Genetic variation and metabolic pathway intricacy govern the active compound content and quality of the Chinese medicinal plant Lonicera japonica thunb
- Yuan Yuan†1,
- Lipu Song†2,
- Minhui Li1, 4,
- Guiming Liu2,
- Yanan Chu2,
- Luyu Ma3,
- Yuanyuan Zhou2,
- Xiao Wang1, 5,
- Wei Gao1,
- Shuangshuang Qin1,
- Jun Yu2,
- Xumin Wang2Email author and
- Luqi Huang1Email author
© Yuan et al.; licensee BioMed Central Ltd. 2012
Received: 9 January 2012
Accepted: 2 May 2012
Published: 20 May 2012
Traditional Chinese medicine uses various herbs for the treatment of various diseases for thousands of years and it is now time to assess the characteristics and effectiveness of these medicinal plants based on modern genetic and molecular tools. The herb Flos Lonicerae Japonicae (FLJ or Lonicera japonica Thunb.) is used as an anti-inflammatory agent but the chemical quality of FLJ and its medicinal efficacy has not been consistent. Here, we analyzed the transcriptomes and metabolic pathways to evaluate the active medicinal compounds in FLJ and hope that this approach can be used for a variety of medicinal herbs in the future.
We assess transcriptomic differences between FLJ and L. japonica Thunb. var. chinensis (Watts) (rFLJ), which may explain the variable medicinal effects. We acquired transcriptomic data (over 100 million reads) from the two herbs, using RNA-seq method and the Illumina GAII platform. The transcriptomic profiles contain over 6,000 expressed sequence tags (ESTs) for each of the three flower development stages from FLJ, as well as comparable amount of ESTs from the rFLJ flower bud. To elucidate enzymatic divergence on biosynthetic pathways between the two varieties, we correlated genes and their expression profiles to known metabolic activities involving the relevant active compounds, including phenolic acids, flavonoids, terpenoids, and fatty acids. We also analyzed the diversification of genes that process the active compounds to distinguish orthologs and paralogs together with the pathways concerning biosynthesis of phenolic acid and its connections with other related pathways.
Our study provides both an initial description of gene expression profiles in flowers of FLJ and its counterfeit rFLJ and the enzyme pool that can be used to evaluate FLJ quality. Detailed molecular-level analyses allow us to decipher the relationship between metabolic pathways involved in processing active medicinal compounds and gene expressions of their processing enzymes. Our evolutionary analysis revealed specific functional divergence of orthologs and paralogs, which lead to variation in gene functions that govern the profile of active compounds.
KeywordsRNA-seq Transcriptome Active compounds Synthetic pathways Flos Lonicerae Japonicae
Flos Lonicerae Japonicae (FLJ, Lonicera japonica Thunb.) is used as a herbal medicine with anti-inflammatory effect . The first record in the literature on this herb is found in one of the world earliest pharmacopoeias, the Shen-Nong’s Herbals. The commercial value of FLJ in herbal medicine trading markets has increased over 400% in recent years, and over 30% of current traditional Chinese medicine prescriptions contain FLJ. This important herb is used to treat various diseases, such as severe acute respiratory syndromes, H1N1 influenza, and hand-foot-and-mouth disease. FLJ extracts also show other biological and pharmaceutical properties, including, anti-bacterial, anti-inflammatory, anti-viral, liver protection , anti-angiogenic, and antinociceptive activities . However, the quality of FLJ as a medicinal herb is rather inconsistent and largely due to its uncharacterized active compound content.
Chlorogenic acid and luteoloside are biomarkers used by the Chinese Pharmacopoeia (Chinese Pharmacopoeia Commission, 2010) for evaluating the quality of FLJ. Pharmacological experiments show that luteolin has a spectrum of biological activities, particularly antioxidative and anti-inflammatory properties. Luteolin has a direct inhibitory effect on lung fibrosis . Other phenolic compounds, including phenolic acids, have been identified in the methanolic extract of FLJ by liquid chromatography with time-of-flight mass spectrometry . Among these compounds, loganin and sweroside also exhibit anti-inflammatory and analgesic activities, which are candidate active components of the FLJ extract . Rutin is demonstrated to reduce oxidative stress-mediated myocardial damage in both in vitro and in vivo models and may prove beneficial in the treatment of myocardial infarction .
FLJ has other commercial applications, such as scent additive used in foods and cosmetics . The main volatile component is linalool, but other floral volatile compounds, including germacrene D, cis-jasmone, E,E-α-farnesene, nerolidol, cis-3-hexenyl acetate, hexyl acetate, cis-hexenyl tiglate, and indole, have been detected based on headspace analysis and gas chromatography–mass spectrometry (GC-MS) .
The concentrations of active components and volatile compounds are closely correlated with floral developmental stages . The content is higher in the early stages of cell differentiation status and the flower buds often show the highest medical value, whereas the chlorogenic acid content is significantly reduced in blooming flowers. However, the budding period is both short and not synchronized among individual plants, making it problematic for mass harvest.
A limited number of publications have assessed the relationship between the abundance of active compounds and floral development of FLJ based on molecular biological techniques. One of the studied showed that hydroxycinnamoyl-CoA quinate hydroxycinnamoyl transferase (HQT) gene, encoding a protein of 439 amino acids and identified in FLJ, has a tissue distribution that correlated with the pattern of chlorogenic acid abundance . Another study cloned multi-copied allene oxide synthase LjAOS (GenBank accession: DQ303120) from FLJ and demonstrated that LjAOS mRNA is most abundant in flower buds, and its expression correlates with the concentration of chlorogenic acid .
The content of active compounds also differs significantly among the species and varieties  of the Lonicerae family. L. japonica Thunb. var. chinensis (Watts) (rFLJ) is a Chinese local variety and its corolla has purple outer (upper) and white inner (lower) portions and the whole flower has different active compound contents when compared with FLJ . Qin et al. reported different chlorogenic acid contents between FLJ and rFLJ. Changes of the active compound contents often result in different pharmacological activity and medicinal quality . However, a systematic study to compare gene expression and active compound variations in the active parts among varieties and closely related plant species is of essence.
The high-throughput of the next generation RNA sequencing (RNA-seq) technologies offers rapid genome-wide transcriptomic studies and is widely used to define gene structure and expression profiles in model organisms [17–19]. The assembly of de novo transcriptomes based on short reads generated from RNA-seq method allows gene discovery in organisms without reference genomes. In this study, we applied RNA-seq to the study of floral transcriptomes of FLJ and rFLJ. We generated over 100 million reads using the Illumina GAII platform, and analyzed over 6,000 expressed genes from each of the three flowering stages: bud, blooming, and bloomed. We correlated the transcriptomic profiles with metabolic activities of the relevant active compounds, including phenolic acids, flavonoids, terpenoids, and fatty acids, to elucidate effects of enzymatic divergence on biosynthetic pathways.
RNA isolation and sequence acquisition
Total RNA was extracted from flower samples by using Concert Plant RNA Reagent (Invitrogen, Cat. 12322–012) according to the manufacturer’s protocol. RNA integrity was measured by using gel electrophoresis and spectrophotometer (Nonodrop). An Oligotex-dT30 Super mRNA Purification Kit from TaKaRa (D9086) was used to extract mRNA.
De novo sequence assembly and contig clustering
Prior to assembly and mapping, we removed low quality reads (such as >30% “N” in a read and polyA tails) from the raw data and assembled the processed reads into contigs using ABySS http://www.bcgsc.ca/platform/bioinfo/software/abyss; . We used contigs longer than 100 bp for further annotations. Since the genome sequence of FLJ has not been available, we used BLAST  to align the contigs to the NCBI non-redundant sequence database. Because V. vinifera full-length cDNA sequences provided the most annotations, we clustered the FLJ/rFLJ contigs in reference to the Vitis vinifera cDNA sequences.
Gene annotation and expression analysis
We used BLASTX  to search against the NCBI non-redundant (nr) database to identify transcripts and annotated the transcripts using KEGG  and COG  with an E value cut-off of 10−5. We applied InterPro  and Blast2GO  to the annotation of protein motifs/domains and Gene Ontology (GO) terms. GO annotation enrichment analyses were conducted based on a Benjamini and Hochberg false discovery rate correction with significance set at p < 0.05 by using the Cytoscape plug-in BiNGO.
We mapped the sequence reads and contigs using SOAP http://soap.genomics.org.cn/soapaligner.html;  and handled isoforms/spliced variants with cautions . We used sequence similarity information and the Vitis vinifera full-length cDNAs for transcriptome mapping and tag counting using LASTZ  after clustering the contigs into ESTs. Only uniquely mapped reads were counted. The expression profiling was done by normalizing the total mapped reads and contig length as RPKM (reads per kilobase of transcript sequence per million mapped reads; 19). The effective size was used to adjust RPKM values in subsequent analyses. DEGseq1.2.2  was used to roughly identify the differentially expressed genes (DEGs) via the p value and the RPKM fold-change value. The DEGs were further studied based on pathway/expression analyses and real-time PCR.
Gas chromatography–mass spectrometry profiling
The concentrations of ethanol, acetate, alkane, and terpene in the flower samples were determined based on gas chromatography–mass spectrometry (GC-MS) ( Additional file 1: Figure S1). Fresh flower samples (1.00 g, n = 3) were washed twice with distilled water, subjected to ultrasonic extraction (90 Hz) with 10 ml ethyl acetate for 40 minutes, and filtered through a microfiltration membrane (0.45 μm). Extracted metabolites were analyzed as follows: 1 μl of sample was injected at a split ratio of 10:1 into a Shimadzu GCMS-QP2010 instrument. A VF-5MS capillary column coated with 5% phenyl and 95% dimethylpolysiloxane (30 m × 0.25 mm i.d. and 0.1 μm film thickness; Varian, USA) was employed for separation. Injection temperature was 230°C and the interface temperature was set to 250°C. The ion source was adjusted to 230°C and the solvent cut-time was set to 3 minutes. Helium was the carrier gas at a flow-rate of 1.05 ml minute−1. The temperature program was: an initial temperature of 50°C, programmed at 5°C minutes−1 to 150°C and held for 10 minutes, then ramped at 10°C minute−1 to 260°C and held for 20 minutes. The mass spectrometric detector was operated in the electron impact ionization mode with an ionizing energy of 70 eV, scanning from 40–400 m/z. Peak identification was performed by employing AMDIS (NIST, Gaithersburg, MD, USA) and WILEY7n (Palisade Corporation, NY, USA) databases with a spectral match quality >90%. An internal standard of pentadecanol was added to correct for differences in derivatization efficiency and changes in sample volume during heating. Peaks were quantified by area integration and concentrations were normalized to the quantity of the internal standard recovered. Two technical replicates were analyzed for three biological samples from each flowering stage.
The dried flowers were separately comminuted with a miller. Each solid sample (40 mesh, 0.50 g) was accurately weighed and extracted with 50 ml of 70% aqueous ethanol by ultrasonication for 30 minutes. The extract was cooled to 25°C and diluted to 50 ml with 70% aqueous ethanol, filtered with a 0.45 μm Millipore filter membrane. Then, 10 μl of the filtrate was injected into the HPLC system for analysis ( Additional file 1: Figure S2).
The HPLC system was an Agilent 1200LC series (Agilent Technologies, Palo Alto CA, USA), consisting of an online vacuum degasser (G1379B), a Bin pump SL (G1312B), an auto-sampler (GB67C), a thermostatic column compartment (G1316B), and a diode-array detector (DAD) (G1315C) coupled with an analytical workstation. The column configuration was an Agilent TC-C18 reserved-phase column (5 μm, 250 mm × 4.6 mm). The sample injection volume was 10 μl. The detection wavelength was set at 242 nm for analysis with the flow rate at 1.0 ml minute−1, and the column temperature remaining at 25°C. The mobile phase contained deionized water, formic acid (A; 99:1, v/v), and methanol (B). The elution conditions are shown in Additional file 2: Table S1. To determine the linearity of the chromatographic techniques, calibration plots of eight standards were constructed on the basis of peak areas (y) using seven different concentration solutions ( x). All plots were linear in the examined ranges, and linear ranges were shown at different concentrations for the standard compounds (μg ml−1). The r value in Additional file 2: Table S2 refers to the correlation coefficient of the equation for calculating the content of compound. All the standard compounds showed good linearity (r > 0.9994) in a relatively wide concentration range. The standard compounds, chlorogenic acid (110753), caffeic acid (110885), ferulic acid (110773), rutin (100080), huteoloside (111720), Hyperoside (111521), quercitrin (111538), and quercetin (100081), were purchased from National Institutes for Food and Drug Control in China.
Principal component analysis and statistical assessment of GC-MS and HPLC data
Data sets containing more than two independent biological replicates per samples were statistically analyzed based on the Student’s t test with a significance cutoff of p < 0.05. To assess the metabolic changes or differences between samples and to identify metabolic changes involved in group discrimination, multivariate analyses (PCA, PLS-DA) were performed by using the SIMCA-P + (184.108.40.206.0) program (Umetrics AB, Tvistevdgen 48 Umea 907 19, Sweden).
Phylogeny and identification of paralogs and orthologs
We used the PFAM database  for validating all the gene families and protein sequences and constructed neighbor-joining trees for all sequences (ClustalW2). To identify orthologs, we performed an all-against-all sequence comparison using BLAST (cutoff <10-20) and determined orthologs from the best reciprocal hits >80% alignment length; .
Experimental validation of transcribed sequences
We used RNA samples extracted from the flower samples of FLJ and rFLJ to perform qRT−PCR and M-MLV reverse transcriptase cDNA Synthesis Kit from Takara. The PCR primers are shown in Additional file 2: Table S5. The amplification condition was set as follows: initiated by 1-minute incubation at 95°C, followed by 35 cycles at 95°C for 15 seconds, 57–60°C for 30 seconds, and 68°C for 30 seconds. PCR results were evaluated by using 2–3% NuSieve agarose gels.
Paired-end sequencing and de novo assembly
Summary of sequencing and assembling data
Average Read length (bp)
Number of contigs
Average contigs length (bp)
N50 Of contigs
We employed ABySS (http://www.bcgsc.ca/platform/bioinfo/software/abyss; 20), an assembler developed specifically for the next-generation short-read sequences, to assemble the processed sequences and obtained a total of 180,220 contigs, ranging from 25,232 to 41,796 for each library. We assembled all reads using the SOAP aligner tool , allowing up to two base mismatches. About half (51,607,107 reads) of the total reads are mapped to the contigs, and 49,821,911 reads remain unmapped. Specifically, 11,434,981 (46.01%) reads are mapped to the contigs in the rFLJ bud; 8,202,791 (56.30%) to rFLJ flower2; 17,927,893 (56.27%) to FLJ bud; 8,943,545 (54.21%) to FLJ flower2; and 4,697,897 (36.30%) to FLJ flower1. The average contig lengths are less than 1,000 bp, but the N50 contig sizes are over 1,000 bp for all libraries.
Gene annotation and expression analysis
We used the available public information of plant genes and genomes for annotation and performed a similarity search against the Genbank non-redundant protein database (Genbank nr) using the BLASTx algorithm [21, 33] with an E-value threshold of 10−5 and a size threshold >100 bp. We have 119,965 contigs (66.64%) shown significant similarity to known proteins based on 45,549 unique proteins. Based on the BLAST search, 86% of the contigs show similarities in the six plant species, including Vitis vinifera Ricinus communis Populus trichocarpa Arabidopsis lyrata Glycine max, and Nicotiana tabacum ( Additional file 1: Figure S3), and the fractions of sequences that match to what in V. vinifera are more than 50% for all five libraries. Due to the absence of genome information for FLJ, the full-length cDNA set of V. vinifera (RefSeq project by NCBI) served as the best reference for clustering and combining FLJ and rFLJ data. Moreover, our results indicate that the proportion of the sequences with matches in the Genbank nr database is greater among the longer contigs. For instance, we observed 98.6% matching efficiency for the sequences longer than 2,000 bp but it decreased to 50.8% when the sequence lengths dropped to 100 to 500 bp. The matching efficiencies for the sequences ranging in 500–1,000 bp, 1,000–1,500 bp, and 1,500–2,000 bp, are 90.5%, 96.6%, and 98.2%, respectively.
We defined the FLJ/rFLJ genes using LASTZ  and V. vinifera full-cDNAs (15,674 V. vinifera genes and E value of 10−10) as the reference. Fragmented genes were also identified and joined as ESTs. The FLJ/rFLJ transcriptomes were defined based on the criterion: at least one contig mapped to a reference gene. Nearly 30% of the total reference genes have matches to the FLJ/rFLJ contigs. Finally, we have 5,480, 5,310, 5,818, and 5,131 unigenes identified in rFLJ bud, rFLJ flower2, FLJ bud, and FLJ flower2, respectively. Only the FLJ flower1library (4,483 genes) has less than 5,000 unigenes identified.
We carried out functional and pathway analyses using the Kyoto Encyclopedia of Genes and Genomes (KEGG; http://www.genome.jp/kaas-bin/kaas_main?mode=est_b;22), and 180,020 sequences with significant matches were assigned to 276 KEGG pathways. Of the total, 21,692 unigenes have enzyme commission (EC) numbers ( Additional file 2: Table S3). We attempted to map major compounds that are involved in the biosynthesis of phenylalanine, terpenoid backbone, and fatty acid to the citric acid cycle, glycolysis, and sucrose metabolic pathways based on sequence homologies to the known plant genes ( Additional file 1: Figure S4). We categorized a total of 1,321 unigenes involved in the biosynthetic pathways. In addition to genes assigned to metabolic pathways, 18,417 unigenes are attributable to functions in genetic information processing, membrane transport, signal transduction, immune system, and environmental adaptation. These results demonstrated the power of high-throughput sequencing in identifying novel genes in non-model organisms, providing a valuable resource for investigating specific processes, functions, and pathways involved in active compound formation and their accumulation in FLJ flowers.
Analysis of differentially-expressed genes (DEGs)
We calculated gene expression level based on unique read counts and RPKM values (reads per kilobase of exon model per million mapped reads) for each contig and ESTs (the length of the ESTs was the exon length for the contigs). Two methods were used to define DEGs: DEGseq software based on p value estimates and 2-fold RPKM differences between the two libraries as a threshold for each gene. The numbers of DEGs between different datasets are: (1) 2,316 between the FLJ bud and flower1, (2) 1,713 between the FLJ bud and flower2, (3) 1,163 between FLJ flower1 and flower2, (4) 666 between rFLJ and FLJ bud, and (5) 692 between rFLJ and FLJ flower2.
We identified 262 (26%) up-regulated and 663 (67%) down-regulated genes in the flower bud in comparison with flower1 and flower2 in FLJ (the group 1 comparison; Additional file 1: Figure S5). These DEGs are concentrated on certain pathways, such as biosynthesis of plant hormones (41 DEGs), biosynthesis of terpenoids and steroids (28 DEGs), ribosomes (25 DEGs), biosynthesis of phenylpropanoids (24 DEGs), and biosynthesis of alkaloids derived from terpenoids and polyketides (21 DEGs). Up-regulated genes are absent in certain pathways, such as fatty acid and unsaturated biosyntheses. The total number of down-regulated genes in the three fatty acid-related pathways is 12, which is higher than that found in other pathways without up-regulated genes ( Additional file 1: Figure S6).
One of the down-regulated DEGs in pathogenesis (GO:0009405) is glyceraldehyde-3-phosphate dehydrogenase (G3PD, EC:220.127.116.11; Additional file 1: Figure S7). G3PD catalyzes the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate in glycolysis and plays a critical role in the control of plant metabolism and development . Munoz-Bertomeu reported  that the expression and catalytic activity of G3PD in anthers are necessary for mature pollen development of Arabidopsis. Exine formation in developing pollen exhibits an intricate pattern, primarily comprised of a polymer of fatty acids and phenolic compounds . We show here that the transcription level of sc_FLJ_007660 is 9.57-fold and 27.54-fold higher in the group 1 comparison.
The up-regulated DEGs are involved in transport (GO:0006810), transmembrane transporter activity (GO:0022857), and substrate-specific transporter activity (GO:0022892). One of the DEGs involved in localization (GO:0051234) is a regulator of Vps4 activity in the MVB protein pathway and related to pollen tube growth  ( Additional file 1: Figure S7). Rapid pollen tube growth requires a high rate of sugar metabolism to meet energetic and biosynthetic demands . The transcription level of sc_FLJ_015256 is 0.35-fold and 0.43-fold in the group 1 comparison and the result suggests a distinct sucrose metabolism.
DEGs and their related metabolic pathways
Phenolic acid pathway
In the group 1 comparison, we obtained five DEGs in terpenoid backbone biosynthesis. The transcription levels of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR, [EC:18.104.22.168]) in the mevalonate pathway, and 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase [EC:22.214.171.124], isopentenyl-diphosphate delta-isomerase [EC:126.96.36.199], (E)-4-hydroxy-3-methylbut-2-enyl-diphosphate synthase [EC:188.8.131.52], and 4-hydroxy-3-methylbut-2-enyl diphosphate reductase [EC:184.108.40.206] in the plastid MEP/DXP pathway are all down-regulated. Furthermore, the transcription levels of six DEGs (EC:220.127.116.11, EC:18.104.22.168, EC:22.214.171.124, EC:126.96.36.199, EC:188.8.131.52) are down-regulated but farnesyl diphosphate synthase [EC:184.108.40.206 220.127.116.11] is up-regulated. The combination of the different trends results in an increase in germacrene D content in FLJ buds.
Fatty acid pathway
The key enzymes involved in fatty acid metabolism, aldehyde dehydrogenase [EC:18.104.22.168,1 22.214.171.124], acyl-CoA oxidase [EC:126.96.36.199], and long-chain acyl-CoA synthetase [EC:188.8.131.52], are all down-regulated in the group 1 comparison. Other DEGs, such as 3R-hydroxymyristoyl ACP dehydrase [EC:4.2.1.-], acyl-ACP desaturase [EC:184.108.40.206], fatty acyl-ACP thioesterase B [EC:220.127.116.11 3.1.2.-], acyl-CoA oxidase [EC:18.104.22.168], and omega-3 fatty acid desaturase (delta-15 desaturase) [EC:1.14.19.-], involved in the biosynthesis of fatty acids and unsaturated fatty acids, are also down-regulated. In addition, five DEGs (EC:1.14.19.-, EC:1.3.1.-, EC:22.214.171.124/3.1.2.-) involved in fatty acid and unsaturated fatty acid biosynthesis are also down-regulated in the group 2 comparison. Our expression data again are in complete agreement with the metabolic data (Figure 4).
Our group 1 comparison results lead to the identifications of down-regulated DEGs: eight (EC:126.96.36.199, EC:188.8.131.52, EC:184.108.40.206, EC:220.127.116.11, EC:18.104.22.168, EC:22.214.171.124, EC:126.96.36.199/188.8.131.52) in the citrate cycle, eleven (EC:184.108.40.206, EC:220.127.116.11, EC:18.104.22.168, EC:22.214.171.124, EC:126.96.36.199, EC:188.8.131.52, EC:184.108.40.206, EC:220.127.116.11/18.104.22.168) in glycolysis/gluconeogenesis, and five (EC:22.214.171.124, EC:126.96.36.199, EC:188.8.131.52, EC:184.108.40.206 and EC:220.127.116.11) in starch and sucrose metabolism. In addition, we found five up-regulated genes in the group 1 comparison: succinate dehydrogenase (ubiquinone) iron-sulfur protein [EC:18.104.22.168], pyruvate kinase [EC:22.214.171.124], α, α-trehalose-phosphate synthase (UDP-forming) [EC:126.96.36.199], 4-α-glucanotransferase [EC:188.8.131.52], and beta-amylase [EC:184.108.40.206]. In the group 2 comparison, we defined five down-regulated genes (EC:220.127.116.11, EC:18.104.22.168, EC:22.214.171.124, EC:126.96.36.199, EC:188.8.131.52, and EC:184.108.40.206) and they are involved in the citrate cycle, glycolysis/gluconeogenesis, and starch and sucrose metabolism. In contrast, we have eight up-regulated genes: EC:220.127.116.11, EC:18.104.22.168, EC:22.214.171.124, EC:126.96.36.199, EC:188.8.131.52, EC:184.108.40.206, EC:220.127.116.11, EC:18.104.22.168, and EC:22.214.171.124.
We also validated some less-abundant DEGs as representatives for most of the above-mentioned pathways using qRT-PCRs, and the results are consistent with the RNA-seq data ( Additional file 2: Table S5).
Integrated analysis of active compound abundance and gene expression
The MEP/DXP and MVA pathways are thought to be interdependent during the biosynthesis of isoprenoids . Sesquiterpene is synthesized by the MVA pathway during the isoprenoid biosynthetic system in plants and the HMGR multi-gene family catalyzes the synthesis of mevalonate, which is a precursor in this pathway . The inducible HMGR enzyme activity is required for sesquiterpene accumulation in tobacco cell suspension cultures . The transcription level of HMGR has no effect and farnesyl diphosphate sythase (FPS) is only transiently induced after emission of (−)-germacrene D in poplar . However, our results suggest that the content of sesquiterpene may be negatively controlled by HMGR and the up-regulation of FPS may increase the production of germacrene D.
Analysis of active compounds in FLJ and rFLJ
GC-MS analysis of volatile compounds
Methyl glycol phthalate
Docosanoic acid, methyl ester
Tetracosanoic acid, methyl ester
Interactions of secondary and primary/central metabolisms
The production of secondary metabolites is tightly associated with pathways of primary/central metabolism, such as glycolysis, the shikimate pathway, the production of aromatic amino acids, and aliphatic amino acids . GC-MS and HPLC analysis detected three compound groups produced from primary metabolisms in FLJ and rFLJ: phenolic acids, terpenoids, and fatty acids ( Additional file 1: Figure S4).
Phosphoenolpyruvate (PEP) is an example of glycolysis intermediates. It is indispensable for energy metabolism in the cytosol and delivers ATP and pyruvate catalyzed by cytosolic pyruvate kinase (PK) . Inside the plastids, PEP acts as a precursor for at least four metabolic pathways: fatty acids, isoprenoids, branched chain amino acids, and the shikimate pathway . However, chloroplasts and most non-green plastids lack the ability to produce PEP via glycolysis, because their enolase is either absent or has low activity . In this study, we found that enolase (sc_FLJ_010870) transcription is 1232.8-fold higher in the FLJ flower buds when compared with flower1, and no detectable expression of the enzyme in flower2. Therefore, the formation of PEP is negatively correlated with FLJ flowering. In contrast, the transcription level of PK (sc_FLJ_000591), a competing enzyme, is 0.11-fold and 0.54-fold in the group 1 comparison. We neither observed significant changes in the flavonoid content nor in ABA and fatty acids contents as what was reported in an eno1 mutant when compared with the wild type of A. thaliana. We did found that the transcription level of enolase is up-regulated in buds as compared with the two flower stages, as well as in the group 2 comparison, but showed no correlation with the phenolic acid content. In contrast, the transcription level of PK is higher in the same comparison. Our combined analysis of biosynthesis of phenolic acid, terpenoids, and fatty acids did indicate that there is a potential correlation between PK expression level and the contents of flavonoids, terpenoids, and fatty acids in FLJ.
ATP synthesis could also have influences on the interaction of primary and secondary metabolisms. We assessed the predicted phosphorylation sites of the relevant enzymes ( Additional file 2: Table S4) and found that all are candidates for phosphorylation-based regulation. Of the defined DEGs, a study in animals showed that HMGR exists in both active (dephosphorylated) and inactive (phosphorylated) forms . PK has also been shown to be more susceptible to inhibition by ATP . We analyzed the differential expression of ATP synthesis-coupled electron transporters and found that this gene family is down-regulated in the group 1 comparison and up-regulated in the group 2 comparison. Prediction of phosphorylation sites from protein sequences indicates that HMGR (sc_FLJ_012288) has nine phosphorylation sites, whereas pyruvate kinase (sc_FLJ_000591) has four phosphorylation sites.
Previous studies on primary and secondary metabolisms have indicated that they are linked via the phenylalanine pool [50, 51]. Therefore, it is possible that a reduced carbon flux in the phenylpropanoid pathway may affect carbohydrate metabolism.
As Matt et al.  showed that an increase in the sugar/amino acid ratio resulted in an elevated production of carbon-rich phenylpropanoids, we found that biosynthesis of phenylpropanoids is down-regulated in both group 1 and group 2 comparisons, and the transcription level of β-glucosidase that generates β-D-glucose and α-D-glucose is down-regulated in the group 2 comparison. Furthermore, sucrose synthase that creates sucrose is down-regulated in the group 1 comparison but β-amylase that generates maltose is up-regulated in the group 1 comparison and down-regulated in the group 2 comparison.
Moriizumi  reported that glucose-regulated transcription of pyruvate kinase is mediated by its glucose response element; the carbohydrate response elements are composed of two E box-like motifs separated by 5 bp and is recognized by two basic helix-loop-helix/leucine zipper (bHLH/LZ) proteins [54, 55]. In the group 2 context, down-regulation of pyruvate kinase may be mediated by down-regulating glucose biosynthesis, and in the group 1 context, however, the transcription level of glucose biosynthetic enzymes is insignificant albeit up-regulated pyruvate kinase. Finally, two bHLH (sc_FLJ_008421 and sc_FLJ_006390) proteins are seen up-regulated and down-regulated in the group 1 and 2 comparisons, respectively.
A putative sequence for a carbohydrate-response-element binding protein (sc_FLJ_004075) is obtained based on sequence homology (a homolog in Norway rat; EMBLCDS:BAB77523 and in chicken GenBank: ABV72703.1). The transcript is up-regulated in the group 1 comparison, suggesting that bHLH transcription factors may be involved in regulating the response of PK to glucose in FLJ.
The complexity of defining orthologs and paralogs for key metabolic pathways
Since gene duplication is very common in plant genomes, we made an effort to differentiate orthologs and paralogs from all homologs. The overall sequence identity between FLJ and rFLJ contigs is 99.0%. In our analysis, we selected 55 DEGs sequences from all pathway-related genes, where six of the selected genes have slightly lower identities, about 97.4% ( Additional file 2: Table S6).
Gene expression data provide more comprehensive understanding of FLJ as medicinal plant
Although several studies have reported that the content of chlorogenic acid is higher in the flower buds as compared to that in flowers, it has long been disputed as to which organ has the highest medical value due to the applications of controversial evaluation methods. In this study, we obtained a sufficient amount of transcriptomic data from both young buds and mature flowers, and carried out an integrated analysis on the variations of gene expression and the contents of active compounds. Our data from different flowering stages indicate that the contents of eight major active compounds either decreased (five of them) or remain unchanged (three of them) and that the biosynthesis of the active compounds is overall higher in the buds than in the flowers. In addition, many key enzymes, such as PAL, CHS, and CHI, are up-regulated in the FLJ flower buds (Figure 4). PAL is a key enzyme in the synthesis of phenylpropanoid phytoalexins and other phenolics . Previous data have shown that the PAL expression levels decline during flowering in Nicotiana, and the decrease enzyme activity corresponds to the decreased major phenylpropanoid compounds. Our data add further support for this notion, as we found that the CHS and CHI genes are also significantly down-regulated in the later flowering stages. CHS functions to produce flavonoid precursors, and CHI plays a major part in the cyclization reaction from chalcone to flavanone .
The quality of herbal medicine has been very difficult to control and to evaluate primarily because of the complexity and incomplete knowledge of the active medicinal compounds. The primary methods that have been used for quality evaluation of Chinese herbal medicines are chemical and pharmacological analyses. Chemical evaluations showed that chlorogenic acid and luteoloside are two common active compounds found in FLJ. However, content and fingerprint analysis of one or more of these compounds are not indicative for the medicinal value of the plant, and our genomic approach provides a comprehensive survey. Our study generated gene expression data for terpenoids and fatty acid biosynthesis and increased valuable knowledge on other FLJ compounds.
Both fatty and phenolic acid biosyntheses may depend on the interaction between the two pathways [59, 60]. In FLJ, the transcription level of long-chain acyl-CoA synthetase (sc_FLJ_011431) and 4-coumarate-CoA ligase (sc_FLJ_001317) decreases after flowering. Since the contents of phenolic compounds and fatty acids are down-regulated in both the group 1 and 2 comparisons, some of the biosynthetic genes for phenypropanoid compounds and fatty acids may be regulated in a similar way or function as similar enzymes (such as sharing catalytic domains).
Changes in phenolic acids, fatty acid biosynthesis, and the MEP/DXP pathway show a decreased trend in gene expression from the medicinal organ (buds) to the non-medicinal organs (flower1 and flower2). In addition, we used phylogenetic tools for paralog analysis and revealed that the mRNA levels of these enzymes have higher expressions in the flower buds of both FLJ and rFLJ. We found that a total of eighteen enzymes in these biosynthetic networks are differentially expressed ( Additional file 2: Table S9).
Enzymatic divergence of orthologs and paralogs results in gene function variation and active compound content
Secondary metabolism varies intensively, even between two closely related taxonomic groups, and the underlining functional variations, such as enzyme activities, often lead to the production of unique compounds. In this study, we identified several orthologous enzymes as well as their related paralogs and evaluated their evolutionary relatedness between FLJ and rFLJ. We observed that the RPKM values of PAL, CHS, and HMGR are higher in rFLJ than FLJ, and these variations in expression may lead to alternations in the active compound contents between in the two plants.
In contrast to specific evolutionary changes to individual enzymes during speciation, most of the functional variations appear to be related to gene or genome duplications . After gene duplication events, most paralogs are lost over time, but those have survived often gain new functions (neo-functionalization), partition the original function into different time and tissues (subfunctionalization), or have lost their functions (nonfunctionalization). Our data indicate that gene duplication in FLJ may provide an opportunity for neo-functionalization, whereby the PK gene and its orthologs and paralogs may evolve to have complementary enzyme functions. For instance, an FLJ PK paralog turns out to have a high RPKM value and may undergo neofunctionalization, whereas the expression of another paralog in FLJ is higher than that of rFLJ. There is a third PK paralog whose expression levels are quite similar in both plants. Nevertheless, the divergence of gene expression due to duplicated genes appears to play direct roles in the production of active compounds in FLJ and rFLJ.
Lonicera japonica Thunb
Lonicera japonica Thunb. var. chinensis (Wats.)
Quantitative reverse transcriptase PCR
Reverse transcriptase PCR
Flowers were provided by Chinese Shandong YATE Ecological Co. and auth by Dr. Qingjun Yuan. We thank Dr. Haifang Li for helping the GC-MS analysis. The work received financial support from the National Natural Science Foundation of China (81001605).
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