Identification of the molecular basis for the functional difference between flavonoid 3'-hydroxylase and flavonoid 3',5'-hydroxylase

Flavonoid 3'-hydroxylase (F3'H) and flavonoid 3',5'-hydroxylase (F3'5'H) are cytochrome P450 enzymes and determine the B-ring hydroxylation pattern of flavonoids by introducing hydroxyl groups at the 3'- or the 3'- and 5'-position, respectively. Sequence identity between F3'H and F3'5'H is generally low since their divergence took place early in the evolution of higher plants. However, in the Asteraceae the family-specific evolution of an F3'5'H from an F3'H precursor occurred, and consequently sequence identity is substantially higher. We used this phenomenon for alignment studies, in order to identify regions which could be involved in determining substrate specificity and functionality. Subsequent construction and expression of chimeric genes indicated that substrate specificity of F3'H and F3'5'H is determined near the N-terminal end and the functional difference between these two enzymes near the C-terminal end. The impact on function of individual amino acids located in substrate recognition site 6 (SRS6) was further tested by site-directed mutagenesis. Most interestingly, a conservative Thr to Ser exchange at position 487 conferred additional 5'-hydroxylation activity to recombinant Gerbera hybrida F3'H, whereas the reverse substitution transformed recombinant Osteospermum hybrida F3'5'H into an F3'H with low remaining 5'-hydroxylation activity. Since the physicochemical properties of Thr and Ser are highly similar, the difference in size appears to be the main factor contributing to functional difference. The results further suggest that relatively few amino acids exchanges were required for the evolutionary extension of 3'- to 3',5'-hydroxylation activity.     

A rationale for the shift in colour towards blue in transgenic carnation flowers expressing the flavonoid 3',5'-hydroxylase gene

Recently marketed genetically modified violet carnations cv. Moondust and Moonshadow (Dianthus caryophyllus) produce a delphinidin type anthocyanin that native carnations cannot produce and this was achieved by heterologous flavonoid 3',5'-hydroxylase gene expression. Since wild type carnations lack a flavonoid 3',5'-hydroxylase gene, they cannot produce delphinidin, and instead accumulate pelargonidin or cyanidin type anthocyanins, such as pelargonidin or cyanidin 3,5-diglucoside-6"-O-4, 6"'-O-1-cyclic-malyl diester. On the other hand, the anthocyanins in the transgenic flowers were revealed to be delphinidin 3,5-diglucoside-6"-O-4, 6"'-O-1-cyclic-malyl diester (main pigment), delphinidin 3,5-diglucoside-6"-malyl ester, and delphinidin 3,5-diglucoside-6",6"'- dimalyl ester. These are delphinidin derivatives analogous to the natural carnation anthocyanins. This observation indicates that carnation anthocyanin biosynthetic enzymes are versatile enough to modify delphinidin. Additionally, the petals contained flavonol and flavone glycosides. Three of them were identified by spectroscopic methods to be kaempferol 3-(6"'-rhamnosyl-2"'-glucosyl-glucoside), kaempferol 3-(6"'-rhamnosyl-2"'-(6-malyl-glucosyl)-glucoside), and apigenin 6-C-glucosyl-7-O-glucoside-6"'-malyl ester. Among these flavonoids, the apigenin derivative exhibited the strongest co-pigment effect. When two equivalents of the apigenin derivative were added to 1 mM of the main pigment (delphinidin 3,5-diglucoside-6"-O-4,6"'-O-1-cyclic-malyl diester) dissolved in pH 5.0 buffer solution, the lambda(max) shifted to a wavelength 28 nm longer. The vacuolar pH of the Moonshadow flower was estimated to be around 5.5 by measuring the pH of petal. We conclude that the following reasons account for the bluish hue of the transgenic carnation flowers: (1). accumulation of the delphinidin type anthocyanins as a result of flavonoid 3',5'-hydroxylase gene expression, (2). the presence of the flavone derivative strong co-pigment, and (3). an estimated relatively high vacuolar pH of 5.5.     

The B gene of pea encodes a defective flavonoid 3',5'-hydroxylase, and confers pink flower color

The inheritance of flower color in pea (Pisum sativum) has been studied for more than a century, but many of the genes corresponding to these classical loci remain unidentified. Anthocyanins are the main flower pigments in pea. These are generated via the flavonoid biosynthetic pathway, which has been studied in detail and is well conserved among higher plants. A previous proposal that the Clariroseus (B) gene of pea controls hydroxylation at the 5' position of the B ring of flavonoid precursors of the anthocyanins suggested to us that the gene encoding flavonoid 3',5'-hydroxylase (F3'5'H), the enzyme that hydroxylates the 5' position of the B ring, was a good candidate for B. In order to test this hypothesis, we examined mutants generated by fast neutron bombardment. We found allelic pink-flowered b mutant lines that carried a variety of lesions in an F3'5'H gene, including complete gene deletions. The b mutants lacked glycosylated delphinidin and petunidin, the major pigments present in the progenitor purple-flowered wild-type pea. These results, combined with the finding that the F3'5'H gene cosegregates with b in a genetic mapping population, strongly support our hypothesis that the B gene of pea corresponds to a F3'5'H gene. The molecular characterization of genes involved in pigmentation in pea provides valuable anchor markers for comparative legume genomics and will help to identify differences in anthocyanin biosynthesis that lead to variation in pigmentation among legume species.     

Redirection of anthocyanin synthesis in Osteospermum hybrida by a two-enzyme manipulation strategy

Modern biotechnology has developed powerful tools for genetic engineering and flower colours are an excellent object to study possibilities and limitations of engineering strategies. Osteospermum hybrida became a popular ornamental plant within the last 20 years. Many cultivars display rose to lilac flower colours mainly based on delphinidin-derived anthocyanins. The predominant synthesis of delphinidin derivatives is referred to a strong endogenous flavonoid 3',5'-hydroxylase (F3'5'H) activity. Furthermore, since dihydroflavonol 4-reductase (DFR) of Osteospermum does not convert dihydrokaempferol (DHK) to leucopelargonidin, synthesis of pelargonidin-based anthocyanins is naturally not realised. In order to redirect anthocyanin biosynthesis in Osteospermum towards pelargonidin derivatives, we introduced cDNAs coding for DFRs which efficiently convert DHK to LPg. But neither the expression of Gerbera hybrida DFR nor of Fragaria x ananassa DFR - the latter is characterised by an unusual high substrate preference for DHK - altered anthocyanin composition in flowers of transgenic plants. However, chemical inhibition of F3'5'H activity in ray florets of dfr transgenic plants resulted in the accumulation of pelargonidin derivatives. Accordingly, retransformation of a transgenic plant expressing Gerbera DFR with a construct for RNAi-mediated suppression of F3'5'H activity resulted in double transgenic plants accumulating predominantly pelargonidin derivatives in flowers.     

Molecular regulation of catechins biosynthesis in tea [Camellia sinensis (L.) O. Kuntze

Catechins are bioprospecting molecules present in tea and any effort towards metabolic engineering of this important moiety would require knowledge on gene regulation. These are synthesized through the activities of phenylpropanoid and flavonoid pathways. Expression regulation of various genes of these pathways namely phenylalanine ammonia-lyase (CsPAL), cinnamate 4-hydroxylase (CsC4H), p-coumarate:CoA ligase (Cs4CL), flavanone 3-hydroxylase (CsF3H), dihydroflavonol 4-reductase (CsDFR) and anthocyanidin reductase (CsANR) was accomplished previously. In depth analyses of the remaining genes namely, chalcone synthase (CsCHS), chalcone isomerase (CsCHI), flavonoid 3'5'-hydroxylase (CsF3'5'H) and anthocyanidin synthase (CsANS) were lacking. The objective of the work was to clone and analyze these genes so as to generate a comprehensive knowledge on the critical genes of catechins biosynthesis pathway. Gene expression analysis was carried out in response to leaf age and external cues (drought stress, abscisic acid, gibberellic acid treatments and wounding). A holistic analysis suggested that CsCHI, CsF3H, CsDFR, CsANS and CsANR were amongst the critical regulatory genes in regulating catechins content.     

Differential expression of two flavonoid 3'-hydroxylase cDNAs involved in biosynthesis of anthocyanin pigments and 3-deoxyanthocyanidin phytoalexins in sorghum

Three unique sorghum flavonoid 3'-hydroxylase (F3'H) cDNAs (SbF3'H1, SbF3'H2 and SbF3'H3) were discovered through bioinformatics analysis. Their encoded proteins showed >60% identity to the Arabidopsis TT7 (F3'H) protein. Overexpression of SbF3'H1 or SbF3'H2 restored the ability of tt7 mutants to produce 3'-hydroxylated flavonoids, establishing their roles as functional F3'H enzymes. In sorghum mesocotyls, SbF3'H1 expression was involved in light-specific anthocyanin accumulation while SbF3'H2 expression was involved in pathogen-specific 3-deoxyanthocyanidin synthesis. No SbF3'H3 expression was detected in all tissues examined. The sorghum mesocotyls represent a good system for investigation of differential regulation of F3'H genes/alleles responding to different external stimuli.     

Identification and characterisation of CYP75A31, a new flavonoid 3'5'-hydroxylase,isolated from <Emphasis Type="Italic">Solanum lycopersicum</Emphasis>

Background  

Understanding the regulation of the flavonoid pathway is important for maximising the nutritional value of crop plants and possibly enhancing their resistance towards pathogens. The flavonoid 3'5'-hydroxylase (F3'5'H) enzyme functions at an important branch point between flavonol and anthocyanin synthesis, as is evident from studies in petunia (Petunia hybrida), and potato (Solanum tuberosum). The present work involves the identification and characterisation of a F3'5'H gene from tomato (Solanum lycopersicum), and the examination of its putative role in flavonoid metabolism.     

Identification of the Arabidopsis thaliana flavonoid 3'-hydroxylase gene and functional expression of the encoded P450 enzyme

The phenylpropanoid pathway results in the synthesis of thousands of compounds, including flavonoids like flavonols, anthocyanidins and tannins. In Arabidopsis thaliana, the lack of tannins in the seed coat (testa) causes the transparent testa (tt) phenotype. In the present study, we identified the gene responsible for the tt7 mutation. We show that TT7 encodes the enzyme flavonoid 3'-hydroxylase (F3'H), and demonstrate that this P450-dependent monooxygenase has F3'H activity. The availability of the AtF3'H gene and promoter sequence will allow us to study the coregulation of a complete set of flavonol and anthocyanidin biosynthesis genes in A. thaliana, and makes in vitro synthesis of hydroxylated flavonoids more feasible.     

梅花‘南京红须’F3'H全长基于gDNA的TAIL-PCR法克隆

根据从基因组DNA扩增到的梅花‘南京红须’类黄酮3’-羟化酶基因片段（469bp）设计3条嵌套的特异性引物．与6条短的随机简并引物组成的引物库分别用热不对称交错PCR法从‘南京红须’基因组DNA扩增该片段的5’和3’旁侧序列。获得的5’和3’旁侧序列分别长1443bp和1200bp。将两个旁侧序列在469bp片段的基础上拼接得到‘南京红须’全长为2lrl4bp的类黄酮3’-羟化酶基因,被命名为pmhxF3’H。序列分析表明：该基因与11条正式发表的、已递交到GenBank的类黄酮3’-羟化酶基因的eDNA序列在总体上有52．21％的一致性．具有3个内含子。其启动子含有1个“AGGA盒”、1个“GC盒”和3个“TATA盒”。这是首次用热不对称交错PCR法从木本植物的基因组DNA克隆到类黄酮3’-羟化酶基因。本研究将为梅花花色的分子生物学机理探索、花色的基因工程改良提供参考。     

Flavonoid hydroxylase from Catharanthus roseus: cDNA, heterologous expression, enzyme properties and cell-type specific expression in plants

We investigated the P450 dependent flavonoid hydroxylase from the ornamental plant Catharanthus roseus. cDNAs were obtained by heterologous screening with the CYP75 Hf1 cDNA from Petunia hybrida. The C. roseus protein shared 68-78% identity with other CYP75s, and genomic blots suggested one or two genes. The protein was expressed in Escherichia coli as translational fusion with the P450 reductase from C. roseus. Enzyme assays showed that it was a flavonoid 3', 5'-hydroxylase, but 3'-hydroxylated products were also detected. The substrate specificity was investigated with the C. roseus enzyme and a fusion protein of the Petunia hybrida CYP75 with the C. roseus P450 reductase. Both enzymes accepted flavanones as well as flavones, dihydroflavonols and flavonols, and both performed 3'- as well as 3'5'-hydroxylation. Kinetics with C. roseus cultures on the level of enzyme activity, protein and RNA showed that the F3'5'H was present in dark-grown cells and was induced by irradiation. The same results were obtained for cinnamic acid 4-hydroxylase and flavanone 3beta-hydroxylase. In contrast, CHS expression was strictly dependent on light, although CHS is necessary in the synthesis of the F3'5'H substrates. Immunohistochemical localization of F3'5'H had not been performed before. A comparison of CHS and F3'5'H in cotyledons and flower buds from C. roseus identified CHS expression preferentially in the epidermis, while F3'5'H was only detected in the phloem. The cell-type specific expression suggests that intercellular transport may play an important role in the compartmentation of the pathways to the different flavonoids.     

Induction and characterization of a microsomal flavonoid 3'-hydroxylase from parsley cell cultures

A microsomal preparation from irradiated parsley cell cultures catalyses the NADPH and dioxygen-dependent hydroxylation of (S)-naringenin [(S)-5, 7, 4'-trihydroxyflavanone] to eriodictyol (5, 7, 3', 4'-tetrahydroxyflavanone). Dihydrokaempferol, kaempferol, and apigenin were also substrates for the 3'-hydroxylase reaction. In contrast prunin (naringenin 7-O-beta-glucoside) was not converted by the enzyme. The microsomal preparation, which also contains cinnamate 4-hydroxylase, did not catalyse hydroxylation of 4-coumaric acid to caffeic acid. 3'-Hydroxylase activity is partially inhibited by carbon monoxide in the presence of oxygen as well as by cytochrome c and NADP+. These properties suggest that the enzyme is a cytochrome P-450-dependent flavonoid 3'-monooxygenase. Pronounced differences in the inhibition of flavonoid 3'-hydroxylase and cinnamate 4-hydroxylase were found with EDTA, potassium cyanide and N-ethylmaleimide. Irradiation of the cell cultures led to increase of flavonoid 3'-hydroxylase activity with a maximum at about 23 h after onset of irradiation and subsequent decrease. This is similar to light-induction of phenylalanine ammonialyase and cinnamate 4-hydroxylase. In contrast, treatment of the cell cultures with a glucan elicitor from Phytophthora megasperma f. sp. glycinea did not induce flavonoid 3'-hydroxylase nor chalcone isomerase but caused a strong increase in the activities of phenylalanine ammonia-lyase, cinnamate 4-hydroxylase, and NADPH--cytochrome reductase. The results prove that flavonoid 3'-hydroxylase and cinnamate 4-hydroxylase are two different microsomal monooxygenases.     

Functional analysis of soybean genes involved in flavonoid biosynthesis by virus-induced gene silencing

Virus-induced gene silencing (VIGS) is a powerful tool for functional analysis of genes in plants. A wide-host-range VIGS vector, which was developed based on the Cucumber mosaic virus (CMV), was tested for its ability to silence endogenous genes involved in flavonoid biosynthesis in soybean. Symptomless infection was established using a pseudorecombinant virus, which enabled detection of specific changes in metabolite content by VIGS. It has been demonstrated that the yellow seed coat phenotype of various cultivated soybean lines that lack anthocyanin pigmentation is induced by natural degradation of chalcone synthase ( CHS ) mRNA. When soybean plants with brown seed coats were infected with a virus that contains the CHS gene sequence, the colour of the seed coats changed to yellow, which indicates that the naturally occurring RNA silencing is reproduced by VIGS. In addition, CHS VIGS consequently led to a decrease in isoflavone content in seeds. VIGS was also tested on the putative flavonoid 3'-hydroxylase ( F3'H ) gene in the pathway. This experiment resulted in a decrease in the content of quercetin relative to kaempferol in the upper leaves after viral infection, which suggests that the putative gene actually encodes the F3'H protein. In both experiments, a marked decrease in the target mRNA and accumulation of short interfering RNAs were detected, indicating that sequence-specific mRNA degradation was induced. The present report is a successful demonstration of the application of VIGS for genes involved in flavonoid biosynthesis in plants; the CMV-based VIGS system provides an efficient tool for functional analysis of soybean genes.     

Regulation of gene expression involved in flavonol and anthocyanin biosynthesis during petal development in lisianthus (Eustoma grandiflorum)

To elucidate gene regulation of flower colour formation, the gene expressions of the enzymes involved in flavonoid biosynthesis were investigated in correlation with their product during floral development in lisianthus. Full-length cDNA clones of major responsible genes in the central flavonoid biosynthetic pathway, including chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 3',5'-hydroxylase (F3'5'H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), and flavonol synthase (FLS), were isolated and characterized. In lisianthus, the stage of the accumulation of flavonols and anthocyanins was shown to be divided clearly. The flavonol content increased prior to anthocyanin accumulation during floral development and declined when anthocyanin began to accumulate. CHS, CHI, and F3H were necessary for both flavonol and anthocyanin biosynthesis and were coordinately expressed throughout all stages of floral development; their expressions were activated independently at the stages corresponding to flavonol accumulation and anthocyanin accumulation, respectively. Consistent with flavonol and anthocyanin accumulation patterns, FLS, a key enzyme in flavonol biosynthesis, was expressed prior to the expression of the genes involved in anthocyanin biosynthesis. The genes encoding F3'5'H, DFR, and ANS were expressed at later stages, just before pigmentation. The genes responsible for the flavonoid pathways branching to anthocyanins and flavonols were strictly regulated and were coordinated temporally to correspond to the biosynthetic order of their respective enzymes in the pathways, as well as in specific organs. In lisianthus, FLS and DFR, at the position of branching to flavonols and anthocyanins, were supposed to play a critical role in regulation of each biosynthesis.     

Selective accumulation of delphinidin derivatives in tobacco using a putative flavonoid 3',5'-hydroxylase cDNA from Campanula medium

Blue flowers generally contain 3',5'-hydroxylated anthocyanins (delphinidin derivatives) as pigments, which are formed only in the presence of flavonoid 3',5'-hydroxylases (F3'5'H). Heterologous expression of a F3'5'H gene therefore provides an opportunity to produce novel blue flowers for a number of ornamental plants missing blue flowering varieties. However, our previous study indicated difficulties in obtaining good accumulation of delphinidin derivatives in plants expressing F3'5'H. Here we report the isolation of a putative F3'5'H cDNA (Ka1) from canterbury bells (Campanula medium) and its expression in tobacco. Surprisingly, compared with other F3'5'H cDNAs, Ka1 encoded a protein with a unique primary structure that conferred high competence in the accumulation of delphinidin derivatives (up to 99% of total anthocyanins) and produced novel purple flowers. These results suggest that, among F3'5' H cDNAs, Ka1 is the best genetic resource for the creation of fine blue flowers by genetic engineering.