key: cord-0716800-jop1hxox
authors: Liu, Qun; Li, Li; Cheng, Haitao; Yao, Lixiang; Wu, Jie; Huang, Hui; Ning, Wei; Kai, Guoyin
title: The basic helix-loop-helix transcription factor TabHLH1 increases chlorogenic acid and luteolin biosynthesis in Taraxacum antungense Kitag
date: 2021-09-01
journal: Hortic Res
DOI: 10.1038/s41438-021-00630-y
sha: 1e3c97211eda744d92b17776ce264cafe0e3f93d
doc_id: 716800
cord_uid: jop1hxox

Polyphenols are the main active components of the anti-inflammatory compounds in dandelion, and chlorogenic acid (CGA) is one of the primary polyphenols. However, the molecular mechanism underlying the transcriptional regulation of CGA biosynthesis remains unclear. Hydroxycinnamoyl-CoA:quinate hydroxycinnamoyl transferase (HQT2) is the last rate-limiting enzyme in chlorogenic acid biosynthesis in Taraxacum antungense. Therefore, using the TaHQT2 gene promoter as a probe, a yeast one-hybrid library was performed, and a basic helix-loop-helix (bHLH) transcription factor, TabHLH1, was identified that shared substantial homology with Gynura bicolor DC bHLH1. The TabHLH1 transcript was highly induced by salt stress, and the TabHLH1 protein was localized in the nucleus. CGA and luteolin concentrations in TabHLH1-overexpression transgenic lines were significantly higher than those in the wild type, while CGA and luteolin concentrations in TabHLH1-RNA interference (RNAi) transgenic lines were significantly lower. Quantitative real-time polymerase chain reaction demonstrated that overexpression and RNAi of TabHLH1 in T. antungense significantly affected CGA and luteolin concentrations by upregulating or downregulating CGA and luteolin biosynthesis pathway genes, especially TaHQT2, 4-coumarate-CoA ligase (Ta4CL), chalcone isomerase (TaCHI), and flavonoid-3′-hydroxylase (TaF3′H). Dual-luciferase, yeast one-hybrid, and electrophoretic mobility shift assays indicated that TabHLH1 directly bound to the bHLH-binding motifs of proTaHQT2 and proTa4CL. This study suggests that TabHLH1 participates in the regulatory network of CGA and luteolin biosynthesis in T. antungense and might be useful for metabolic engineering to promote plant polyphenol biosynthesis.

Dandelions (Taraxacum spp.) have been used as medicinal herbs and functional foods for several centuries 1,2 . The increasing demand for dandelion products, such as tea, wine, syrup, and coffee, has promoted the industrialization of dandelion production 3 . The total phenolic compound concentrations in different tissues of Taraxacum mongolicum ranged from 37.12 to 68.89 mg GAE/g [4] [5] [6] [7] . However, these levels in Taraxacum antungense have not been tested in previous studies. Polyphenolic compounds, including phenolic acids, flavonoids, and anthocyanins, have many biological activities [8] [9] [10] . In Taraxacum antungense Kitag, chlorogenic acid (CGA) and caffeic acid (CA) have antioxidative benefits, being hepatoprotective and having diuretic activities [11] [12] [13] ; rutin and luteolin are used to treat several diseases, such as Parkinson's disease, severe acute respiratory syndrome, hepatitis, and cancer [14] [15] [16] . However, the concentration of these functional active constituents in T. antungense is lower than that in other medicinal plants, such as Lonicera japonica and Eucommia ulmoides, which restricts dandelion industrialization 14 . Bioengineering strategies could potentially increase the polyphenolic compounds in Taraxacum; however, a better understanding of the polyphenolic compound biosynthesis pathway is required.

Since the 1990s, the polyphenol biosynthesis pathway has been reported in several medicinal model plants, such as Salvia miltiorrhiza, L. japonica, and Dendranthema morifolium 17, 18 . In the first three steps of polyphenol biosynthesis, the key enzymes are phenylalanine ammonia-lyase (PAL), 4-coumarate-CoA ligase (4CL), and cinnamate 4-hydroxylase (C4H), which catalyze the synthesis of p-coumaroyl-CoA from phenylalanine 19 . CA is directly catalytically synthesized by coumarate 3-hydroxylase from p-coumaroyl-CoA, whereas CGA is catalytically synthesized by hydroxycinnamoyl CoA quinate hydroxycinnamoyl transferase (HQT) and hydroxycinnamoyl-CoA shiki-mate/quinate hydroxycinnamoyl transferase (HCT) 20 . However, in T. antungense, only HQTs (TaHQT1/2) have been isolated, identified, and assessed, and TaHQT2 is the last ratelimiting enzyme in chlorogenic acid biosynthesis 21 . For flavonoids, p-coumaroyl-CoA is a branch and precursor compound. The first two steps in the catalytic synthesis of naringenin from p-coumaroyl-CoA are via chalcone synthase and chalcone isomerase (CHS and CHI, respectively) 8 . Naringenin is catalyzed by flavonoid synthase and flavonoid-3′-hydroxylase (FNS and F3′H, respectively) in plants to synthesize luteolin 8, 22 . Naringenin is also catalyzed by flavonol-3-dehydrogenase (F3H), flavonol synthetase (FLS), and UDP-glucoronosyl/UDP-glucosyl transferase (UFGT) to synthesize rutin [23] [24] [25] . PAL, CHS, CHI, and F3′H increase CGA and luteolin concentrations in L. japonica, whereas in Fagopyrum tataricum, the rutin concentration is increased by upregulation of FLS and UFGT gene expression 25, 26 . Identification of these polyphenol biosynthesis pathway genes will lay the foundation for further genetic engineering research in T. antungense (Fig. 1) .

Plant polyphenolic compounds are important components acting against biotic and abiotic stresses 21, 25 . Plant transcription factor (TF) family members, such as MYB11/12/111 and WRKY18/40/60, respond to biotic and abiotic stress to regulate the expression of downstream structural genes and ultimately promote the biosynthesis of polyphenols 18, 19, 27 . Coexpression analysis of TFs and biosynthesis pathway gene expression levels often showed a highly linear relationship 27, 28 . Thus, TFs may be used as a tool not only to improve a plant's ability to adapt to the environment but also to increase polyphenolic compound production in plants 21 .

Basic helix-loop-helix (bHLH) TFs are one of the largest families that regulate the expression of key enzyme genes Phenylalanine ammonia-lyase (PAL); cinnamate 4-hydroxylase (C4H); 4-coumarate-CoA ligase (4CL); Coumarate 3-hydroxylase (C3H); 5-O-(4coumaroyl)-D-quinate 3′-monooxygenase (C3′H); hydroxycinnamoyl-coenzyme (Co)A shikimate/quinate hydroxycinnamoyl transferase (HCT); hydroxycinnamoyl-CoA quinate hydroxycinnamoyl transferase (HQT); caffeoylshikimate esterase (CSE); UDP-glucosyltransferase (UGT84); Chalcone synthase (CHS); flavonoid synthase (FNS); Chalcone isomerase (CHI); and hydroxycinnamoyl D-glucose quinate hydroxycinnamoyl transferase (HCGQT). Three arrows indicate multi-steps & routes. Blue indicates what has been reported in this article, and gray indicates information from other plants in plants 28 . Among these TFs, myelocytomatosis oncogene (MYC) TFs (representative bHLH TFs) often participate in secondary metabolite accumulation: AaMYC2 (Artemisia annua L) regulates artemisinin biosynthesis, NtMYC2 (Nicotiana tabacum) regulates nicotine biosynthesis, and CrMYC2 (Catharanthus roseus) regulates the expression of alkaloid biosynthesis genes that respond to methyl jasmonate (MeJA) 29, 30 . In the medicinal model plant S. miltiorrhiza, SmMYC2a/2b regulates key enzyme genes for phenolic acid biosynthesis 31 . Upregulation of SmPAL by SmbHLH37 leads to increased phenolic acid accumulation 32 . Moreover, the MeJA-responsive SmbHLH53 TF regulates enzymatic genes involved in the salvianolic acid B biosynthesis pathway in S. miltiorrhiza 33 . However, the function of bHLH TFs in T. antungense polyphenol biosynthesis and their regulatory models have been poorly reported 34 .

TFs regulate gene expression levels by combining with cis-acting elements of functional gene promoters; bHLH TFs specifically bind to E-box 28, 35 . In T. antungense, the TaHQT2 gene promoter was obtained and was found to contain various cis-acting elements, specifically four E-boxes (CANNTG) (Supplementary Fig. S1 and Table  S2 ). E-boxes are widely distributed in the promoter region of key enzymes in polyphenol biosynthesis 36 . Therefore, it was speculated that bHLH TFs bind to the TaHQT2 promoter to participate in polyphenol biosynthesis in T. antungense.

In this study, a T. antungense bHLH TF, TabHLH1, which was obtained through yeast one-hybrid (Y1H) screening, had high homology to bHLH1 from the Gerbera hybrid cultivar. Polyphenol concentration analysis and quantitative real-time polymerase chain reaction (qRT-PCR) results showed that TabHLH1 increased CGA and luteolin biosynthesis by increasing TaHQT2, Ta4CL, TaCHI, and TaF3′H gene expression levels in T. antungense transgenic lines. Functional analysis of TabHLH1 suggested that it regulates the biosynthesis of CGA and luteolin, enhancing the understanding of the routes of polyphenol biosynthesis and providing a structure for future metabolic engineering of T. antungense.

To identify bHLH TFs involved in CGA biosynthesis of T. antungense, Y1H assays were applied to screen the T. antungense cDNA library, and the TaHQT2 promoter was used as bait. Approximately 860 bp of the TaHQT2 promoter sequence (proTaHQT2) was cloned after two rounds of amplification, and several elements were identified, including an androgen response element, TATAbox, CAAT-box, CGTCA/TGACG-motif, estrogenresponsive element, long terminal repeat, E-box, P-box, TGA-box, and light-responsive elements ( Fig. S1 and Table S2 ). A Y1H cDNA library of T. antungense was created with a titer of approximately 5 × 10 7 colonyforming units/mL. PCR results showed that the length of T. antungense cDNA ranged from 200-2000 bp (Fig. S2) . A 200 bp DNA fragment containing four E-boxes (pro-TaHQT2-E-box-1, -2, -3, and -4) that were identified in proTaHQT2 (from -685 to -810) within 860 bp of the ATG start codon was cloned. The isolated gene coding protein was able to bind to the proTaHQT2 CATGTG motif (Fig. 2) . The results also showed that pMutant-TaHQT2 interacted with the isolated gene coding protein.

The isolated protein acquired from the nonredundant protein database (using the Basic Local Alignment Search Tool for protein [BLASTP] ) showed high homology to the Gerbera hybrid cultivar bHLH1 (GhbHLH1: 69% identity and 78% positivity) and Gynura bicolor bHLH1 (GbbHLH: 66% identity and 76% positivity) (Fig. 3A) . The full-length TabHLH1 cDNA was 1566 bp and encoded a 521-amino acid protein (accession number: MH683054) (Fig. 3B ). Multiple sequence alignment identified a conserved bHLH-MYC_N domain and an HLH domain in TabHLH1 (pfam 14215 and pfam 00010) using the Pfam online tool (http://pfam.xfam.org/search/sequence). Based on these results, we deduced that TabHLH1 was a bHLH TF. Subcellular localization showed that the yellow fluorescent protein (YFP) (control group; empty vector) was distributed uniformly throughout the cell, whereas TabHLH1-YFP fluorescence (treatment group; TabHLH1-YFP vector) was observed in the nucleus. This was confirmed using the positive control nuclear stain 4′,6-diamidino-2-phenylindole (DAPI) (Fig. 3C ).

Biochemical analyses of the main polyphenol compounds in T. antungense and expression profiling of TabHLH1

The total phenolic concentrations of T. antungense in different tissue samples ranged from 32.37-66.23 mg GAE/g, and the highest concentrations of total phenolics were found in flowers, followed by leaves, roots, and stems ( Fig. 4A, B) . The four phenolic acids (CGA, CA, rutin, luteolin) in different tissues of T. antungense showed significant differences. CGA concentrations were highest in the roots, followed by flowers, stems, and leaves; CA concentrations were highest in the flowers, followed by leaves and flowers; rutin concentrations were highest in roots and flowers, followed by leaves and stems; luteolin concentrations were highest in leaves, followed by flowers and roots, while CA and luteolin were not detected in stems (Fig. 4C) .

To study the correlation between key polyphenol biosynthesis enzyme genes and TabHLH1 TF in different tissues, one-year-old wild flowering T. antungense was used for expression analysis by qRT-PCR. The results showed that HQT1 was highly expressed in roots; CHS, CHI, 4CL, FNS, and F3′H were highly expressed in flowers followed by leaves; HCT, PAL, HQT2, and C4H were highly expressed in leaves followed by flowers. For TabHLH1, the expression level was highest in leaves, followed by flowers, roots, and stems (Fig. 4D) .

To study the effects of various biotic and abiotic stresses on the expression level of TabHLH1, NaCl, MeJA, salicylic acid (SA), abscisic acid (ABA), ethephon (ET), and gibberellic acid (GA3) treatments were performed at different time points in T. antungense leaves, and then qRT-PCR was used for expression analysis. Under NaCl treatment (500 ng/mL), TabHLH1 expression increased more than 40-fold in 3 h, indicating that TabHLH1 was most responsive to NaCl treatment. After 3 h of ABA treatment, the expression level of TabHLH1 reached 13-fold and had the same tendency as its expression under NaCl treatment. The expression level of TabHLH1 was also affected by 100 μM MeJA; TabHLH1 expression in leaves increased at 3 h and then slowly declined to approximately 1.3-fold at 24 h. Similar results were obtained with SA, ET, and GA3 hormone treatments (Fig. 4E ).

To evaluate the regulatory mechanism of TabHLH1 in T. antungense polyphenol biosynthesis, the overexpression vector pRI101-TabHLH1-YFP was constructed and transformed into T. antungense leaves according to Liu et al. (2018) 14 . Seventeen independent transgenic lines were identified using p35SF as the forward primer (according to the 35S promoter sequence) pAbAi-E-box grew normally on 300 ng/mL aureobasidin A (AbA); however, pAbAi-pMutant-E-box was inhibited by 300 ng/mL AbA. Therefore, 400 ng/mL was the minimum concentration of AbA used to examine the interaction between TabHLH1 and the E-boxes. We used the pAbAi-E-box and pAbAi-pMutant-E-box to transform Y1H gold yeast and then made new Y1H competent cells. Subsequently, the pGADT7-TabHLH1 recombinant vector was transformed into new Y1H competent cells. The cotransformed pAbAi-E-box + pGADT7-TabHLH1 grew on 400-500 ng/mL AbA; however, pAbAi-pMutant-E-box + pGADT7-TabHLH1 was unable to grow and needed a higher concentration of AbA. The difference between the pAbAi-E-box and pAbAi-pMutant-E-box was that the E-box pAbAi-E-box contained CAAATG, CATTTG, CATGTG, and CAATTG motifs. In the pAbAi-pMutant-E-box, AAAAAA was used instead of the CATGTG motif and TabHLH1R as the reverse primer through genomic PCR (Table S1 ). Three transgenic lines (TabHLH1-OE2, TabHLH1-OE7, and TabHLH1-OE13) with higher expression levels of OE-TabHLH1 than the control lines were selected for further experiments ( were significantly higher than those in the wild-type (WT) line, while the expression levels of RNAi-TabHLH1 were lower than those in the WT transgenic lines (Fig. 5B) .

The concentrations of CGA, CA, rutin, and luteolin in the TabHLH1 transgenic lines were determined by highperformance liquid chromatography (HPLC). CGA concentrations increased in the transgenic lines compared to those in the control lines (0.73 ± 0.054 mg/g dry weight [DW]), with the highest concentration in OE-7 (1.18 ± 0.113 mg/g DW) and the lowest in RNAi-19 (0.34 ± 0.141 mg/g DW). There were no significant differences in CA and rutin concentrations between the transgenic and control lines (Fig. 5C ). Luteolin concentrations in OE-13 (7.07 ± 0.687 mg/g DW) increased compared to those in the control lines (4.83 ± 0.345 mg/g DW) and were lowest in RNAi-19 (3.84 ± 0.441 mg/g DW). To identify the CGA and luteolin biosynthetic genes regulated by TabHLH1, the expression levels of key enzyme genes in the CGA and luteolin biosynthesis pathways in T. antungense were first determined. Ta4CL, TaHQT2, TaCHI, and TaF3′H were all upregulated in the TabHLH1-overexpression lines to various degrees. TaHQT2 expression was the most significantly increased among the four upregulated genes (Fig. 5D ). In vivo and in vitro evaluation showed that TabHLH1 increased TaHQT2 and Ta4CL expression

The gene promoters of TaPAL, TaC4H, Ta4CL, TaHCT, and TaHQT1/2 from the CGA biosynthetic pathway all contain bHLH cis-acting elements 33 . Herein, dual-luciferase (LUC) assays were performed to investigate whether TabHLH1 increased the expression of these genes. Reporter and effector vector construction are shown in Fig. 6A . Fluorescence analysis indicated the intensity of gene expression (Fig. 6A ). Of the six examined genes, the ratio of LUC/Renilla (REN) was detected only for Ta4CL and TaHQT2 and was significantly higher than that of the control, with a 6.26-and 10.08-fold increase, respectively (Fig. 6B) . Furthermore, TabHLH1 binding sites (bHLH-responsive cis-elements) were detected using Y1H assays and electrophoretic mobility shift assays (EMSAs). TabHLH1 directly combined with the TaHQT2 and Ta4CL promoters through the E-box motif (CATGTG) (Fig. 6C, D) . These results indicated that TabHLH1 directly increased TaHQT2 and Ta4CL gene expression, thereby modulating CGA accumulation.

Luteolin is also produced in plants using phenylalanine as a precursor 16 . Hence, we measured luteolin concentrations in the transgenic lines. Luteolin concentrations were significantly higher in the OE-TabHLH1 transgenic line and lower in the RNAi-TabHLH1 line than in the control groups (Fig. 5C ). In addition, two of the luteolin biosynthetic pathway genes, TaCHI and TaF3′H, were strongly upregulated in the OE-TabHLH1 line, while TaCHI and TaF3′H were significantly downregulated in the RNAi-TabHLH1 line compared to that in the control groups (Fig. 5D ). Dual-LUC assays showed that TabHLH1 significantly increased TaCHI (harboring the CATGTG motif in their promoter) and TaF3′H (without the CATGTG motif) expression levels. The ratio of LUC/REN increased by 7.94-fold for TaCHI and 6.11-fold for TaF3′ H compared to that in the control groups (Fig. 6B) . However, Y1H assay results indicated that the E-box (CATGTG motif) in the TaCHI promoter cannot directly interact with TabHLH1 (Fig. 6C) . Together, these results indicated that TabHLH1 increased both CGA and luteolin concentrations in T. antungense transgenic lines.

In different tissues of T. antungense, the total polyphenol concentrations are highest among flowers, followed by leaves, roots, and stems, showing the same tendency as in T. mongolicum 4 . Total polyphenols, including chlorogenic, caffeic, quinic, caffeoylquinic, ferulic, cinnamic, caftaric, benzoic, vanillic, protocatechuic, gallic, cumaric acid, Fig. 6 Dual-LUC and EMSAs proved that TabHLH1 activates the expression of Ta4CL, TaHQT2, TaCHI, and TaF3′H. A Dual-LUC assays indicating that TabHLH1 increases the expression levels of Ta4CL, TaHQT2, TaCHI, and TaF3′H. B The relative fold change of LUC/REN (Renilla) indicates that the expression levels of Ta4CL, TaHQT2, TaCHI, and TaF3′H can be activated by TabHLH1. C Y1H assay results indicated that TabHLH1 directly binds to Ta4CL and TaHQT2 in yeast cells. The CATGTG motif has not been found in the promoter of TaF3′H. D EMSA results indicate that TabHLH1 directly binds to the E-box elements (CATGTG) from the TaHQT2 and Ta4CL promoters. The cold-probe sequences were the same as the labeled probes but without biotin labeling chrysoeriol, and vanillin, have been found in Taraxacum species 1 . Flowers and leaves have higher polyphenol concentrations than roots and stems. The CGA and CA levels in T. mongolicum were generally lower than those in T. antungense, while luteolin levels were significantly higher 4 . The possible reasons are various factors, including growth environments, sample extraction, and HPLC methods, which together cause significant differences in CGA, CA, rutin, and luteolin concentrations in T. antungense and closely related species 4, 21, 37 . The relationship among the four compounds and total polyphenol concentrations should be further studied.

First, the HQT2 gene promoter was used as bait to screen functional TFs. CGA is widely distributed in many plants, such as T. antungense, L. japonica, Solanum lycopersicum, and Solanum tuberosum 20, 21, 38 . In the CGA biosynthesis pathway, key enzyme genes, such as PAL, C4H, 4CL, and HCT/HQT, are widely reported 14, 20 . However, the regulation of CGA biosynthesis targeting the key enzyme HQT has not been previously reported 20, 21 . Gene promoters are the core link between TFs and structural gene expression [39] [40] [41] . Through analysis of the TaHQT2 promoter (four CANNTG motifs), it is speculated that TaHQT2 is regulated by bHLH TFs (Fig.  S1 and Table S2) 33, 41 . Herein, using proHQT2 as a probe, we first obtained a bHLH TF named TabHLH1 through Y1H assays.

Second, coexpression analysis can be used to screen and preliminarily identify the correlation between transcription factors and key enzyme gene expression 34, 40 . Under ABA treatment, SmbZIP1 and salvianolic acid biosynthesis pathway gene expression levels significantly increased, and coexpression analysis showed a highly linear relationship 40 . By analyzing TabHLH1 together with polyphenol biosynthesis pathway gene expression levels in T. antungense different tissues, it can be concluded that TabHLH1 potentially regulates the expression of key enzyme genes in polyphenol biosynthesis and thus affects the concentration of polyphenols in T. antungense 32, 33 . Therefore, polyphenol analysis, Y1H assays, and coexpression analysis provide a theoretical basis for screening related TFs for designing molecular breeding strategies to improve T. antungense quality.

In this study, full-length TabHLH1 was isolated and cloned from T. antungense and showed the highest identity with the Gerbera hybrid cultivar GhbHLH1. Both GbbHLH1 and GhbHLH1 play a vital role in the regulation of anthocyanin and dihydroflavonol accumulation 37, 42, 43 . In plants, anthocyanin and CGA have the same precursor and share the first three key enzymes 41 . In this study, salt stress significantly increased TabHLH1 gene expression levels, with the same tendency as that of GhbHLH1, which is consistent with earlier findings 42 . However, under ABA treatment, TabHLH1 expression increased more than 10-fold, which has not been reported in GbbHLH1 42 . A possible reason for this finding may be species evolution and functional redundancy in multiple gene families 31 . TabHLH1 is located in the nucleus, similar to other nucleus-localized bHLH TFs, such as AaMYC2-Like and MdMYC2 44, 45 . Therefore, TabHLH1 plays a vital role in hormone and salt stress signal regulation, which ultimately assists plants in their response to a variety of biotic and abiotic stresses in their natural environment 14, 26 . The relationship among hormones, salt stress signal regulation, and TabHLH1 expression should be further studied.

Multiple TFs, such as SmbHLH37 and SmbHLH53, increase polyphenol concentrations through PAL or other key enzymes 32, 33 . By analyzing the CGA concentrations in TabHLH1 transgenic lines (OE-7 and RNAi-19), CGA concentrations in the OE-7 transgenic line increased nearly 63.6% compared to that in the WT, while CGA concentrations in RNAi-19 decreased to 53.2%, suggesting that TabHLH1 significantly increased CGA concentrations in T. antungense. TaHQT2 was identified as the key enzyme that can directly synthesize CGA in previous studies 21 . In this study, Ta4CL and TaHQT2 gene expression levels were positively correlated with the CGA concentration. Furthermore, dual-LUC and EMSA demonstrated that TabHLH1 bound directly to the TaHQT2 promoter region. These results together demonstrated that TaHQT2 was a target of TabHLH1; thus, TabHLH1 played a positive role in regulating CGA biosynthesis.

p-Coumarin-CoA is a precursor compound for the synthesis of downstream phenolic acids, flavonoids, and anthocyanins 7, 42 . 4CL participates in the front-end enzymatic reaction of polyphenol biosynthesis and directly catalyzes the biosynthesis of p-coumarin-CoA (Fig. 1) . In this study, TabHLH1 increased the expression level of Ta4CL, thereby regulating metabolic flow in plants to accumulate polyphenols, which is consistent with CsbHLH1 directly activating the Cs4CL promoter 44, 46, 47 . Dual-LUC, Y1H, and EMSA demonstrated that TabHLH1 bound directly to the Ta4CL promoter region. These results suggested that Ta4CL was another target of TabHLH1.

Flavonoid biosynthesis regulation has been studied indepth, particularly in the medicinal plant S. miltiorrhiza 19, 33, 48 . Functional genes, including key enzyme genes and numerous TFs, are required for luteolin biosynthesis 29, 44 . In this study, TabHLH1 significantly increased luteolin concentrations (OE-7 increased to 1.46-fold, and RNAi-19 decreased to 0.78-fold) in transgenic lines compared to that in the WT. In addition, concentrations of the key luteolin biosynthesis enzymes TaCHI and TaF3′H were directly influenced by overexpression/RNAi of TabHLH1, suggesting that TabHLH1 may interact with TaCHI and TaF3′H. Dual-LUC results showed that TabHLH1 increased the expression levels of two pathway genes involved in luteolin biosynthesis. However, TabHLH1 binds indirectly to the promoters of TaCHI and TaF3′H (Fig. 6C) . The possible reason may be that TabHLH1 interacts with other proteins, such as MYB partners, which can directly interact with TaCHI and TaF3′H. These results indicated that TabHLH1 was positively involved in regulating luteolin biosynthesis and metabolic flow, and the direct target was Ta4CL, while the potential targets were TaCHI and TaF3′H. The MYB-bHLH-WD40 complex often increases the expression level of flavonoid biosynthesis genes to control the production of anthocyanins 49 . CsbHLH1 (CsMYC1) interacts with CsbHLH42, CsWD40, CsMYB60 and itself to regulate flavonoid biosynthesis in cucumber 47 . In T. antungense, further studies should focus on proteins that interact with TabHLH1 to regulate polyphenol biosynthesis.

Based on these results, a functional model for the role of TabHLH1 in polyphenol biosynthetic regulation in T. antungense is provided (Fig. 7) . In summary, TabHLH1 increased TaHQT2 and Ta4CL expression levels, leading to increased CGA concentrations. This is the first dandelion bHLH protein identified as being involved in CGA pathway regulation. In addition, TabHLH1 promoted the expression of luteolin biosynthesis genes (TaCHI and TaF3′H) to increase the accumulation of luteolin. These studies provide new insights into the role of TabHLH1 in the regulation of polyphenol biosynthesis. In addition, these findings lay the foundation for further exploration of the molecular mechanisms and potential functional genes of secondary metabolite biosynthesis in T. antungense.

T. antungense biomaterials were collected and transplanted in the greenhouse of our laboratory, as previously reported 34 . N. benthamiana and T. antungense seeds were sown in substrate/vermiculite (3/1)-admixture soil and transplanted in pots for 4-6 weeks for hormone treatment experiments. Plants were maintained at a constant temperature of 25°C under 16/8 h light/dark cycles for use in transient expression analysis 14 .

The CTAB method was used to extract dandelion plant DNA, and RNase was used to remove the remaining RNA.

Fusion primers and nested integrated PCR were used to obtain the TaHQT2 5′-end promoter 47, 50 . All primers are listed in Table S1 . PlantCARE (http://bioinformatics.psb. ugent.be/webtools/plantcare/html/) was used to analyze the promoter cis-element sequences.

The TaHQT2 promoter (four E-boxes located from -691 bp to -806 bp, Fig. S1 ) was constructed in pABAi to create the pAbAi-proTaHQT2 recombinant vector using the BamH I and Hind III restriction sites. BstBI was used to digest recombinant plasmids, linearized pAbAi-proTaHQT2 plasmids were transformed into the yeast strain (Y1H), and then, the resulting strains were tested on SD/-Ura media containing aureobasidin A (AbA) at concentrations ranging from 100-500 ng/mL.

A Y1H cDNA library of T. antungense was constructed using the Matchmaker™ one-hybrid library construction & screening kit PT3529-1 (PR732190, Takara Biomedical Technology Co., Ltd., Beijing, China). Yeast recombinant vector (GAL4-AD-sec) was extracted from the primary library and transformed into Y1H containing pAbAi-proTaHQT2. After rescreening on SD/-Leu-Ura medium with higher AbA concentrations than listed above, Fig. 7 Proposed model of the role of TabHLH1 in regulating polyphenol biosynthesis. Using the HQT2 promoter E-box region as a probe, a bHLH transcription factor (named TabHLH1) was obtained by yeast one-hybrid screening. TabHLH1 significantly promoted the expression levels of key enzyme genes in the polyphenol biosynthesis pathway, including 4CL, HQT, CHI, and F3′H, thus increasing CGA and luteolin accumulation positive pGADT7-sequence yeast strains were used for sequencing analysis. The PCR thermocycler program used was as follows: 94°C for 10 min, 40 cycles of denaturation, annealing, and extension (94°C for 30 s, 50°C for 45 s, and 72°C for 90 s, respectively), and a final extension at 72°C for 10 min.

A bHLH TF protein isolated from T. antungense, which was designated TabHLH1, was found to interact with the E-boxes of TaHQT2. The gene sequence was compared to Taraxacum kok-saghyz Rodin (accession number: GWHAAAAM043215) 36 . The complete coding sequence was obtained using homologous cloning. The target cDNA fragment was connected to the 18 T vector (Takara Biomedical Technology Co., Ltd., Beijing, China) for sequencing analysis. BLAST alignment (http://www.ncbi. nlm.nih.gov/BLAST/) was used to search for orthologs. ClustalX (version 1.81) was used for multiple sequence alignment through the neighbor-joining method using 1000 repetitions. A molecular phylogenetic tree was constructed using the MEGA program (version 8.0) 14 .

Methyl jasmonate (MeJA), salicylic acid (SA), ethephon (ET), gibberellin (GA), and abscisic acid (ABA) purchased from Sigma-Aldrich (Shanghai, China) were directly dissolved in distilled water at a final concentration of 100 mM. NaCl (Aladdin, Shanghai, China) was dissolved at a concentration of 500 mM, and distilled water was used as a control. Two-month-old well-grown T. antungense transgenic and WT plants were chosen for different treatments. The above elicitors were filter-sterilized through a 0.45 μm filter membrane (Pall Corporation, NY, USA) and added to the cultures at a final concentration of 100 μM. Tissues from different parts of the plants were collected after 0, 1, 3, 6, 9, 12, 16, and 24 h of treatment.

To identify the in vivo subcellular location of TabHLH1, the coding sequence of TabHLH1 was fused with a reporter sequence. The complete coding sequence of TabHLH1 (without the TAG stop codon), including the restriction sites Nde I (in the 5′-end) and Sal I (in the 3′end), was amplified and subcloned into the pMD19-T simple vector (Takara Biomedical Technology Co., Ltd., Beijing, China). The plasmid pRI101-YFP (containing the yellow fluorescent protein gene) was double-digested using the same enzymes to create a recombinant vector termed pRI101-TabHLH1-YFP, and the insert was sequenced using the 35S (forward) and TabHLH1R (reverse) primers ( Fig. S2 and Table S1 ). The fused recombinant expression plasmid was transformed into N. tabacum. pRI101-YFP was used as a control to perform the transient expression assay 21, 40 .

The pCAMBIA1300-35S-X (restriction sites: BamH I/ Spe I for sense and Kpn I/Sac I for antisense sequences) RNAi expression vector was used for RNAi-TabHLH1 recombinant plasmid creation 21 . The SPLRNAi gene was used as intron X. The middle region of TabHLH1 (631-839 bp) was used for vector construction (nonconserved region). Both pCAMBIA1300-35S-TabHLH1 and pRI101-TabHLH1-YFP were transformed into Agrobacterium tumefaciens strain GV3101. pCAMBIA1300-35S-X and the pRI101-YFP vector were used as controls. After positive identification, Agrobacterium harboring different recombinant plasmids were used for injection into plants. Following a previously published protocol 14 , Agrobacterium infection was used for genetic transformation to obtain T. antungense transgenic plants.

Different tissues or transgenic lines of T. antungense were used for total RNA extraction, followed by cDNA synthesis, which was performed following the abovementioned methods 50, 51 . qRT-PCR was performed using gene-specific primer pairs for PAL, C4H, 4CL, HCT, HQT2, CHS, CHI, and F3′H (Supplemental Table S1 ) using three technical replicates. Based on the 2 −ΔΔCt method, qRT-PCR was performed, and relative expression levels were calculated using β-actin as a reference gene 14 .

HPLC was used to investigate the concentrations of four polyphenols (CGA, CA, rutin, and luteolin) in the T. antungense plant materials from the control groups and transgenic lines (Table S3) . For transgenic lines, 3-monthold whole plants (containing roots and leaves) were dried and dehydrated at −20°C to constant weight, ground into powder, and then used as samples. Samples were ultrasonically extracted for polyphenol compounds and passed through a 0.22 μm filter membrane for HPLC, as previously described 14, 23 . HPLC conditions for polyphenol detection were as described in the previous reports 34 .

The total phenolic concentrations of T. antungense in different tissues were extracted with Folin-Ciocalteu reagent as previously reported 4, 5 . Then, 500 µL of T. antungense extract was added to 1.5 mL FC reagent (0.2 mg/mL) and mixed. Two milliliters of 7.5% Na 2 CO 3 reagent and 2 mL distilled water were added. Then, the mixture was incubated at 25°C for 1 h (in the dark). The absorbance of the mixture was recorded at 727 nm, and 60% methanol was used as a control. The total phenolic concentrations of T. antungense samples were calculated according to milligrams of gallic acid equivalents per gram dry weight of the sample (mg GAE/g) 4

The pCAMBIA2300 + -TabHLH1 vector acted as an effector and was transferred into A. tumefaciens strain GV3101 (pCAMBIA2300 + -vector was used as a control). The promoters of key enzyme genes, including Ta4CL, TaHQT2, TaCHI, and TaF3′H, were cloned separately into the pGreen0800-rec plasmid. The pGreen0800promoter recombinant vectors, separately with the helper vector pSoup19, were cotransformed into GV3101. The Renilla vector was used as an internal control. Both the reporter and effector strains were mixed in equal proportions (3 mL each), slowly cultivated for 2 h, and injected into the leaves of 2-month-old N. benthamiana. After incubation in the dark for 2-3 days, commercial dual-LUC reaction reagents (Promega Biomedical Technology Co., Ltd., Beijing, China) were used to perform dual-LUC assays on leaf samples 18 . Three biological replicates were measured for each sample.

The Y1H assay was performed differently from the Y1H screening and used the pB42AD/pLacZ system, which has been previously described 40 . The full-length TabHLH1 ORF fragment was amplified, sequenced, and cloned into the effector plasmid (pB42AD). For the reporter plasmid pLacZ, a triple tandem copy of the E-box (NNNNCATGTGNNNN) motif near 4 bp from every promoter (TaHQT2, Ta4CL, TaCHI) was inserted by using EcoR I and Xho I as restriction endonuclease sites. After identification by sequencing, recombinant effector plasmids (pB42AD-TabHLH1) and recombinant reporter plasmids (pLacZ-TaHQT2, pLacZ-Ta4CL, pLacZ-TaCHI) were cotransformed into yeast strain EGY48a. Transformants were cultivated on SD/-Ura/-Trp medium for 2 d and then transferred (by using distilled water-diluted 1000-fold) to SD/-Ura/-Trp medium with 5-bromo-4chloro-3-indolyl-β-D-galactopyranoside (X-gal) for another 1-2 d. Empty pB42AD and pLacZ plasmids were used as negative controls and cotransformed into EGY48a strains.

The complete sequence of TabHLH1 was inserted into the BamH I and Sal I sites of the pGEX4T-1 plasmid and then transformed into Escherichia coli (BL21 or DE3 strain). Isopropyl-D-thiogalactoside was used to induce recombinant protein expression overnight (16 h) , and the GSTtagged protein purification kit (Transgen Biotech Co., Ltd., Beijing, China) was used to purify recombinant proteins. Biotin-labeled 5′-and 3′-ends of the TaHQT2 promoter were synthesized by Shanghai Sangon Co. (Shanghai, China), and the two biotin-labeled primers were annealed to form double-stranded DNA fragments. The purified recombinant proteins and DNA fragments were incubated in 10× EMSA binding buffer (Beyotime Biotechnology Co., Ltd., Shanghai, China) at 25°C for 30 min. DNA fragments without biotin labeling were used as an internal control. The DNA-protein complex was electrotransferred to a wet electromembrane and examined following the manufacturer's instructions by using a chemiluminescent nucleic acid detection module kit (Beyotime Biotechnology Co., Ltd., Shanghai, China) 19, 27, 48, 52 .

Statistical comparisons were performed using SPSS v19.0 software. Error bars represent the SE of three biological replicates. All data are presented as the mean ±standard deviation (SD). Statistical significance was assessed using Student's t test (#: decrease, *: increase, P < 0.05) for all the experiments involved in this article (for the different tissues, transgenic lines, and control groups).

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The authors declare no competing interests.

The online version contains supplementary material available at https://doi.org/10.1038/s41438-021-00630-y.