Isolation and antisense suppression of flavonoid 3', 5'-hydroxylasemodifies flower pigments and colour in cyclamen
© Boase et al; licensee BioMed Central Ltd. 2010
Received: 28 October 2009
Accepted: 13 June 2010
Published: 13 June 2010
Cyclamen is a popular and economically significant pot plant crop in several countries. Molecular breeding technologies provide opportunities to metabolically engineer the well-characterized flavonoid biosynthetic pathway for altered anthocyanin profile and hence the colour of the flower. Previously we reported on a genetic transformation system for cyclamen. Our aim in this study was to change pigment profiles and flower colours in cyclamen through the suppression of flavonoid 3', 5'-hydroxylase, an enzyme in the flavonoid pathway that plays a determining role in the colour of anthocyanin pigments.
A full-length cDNA putatively identified as a F3'5'H (CpF3'5'H) was isolated from cyclamen flower tissue. Amino acid and phylogeny analyses indicated the CpF3'5'H encodes a F3'5'H enzyme. Two cultivars of minicyclamen were transformed via Agrobacterium tumefaciens with an antisense CpF3'5'H construct. Flowers of the transgenic lines showed modified colour and this correlated positively with the loss of endogenous F3'5'H transcript. Changes in observed colour were confirmed by colorimeter measurements, with an overall loss in intensity of colour (C) in the transgenic lines and a shift in hue from purple to red/pink in one cultivar. HPLC analysis showed that delphinidin-derived pigment levels were reduced in transgenic lines relative to control lines while the percentage of cyanidin-derived pigments increased. Total anthocyanin concentration was reduced up to 80% in some transgenic lines and a smaller increase in flavonol concentration was recorded. Differences were also seen in the ratio of flavonol types that accumulated.
To our knowledge this is the first report of genetic modification of the anthocyanin pathway in the commercially important species cyclamen. The effects of suppressing a key enzyme, F3'5'H, were wide ranging, extending from anthocyanins to other branches of the flavonoid pathway. The results illustrate the complexity involved in modifying a biosynthetic pathway with multiple branch points to different end products and provides important information for future flower colour modification experiments in cyclamen.
To date there has only been one reported molecular breeding experiment involving flavonoid pigments for cyclamen. It was focused on the generation of yellow flower colours through the production of yellow flavonoid pigments . Our interest is in altering the anthocyanin-based colours . In flower colour modification studies in general, particular attention has been paid to the enzymes responsible for the hydroxylation of the B-ring of the flavonoid molecule, namely F3'H and F3'5'H (Figure. 1) because of their key influence on the colour of anthocyanin pigments . Specific experiments to accumulate delphinidin-derived anthocyanins by over expression of a F3'5'H transgene have been reported for carnation  and rose , while inhibition of both the F3'H and the F3'5'H genes has been used to modify colour and promote cyanidin- and pelargonidin-based pigment accumulation in flowers in the genera Torenia , Nierembergia  and Osteospermum .
Our strategy for modification of flower colour in cyclamen focused on the F3'5'H. Substrate feeding experiments with DHK and the F3'H/F3'5'H inhibitor tetcyclacis indicate that the cyclamen DFR can use DHK and that cyclamen has the ability to make pelargonidin-derived anthocyanins (K. Schwinn, unpublished data). The cloning of a F3'5'H cDNA and our cyclamen genetic transformation system  have allowed us to investigate flower colour formation in cyclamen. In this study we report on the effects of antisense suppression of F3'5'H on flavonoid end-product accumulation and flower colour.
Isolation and sequence analysis of a cyclamen flavonoid 3', 5'-hydroxylase cDNA
A putative full-length cDNA for F3'5'H (CpF3'5'H) was isolated from a cDNA library made from mixed flower bud stages of C. persicum 'Sierra Rose'. The complete nucleotide sequence has 1719 nucleotides with a single major ORF encoding 508 amino acid residues (GenBank accession GQ891056).
Generation of transformed lines and transgene expression analyses
Northern blot analysis of cultivar (cv) 'Purple' transformants showed that eight lines were transgenic for the hygromycin selectable marker (Figure. 3B). RT-PCR analysis of the nptII selectable marker showed the three cv 'Wine-Red' lines were also transgenic as expected (Figure. 3B).
Northern blot analysis with a mixed sense and antisense CpF3'5'H probe, (1.7 kb XbaI-EcoRI fragment, Figure. 3A), showed that two F3'5'H specific transcripts were detected (Figure. 3C). There was a marked reduction in endogenous CpF3'5'H transcript in all antisense lines of both cultivars. Antisense CpF3'5'H transcript was detected only in the transgenic lines and the levels varied between lines.
HPLC-MS2 based identifications of the main anthocyanins detected in petal tissue.
277, 343, 526
277, 348, 531
277, 348, 531
282, 330, 516
282, 330, 521
277, 343, 531
277, 330, 516
There was also a marked reduction in total anthocyanin concentration in petal tissue of the transgenic lines. Lines with modified flower colour showed a decrease in total anthocyanin concentration of up to 80% of that in untransformed controls (Figure. 6B). The difference in anthocyanin concentrations between the transgenic lines and their respective controls were statistically significant at the 5% level.
Flavonol concentration and ratios in petals of transgenic lines (mg.g.DW-1) (Mean ± SEM, n = 2).
Flavonols ± sem (mg.g.DW-1)
Q/K ratio ± sem
2.7 ± 0.05
0.7 ± 0.05
1.6 ± 0.27
0.4 ± 0.06
2.9a ± 0.06
1.3a ± 0.00
2.1 ± 0.05
0.6 ± 0.00
3.9a ± 0.07
2.0a ± 0.01
3.4a ± 0.12
1.2a ± 0.03
4.4a ± *
1.1a ± *
4.0a ± *
0.8 ± *
4.6a ± 0.03
1.6a ± 0.02
3.0a ± 0.12
1.5a ± 0.05
2.1 ± 0.14
5.4 ± 0.01
1.7 ± *
4.4 ± *
4.7a ± 0.30
1.3a ± 0.05
4.3a ± 0.54
1.5a ± 0.05
3.1a ± 0.02
1.9a ± 0.22
2.4 ± 0.13
3.1a ± 0.13
Flower colour analysis
Expression of the introduced antisense CpF3'5'H transgene and resulting flavonoid concentration and profile changes in the transgenic lines were translated into visible flower colour changes (Figure. 4). Cultivar 'Purple' lines showed a loss of purple colour and became pink, while the cv 'Wine-Red' lines remained a similar pinkish hue but with reduced intensity (chroma).
Flower colour characteristics for petal tissue of the control and transgenic lines.
40 ± 0.5
71 ± 0.5
348 ± 0.9
34 ± 0.3
69 ± 0.5
359a ± 0.6
53a ± 6.9
67 ± 3.8
355a ± 1.8
63a ± *
58a ± *
351 ± *
55a ± 1.3
58a ± 0.9
1.2a ± 1.1
64a ± 3.2
57a ± 3.4
352a ± 2.1
57a ± 0.7
65a ± 1.5
359a ± 1.1
59a ± 1.1
66a ± 0.6
51a ± 0.7
5.1a ± 0.6
38 ± 1.1
62 ± 0.5
1.7 ± 2.0
63a ± 0.6
55a ± 1.2
359 ± 0.6
65a ± 1.7
50a ± 1.4
357a ± 0.9
45 ± 2.1
66 ± 0.8
357a ± 0.5
Antisense suppression of CpF3'5'H was successful in changing anthocyanin profiles and flower colour in cyclamen. A shift from predominantly delphinidin-derived pigments to a greater relative proportion of cyanidin-derived pigments was achieved and in general this showed up as a concomitant shift in H°, the parameter indicating colour group. It is interesting that the degree of change in H° did not correlate with the degree of shift in pigments. The fact that the transformants also showed variable drops in total anthocyanin levels and changes in flavonol level and type illustrates both the links between the different pools of flavonoid substrates and the importance of the roles that anthocyanin concentration and flavonol copigmentation play in flower colour.
Similar changes in anthocyanin concentration and the accumulation of cyanidin-derived anthocyanins were seen for the two different minicyclamen cultivars and yet the greatest change in H° was seen in the lines of the purple cultivar. This is most likely due to a reduction in the predominant anthocyanin, malvidin 3-5-di-O-glucoside in these lines. This anthocyanin has been reported as being bluer in colour than malvidin mono-glucosides . The predominant anthocyanin in the 'Wine-Red' cultivar is malvidin 3-O-glucoside and this has been reported to give pink/purple colours, closer to the colour associated with cyanidin and peonidin pigments .
Pelargonidin-based pigments were not detected in the flowers of the transgenics. One explanation for their absence is that suppression of F3'5'H activity was not complete, as evidenced by the presence of delphinidin-derived anthocyanins. This may be either due to inefficiency of the antisense approach (as opposed to hairpin RNA-induced RNAi ), effects due to transgene insertion or copy number [19–21], or the presence of other unaffected F3'5'H family members. The presence of a F3'H enzyme in the petals could have also removed substrate for pelargonidin production. We have searched for a cyclamen F3'H cDNA and found one (GenBank GU808358) with high deduced amino acid similarity to known F3'H sequences of other species (81% identity with the F3'H of gentian). However, transcript levels for this particular F3'H gene were not detectable by northern analysis during cyclamen petal development (unpublished data).
Substrate specificity is an important consideration regarding pelargonidin production. In some species, such as petunia , cymbidium [23, 24] and Osteospermum , synthesis of pelargonidin-based anthocyanins is limited by the substrate specificity of the endogenous DFR. Our substrate feeding experiments (mentioned previously) showed that cyclamen has the ability to make pelargonidin-derived anthocyanins. It is still possible, however, that cyclamen DFR has low substrate specificity for DHK and the action of flavonol synthase (FLS), F3'H and F3'5'H means that the DHK substrate is not used for the synthesis of pelargonidin. Retransformation of an antisense F3'5'H line from this study, with a transgene encoding a DFR known to efficiently catalyse the reduction of DHK to leucopelargonidin [25–27] could result in transgenic plants accumulating pelargonidin derivatives in flowers, as successfully demonstrated for Osteospermum . It remains to be resolved whether there is a F3'H functioning in the flower. The presence of cyanidin-based pigments in the flowers of the antisense CpF3'5'H lines suggests F3'H activity. Thus, inhibition of either F3'H or FLS gene activity to reduce enzymatic competition for DHK substrate may also be necessary to promote pelargonidin production in DFR/antisense F3'5'H transgenics.
In the cyclamen transgenic lines, total anthocyanin levels decreased markedly while flavonol levels increased and the quercetin/kaempferol ratio changed. Similar results were reported for Nierembergia flowers modified with an antisense F3'5'H construct and were suggested to be due to a modified flow through the flavonoid pathway . A block in F3'5'H activity resulted in an increase in pelargonidin precursors. Low F3'H activity coupled with a DFR that putatively does not recognise DHK, was suggested to have led to limited substrate flow toward pigment production and an increase in the sustrate pool for FLS . The flavonoid enzyme kinetics are not known for cyclamen. However, if the cyclamen DFR has a low specificity for pelargonidin or cyanidin precursors (as the reduction in total anthocyanins (Figure. 6B) suggests) this would provide extra substrate for the FLS enzyme and explain the increased flavonol levels. Competition for substrate between FLS and DFR has also been shown to occur in petunia [28, 29].
It is interesting that while flavonol levels generally increased in the transgenics, there were differences in the quercetin/kaempferol ratios between the lines of the different cultivars. Quercetin flavonols increased in cv 'Purple' lines while kaempferol types increased in cv 'Wine-Red' lines. This inverse result and the consistency of the ratio change within lines of each cultivar argues against the suppression of F3'5'H activity directly altering the balance of DHK and DHQ, and thus what is available for the FLS. Furthermore, differing substrate specificities of their respective FLS cannot account for the observed results. Differing specificities of other enzymes are likely to be the cause. The probable candidate is F3'H, which in other species can not only alter the balance between DHK and DHQ, but also convert kaempferol to quercetin . Further studies of cyclamen flower colour would warrant a continued search for a F3'H.
We report here the first successful alteration of cyclamen anthocyanin pigmentation using genetic modification techniques. Our results highlight the intricate interplay between type and concentration of both anthocyanin pigments and flavonol co-pigments in flower colour and illustrate the complexity involved in modifying a biosynthetic pathway with multiple branch points to different end products.
Cloning of F3'5'H cDNA and sequence analysis
A cDNA library from mixed flower stages of C. persicum 'Sierra Rose' petals was made using a lambda ZAPII bacteriophage vector kit (Stratagene, USA). This library was first screened with a heterologous clone of F3'H from petunia (Florigene Flowers, Australia) and a partial F3'5'H cDNA was found. The partial F3'5'H cDNA was used to rescreen the cDNA library to obtain a full length CpF3'5'H cDNA.
The MegAlign programme of Lasergene (DNASTAR Inc., Madison, USA) was used to compare the CpF3'5'H deduced amino acid sequence to ten known F3'5'H sequences (Camellia sinensis AAY23287; Campanula medium BAA03440; Catharanthus roseus CAA09850; Eustoma grandiflorum BAA03439; Glycine max ABQ96218; Gossypium hirsutum AAP31058; Petunia hybrida CAA80266; Phalaenopsis hybrida AAZ79451; Solanum tuberosum AAV85473; Vitis vinifera BAE47007), ten F3'H sequences (Antirrhinum majus ABB53383; Arabidopsis thaliana NP_196416; Glycine max ABW69386; Ipomoea tricolor BAD00192; Matthiola incana AAG49301; Perilla frutescens BAB59005; Petunia hybrida AAD56282; Populus trichocarpa XP_002319761; Sorghum bicolor ABG54321; Vitis vinifera ABH06586), cinnamate 4-hydroxylase from Arabidopsis thaliana (AAC99993) and flavone synthase II from Medicago truncatula (ABC86159).
Construction of binary vectors
The CpF3'5'H cDNA was cloned into the EcoRI multiple cloning site of pART7  in the antisense orientation to form pLN95. The NotI fragment from pLN95, which contains the 35S:antisenseF3'5'H:Ocs expression cassette, was ligated into the binary vectors; pART27  to make pLN96, pMOA33  to make pPN50, pMOA 34  to make pPN51, and BJ49  to make pPN48 (Figure. 3A). These binary vectors carried either the nptII or hpt selectable marker genes under a NOS promoter (Figure. 3A).
Transformation with Agrobacterium tumefaciens
Etiolated hypocotyls of two parental lines of F1 hybrid minicyclamen cv 'Purple' and cv 'Wine-Red' were used as explants for transformation experiments. A. tumefaciens strain EHA105 containing either pLN96, pPN48, pPN50 or pPN51 were used to inoculate explants. The transformation protocol used was that reported by Boase et al.  except that hygromycin was used as the selection agent for cv 'Purple' lines using a range of concentrations: 5mg/l to day 12 after Agrobacterium inoculation, 20mg/l to day 77 after inoculation, then 15mg/l until shoots were recovered.
Northern blot analyses
RNA was extracted from petal tissue for northern blot analysis using a modified hot borate method [33, 34]. RNA was separated by electrophoresis on a 1% agarose RNA gel and subsequently transferred to Hybond XL nylon membranes using a SSC overnight blotting method. The membranes were hybridized with appropriate radioactively-labelled probes. The probe for hpt was a 1.1 kb XhoI fragment digested from pCAMBIA1301, which contained the hpt gene. The probe for F3'5'H was a 1.7 kb XbaI-EcoRI fragment digested from pLN95. Both membranes were also rehybrised to a cDNA probe corresponding to a 25/26S rRNA (pTip6) from Asparagus officinalis, to show RNA loadings. Autoradiography was conducted at -80°C using Kodak Biomax X-ray film.
RT-PCR analysis of nptIImRNA transcripts
To investigate the expression of the introduced nptII selectable marker recombinant gene, RT-PCR analysis was performed on RNA extracted from petals using a modified hot borate method [33, 34]. Three independent transgenic lines of cv 'Wine-Red' (#31691, #31695 and #31698) and one untransformed control (#29009) were tested. First strand cDNA was reverse transcribed from 100ngRNA per sample using Superscript II (Invitrogen USA) and oligo dT primer, and then 1 μl of the resulting cDNA per line was used for the PCR. For PCR, initial denaturation was at 94°C for 2 min followed by 40 cycles of melting (94°C/30 s), annealing (50°C/30 s) and extension (72°C/2 min). The nptII primers used were: forward 5'-ATGACTGGGCACAACAGACCATCGGCTGCT-3' and reverse, 5'-CGGGTAGCCAACGCTATGTCCTGATAGCGG-3'.
PCR products were separated electrophoretically on a 1% (w/v) NaB agarose gel stained with Sybr®safe (Invitrogen USA).
Flavonoids were analysed by high performance liquid chromatography (HPLC) and liquid chromatography mass spectrometry (LC-MS). Freeze dried tissue was used for the analysis. Samples of ground freeze-dried petal tissue (50mg DW) were extracted initially in 2ml of methanol:acetic acid:water (70:3:27) and then re-extracted in 2 ml methanol:acetic acid:water (90:1:9). The combined supernatants were concentrated in vacuo and made up to a final volume of 1ml. HPLC analysis was carried out using a Waters 600 solvent delivery system with a Phenomenex Prodigy (5 μm, 250 × 4.6 mm) RP-18 endcapped column (column temperature 30°C) and a Waters 996 PDA detector. Solvent systems, flow rates and gradients are as described by Bloor et al. . Flavonoids were detected at 350nm and anthocyanins at 530nm. Flavonoid levels were determined as quercetin-3-O-rhamnoglucoside (Apin Chemicals, Abingdon, Oxon, UK) equivalents, and the anthocyanins as cyanidin 3-O-glucoside (Extrasynthese, Genay, France) equivalents. Results are reported as the mean of the two replicates.
Separate extracts were analysed by electrospray mass spectrometry with a Thermo Finnigan LTQ ion-trap mass spectrometer. A Synergi Fusion RP80, 4 μm, 150 × 2.1 mm column with 4 × 2 mm guard cartridge from Phenomenex Ltd was used for separation. The mobile phase consisted of water (A) and acetonitrile (B) both containing 1% formic acid (FA). Extracts were injected at 5 μL volumes with a gradient program from 95% A to 50% A over 50 min. The column was washed by ramping to 90% B for 5 min and then re-equilibrated to the starting conditions for a further 5 min. Compound elution was monitored by PDA detector scanning the range 250-600 nm and by mass scanning from m/z 150-1500 to collect parent, MS2 and MS3 data in positive and negative ion (additional run) selection modes.
Flower colorimeter analysis
Colours in all lines were quantified by measurement of three petals of each flower, three flowers per line with a Minolta CR-200 tristimulus colorimeter, set on CIELab D65 light source and 0° observer angle. Lightness (L) represents the proportion of total incident light that is reflected. Chroma (C) describes the degree to which selective absorption occurs i.e. colour saturation in relative intensity units. Hue angle (H) is derived from a CIELAB colour space wheel with values stepped counterclockwise from red at 0°/360°, yellow at 90°, bluish-green at 180° and blue at 270° .
A one-way ANOVA was performed on each data set shown in Tables 2 & 3 and Figure 6B followed by a comparison of means using either a 5% Fisher's Least Significant Difference (5% LSD) to compare each line with a single control, or contrasts to compare each line with the combined mean of two controls. Lines with values significantly different from their control (or pair of controls) at the 5% level have been indicated by adding a superscript a to the means in Tables 2 and 3, and in Figure 6B. All analyses were performed using GenStat statistical software .
Ms Marshall and Ms Patel are former team members of the New Zealand Institute for Plant and Food Research Ltd.
Dr Arie van Diepen of Goldsmith Seeds BV in the Netherlands is thanked for supplying seed of cv 'Purple' and cv 'Wine-Red'. Nigel Joyce at Plant & Food Research Lincoln carried out the LC-MS analysis. Theresa Lill carried out some subculturing in tissue culture. Deepa Bowatte assisted with tissue culture subculturing and TLC analyses. Ian King transplanted the cyclamen plants to soil in the glasshouse and grew them to a flowering state. Drs Bart Janssen, Andrew Gleave and Phillipa Barrell supplied the binary vectors, BJ49, pART27 and pMOA33 and pMOA34 respectively. Andrew Mullan made up the media used in tissue culture. Dr Andrew McLachlan conducted statistical analyses.
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