxinus angustifolia leaves, we found the metabolites closely related to the differentially expressed genes in two different periods of Fraxinus angustifolia, among which flavonoid compounds were the main differential metabolites. Through transcriptome and metabolomics data association analysis, we screened nine differentially expressed genes related to anthocyanins. Transcriptome and qRT-PCR results showed that these nine genes showed significant expression differences in different stages of the sample, and we speculate that they are likely to be the main regulatory factors in the molecular mechanism of leaf coloration. This is the first time that we have analyzed the transcriptome combination metabolome in the process of leaf coloration of Fraxinus angustifolia, which has important guiding significance for directional breeding of colored-leaf Fraxinus species and will also give new insights for enriching the landscape.

A novel aldo-keto reductase gene is involved in 6'-deoxychalcone biosynthesis in dahlia (Dahlia variabilis)

A novel gene belonging to the aldo-keto reductase 13 family is involved in isoliquiritigenin biosynthesis in dahlia. The yellow pigments of dahlia flowers are derived from 6'-deoxychalcones, which are synthesized via a two-step process, involving the conversion of 3-malonyl-CoA and 4-coumaloyl-CoA into isoliquiritigenin in the first step, and the subsequent generation of butein from isoliquiritigenin. The first step reaction is catalyzed by chalcone synthase (CHS) and aldo-keto reductase (AKR). AKR has been implicated in the isoflavone biosynthesis in legumes, however, isolation of butein biosynthesis related AKR members are yet to be reported. A comparative RNA-seq analysis between two dahlia cultivars, 'Shukuhai' and its butein-deficient lateral mutant 'Rinka', was used in this study to identify a novel AKR gene involved in 6'-deoxychalcone biosynthesis. DvAKR1 encoded a AKR 13 sub-family protein with significant differential expression levels, and was phylogenetically distinct from the chalcone reductases, which belongs to the AKR 4A sub-family in legumes. DNA sequence variation and expression profiles of DvAKR1 gene were correlated with 6'-deoxychalcone accumulation in the tested dahlia cultivars. A single over-expression analysis of DvAKR1 was not sufficient to initiate the accumulation of isoliquiritigenin in tobacco, in contrast, its co-overexpression with a chalcone 4'-O-glucosyltransferase (Am4'CGT) from Antirrhinum majus and a MYB transcription factor, CaMYBA from Capsicum annuum successfully induced isoliquiritigenin accumulation. In addition, DvAKR1 homologous gene expression was detected in Coreopsideae species accumulating 6'-deoxychalcone, but not in Asteraceae species lacking 6'-deoxychalcone production. These results not only demonstrate the involvement of DvAKR1 in the biosynthesis of 6'-deoxychalcone in dahlia, but also show that 6'-deoxychalcone occurrence in Coreopsideae species developed evolutionarily independent from legume species.

First purified recombinant CYP75B including transmembrane helix with unexpected high substrate specificity to (2R)-naringenin

Anthochlor pigments (chalcones and aurones) play an important role in yellow flower colourization, the formation of UV-honey guides and show numerous health benefits. The B-ring hydroxylation of chalcones is performed by membrane bound cytochrome P450 enzymes. It was assumed that usual flavonoid 3′-hydroxlases (F3′Hs) are responsible for the 3,4- dihydroxy pattern of chalcones, however, we previously showed that a specialized F3′H, namely chalcone 3-hydroxylase (CH3H), is necessary for the hydroxylation of chalcones. In this study, a sequence encoding membrane bound CH3H from Dahlia variabilis was recombinantly expressed in yeast and a purification procedure was developed. The optimized purification procedure led to an overall recovery of 30% recombinant DvCH3H with a purity of more than 84%. The enzyme was biochemically characterized with regard to its kinetic parameters on various substrates, including racemic naringenin, as well as its enantiomers (2S)-, and (2R)-naringenin, apigenin and kaempferol. We report for the first time the characterization of a purified Cytochrome P450 enzyme from the flavonoid biosynthesis pathway, including the transmembrane helix. Further, we show for the first time that recombinant DvCH3H displays a higher affinity for (2R)-naringenin than for (2S)-naringenin, although (2R)-flavanones are not naturally formed by chalcone isomerase.

Different Phenylalanine Pathway Responses to Cold Stress Based on Metabolomics and Transcriptomics in Tartary Buckwheat Landraces

Tartary buckwheat (Fagopyrum tataricum) is strongly adapted to growth in adverse environments. In Liangshan, the Yi people cultivate different Tartary buckwheat landraces in different habitats. In this study, we aimed to understand the molecular differences in transcriptomic and metabolomic responses underlying cold tolerance between two Tartary buckwheat landraces (TM and RG) cultivated at different altitudes. After cold treatment, TM showed normal growth in the seedling stage and had significantly higher total flavonoids (16.53 mg/g, 1.47 times), rutin (5.73 mg/g, 1.32 times), and quercetin (0.08 mg/g, 2.67 times), which were higher than those in RG. In addition, TM showed higher-level changes in carbon and nitrogen metabolism than RG. Combined transcriptome and metabolomic analyses showed that phenylpropanoid biosynthesis was upregulated after cold treatment, and in TM, rutin synthesis was upregulated with a higher-level response to cold stress. RG showed higher expression in anthocyanins in response to cold stress. In addition, 24 structural genes involved in flavonoid synthesis, including 6 PAL, 3 C4H, 2 4CL, 2 CHS, 1 CHI, 3 F3H, 3 DFR, 1 FLS, 1 F3'H, and 4 GTR genes, were identified. These results will provide sufficient information for breeding Tartary buckwheat with high cold tolerance and constructing rutin high-yield varieties based on genetic engineering.

Identification of bHLH genes through genome-wide association study and antisense expression of ZjbHLH076/ZjICE1 influence tolerance to low temperature and salinity in Zoysia japonica

Abiotic stress greatly affects plant growth and developmental processes, resulting in poor productivity. A variety of basic helix-loop-helix (bHLH) transcription factors (TFs) that play important roles in plant abiotic stress response pathways have been identified. However, bHLH proteins of Zoysia japonica, one of the warm-season turfgrasses, have not been widely studied. In this study, 141 bHLH genes (ZjbHLHs) were identified and classified into 22 subfamilies. The ZjbHLHs were mapped on 19 chromosomes except for Chr17 and one pair of the tandemly arrayed genes was identified on Chr06. Also, the co-linearity of ZjbHLHs was found to have been driven mostly by segmental duplication events. The subfamily IIIb genes of our present interest, possessed various stress responsive cis-elements in their promoters. ZjbHLH076/ZjICE1, a MYC-type bHLH TF in subfamily IIIb was analyzed by overexpression and its loss-of-function via overexpressing a short ZjbHLH076/ZjICE1 fragment in the antisense direction. The overexpression of ZjbHLH076/ZjICE1 enhanced the tolerance to cold and salinity stress in the transgenic Z. japonica plants. However, the anti-sense expression of ZjbHLH076/ZjICE1 showed sensitive to these abiotic stresses. These results suggest that ZjbHLH076/ZjICE1 would be a promising candidate for the molecular breeding program to improve the abiotic stress tolerance of Z. japonica.

Integrated mRNA and small RNA sequencing reveals a regulatory network associated with flower color in oriental hybrid lily

Anthocyanins are one of the main components of pigments, that are responsible for a wide range of colors in plants. To clarify the regulatory mechanism of anthocyanin biosynthesis in oriental hybrid lily, UPLC/MS analysis was performed to identify the pigments in two cultivars (white and pink). Four major anthocyanins were identified in pink cultivar, and no anthocyanins were detected in white cultivar. Transcriptome and small RNA sequencing (sRNAseq) analyses were performed using tepal tissues at two floral developmental stages from the two cultivars. In total, 55,698 transcripts were assembled, among which 233 were annotated as putative anthocyanin-related transcripts. Differential expression analysis and qRT-PCR results confirmed that most of the anthocyanin-related structural genes had higher expression levels in pink cultivar than in white cultivar. Conversely, LhANR showed a significantly high expression level in white cultivar. Annotated transcription factors (TFs), including MYB activators, MYB repressors and bHLHs, that putatively inhibit or enhance the expression of anthocyanin-related genes were identified. LhMYBA1, an anthocyanin activator, was isolated, and its heterologous expression resulted in a remarkably high level of anthocyanin accumulation. Additionally, 73 differentially expressed microRNAs (miRNAs), including 23 known miRNAs, were detected through sRNAseq. The miRNA target prediction showed that several anthocyanin-related genes might be targeted by miRNAs. Expression profile analysis revealed that these miRNAs showed higher expression levels at later floral developmental stages in white cultivar than in pink cultivar. The results indicated that anthocyanin deficiency in white cultivar might be influenced by multiple levels of suppressive mechanisms, including mRNAs and sRNAs.

Optimum chalcone synthase for flavonoid biosynthesis in microorganisms

Chalcones and the subsequently generated flavonoids, as well as flavonoid derivatives, have been proven to have a variety of physiological activities and are widely used in: the pharmaceutical, food, feed, and cosmetic industries. As the content of chalcones and downstream products in native plants is low, the production of these compounds by microorganisms has gained the attention of many researchers and has a history of more than 20 years. The mining and engineering of chalcone synthase (CHS) could be one of the most important ways to achieve more efficient production of chalcones and downstream products in microorganisms. CHS has a broad spectrum of substrates, and its enzyme activity and expression level can significantly affect the efficiency of the biosynthesis of flavonoids. This review summarizes the recent advances in the: structure, mechanism, evolution, substrate spectrum, transformation, and expression regulation in the flavonoid biosynthesis of this vital enzyme. Future development directions were also suggested. The findings may further promote the research and development of flavonoids and health products, making them vital in the fields of human diet and health.

MaMAPK3-MaICE1-MaPOD P7 pathway, a positive regulator of cold tolerance in banana

BACKGROUND

Banana is a tropical fruit with a high economic impact worldwide. Cold stress greatly affects the development and production of banana.

RESULTS

In the present study, we investigated the functions of MaMAPK3 and MaICE1 involved in cold tolerance of banana. The effect of RNAi of MaMAPK3 on Dajiao (Musa spp. 'Dajiao'; ABB Group) cold tolerance was evaluated. The leaves of the MaMAPK3 RNAi transgenic plants showed wilting and severe necrotic symptoms, while the wide-type (WT) plants remained normal after cold exposure. RNAi of MaMAPK3 significantly changed the expressions of the cold-responsive genes, and the oxidoreductase activity was significantly changed in WT plants, while no changes in transgenic plants were observed. MaICE1 interacted with MaMAPK3, and the expression level of MaICE1 was significantly decreased in MaMAPK3 RNAi transgenic plants. Over-expression of MaICE1 in Cavendish banana (Musa spp. AAA group) indicated that the cold resistance of transgenic plants was superior to that of the WT plants. The POD P7 gene was significantly up-regulated in MaICE1-overexpressing transgenic plants compared with WT plants, and the POD P7 was proved to interact with MaICE1.

CONCLUSIONS

Taken together, our work provided new and solid evidence that MaMAPK3-MaICE1-MaPOD P7 pathway positively improved the cold tolerance in monocotyledon banana, shedding light on molecular breeding for the cold-tolerant banana or other agricultural species.

Genome-wide identification of PbrbHLH family genes, and expression analysis in response to drought and cold stresses in pear (Pyrus bretschneideri)

BACKGROUND

The basic helix-loop-helix (bHLH) transcription factors play important roles in many processes in plant growth, metabolism and responses to abiotic stresses. Although, the sequence of Chinese white pear genome (cv. 'Dangshansuli') has already been reported, there is still a lack of clarity regarding the bHLH family genes and their evolutionary history.

RESULTS

In this work, a genome-wide identification of the bHLH genes in Chinese white pear was performed, and we characterized the functional roles of these PbrbHLH genes in response to abiotic stresses. Based on the phylogenetic analysis and structural characteristics, 197 identified bHLH genes could be well classified into 21 groups. Expansion of PbrbHLH gene family was mainly driven by WGD and dispersed duplication with the purifying selection from the recent WGD. The functional annotation enrichment showed that the majority of PbrbHLHs were enriched in the GO terms and KEGG pathways involved in responds to stress conditions as TFs. Transcriptomic profiles and qRT-PCR revealed that PbrbHLH7, PbrbHLH8, PbrbHLH128, PbrbHLH160, PbrbHLH161 and PbrbHLH195 were significantly up-regulated under cold and drought treatments. In addition, PbrbHLH195-silenced pear seedlings display significant reduced cold tolerance, exhibiting reduced chlorophyll content, as well as increased electrolyte leakage and concentrations of malondialdehyde and H₂O₂.

CONCLUSION

For the first time, a comprehensive analysis identified the bHLH genes in Chinese white pear and demonstrated that PbrbHLH195 is involved in the production of ROS in response to cold stress, suggesting that members of the PbrbHLH family play an essential role in the stress tolerance of pear.

Alternative Splicing of the Basic Helix-Loop-Helix Transcription Factor Gene CmbHLH2 Affects Anthocyanin Biosynthesis in Ray Florets of Chrysanthemum (Chrysanthemum morifolium)

Chrysanthemum is an important ornamental crop worldwide. Some white-flowered chrysanthemum cultivars produce red ray florets under natural cultivation conditions, but little is known about how this occurs. We compared the expression of anthocyanin biosynthetic and transcription factor genes between white ray florets and those that turned red based on cultivation conditions to comprehend the underlying mechanism. Significant differences in the expression of CmbHLH2 were detected between the florets of different colors. CmbHLH2 generated two alternatively spliced transcripts, designated CmbHLH2^Full and CmbHLH2^Short . Compared with CmbHLH2^Full , CmbHLH2^Short encoded a truncated protein with only a partial MYB-interaction region and no other domains normally present in the full-length protein. Unlike the full-length form, the splicing variant protein CmbHLH2^Short localized to the cytoplasm and the nucleus and could not interact with CmMYB6. Additionally, CmbHLH2^Short failed to activate anthocyanin biosynthetic genes and induce pigment accumulation in transiently transfected tobacco leaves, whereas CmbHLH2^Full promoted both processes when simultaneously expressed with CmMYB6. Co-expressing CmbHLH2^Full and CmMYB6 also enhanced the promoter activities of CmCHS and CmDFR. Notably, the Arabidopsis tt8-1 mutant, which lacks red pigmentation in the leaves and seeds, could be complemented by the heterologous expression of CmbHLH2^Full, which restored red pigmentation and resulted in red pigmentation in high anthocyanin and proanthocyanidin contents in the leaves and seeds, respectively, whereas expression of CmbHLH2^Short did not. Together, these results indicate that CmbHLH2 and CmMYB6 interaction plays a key role in the anthocyanin pigmentation changes of ray florets in chrysanthemum. Our findings highlight alternative splicing as a potential approach to modulate anthocyanin biosynthesis in specific tissues.

Transcriptome and metabolome analyses reveal the regulation of peel coloration in green, red Chinese prickly ash (Zanthoxylum L.)

Peel colour is an important external economic characteristic of Chinese prickly ash cultivars (Zanthoxylum bungeanum Maxim.). To gain insight into their coloration mechanisms, we performed an integrated analysis of green and red peels using combined metabolomic and transcriptomic analyses. Pelargonin-O-hexoside-O-rhamnoside-O-hexoside, pelargonidin 3,5-diglucoside, peonidin O-hexoside, cyanidin O-syringic acid and peonidin 3-O-glucoside were found to be the key anthocyanins. Transcriptome data indicated that the anthocyanidin synthase genes and UDP-glucose flavonoid 3-O-glucosytransferase genes were significantly increased to promote the redness of the peels. In addition, we discussed the role of R₂R₃-MYB transcription factors in coloration, of which the c80935 and c226097 genes may be the key regulatory factors for anthocyanin biosynthesis. Generally, this is the first study to identify and reveal the main anthocyanins in Chinese prickly ash peels during different developmental periods. The results of this research lay the foundation for understanding the regulation of coloration in Chinese prickly ash peels.

GREEN STRIPE, encoding methylated TOMATO AGAMOUS-LIKE 1, regulates chloroplast development and Chl synthesis in fruit

Fruit development involves chloroplast development, carotenoid accumulation and fruit coloration. Although genetic regulation of fruit development has been extensively investigated, epigenetic regulation of fruit coloration remains largely unexplored. Here, we report a naturally occurring epigenetic regulation of TAGL1, and its impact on chloroplast development and fruit coloration. We used a genome-wide association study in combination with map-based cloning to identify the GREEN STRIPE (GS) locus, a methylated isoform of TAGL1 regulating diversified chloroplast development and carotenoid accumulation. Nonuniform pigmentation of fruit produced by GS was highly associated with methylation of the TAGL1 promoter, which is linked to a SNP at SL2.50ch07_63842838. High degrees of methylation of the TAGL1 promoter downregulated its expression, leading to green stripes. By contrast, low degrees of methylation led to light green stripes in gs. RNA-seq and ChIP collectively showed that the expression of genes involved with Chl synthesis and chloroplast development were significantly upregulated in green stripes relative to light green stripes. Quantitative PCR and dual luciferase assay confirmed that TAGL1 downregulates expression of SlMPEC, SlPsbQ, and SlCAB, and upregulates expression of PSY1 - genes which are associated with chloroplast development and carotenoid accumulation. Altogether, our findings regarding the GS locus demonstrate that naturally occurring methylation of TAGL1 has diverse effects on plastid development in fruit.

PsbHLH1, a novel transcription factor involved in regulating anthocyanin biosynthesis in tree peony (Paeonia suffruticosa)

Flower color is one of the most important features of ornamental plants. Anthocyanin composition and concentration are usually closely related to flower color formation. The biosynthesis of anthocyanin is regulated by a series of structural genes and regulatory genes. The basic helix-loop-helix proteins (bHLHs) are considered as one of the key transcription factors known as the regulators of anthocyanin biosynthesis. However, the bHLH transcription factor family of tree peony (Paeonia suffruticosa) has not been systematically studied in previous studies, especially for the regulation of petal pigmentation. The aim of this study was to identify bHLH genes and unravel their underlying molecular mechanism involved in the regulation of anthocyanin biosynthesis in tree peony. Based on transcriptome profiling analysis, we identified three bHLHs candidate anthocyanin regulators, PsbHLH1, PsbHLH2, and PsbHLH3. PsbHLH1-3 were phylogenetically clustered in the IIIf bHLH subgroup, which is involved in anthocyanin biosynthesis in other plant species. In addition, three bHLH proteins were localized in the nucleus and displayed transcriptional activation activity in a yeast hybrid system. Through a series of functional experiments, we further demonstrated that PsbHLH1 could transcriptionally activate the expression of PsDFR and PsANS via directly binding to their promoters. These results laid a solid foundation to better understand the regulatory mechanisms of anthocyanin biosynthesis in P. suffruticosa and to benefit molecular breeding of tree peony cultivars with novel color.

Functional MYB transcription factor gene HtMYB2 is associated with anthocyanin biosynthesis in Helianthus tuberosus L

BACKGROUND

Tuber color is an important trait for Helianthus tuberosus L. (Jerusalem artichoke). Usually, purple tubers with high anthocyanin content are more nutritious than white tuber. But, the molecular mechanism underlying it is unknown.

RESULTS

In the current study, high-throughput RNA-sequencing was used to compare the transcriptomes between plants with tubers with red or white epidermis. Compared with the white-skinned tubers of cultivar QY3, anthocyanin biosynthesis structural genes had greater expression in the red-skinned tubers of cultivar QY1, indicating that the anthocyanin biosynthesis pathway was activated in 'QY1'; quantitative PCR confirmed this difference in expression. HtMYB2 (Unigene44371_All) was the only MYB transcription factor, homologous to the MYB transcription factor regulating anthocyanin biosynthesis, expressed in the red tuber epidermis of 'QY1'. The anthocyanin concentration in the root, stem, leaf, flower, and tuber epidermis of 'QY1' was higher than in 'QY3', especially tuber epidermis. Correspondingly, HtMYB2 had greater expression in these tissues of 'QY1' than in 'QY3'. The expression of HtMYB2 was associated with anthocyanin accumulation in the different tissues. Overexpression of HtMYB2 activated the anthocyanin biosynthesis pathway, accumulating the pigment in leaves of transgenic tobacco, supporting the model that HtMYB2 regulated anthocyanin biosynthesis. Further experiments found that HtMYB2 had the same coding sequence and genomic sequence in 'QY1' and 'QY3', but that there were several single nucleotide polymorphisms and one insertion-deletion (indel) mutation of 21 nucleotides in the promoter region between the two alleles. The deletion of three nucleotides "AAA" made the promoter of 'QY1' predicted to contain one more possible promoter region. A specific primer, based on the indel, could differentiate between cultivars with red or white tuber epidermis. The genetic variation in HtMYB2 was associated with the tuber skin color in a natural population.

CONCLUSIONS

RNA-seq can successfully isolate the candidate gene (HTMYB2) controlling anthocyanin biosynthesis in purple epidermis of Jerusalem artichoke tuber. HTMYB2 can regulate anthocyanin biosynthesis in plants and is closely related to the formation of purple phenotype in tubers. This study should be useful in understanding the genetic mechanism underlying different tuber skin colors and in breeding new H. tuberosus cultivars with different tuber skin colors.

A novel basic helix-loop-helix transcription factor, ZjICE2 from Zoysia japonica confers abiotic stress tolerance to transgenic plants via activating the DREB/CBF regulon and enhancing ROS scavenging

ZjICE2 works as a positive regulator in abiotic stress responses and ZjICE2 is a valuable genetic resource to improve abiotic stress tolerance in the molecular breeding program of Zoysia japonica. The basic helix-loop-helix (bHLH) family transcription factors (TFs) play an important role in response to biotic or abiotic stresses in plants. However, the functions of bHLH TFs in Zoysia japonica, one of the warm-season turfgrasses, remain poorly understood. Here, we identified ZjICE2 from Z. japonica, a novel MYC-type bHLH transcription factor that was closely related to ICE homologs in the phylogenetic tree, and its expression was regulated by various abiotic stresses. Transient expression of ZjICE2-GFP in onion epidermal cells revealed that ZjICE2 was a nuclear-localized protein. Also, ZjICE2 bound the MYC cis-element in the promoter of dehydration responsive element binding 1 of Z. japonica (ZjDREB1) using yeast one-hybrid assay. A phenotypic analysis showed that overexpression of the ZjICE2 in Arabidopsis enhanced tolerance to cold, drought, and salt stresses. The transgenic Arabidopsis and Z. japonica accumulated more transcripts of cold-responsive DREB/CBFs and their downstream genes than the wild type (WT) after cold treatment. Furthermore, the transgenic plants exhibited an enhanced Reactive oxygen species (ROS) scavenging ability, which resulted in an efficient maintenance of oxidant-antioxidant homeostasis. In addition, overexpression of the ZjICE2 in Z. japonica displayed intensive cold tolerance with increases in chlorophyll contents and photosynthetic efficiency. Our study suggests that ZjICE2 works as a positive regulator in abiotic stress responses and the ICE-DREB/CBFs response pathway involved in cold stress tolerance is also conserved in Z. japonica. These results provide a valuable genetic resource for the molecular breeding program especially for warm-season grasses as well as other leaf crop plants.

UV-B Induces Distinct Transcriptional Re-programing in UVR8-Signal Transduction, Flavonoid, and Terpenoids Pathways in Camellia sinensis

Plants are known to respond to Ultraviolet-B radiation (UV-B: 280-320 nm) by generating phenolic metabolites which absorbs UV-B light. Phenolics are extraordinarily abundant in Camellia sinensis leaves and are considered, together with pleasant volatile terpenoids, as primary flavor determinants in tea beverages. In this study, we focused on the effects of UV-B exposure (at 35 μW cm^-2 for 0, 0.5, 2, and 8 h) on tea transcriptional and metabolic alterations, specifically related to tea flavor metabolite production. Out of 34,737 unigenes, a total of 18,081 differentially expressed genes (DEGs) due to UV-B treatments were identified. Additionally, the phenylpropanoid pathway was found as one of the most significantly UV-B affected top 20 KEGG pathways while flavonoid and monoterpenoid pathway-related genes were enhanced at 0.5 h. In the UVR8-signal transduction pathway, UVR8 was suppressed at both short and long exposure of UV-B with genes downstream differentially expressed. Divergent expression of MYB4 at different treatments could have differentially altered structural and regulatory genes upstream of flavonoid biosynthesis pathways. Suppression of MYB4-1&3 at 0.5 h could have led to the up-regulation of structural CCOAOMT-1&2, HST-1&2, DFR-4, ANR-2, and LAR-1&3 genes resulting in accumulation of specialized metabolites at a shorter duration of UV-B exposure. Specialized metabolite profiling revealed the correlated alterations in the abundances of catechins and some volatile terpenoids in all the treatments with significant accumulation of specialized metabolites at 0.5 h treatment. A significant increase in specialized metabolites at 0.5 h treatment and no significant alteration observed at longer UVB treatment suggested that shorter exposure to UV-B led to different display in gene expression and accumulation of specialized metabolites in tea shoots in response to UV-B stress. Taken together, our results indicated that the UV-B treatment applied in this study differentially altered the UVR8-signal transduction, flavonoid and terpenoid pathways at transcriptional and metabolic levels in tea plants. Our results show strong potential for UV-B application in flavor improvement in tea at the industrial level.

Transposition of a non-autonomous DNA transposon in the gene coding for a bHLH transcription factor results in a white bulb color of onions (Allium cepa L.)

A DNA transposon was found in the gene encoding a bHLH transcription factor. Genotypes of the marker tagging this DNA transposon perfectly co-segregated with color phenotypes in large F_2:3 populations A combined approach of bulked segregant analysis and RNA-Seq was used to isolate causal gene for C locus controlling white bulb color in onions (Allium cepa L.). A total of 114 contigs containing homozygous single nucleotide polymorphisms (SNPs) between white and yellow bulked RNAs were identified. Four of them showed high homologies with loci clustered in the middle of chromosome 5. SNPs in 34 contigs were confirmed by sequencing of PCR products. One of these contigs showed perfect linkage to the C locus in F_2:3 populations consisting of 2491 individuals. However, genotypes of molecular marker tagging this contig were inconsistent with color phenotypes of diverse breeding lines. A total of 146 contigs showed differential expression between yellow and white bulks. Among them, transcription levels of B2 gene encoding a bHLH transcription factor were significantly reduced in white RNA bulk and F_2:3 individuals, although there was no SNP in the coding region. Phylogenetic analysis showed that onion B2 was orthologous to bHLH-coding genes regulating anthocyanin biosynthesis pathway in other plant species. Promoter regions of B2 gene were obtained by genome walking and a 577-bp non-autonomous DNA transposon designated as AcWHITE was found in the white allele. Molecular marker tagging AcWHITE showed perfect linkage with the C locus. Marker genotypes of the white allele were detected in some white accessions. However, none of tested red or yellow onions contained AcWHITE insertion, implying that B2 gene was likely to be a casual gene for the C locus.

Zoysia japonica MYC type transcription factor ZjICE1 regulates cold tolerance in transgenic Arabidopsis

ICE1 (Inducer of CBF Expression 1) is a regulator of cold-induced transcriptome, which plays an important role in plant cold response pathway. To enhance the cold tolerance of Zoysia japonica, one of the warm-season turfgrasses, it is helpful to understand the cold response mechanism in Zoysia japonica. We identified stress-responsive ZjICE1 from Zoysia japonica and characterized its function in cold stress. Our results showed that ZjICE1 shared the typical feature of ICE homolog proteins belonging to a nucleic protein. Transactivation activity assay revealed that ZjICE1 bound to the MYC cis-element in the ZjDREB1's promotor. The ZjICE1 overexpressed transgenic Arabidopsis showed enhanced tolerance to cold stress with an increases in SOD, POD, and free proline content and reduction in MDA content. They also induced the transcripts abundance of cold-responsive genes (CBF1, CBF2, CBF3, COR47A, KIN1, and RD29A) after cold treatment. These results suggest that ZjICE1 is a positive regulator in Zoysia japonica plant during cold stress and can be a useful gene for the molecular breeding program to develop the cold tolerant zoysiagrass. Furthermore, the ZjICE1 also conferred resistance to salt and drought stresses, providing the better understanding of the basic helix-loop-helix (bHLH) gene family in abiotic stress responses.

Comparative metabolomic analysis reveals the variations in taxoids and flavonoids among three Taxus species

BACKGROUND

Trees of the genus Taxus are highly valuable medicinal plants with multiple pharmacological effects on various cancer treatments. Paclitaxel from Taxus trees is an efficient and widely used anticancer drug, however, the accumulation of taxoids and other active ingredients can vary greatly among Taxus species. In our study, the metabolomes of three Taxus species have been investigated.

RESULTS

A total of 2246 metabolites assigned to various primary and secondary metabolic pathways were identified using an untargeted approach. Analysis of differentially accumulated metabolites identified 358 T. media-, 220 T. cuspidata-, and 169 T. mairei-specific accumulated metabolites, respectively. By searching the metabolite pool, 7 MEP pathway precursors, 11 intermediates, side chain products and derivatives of paclitaxel, and paclitaxel itself were detected. Most precursors, initiated intermediates were highly accumulated in T. mairei, and most intermediate products approaching the end point of taxol biosynthesis pathway were primarily accumulated in T. cuspidata and T. media. Our data suggested that there were higher-efficiency pathways to paclitaxel in T. cuspidata and T. media compared with in T. mairei. As an important class of active ingredients in Taxus trees, a majority of flavonoids were predominantly accumulated in T. mairei rather than T. media and T. cuspidata. The variations in several selected taxoids and flavonoids were confirmed using a targeted approach.

CONCLUSIONS

Systematic correlativity analysis identifies a number of metabolites associated with paclitaxel biosynthesis, suggesting a potential negative correlation between flavonoid metabolism and taxoid accumulation. Investigation of the variations in taxoids and other active ingredients will provide us with a deeper understanding of the interspecific differential accumulation of taxoids and an opportunity to accelerate the highest-yielding species breeding and resource utilization.

Functional characterization of a liverworts bHLH transcription factor involved in the regulation of bisbibenzyls and flavonoids biosynthesis

BACKGROUND

The basic helix-loop-helix (bHLH) transcription factors (TFs), as one of the largest families of TFs, play important roles in the regulation of many secondary metabolites including flavonoids. Their involvement in flavonoids synthesis is well established in vascular plants, but not as yet in the bryophytes. In liverworts, both bisbibenzyls and flavonoids are derived through the phenylpropanoids pathway and share several upstream enzymes.

RESULTS

In this study, we cloned and characterized the function of PabHLH1, a bHLH family protein encoded by the liverworts species Plagiochasma appendiculatum. PabHLH1 is phylogenetically related to the IIIf subfamily bHLHs involved in flavonoids biosynthesis. A transient expression experiment showed that PabHLH1 is deposited in the nucleus and cytoplasm, while the yeast one hybrid assay showed that it has transactivational activity. When PabHLH1 was overexpressed in P. appendiculatum thallus, a positive correlation was established between the content of bibenzyls and flavonoids and the transcriptional abundance of corresponding genes involved in the biosynthesis pathway of these compounds. The heterologous expression of PabHLH1 in Arabidopsis thaliana resulted in the activation of flavonoids and anthocyanins synthesis, involving the up-regulation of structural genes acting both early and late in the flavonoids synthesis pathway. The transcription level of PabHLH1 in P. appendiculatum thallus responded positively to stress induced by either exposure to UV radiation or treatment with salicylic acid.

CONCLUSION

PabHLH1 was involved in the regulation of the biosynthesis of flavonoids as well as bibenzyls in liverworts and stimulated the accumulation of the flavonols and anthocyanins in Arabidopsis.

Transcriptomic analysis reveals flavonoid biosynthesis of Syringa oblata Lindl. in response to different light intensity

BACKGROUND

Hazy weather significantly increase air pollution and affect light intensity which may also affect medicinal plants growth. Syringa oblata Lindl. (S. oblata), an effective anti-biofilm medicinal plants, is also vulnerable to changes in plant photoperiods and other abiotic stress responses. Rutin, one of the flavonoids, is the main bioactive ingredient in S. oblata that inhibits Streptococcus suis biofilm formation. Thus, the present study aims to explore the biosynthesis and molecular basis of flavonoids in S. oblata in response to different light intensity.

RESULTS

In this study, it was shown that compared with natural (Z0) and 25% ~ 35% (Z2) light intensities, the rutin content of S. oblata under 50% ~ 60% (Z1) light intensity increased significantly. In addition, an integrated analysis of metabolome and transcriptome was performed using light intensity stress conditions from two kinds of light intensities which S. oblata was subjected to: Z0 and Z1. The results revealed that differential metabolites and genes were mainly related to the flavonoid biosynthetic pathway. We found out that 13 putative structural genes and a transcription factor bHLH were significantly up-regulated in Z1. Among them, integration analysis showed that 3 putative structural genes including 4CL1, CYP73A and CYP75B1 significantly up-regulated the rutin biosynthesis, suggesting that these putative genes may be involved in regulating the flavonoid biosynthetic pathway, thereby making them key target genes in the whole metabolic process.

CONCLUSIONS

The present study provided helpful information to search for the novel putative genes that are potential targets for S. oblata in response to light intensity.

Overexpression of a basic helix-loop-helix transcription factor gene, SlbHLH22, promotes early flowering and accelerates fruit ripening in tomato (Solanum lycopersicum L.)

The overexpression of SlbHLH22 functioned in controlling flowering time, accelerated fruit ripening, and produced more ethylene-producing phenotypes in tomato. Flowering and fruit ripening are two complex transition processes regulated by various internal and external factors that ultimately lead to fruit maturation and final seed dispersal. The basic helix-loop-helix (bHLH) transcription factor is the largest TF gene family in plants that controls various biological and developmental aspects, but the actual roles of these genes have not been fully studied. Here, we performed a functional characterization of the bHLH gene SlbHLH22 in tomato. SlbHLH22 was fully expressed in tomato flowers, while a moderate expression level was also observed in fruits at different developmental stages. Overexpression of the SlbHLH22 gene revealed that it is highly involved in controlling flowering time, through the activation of the SlSFT or SlLFY genes, and promoting fruit ripening and improved carotenoid accumulation. The expression patterns of carotenoid-related genes (SlPYS1) were also upregulated in transgenic tomato fruits. In transgenic tomato fruit, we observed clear changes in colour from green to orange with enhanced expression of the SlbHLH22 gene. SlbHLH22 was upregulated under exogenous ACC, IAA, ABA, and ethephon. Overexpression of SlbHLH22 also promotes ethylene production. Moreover, ethylene biosynthesis and perception genes (SlACO3, SlACS1, SlACS2, SlACS4, SlACS1a, SlEIN1, SlEIN2, SlEIN3, SlEIN4, SlETR2, SlETR3, SlSAM3, and SlSAMS) were upregulated. Ripening-related genes (SlAP2a, SlCNR, SlNOR, SlMYB, and SlTAG) were consistent in their expression pattern in transgenic plants. Finally, our study provides evidence that tomato bHLH genes play an important role in flowering, fruit ripening, and development.

CmMYB#7, an R3 MYB transcription factor, acts as a negative regulator of anthocyanin biosynthesis in chrysanthemum

'Jimba', a well-known white flowered chrysanthemum cultivar, occasionally and spontaneously produces red colored petals under natural cultivation, but there is little information about the molecular regulatory mechanism underlying this process. We analysed the expression patterns of 91 MYB transcription factors in 'Jimba' and 'Turning red Jimba' and identified an R3 MYB, CmMYB#7, whose expression was significantly decreased in 'Turning red Jimba' compared with 'Jimba', and confirmed it is a passive repressor of anthocyanin biosynthesis. CmMYB#7 competed with CmMYB6, which together with CmbHLH2 is an essential component of the anthocyanin activation complex, for interaction with CmbHLH2 through the bHLH binding site in the R3 MYB domain. This reduced binding of the CmMYB6-CmbHLH2 complex and inhibited its ability to activate CmDFR and CmUFGT promoters. Moreover, using transient expression assays we demonstrated that changes in the expression of CmMYB#7 accounted for alterations in anthocyanin content. Taken together, our findings illustrate that CmMYB#7 is a negative regulator of anthocyanin biosynthesis in chrysanthemum.

Differential accumulation of pelargonidin glycosides in petals at three different developmental stages of the orange-flowered gentian (Gentiana lutea L. var. aurantiaca)

Corolla color in Gentiana lutea L. exhibits a yellow/orange variation. We previously demonstrated that the orange petal color of G. lutea L. var. aurantiaca is predominantly caused by newly synthesized pelargonidin glycosides that confer a reddish hue to the yellow background color, derived from the carotenoids. However, the anthocyanin molecules of these pelargonidin glycosides are not yet fully identified and characterized. Here, we investigated the regulation, content and type of anthocyanins determining the petal coloration of the orange-flowered G. lutea L. var. aurantiaca. Anthocyanins from the petals of G. lutea L. var. aurantiaca were characterized and quantified by HPLC-ESI-MS/MS (High-performance liquid chromatography-electrospray ionization-tandem mass spectrometry) coupled with a diode array detector in flowers at three different stages of development (S1, S3 and S5). Eleven pelargonidin derivatives were identified in the petals of G. lutea L. var. aurantiaca for the first time, but quantitative and qualitative differences were observed at each developmental stage. The highest levels of these pelargonidin derivatives were reached at the fully open flower stage (S5) where all anthocyanins were detected. In contrast, not all the anthocyanins were detected at the budlet stage (S1) and mature bud stage (S3) and those corresponded to more complex pelargonidin derivatives. The major pelargonidin derivatives found at all the stages were pelargonidin 3-O-glucoside, pelargonidin 3,5-O-diglucoside and pelargonidin 3-O-rutinoside. Furthermore, the expression of DFR (dihydroflavonol 4-reductase), ANS (anthocyanidin synthase), 3GT (UDP-glucose:flavonoid 3-O-glucosyltransferase), 5GT (UDP-glucose:flavonoid 5-O-glucosyltransferase) and 5AT (anthocyanin 5-aromatic acyltransferase) genes was analyzed in the petals of three developmental stages, showing that the expression level of DFR, ANS and 3GT parallels the accumulation of the pelargonidin glucosides. Overall, this study enhances the knowledge of the biochemical basis of flower coloration in Gentiana species, and lays a foundation for breeding of flower color and genetic variation studies on Gentiana varieties.

Molecular Regulation of Catalpol and Acteoside Accumulation in Radial Striation and non-Radial Striation of Rehmannia glutinosa Tuberous Root

Rehmannia glutinosa L., a perennial plant of Scrophulariaceae, is one of the most commonly used herbs in traditional Chinese medicine (TCM) that have been widely cultivated in China. However, to date, the biosynthetic pathway of its two quality-control components, catalpol and acteoside, are only partially elucidated and the mechanism for their tissue-specific accumulation remains unknown. To facilitate the basic understanding of the key genes and transcriptional regulators involved in the biosynthesis of catalpol and acteoside, transcriptome sequencing of radial striation (RS) and non-radial striation (nRS) from four R. glutinosa cultivars was performed. A total of 715,158,202 (~107.27 Gb) high quality reads obtained using paired-end Illumina sequencing were de novo assembled into 150,405 transcripts. Functional annotation with multiple public databases identified 155 and 223 unigenes involved in catalpol and acteoside biosynthesis, together with 325 UGTs, and important transcription factor (TF) families. Comparative analysis of the transcriptomes identified 362 unigenes, found to be differentially expressed in all RS vs. nRS comparisons, with 143 upregulated unigenes, including those encoding enzymes of the catalpol and acteoside biosynthetic pathway, such as geranyl diphosphate synthase (RgGPPS), geraniol 8-hydroxylase (RgG10H), and phenylalanine ammonia-lyase (RgPAL). Other differentially expressed unigenes predicted to be related to catalpol and acteoside biosynthesis fall into UDP-dependent glycosyltransferases (UGTs), as well as transcription factors. In addition, 16 differentially expressed genes were selectively confirmed by real-time PCR. In conclusion, a large unigene dataset of R. glutinosa generated in the current study will serve as a resource for the identification of potential candidate genes for investigation of the tuberous root development and biosynthesis of active components.

A bHLH Transcription Factor Regulates Bisbibenzyl Biosynthesis in the Liverwort Plagiochasma appendiculatum

Liverworts, a section of the bryophyte plants which pioneered the colonization of terrestrial habitats, produce cyclic bisbibenzyls as secondary metabolites. These compounds are generated via the phenylpropanoid pathway, similar to flavonoid biosynthesis, for which basic helix-loop-helix (bHLH) transcription factors have been identified as one of the important regulators in higher plants. Here, a bHLH gene homolog (PabHLH) was isolated from the liverwort species Plagiochasma appendiculatum and its contribution to bisbibenzyl biosynthesis was explored. Variation in the abundance of PabHLH transcript mirrored that of tissue bisbibenzyl content in three different liverwort tissues. A phylogenetic analysis based on the bHLH domain sequence suggested that the gene encodes a member of bHLH subgroup IIIf, which clusters proteins involved in flavonoid synthesis. The gene's transient expression in onion epidermal cells implied that its product localized to the nucleus, and a transactivation assays in yeast showed that it was able to activate transcription. In both callus and thallus, the overexpression of PabHLH boosted bisbibenzyl accumulation, while also up-regulating PaPAL, Pa4CL1, PaSTCS1 and two genes encoding P450 cytochromes, and its RNA interference (RNAi)-induced suppression down-regulated the same set of genes and reduced the accumulation of bisbibenzyls. The abundance of PaCHS and PaFNSI transcript was related to flavonoid accumulation in transgenic thallus. PabHLH represents a candidate for the metabolic engineering of bisbibenzyl content.

Revisiting the Role of Plant Transcription Factors in the Battle against Abiotic Stress

Owing to diverse abiotic stresses and global climate deterioration, the agricultural production worldwide is suffering serious losses. Breeding stress-resilient crops with higher quality and yield against multiple environmental stresses via application of transgenic technologies is currently the most promising approach. Deciphering molecular principles and mining stress-associate genes that govern plant responses against abiotic stresses is one of the prerequisites to develop stress-resistant crop varieties. As molecular switches in controlling stress-responsive genes expression, transcription factors (TFs) play crucial roles in regulating various abiotic stress responses. Hence, functional analysis of TFs and their interaction partners during abiotic stresses is crucial to perceive their role in diverse signaling cascades that many researchers have continued to undertake. Here, we review current developments in understanding TFs, with particular emphasis on their functions in orchestrating plant abiotic stress responses. Further, we discuss novel molecular mechanisms of their action under abiotic stress conditions. This will provide valuable information for understanding regulatory mechanisms to engineer stress-tolerant crops.

Overexpression of Fagopyrum tataricum FtbHLH2 enhances tolerance to cold stress in transgenic Arabidopsis

bHLH transcription factors play important roles in the abiotic stress response in plants, but their characteristics and functions in Tartary buckwheat (Fagopyrum tataricum), a traditional coarse cereal with a strong stress tolerance, haven't been sufficiently studied. Here, we found that the expression of a bHLH gene, FtbHLH2, was induced significantly by cold treatments in Tartary buckwheat seedlings. Subcellular localization indicated that FtbHLH2 localized in nucleus. Its overexpression in Arabidopsis increased tolerance to cold. The Arabidopsis plants overexpressing FtbHLH2 displayed higher root length and photosynthetic efficiency, and had lower malondialdehyde (MDA) and reactive oxygen species (ROS) after cold treatment compared to wild type (WT) plants. Meanwhile, the expression levels of some stress-related genes in transgenic plants were remarkably higher than that in wild type under normal and/or stress conditions. Furthermore, transgenic Arabidopsis lines with the FtbHLH2 promoter had higher GUS activity after cold stress. On the whole, the results suggest that FtbHLH2 may play a positive regulatory in cold stress of Tartary buckwheat.

Identification of 'Xinlimei' radish candidate genes associated with anthocyanin biosynthesis based on a transcriptome analysis

Radish is an economically important vegetable crop belonging to the family Brassicaceae. The high anthocyanin content of the 'Xinlimei' radish roots has been associated with diverse health benefits. However, there is a lack of transcript-level information regarding anthocyanin biosynthesis. In the present study, the 'Xinlimei' radish root transcriptome was analyzed by RNA sequencing at five developmental stages. A total of 222,384,034 clean reads were obtained and 32,253 unigenes were annotated. Expression profiles revealed 10,890 differentially expressed genes (DEGs) among the five analyzed libraries. The DEGs were predominantly involved in KEGG pathways related to the biosynthesis of phenylpropanoids, flavonoids, flavone, and flavonol. The transcriptome data revealed 44 structural and 182 transcription factor genes (TFs) associated with the anthocyanin biosynthetic pathway. Ten structural genes (i.e., 4CL3, CHSB2, CHS1, CHS3, F3H1, F3'H, DFR, DFR1, ANS, and UFGT) and two MYB genes, which were highly and differentially expressed during root development, may be critical for anthocyanin biosynthesis. Additionally, the co-expression of TFs and structural genes was analyzed. Three structural genes (i.e., DFR, ANS, and UFGT) were validated by molecular cloning. The qRT-PCR results indicated that the expression profiles of DEGs were generally consistent with the high-throughput sequencing results. These findings helped identify candidate genes involved in anthocyanin biosynthesis and may be useful for clarifying the molecular mechanism underlying the accumulation of anthocyanins in radish roots.

Post-transcriptional silencing of chalcone synthase is involved in phenotypic lability in petals and leaves of bicolor dahlia (Dahlia variabilis) 'Yuino'

Post-transcriptional gene silencing (PTGS) of a chalcone synthase ( DvCHS2 ) occurred in the white part of bicolor petals and flavonoid-poor leaves; however, it did not in red petals and flavonoid-rich leaves. Petal color lability is a prominent feature of bicolor dahlia cultivars, and causes plants to produce not only original bicolor petals with colored bases and pure white tips, but also frequently single-colored petals without white tips. In this study, we analysed the molecular mechanisms that are associated with petal color lability using the red-white bicolor cultivar 'Yuino'. Red single-colored petals lose their white tips as a result of recover of flavonoid biosynthesis. Among flavonoid biosynthetic genes including four chalcone synthase (CHS)-like genes (DvCHS1, DvCHS2, DvCHS3, and DvCHS4), DvCHS1 and DvCHS2 had significantly lower expression levels in the white part of bicolor petals than in red petals, while DvCHS3, DvCHS4, and other flavonoid biosynthetic genes had almost the same expression levels. Small RNAs from the white part of a bicolor petal were mapped onto DvCHS1 and DvCHS2, while small RNAs from a red single-colored petal were not mapped onto any of the four CHS genes. A relationship between petal color and leaf flavonoid accumulation has previously been demonstrated, whereby red petal-producing plants accumulate flavonoids in their leaves, while bicolor petal-producing plants tend not to. The expression level of DvCHS2 was down-regulated in flavonoid-poor leaves and small RNAs from flavonoid-poor leaves were mapped onto DvCHS2, suggesting that the down-regulation of DvCHS2 in flavonoid-poor leaves occurs post-transcriptionally. Genomic analysis also suggested that DvCHS2 is the key gene involved in bicolor formation. Together, these results suggest that post-transcriptional gene silencing of DvCHS2 plays a key role in phenotypic lability in this bicolor dahlia.

Wheat bHLH-type transcription factor gene TabHLH1 is crucial in mediating osmotic stresses tolerance through modulating largely the ABA-associated pathway

KEY MESSAGE

Wheat bHLH family gene TabHLH1 is responsive to drought and salt stresses, and it acts as one crucial regulator in mediating tolerance to aforementioned stresses largely through an ABA-associated pathway. Osmotic stresses are adverse factors for plant growth and crop productivity. In this study, we characterized TabHLH1, a gene encoding wheat bHLH-type transcription factor (TF) protein, in mediating plant adaptation to osmotic stresses. TabHLH1 protein contains a conserved basic-helix-loop-helix (bHLH) domain shared by its plant counterparts. Upon PEG-simulated drought stress, salt stress, and exogenous abscisic acid (ABA), the TabHLH1 transcripts in roots and leaves were induced. Under PEG-simulated drought stress and salt stress treatments, the tobacco seedlings with TabHLH1 overexpression exhibited improved growth and osmotic stress-associated traits, showing increased biomass and reduced leaf water loss rate (WLR) relative to wild type (WT). The transgenic lines also possessed promoted stomata closure under drought stress, salt stress, and exogenous ABA and increased proline and soluble sugar contents and reduced hydrogen peroxide (H2O2) amount under osmotic stress conditions, indicating that TabHLH1-mediated osmolyte accumulation and cellular ROS homeostasis contributed to the drought stress and salt stress tolerance. NtPYL12 and NtSAPK2;1, the genes encoding ABA receptor and SnRK2 family kinase, respectively, showed up-regulated expression in lines overexpressing TabHLH1 under osmotic stress and exogenous ABA conditions; overexpression of them conferred plants modified stomata movement, leaf WLR, and growth feature under drought and high salinity, suggesting that these ABA-signaling genes are mediated by wheat TabHLH1 gene and involved in regulating plant responses to simulated drought and salt stresses. Our investigation indicates that the TabHLH1 gene plays critical roles in plant tolerance to osmotic stresses largely through an ABA-dependent pathway.

De Novo Transcriptome Assembly and Annotation of the Leaves and Callus of Cyclocarya Paliurus (Bata1) Iljinskaja

Cyclocarya Paliurus (Bata1) Iljinskaja contains various bioactive secondary metabolites especially in leaves, such as triterpenes, flavonoids, polysaccharides and alkaloids, and its leaves are widely used as an hyperglycemic tea in China. In the present paper, we sequenced the transcriptome of the leaves and callus of Cyclocarya Paliurus using Illumina Hiseq 4000 platform. After sequencing and de novo assembly, a total of 65,654 unigenes were generated with an N50 length of 1,244bp. Among them, 35,041 (53.37%) unigenes were annotated in NCBI Non-Redundant database, 19,453 (29.63%) unigenes were classified into Gene Ontology (GO) database, and 7,259 (11.06%) unigenes were assigned to Clusters of Orthologous Group (COG) categories. Furthermore, 11,697 (17.81%) unigenes were mapped onto 335 pathways in Kyoto Encyclopedia of Genes and Genomes (KEGG), among which 1,312 unigenes were identified to be involved in biosynthesis of secondary metabolites. In addition, a total of 11,247 putative simple sequence repeats (SSRs) were detected. This transcriptome dataset provides a comprehensive sequence resource for gene expression profiling, genetic diversity, evolution and further molecular genetics research on Cyclocarya Paliurus.

TabHLH1, a bHLH-type transcription factor gene in wheat, improves plant tolerance to Pi and N deprivation via regulation of nutrient transporter gene transcription and ROS homeostasis

Basic helix-loop-helix (bHLH) transcription factors (TFs) comprise a large TF family and act as crucial regulators in various biological processes in plants. Here, we report the functional characterization of TabHLH1, a bHLH TF member in wheat (Triticum aestivum). TabHLH1 shares conserved bHLH domain and targets to nucleus with transactivation activity. Upon Pi and N deprivation, the expression of TabHLH1 was up-regulated in roots and leaves, showing a pattern to be gradually increased within 23-h treatment regimes. The lines with overexpression of TabHLH1 exhibited drastically improved tolerance to Pi and N deprivation, showing larger plant phenotype, more biomass, higher concentration and more accumulation of P and N than wild type (WT) upon the Pi- and N-starvation stresses. NtPT1 and NtNRT2.2, the genes encoding phosphate transporter (PT) and nitrate transporter (NRT) in tobacco, respectively, showed up-regulated expression in TabHLH1-overexpressing plants; knockdown expression of them led to deteriorated growth feature, lowered biomass, and decreased nutrient accumulation of plants under Pi- and N-deficient conditions. Compared with WT, the TabHLH1-overexpressing plants also showed lowered reactive oxygen species (ROS) accumulation and improved antioxidant enzyme (AE) activities, such as those of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). NtSOD1, NtCAT1, and NtPOD1;6 that encode SOD, CAT, and POD, respectively, were up-regulated in TabHLH1-overexpressing plants. Further knockdown of these AE gene expression caused reduced antioxidant enzymatic activities, indicative of their crucial roles in mediating cellular ROS homeostasis in Pi- and N-starvation conditions. Together, TabHLH1 plays an important role in mediating adaptation to the Pi- and N-starvation stresses through transcriptional regulation of a set of genes encoding PT, NRT and AEs that mediate the taken up of Pi and N and the cellular homeostasis of ROS initiated by the nutrient stresses.

Two LcbHLH Transcription Factors Interacting with LcMYB1 in Regulating Late Structural Genes of Anthocyanin Biosynthesis in Nicotiana and Litchi chinensis During Anthocyanin Accumulation

Anthocyanin biosynthesis requires the MYB-bHLH-WD40 protein complex to activate the late biosynthetic genes. LcMYB1 was thought to act as key regulator in anthocyanin biosynthesis of litchi. However, basic helix-loop-helix proteins (bHLHs) as partners have not been identified yet. The present study describes the functional characterization of three litchi bHLH candidate anthocyanin regulators, LcbHLH1, LcbHLH2, and LcbHLH3. Although these three litchi bHLHs phylogenetically clustered with bHLH proteins involved in anthcoyanin biosynthesis in other plant, only LcbHLH1 and LcbHLH3 were found to localize in the nucleus and physically interact with LcMYB1. The transcription levels of all these bHLHs were not coordinated with anthocyanin accumulation in different tissues and during development. However, when co-infiltrated with LcMYB1, both LcbHLH1 and LcbHLH3 enhanced anthocyanin accumulation in tobacco leaves with LcbHLH3 being the best inducer. Significant accumulation of anthocyanins in leaves transformed with the combination of LcMYB1 and LcbHLH3 were noticed, and this was associated with the up-regulation of two tobacco endogenous bHLH regulators, NtAn1a and NtAn1b, and late structural genes, like NtDFR and NtANS. Significant activity of the ANS promoter was observed in transient expression assays either with LcMYB1-LcbHLH1 or LcMYB1-LcbHLH3, while only minute activity was detected after transformation with only LcMYB1. In contrast, no activity was measured after induction with the combination of LcbHLH2 and LcMYB1. Higher DFR expression was also oberseved in paralleling with higher anthocyanins in co-transformed lines. LcbHLH1 and LcbHLH3 are essential partner of LcMYB1 in regulating the anthocyanin production in tobacco and probably also in litchi. The LcMYB1-LcbHLH complex enhanced anthocyanin accumulation may associate with activating the transcription of DFR and ANS.

ICE1 of Pyrus ussuriensis functions in cold tolerance by enhancing PuDREBa transcriptional levels through interacting with PuHHP1

ICE1 transcription factor plays an important role in plant cold stress via regulating the expression of stress-responsive genes. In this study, a PuICE1 gene isolated from Pyrus ussuriensis was characterized for its function in cold tolerance. The expression levels of the PuICE1 were induced by cold, dehydration and salt, with the greatest induction under cold conditions. PuICE1 was localized in the nucleus and could bind specifically to the MYC element in the PuDREBa promoter. The PuICE1 fused to the GAL4 DNA-binding domain to have transcriptional activation activity. Ectopic expression of the PuICE1 in tomato conferred enhanced tolerance to cold stress at cold temperatures, less electrolyte leakage, less MDA content, higher chlorophyll content, higher survival rate, higher proline content, higher activities of enzymes. In additon, steady-state mRNA levels of six stress-responsive genes coding for either functional or regulatory genes were induced to higher levels in the transgenic lines by cold stress. Yeast two-hybrid, transient assay, split luciferase complementation and BiFC assays all revealed that PuHHP1 protein can physically interact with PuICE1. Taken together, these results demonstrated that PuICE1 plays a positive role in cold tolerance, which may be due to enhancement of PuDREBa transcriptional levels through interacting with the PuHHP1.

ICE1 of Pyrus ussuriensis functions in cold tolerance by enhancing PuDREBa transcriptional levels through interacting with PuHHP1

ICE1 transcription factor plays an important role in plant cold stress via regulating the expression of stress-responsive genes. In this study, a PuICE1 gene isolated from Pyrus ussuriensis was characterized for its function in cold tolerance. The expression levels of the PuICE1 were induced by cold, dehydration and salt, with the greatest induction under cold conditions. PuICE1 was localized in the nucleus and could bind specifically to the MYC element in the PuDREBa promoter. The PuICE1 fused to the GAL4 DNA-binding domain to have transcriptional activation activity. Ectopic expression of the PuICE1 in tomato conferred enhanced tolerance to cold stress at cold temperatures, less electrolyte leakage, less MDA content, higher chlorophyll content, higher survival rate, higher proline content, higher activities of enzymes. In additon, steady-state mRNA levels of six stress-responsive genes coding for either functional or regulatory genes were induced to higher levels in the transgenic lines by cold stress. Yeast two-hybrid, transient assay, split luciferase complementation and BiFC assays all revealed that PuHHP1 protein can physically interact with PuICE1. Taken together, these results demonstrated that PuICE1 plays a positive role in cold tolerance, which may be due to enhancement of PuDREBa transcriptional levels through interacting with the PuHHP1.

Tobacco streak virus (strain dahlia) suppresses post-transcriptional gene silencing of flavone synthase II in black dahlia cultivars and causes a drastic flower color change

Tobacco streak virus suppressed post-transcriptional gene silencing and caused a flower color change in black dahlias, which supported the role of cyanidin-based anthocyanins for black flower appearance. Black flower color of dahlia (Dahlia variabilis) has been attributed, in part, to the high accumulation of cyanidin-based anthocyanins that occurs when flavone synthesis is reduced because of post-transcriptional gene silencing (PTGS) of flavone synthase II (DvFNS). There are also purple-flowering plants that have emerged from a black cultivar 'Kokucho'. We report that the purple color is not caused by a mutation, as previously thought, but by infection with tobacco streak virus (TSVdahlia), which suppresses the PTGS of DvFNS. When TSVdahlia was eliminated from the purple-flowering 'Kokucho' by leaf primordia-free shoot apical meristem culture, the resulting flowers were black. TSVdahlia-infected purple flowers had lower numbers of siRNAs to DvFNS than black flowers, suggesting that TSVdahlia has a silencing suppressor. The graft inoculation of other black cultivars with TSVdahlia altered their flower color drastically except for 'Fidalgo Blacky', a very deep black cultivar with the highest amount of cyanidin-based anthocyanins. The flowers of all six TSVdahlia-infected cultivars accumulated increased amounts of flavones and reduced amounts of cyanidin-based anthocyanins. 'Fidalgo Blacky' remained black despite the change in pigment accumulation, and the amounts of cyanidin-based anthocyanins in its TSVdahlia-infected plants were still higher than those of other cultivars. We propose that black flower color in dahlia is controlled by two different mechanisms that increase the amount of cyanidin-based anthocyanins: DvFNS PTGS-dependent and -independent mechanisms. If both mechanisms occur simultaneously, the flower color will be blacker than if only a single mechanism is active.