A novel red clover hydroxycinnamoyl transferase has enzymatic activities consistent with a role in phaselic acid biosynthesis

Complete biosynthesis of the phenylethanoid glycoside verbascoside

Verbascoside, which was first discovered in 1963, is a well-known phenylethanoid glycoside (PhG) that exhibits antioxidant, anti-inflammatory, antimicrobial, and neuroprotective activities and contributes to the therapeutic effects of many medicinal plants. However, the biosynthetic pathway of verbascoside remains to be fully elucidated. Here, we report the identification of two missing enzymes in the verbascoside biosynthesis pathway by transcriptome mining and in vitro enzymatic assays. Specifically, a BAHD acyltransferase (hydroxycinnamoyl-CoA:salidroside hydroxycinnamoyltransferase [SHCT]) was shown to catalyze the regioselective acylation of salidroside to form osmanthuside A, and a CYP98 hydroxylase (osmanthuside B 3,3'-hydroxylase [OBH]) was shown to catalyze meta-hydroxylations of the p-coumaroyl and tyrosol moieties of osmanthuside B to complete the biosynthesis of verbascoside. Because SHCTs and OBHs are found in many Lamiales species that produce verbascoside, this pathway may be general. The findings from the study provide novel insights into the formation of caffeoyl and hydroxytyrosol moieties in natural product biosynthetic pathways. In addition, with the newly acquired enzymes, we achieved heterologous production of osmanthuside B, verbascoside, and ligupurpuroside B in Escherichia coli; this work lays a foundation for sustainable production of verbascoside and other PhGs in micro-organisms.

ROS scavenging and ion homeostasis is required for the adaptation of halophyte Karelinia caspia to high salinity

The halophyte Karelinia caspia has not only fodder and medical value but also can remediate saline-alkali soils. Our previous study showed that salt-secreting by salt glands is one of main adaptive strategies of K. caspia under high salinity. However, ROS scavenging, ion homeostasis, and photosynthetic characteristics responses to high salinity remain unclear in K. caspia. Here, physio-biochemical responses and gene expression associated with ROS scavenging and ions transport were tested in K. caspia subjected to 100-400 mM NaCl for 7 days. Results showed that both antioxidant enzymes (SOD, APX) activities and non-enzymatic antioxidants (chlorogenic acid, α-tocopherol, flavonoids, polyamines) contents were significantly enhanced, accompanied by up-regulating the related enzyme and non-enzymatic antioxidant synthesis gene (KcCu/Zn-SOD, KcAPX6, KcHCT, KcHPT1, Kcγ-TMT, KcF3H, KcSAMS and KcSMS) expression with increasing concentrations of NaCl. These responses are beneficial for removing excess ROS to maintain a stable level of H2O2 and O2 - without lipid peroxidation in the K. caspia response to high salt. Meanwhile, up-regulating expression of KcSOS1/2/3, KcNHX1, and KcAVP was linked to Na+ compartmentalization into vacuoles or excretion through salt glands in K. caspia. Notably, salt can improve the function of PSII that facilitate net photosynthetic rates, which is helpful to growing normally in high saline. Overall, the findings suggested that ROS scavenging systems and Na+/K+ transport synergistically contributed to redox equilibrium, ion homeostasis, and the enhancement of PSII function, thereby conferring high salt tolerance.

Substrate promiscuity of acyltransferases contributes to the diversity of hydroxycinnamic acid derivatives in purple coneflower

High pliability and promiscuity are observed widely exist in plant specialized metabolism, especially the hydroxycinnamic acid metabolism. Here, we identified an addition BAHD acyltransferase (EpHMT) that catalyzes phaselic acid biosynthesis and found that the substrate promiscuities of identified BAHD and SCPL acyltransferases are responsible for the diversity of hydroxycinnamic acid derivatives in purple coneflower.

Near-real time determination of BAHD acyl-coenzyme A transferase reaction rates and kinetic parameters using Ellman's reagent

BAHD acyl-coenzyme A (CoA) acyltransferases play key roles in a large number of biosynthetic reactions involved in plant specialized metabolism. One approach to measure reaction rates for these enzymes is to quantify the amide or ester reaction products following chromatographic separation of reaction components, an approach that can be labor intensive and time consuming, and complicated by a lack of pure standards. We previously developed and validated an alternative approach using 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB, Ellman's reagent) to spectrophotometrically monitor reaction progress by the release of free CoA in the reaction. This approach allows near-real time measurement of reaction rates, permitting reaction conditions (buffer, reactant, and enzyme concentrations, etc.) to be changed "on the fly." The ease and rapidity of data collection allows a high density of data points to be collected for determination of kinetic parameters. Here we provide a detailed procedure for using DTNB to measure BAHD acyl-CoA acyltransferase reaction rates, and as an example, use it to determine kinetic parameters for red clover hydroxycinnamoyl-CoA:shikimate hydroxycinnamoyltransferase, a BAHD acyl-CoA hydroxycinnamoyltransferase not previously characterized with respect to kinetic parameters. This approach may be more generally applicable to transferases using CoA donors.

Red Clover HDT, a BAHD Hydroxycinnamoyl-Coenzyme A:L-3,4-Dihydroxyphenylalanine (L-DOPA) Hydroxycinnamoyl Transferase That Synthesizes Clovamide and Other N-Hydroxycinnamoyl-Aromatic Amino Acid Amides

Red clover leaves accumulate high levels (up to 1 to 2% of dry matter) of two caffeic acid derivatives: phaselic acid (2-O-caffeoyl-L-malate) and clovamide [N-caffeoyl-L-3,4-dihydroxyphenylalanine (L-DOPA)]. These likely play roles in protecting the plant from biotic and abiotic stresses but can also help preserve protein during harvest and storage of the forage via oxidation by an endogenous polyphenol oxidase. We previously identified and characterized, a hydroxycinnamoyl-coenzyme A (CoA):malate hydroxycinnamoyl transferase (HMT) from red clover. Here, we identified a hydroxycinnamoyl-CoA:L-DOPA hydroxycinnamoyl transferase (HDT) activity in unexpanded red clover leaves. Silencing of the previously cloned HMT gene reduced both HMT and HDT activities in red clover, even though the HMT enzyme lacks HDT activity. A combination of PCR with degenerate primers based on BAHD hydroxycinnamoyl-CoA transferase sequences and 5' and 3' rapid amplification of cDNA ends was used to clone two nearly identical cDNAs from red clover. When expressed in Escherichia coli, the encoded proteins were capable of transferring hydroxycinnamic acids (p-coumaric, caffeic, or ferulic) from the corresponding CoA thioesters to the aromatic amino acids L-Phe, L-Tyr, L-DOPA, or L-Trp. Kinetic parameters for these substrates were determined. Stable expression of HDT in transgenic alfalfa resulted in foliar accumulation of p-coumaroyl- and feruloyl-L-Tyr that are not normally present in alfalfa, but not derivatives containing caffeoyl or L-DOPA moieties. Transient expression of HDT in Nicotiana benthamiana resulted in the production of caffeoyl-L-Tyr, but not clovamide. Coexpression of HDT with a tyrosine hydroxylase resulted in clovamide accumulation, indicating the host species' pool of available amino acid (and hydroxycinnamoyl-CoA) substrates likely plays a major role in determining HDT product accumulation in planta. Finally, that HDT and HMT proteins share a high degree of identity (72%), but differ substantially in substrate specificity, is promising for further investigation of structure-function relationships of this class of enzymes, which could allow the rational design of BAHD enzymes with specific and desirable activities.

Catalytic function, mechanism, and application of plant acyltransferases

Acyltransferases (ATs) are important tailoring enzymes that contribute to the diversity of natural products. They catalyze the transfer of acyl groups to the skeleton, which improves the lipid solubility, stability, and pharmacological activity of natural compounds. In recent years, a number of ATs have been isolated from plants. In this review, we have summarized 141 biochemically characterized ATs during the period July 1997 to October 2020, including their function, heterologous expression systems, and catalytic mechanisms. Their catalytic performance and application potential has been further discussed.

Engineering Alfalfa to Produce 2-O-Caffeoyl-L-Malate (Phaselic Acid) for Preventing Post-harvest Protein Loss via Oxidation by Polyphenol Oxidase

Many plants accumulate high levels of hydroxycinnamoyl esters and amides in their tissues, presumably to protect against biotic and abiotic stress. Red clover (Trifolium pretense) leaves accumulate high levels [5-15 mmol/kg fresh weight (FW)] of caffeic acid derivatives, including phaselic acid (2-O-caffeoyl-L-malate). Oxidation of caffeoyl-malate by an endogenous polyphenol oxidase (PPO) has been shown to help preserve forage protein after harvest and during storage as silage, which should improve N use efficiency in dairy and other ruminant production systems. The widely grown forage alfalfa lacks both PPO and PPO substrates and experiences substantial loss of protein following harvest. We previously identified a hydroxycinnamoyl-coenzyme A (CoA):malate hydroxycinnamoyl transferase (HMT, previously called HCT2) responsible for phaselic accumulation in red clover. With the goal of producing PPO-oxidizable compounds in alfalfa to help preserve forage protein, we expressed red clover HMT in alfalfa. Leaves of these alfalfa accumulated mainly p-coumaroyl- and feruloyl-malate (up to 1.26 and 0.25 mmol/kg FW, respectively). Leaves of HMT-expressing alfalfa supertransformed with an RNA interference (RNAi) construct to silence endogenous caffeoyl-CoA acid O-methyltransferase (CCOMT) accumulated high levels of caffeoyl-malate, as well as the p-coumaroyl and feruloyl esters (up to 2.16, 2.08, and 3.13 mmol/kg FW, respectively). Even higher levels of caffeoyl- and p-coumaroyl-malate were seen in stems (up to 8.37 and 3.15 mmol/kg FW, respectively). This level of caffeoyl-malate accumulation was sufficient to inhibit proteolysis in a PPO-dependent manner in in vitro experiments, indicating that the PPO system of post-harvest protein protection can be successfully adapted to alfalfa.

A metabolomics characterisation of natural variation in the resistance of cassava to whitefly

BACKGROUND

Cassava whitefly outbreaks were initially reported in East and Central Africa cassava (Manihot esculenta Crantz) growing regions in the 1990's and have now spread to other geographical locations, becoming a global pest severely affecting farmers and smallholder income. Whiteflies impact plant yield via feeding and vectoring cassava mosaic and brown streak viruses, making roots unsuitable for food or trading. Deployment of virus resistant varieties has had little impact on whitefly populations and therefore development of whitefly resistant varieties is also necessary as part of integrated pest management strategies. Suitable sources of whitefly resistance exist in germplasm collections that require further characterization to facilitate and assist breeding programs.

RESULTS

In the present work, a hierarchical metabolomics approach has been employed to investigate the underlying biochemical mechanisms associated with whitefly resistance by comparing two naturally occurring accessions of cassava, one susceptible and one resistant to whitefly. Quantitative differences between genotypes detected at pre-infestation stages were consistently observed at each time point throughout the course of the whitefly infestation. This prevalent differential feature suggests that inherent genotypic differences override the response induced by the presence of whitefly and that they are directly linked with the phenotype observed. The most significant quantitative changes relating to whitefly susceptibility were linked to the phenylpropanoid super-pathway and its linked sub-pathways: monolignol, flavonoid and lignan biosynthesis. These findings suggest that the lignification process in the susceptible variety is less active, as the susceptible accession deposits less lignin and accumulates monolignol intermediates and derivatives thereof, differences that are maintained during the time-course of the infestation.

CONCLUSIONS

Resistance mechanism associated to the cassava whitefly-resistant accession ECU72 is an antixenosis strategy based on reinforcement of cell walls. Both resistant and susceptible accessions respond differently to whitefly attack at biochemical level, but the inherent metabolic differences are directly linked to the resistance phenotype rather than an induced response in the plant.

A BAHD hydroxycinnamoyltransferase from Actaea racemosa catalyses the formation of fukinolic and cimicifugic acids

The nucleotide sequence of a BAHD hydroxycinnamoyltransferase was amplified from Actaea racemosa (Ranunculaceae) and expressed in E. coli. The protein catalysed the formation of cimicifugic acids and thus is named hydroxycinnamoyl-CoA:piscidic acid hydroxycinnamoyltransferase (ArHPT1; cimicifugic acid synthase). Actaea racemosa (syn. Cimicifuga racemosa) is known to contain triterpene lactone glycosides and cimicifugic acids. The latter are esters of various hydroxycinnamic or benzoic acids with piscidic or fukiic acid. Amplification of a nucleotide sequence from A. racemosa, that was already known as HCT1 from an EST approach, and its expression in E. coli resulted in a protein that was able to catalyse the formation of several cimicifugic acids. For the characterisation of this hydroxycinnamoyltransferase (hydroxy)cinnamoyl-coenzyme A thioesters were synthesised as donor substrates and piscidic acid isolated as acceptor substrate. The lowest K_m-value with 6.8 µM was determined for p-coumaroyl-CoA. More than 30 possible acceptor substrates were tested, but only piscidic acid and putatively fukiic acid were accepted. The apparent K_m-value for piscidic acid was 32.3 µM. High expression of the hydroxycinnamoyltransferase gene was found in roots, but the content of cimicifugic acids was higher in leaves and flowers than in roots. This work describes for the first time a biosynthetic step in the formation of cimicifugic acids catalysed by a so far uncharacterised hydroxycinnamoyltransferase accepting piscidic acid as acceptor substrate thus being a hydroxycinnamoyl-CoA:piscidic acid hydroxycinnamoyltransferase (ArHPT1; cimicifugic acid synthase).

Transcriptomic analysis of key genes involved in chlorogenic acid biosynthetic pathway and characterization of MaHCT from Morus alba L

Mulberry leaves (Morus alba L.) are of high medicinal value in traditional Chinese medicine with chlorogenic acid (CGA) as its major biologically active constituent. Mulberry leaves require that they be harvested after frost; previous studies have shown CGA accumulation significantly increased after frost. However, the molecular mechanism of how frost changes the CGA content in mulberry leaves is unclear. Additionally, the mechanism of CGA biosynthesis and key genes in mulberry leaves are not well-understood. In this study, transcriptome sequencing was performed on two mulberry leaf samples with different CGA contents (before and after frost). Fifty-eight genes were annotated in the CGA biosynthetic pathway. Compared to those in pre-frost mulberry leaves, 12 and 5 genes were upregulated and downregulated, respectively, in post-frost leaves. Correlation analysis showed that the expression levels of four genes were significantly positively correlated with CGA content, including those encoding phenylalanine ammonia-lyase, 4-coumarate-CoA ligase, hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyltransferase (HCT), and coumaroyl quinate/shikimate 3'-hydroxylase, and may be key genes in the CGA biosynthetic pathway. We cloned MaHCT4 (GenBank accession no. MH476577) from mulberry leaves. Multiple sequence alignment suggested that MaHCT4 contains the conserved domains HXXXD and DFGWG. Enzymatic assays indicated that MaHCT4 catalyzes the formation of p-coumaroyl shikimic acid, p-coumaroyl quinic acid, and CGA. The K_m values of quinic acid and shikimic acid were 10 ± 1.0 and 31 ± 1.7 μM, respectively, suggesting that MaHCT4 favored quinic acid over shikimic acid as its acyl acceptor. Using quinic acid as an acyl acceptor, MaHCT4 showed a preference for p-coumaroyl-CoA over caffeoyl-CoA. Our results provide insight into the molecular mechanism of how frost alters the CGA content and roles of key genes involved in the CGA biosynthetic pathway in mulberry leaves.

Molecular Identification and Characterization of Hydroxycinnamoyl Transferase in Tea Plants (Camellia sinensis L.)

Tea (Camellia sinensis L.) contains abundant secondary metabolites, which are regulated by numerous enzymes. Hydroxycinnamoyl transferase (HCT) is involved in the biosynthesis pathways of polyphenols and flavonoids, and it can catalyze the transfer of hydroxyconnamoyl coenzyme A to substrates such as quinate, flavanol glycoside, or anthocyanins, thus resulting in the production of chlorogenic acid or acylated flavonol glycoside. In this study, the CsHCT gene was cloned from the Chin-Shin Oolong tea plant, and its protein functions and characteristics were analyzed. The full-length cDNA of CsHCT contains 1311 base pairs and encodes 436 amino acid sequences. Amino acid sequence was highly conserved with other HCTs from Arabidopsis thaliana, Populus trichocarpa, Hibiscus cannabinus, and Coffea canephora. Quantitative real-time polymerase chain reaction analysis showed that CsHCT is highly expressed in the stem tissues of both tea plants and seedlings. The CsHCT expression level was relatively high at high altitudes. The abiotic stress experiment suggested that low temperature, drought, and high salinity induced CsHCT transcription. Furthermore, the results of hormone treatments indicated that abscisic acid (ABA) induced a considerable increase in the CsHCT expression level. This may be attributed to CsHCT involvement in abiotic stress and ABA signaling pathways.

A BAHD neofunctionalization promotes tetrahydroxycinnamoyl spermine accumulation in the pollen coat of the Asteraceae family

In eudicotyledons, accumulation of trihydroxycinnamoyl spermidine that is restricted to the pollen wall constitutes an evolutionary conserved trait. However, the role of this compound, which is synthetized by the BAHD enzyme spermidine hydroxycinnamoyl transferase (SHT), is still a matter of debate. Here, we show that this particular phenolamide is replaced by tetrahydroxycinnamoyl spermine in the pollen coat of the Asteraceae. Phylogenetic analyses combined with quantitative RT-PCR experiments allowed the identification of two homologous genes from Cichorium intybus (chicory) putatively involved in its metabolism. In vitro biochemical characterization of the two enzymes, named CiSHT1 and CiSHT2, confirmed the capability of recombinant proteins to synthesize spermine as well as spermidine derivatives. The wild-type metabolic phenotype was partially restored in an Arabidopsis sht mutant expressing CiSHT2. Strikingly, the transgenic plants also accumulated spermine derivatives that were absent in the wild-type. Overexpression of CiSHT2 in chicory hairy roots led to the accumulation of spermine derivatives, confirming its in vivo function. Complementary sequence analyses revealed the presence of an amino acid motif typical of the SHTs among the BAHD enzyme family. Our results highlight a recent neofunctionalization among the SHTs that has promoted the emergence of new phenolamides in the Asteraceae, which could potentially have contributed to the evolutionary success of this family.

Focus on putative serine carboxypeptidase-like acyltransferases in grapevine

Grapevine (Vitis vinifera L.) berry synthesizes and accumulates a large array of phenolic compounds (e.g. flavonoids and hydroxycinnamic acid derivatives), some of which result from acylation mechanisms. In grapevine, the genes encoding enzymes responsible for such acylation are largely unknown. Enzymes classified as serine carboxypeptidases (SCPs), able to transfer acyl moieties from a glucose ester, have previously been characterized in plants, and named serine carboxypeptidase-like acyltransferases (SCL-ATs). We performed genome-wide identification of SCP sequences in V. vinifera. Phylogenetic analysis revealed that only 12 grapevine SCPs, grouped in clade IA with previously characterized SCPL-AT could have an acylation function. Interestingly, seven putative SCP-ATs are grouped in a 400 kb cluster in chromosome 3. The expression level of putative SCPL-ATs has been evaluated at key stages of grape berry development in the main tissues and compared with the content of acylated phenolic compounds in the corresponding samples. The expression levels of VvGAT1 and VvGAT2 and that of VvSCP5 were increased in hairy-roots overexpressing transcription factors inducing the biosynthesis of proanthocyanidins and anthocyanins, respectively. These findings open the way for the functional characterization of the identified putative SCPL-AT from grapevine.

Discovery of UDP-Glycosyltransferases and BAHD-Acyltransferases Involved in the Biosynthesis of the Antidiabetic Plant Metabolite Montbretin A

Plant specialized metabolism serves as a rich resource of biologically active molecules for drug discovery. The acylated flavonol glycoside montbretin A (MbA) and its precursor myricetin 3-O-(6'-O-caffeoyl)-glucosyl rhamnoside (mini-MbA) are potent inhibitors of human pancreatic α-amylase and are being developed as drug candidates to treat type-2 diabetes. MbA occurs in corms of the ornamental plant montbretia (Crocosmia x crocosmiiflora), but a system for large-scale MbA production is currently unavailable. Biosynthesis of MbA from the flavonol myricetin and MbA accumulation occur during early stages of corm development. We established myricetin 3-O-rhamnoside (MR), myricetin 3-O-glucosyl rhamnoside (MRG), and mini-MbA as the first three intermediates of MbA biosynthesis. Contrasting the transcriptomes of young and old corms revealed differentially expressed UDP-sugar-dependent glycosyltransferases (UGTs) and BAHD-acyltransferases (BAHD-ATs). UGT77B2 and UGT709G2 catalyze the consecutive glycosylation of myricetin to produce MR and of MR to give MRG, respectively. In addition, two BAHD-ATs, CcAT1 and CcAT2, catalyze the acylation of MRG to complete the formation of mini-MbA. Transcript profiles of UGT77B2, UGT709G2, CcAT1, and CcAT2 during corm development matched the metabolite profile of MbA accumulation. Expression of these enzymes in wild tobacco (Nicotiana benthamiana) resulted in the formation of a surrogate mini-MbA, validating the potential for metabolic engineering of mini-MbA in a heterologous plant system.

Isolation and functional characterization of hydroxycinnamoyltransferases from the liverworts Plagiochasma appendiculatum and Marchantia paleacea

Hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferase (HCT, EC: 2.3.1.133) is a key metabolic entry point for the synthesis of monolignols in vascular plants; however, little is known about HCT in liverworts. Here, the isolation and characterization of HCTs encoded by the two liverwort species, Plagiochasma appendiculatum and Marchantia paleacea, are described. The sequences of the two enzymes harbor features typical of BAHD family members, except for the presence of a stretch of >100 residues that are not represented in higher plant HCTs. When truncated versions of both genes, which were constructed to clarify the significance of these extra residues, were investigated, it became apparent that the full-length and the truncated gene products shared similar catalytic activity and recognized the same substrates in vitro. They also functioned equivalently in vivo either when transiently expressed in tobacco to cause a higher total production of CGA (5-CQA) and 4-CQA or stably expressed in liverworts to accumulate the lignin-like contents. A structural model of MpHCT suggests that its active site bind to its substrate similar to that of Arabidopsis thaliana HCT. While truncated forms of HCT were deposited in the nucleocytoplasm, the full-length versions occurred exclusively in the cytoplasm. The conclusion is that liverworts produce bona fide HCTs that represent a point of departure in studying the evolution of lignin synthesis in plants.

Spectrophotometric determination of reaction rates and kinetic parameters of a BAHD acyltransferase using DTNB (5,5'-dithio-bis-[2-nitrobenzoic acid])

Hydroxycinnamoyl-Coenzyme A (CoA) hydroxycinnamoyl transferases are BAHD family acyltransferases that transfer hydroxycinnamoyl moieties from a CoA-thioester to an acceptor amine or alcohol to form an N-hydroxycinnamoyl amide or O-hydroxycinnamoyl ester, respectively, with the concomitant release of free CoA. One approach to measure reaction rates for these enzymes is to quantify the hydroxycinnamoyl amide or ester reaction product following chromatographic separation of reaction components. This approach can be labor-intensive and time-consuming. As an alternative, we examined the use of 5,5'-dithio-bis-(2-nitrobenzoic acid) (DTNB, Ellman's reagent) to spectrophotometrically quantify, in real time, the release of free CoA during the transferase reaction. Using a hydroxycinnamoyl-CoA:l-DOPA hydroxycinnamoyl transferase as a model, we show that DTNB has little to no effect on the transferase reaction and can be used to provide a good estimate of hydroxycinnamoyl amide formation, thus allowing for the quick and easy collection of reaction rate data and determination of transferase kinetic parameters. This approach should be applicable to a wide range of hydroxycinnamoyl-CoA and other BAHD acyltransferases.

Detection of Potential Chloroplastic Substrates for Polyphenol Oxidase Suggests a Role in Undamaged Leaves

Polyphenol oxidases (PPOs) have a recognized role during pathogen and arthropod attack. As an immediate consequence of such wounding, cellular compartmentation is destroyed allowing the chloroplastic PPO enzyme to interact with vacuolar substrates catalyzing the oxidation of monophenols and/or o-diphenols to o-diquinones. This ultimately results in a reduction in the nutritional value of wounded tissue through the formation of non-digestible secondary melanin pigments. However, the chloroplastic location of PPO enzyme could indicate a role for PPO in undamaged tissues. In this study, a wild-type red clover population exhibiting high leaf PPO activity had significantly higher yield than a low leaf PPO mutant population while leaf isoflavonoids and hydroxycinnammates (PPO substrates) accumulated at similar levels in these plants. These data suggest that the presence of leaf PPO activity affects plant vigor. Understanding how this advantage is conferred requires knowledge of the cellular mechanism, including intra-organellar substrates. Here we present evidence of candidate PPO substrates within chloroplasts of wild-type red clover, including the monophenolic acid, coumaroyl malate, and low levels of the diphenolic acid, phaselic acid (caffeoyl malate). Interestingly, chloroplastic phaselic acid concentration increased significantly under certain growth conditions. We discuss the implications of this in regard to a potential role for chloroplastic PPO in undamaged leaves.

Identification of bean hydroxycinnamoyl-CoA:tetrahydroxyhexanedioate hydroxycinnamoyl transferase (HHHT): use of transgenic alfalfa to determine acceptor substrate specificity

Transgenic alfalfa ( Medicago sativa L.) provides a useful reverse genetics platform to elucidate acceptor substrate specificity for uncharacterized BAHD family hydroxycinnamoyl-CoA hydroxycinnamoyl transferases. Tissues of many plant species accumulate hydroxycinnamoyl derivatives, often esters, thought to serve in protection against biotic and abiotic stresses. In many cases, these specialized metabolites are produced by BAHD family hydroxycinnamoyl-CoA hydroxycinnamoyl transferases (HCTs). Bean (Phaseolus vulgaris) leaves contain both hydroxycinnamoyl-malate esters and an HCT activity capable of making them. In seeking to identify this HCT from bean, we identified a gene whose predicted protein showed a high degree of sequence similarity (75%) to the Trifolium pratense (red clover) enzyme that carries out this reaction. The encoded bean protein, however, failed to carry out the malate transfer reaction when expressed in Escherichia coli. Expression of the gene in alfalfa (Medicago sativa) resulted in accumulation of several new hydroxycinnamates not present in nontransformed alfalfa, many of which corresponded to phenolics present in bean. Using accurate mass and UV absorption spectral data, we identified the acceptor substrate for this HCT as tetrahydroxyhexanedioic acids and demonstrated this predicted transferase activity with the E. coli-expressed protein. This finding adds to the growing number of BAHD family HCTs that have been characterized with respect to substrate specificity. Such data, combined with primary sequence and protein structural data will allow for a better understanding of the structure/function relationships of these enzymes and may eventually aid the rational design of such enzymes for altered substrate specificities. Additionally, expression of HCTs of unknown substrate specificity in alfalfa and characterization of the resulting accumulated novel metabolites could be a useful approach to characterizing putative BAHD HCT enzymes.

Exploiting members of the BAHD acyltransferase family to synthesize multiple hydroxycinnamate and benzoate conjugates in yeast

BACKGROUND

BAHD acyltransferases, named after the first four biochemically characterized enzymes of the group, are plant-specific enzymes that catalyze the transfer of coenzyme A-activated donors onto various acceptor molecules. They are responsible for the synthesis in plants of a myriad of secondary metabolites, some of which are beneficial for humans either as therapeutics or as specialty chemicals such as flavors and fragrances. The production of pharmaceutical, nutraceutical and commodity chemicals using engineered microbes is an alternative, green route to energy-intensive chemical syntheses that consume petroleum-based precursors. However, identification of appropriate enzymes and validation of their functional expression in heterologous hosts is a prerequisite for the design and implementation of metabolic pathways in microbes for the synthesis of such target chemicals.

RESULTS

For the synthesis of valuable metabolites in the yeast Saccharomyces cerevisiae, we selected BAHD acyltransferases based on their preferred donor and acceptor substrates. In particular, BAHDs that use hydroxycinnamoyl-CoAs and/or benzoyl-CoA as donors were targeted because a large number of molecules beneficial to humans belong to this family of hydroxycinnamate and benzoate conjugates. The selected BAHD coding sequences were synthesized and cloned individually on a vector containing the Arabidopsis gene At4CL5, which encodes a promiscuous 4-coumarate:CoA ligase active on hydroxycinnamates and benzoates. The various S. cerevisiae strains obtained for co-expression of At4CL5 with the different BAHDs effectively produced a wide array of valuable hydroxycinnamate and benzoate conjugates upon addition of adequate combinations of donors and acceptor molecules. In particular, we report here for the first time the production in yeast of rosmarinic acid and its derivatives, quinate hydroxycinnamate esters such as chlorogenic acid, and glycerol hydroxycinnamate esters. Similarly, we achieved for the first time the microbial production of polyamine hydroxycinnamate amides; monolignol, malate and fatty alcohol hydroxycinnamate esters; tropane alkaloids; and benzoate/caffeate alcohol esters. In some instances, the additional expression of Flavobacterium johnsoniae tyrosine ammonia-lyase (FjTAL) allowed the synthesis of p-coumarate conjugates and eliminated the need to supplement the culture media with 4-hydroxycinnamate.

CONCLUSION

We demonstrate in this study the effectiveness of expressing members of the plant BAHD acyltransferase family in yeast for the synthesis of numerous valuable hydroxycinnamate and benzoate conjugates.

Identification and Characterization of Five BAHD Acyltransferases Involved in Hydroxycinnamoyl Ester Metabolism in Chicory

Chicory (Cichorium intybus) accumulates caffeic acid esters with important significance for human health. In this study, we aim at a better understanding of the biochemical pathway of these bioactive compounds. Detailed metabolic analysis reveals that C. intybus predominantly accumulates caftaric and chicoric acids in leaves, whereas isochlorogenic acid (3,5-diCQA) was almost exclusively accumulated in roots. Chlorogenic acid (3-CQA) was equally distributed in all organs. Interestingly, distribution of the four compounds was related to leaf age. Induction with methyljasmonate (MeJA) of root cell suspension cultures results in an increase of 3-CQA and 3,5-diCQA contents. Expressed sequence tag libraries were screened using members of the BAHD family identified in Arabidopsis and tobacco as baits. The full-length cDNAs of five genes were isolated. Predicted amino acid sequence analyses revealed typical features of BAHD family members. Biochemical characterization of the recombinant proteins expressed in Escherichia coli showed that two genes encode HCTs (hydroxycinnamoyl-CoA:shikimate/quinate hydroxycinnamoyltransferases, HCT1 and HCT2) whereas, three genes encode HQTs (hydroxycinnamoyl-CoA:quinate hydroxycinnamoyltransferases, HQT1, HQT2, and HQT3). These results totally agreed with the phylogenetic analysis done with the predicted amino acid sequences. Quantitative real-time polymerase chain reaction analysis of gene expression indicated that HQT3, HCT1, and HCT2 might be more directly associated with CQA accumulation in cell culture in response to MeJA elicitation. Transient expression of HCT1 and HQT1 in tobacco resulted in a higher production of 3-CQA. All together these data confirm the involvement of functionally redundant genes in 3-CQA and related compound synthesis in the Asteraceae family.

BAHD or SCPL acyltransferase? What a dilemma for acylation in the world of plant phenolic compounds

Phenolic compounds are secondary metabolites involved in several plant growth and development processes, including resistance to biotic and abiotic stresses. The biosynthetic pathways leading to the vast diversity of plant phenolic products often include an acylation step, with phenolic compounds being the donor or acceptor molecules. To date, two acyltransferase families using phenolic compounds as acceptor or donor molecules have been described, with each using a different 'energy-rich' acyl donor. BAHD-acyltransferases, named after the first four biochemically characterized enzymes of the group, use acyl-CoA thioesters as donor molecules, whereas SCPL (Serine CarboxyPeptidase Like)-acyltransferases use 1-O-β-glucose esters. Here, common and divergent specifications found in the literature for both enzyme families were analyzed to answer the following questions. Are both acyltransferases involved in the synthesis of the same molecule (or same group of molecules)? Are both acyltransferases recruited in the same plant? How does the subcellular localization of these enzymes impact metabolite trafficking in plant cells?

Soluble Phenolic Compounds in Different Cultivars of Red Clover and Alfalfa, and their Implication for Protection against Proteolysis and Ammonia Production in Ruminants

Red clover ( Trifolium pratense) contains soluble phenolic compounds with roles in inhibiting proteolysis and ammonia production. Alfalfa ( Medicago sativa) has been found to have a low phenolic content, but few alfalfa and red clover cultivars have been compared for phenolic content. Total soluble phenolics were quantified by a Folin-Ciocalteu colorimetric assay in nine red clover and 27 alfalfa cultivars. Mean total phenolic contents of red clover and alfalfa were 36.5 ± 4.3 mg/gdw and 15.8 ± 1.4 mg/gdw, respectively, with the greater standard deviation of red clover possibly indicating more diversity in phenolic content. Because different phenolic standards had different response factors in the colorimetric assay, the red clover and 11 alfalfa cultivars were analyzed by HPLC to determine if the differences in total soluble phenolics between genera reflected differences in the amounts of phenolics or in the classes of phenolics responding to the colorimetric assay. Two red clover cultivars differed in total phenolics and phaselic acid. Alfalfa produced different phenolic compounds from red clover, at lower concentrations. Extracts of two red clover cultivars were separated by thin-layer chromatography (TLC), and the bands were assayed for activity against Clostridium sticklandii, a bovine ruminal hyper ammonia-producing bacterium (HAB). Only biochanin A had anti-HAB activity. Inhibitory amounts indicated that five red clover cultivars could be suitable sources of anti-HAB activity.

Cell wall composition and digestibility alterations in Brachypodium distachyon achieved through reduced expression of the UDP-arabinopyranose mutase

Nucleotide-activated sugars are essential substrates for plant cell-wall carbohydrate-polymer biosynthesis. The most prevalent grass cell wall (CW) sugars are glucose (Glc), xylose (Xyl), and arabinose (Ara). These sugars are biosynthetically related via the UDP-sugar interconversion pathway. We sought to target and generate UDP-sugar interconversion pathway transgenic Brachypodium distachyon lines resulting in CW carbohydrate composition changes with improved digestibility and normal plant stature. Both RNAi-mediated gene-suppression and constitutive gene-expression approaches were performed. CWs from 336 T0 transgenic plants with normal appearance were screened for complete carbohydrate composition. RNAi mutants of BdRGP1, a UDP-arabinopyranose mutase, resulted in large alterations in CW carbohydrate composition with significant decreases in CW Ara content but with minimal change in plant stature. Five independent RNAi-RGP1 T1 plant lines were used for in-depth analysis of plant CWs. Real-time PCR analysis indicated that gene expression levels for BdRGP1, BdRGP2, and BdRGP3 were reduced in RNAi-RGP1 plants to 15-20% of controls. CW Ara content was reduced by 23-51% of control levels. No alterations in CW Xyl and Glc content were observed. Corresponding decreases in CW ferulic acid (FA) and ferulic acid-dimers (FA-dimers) were observed. Additionally, CW p-coumarates (pCA) were decreased. We demonstrate the CW pCA decrease corresponds to Ara-coupled pCA. Xylanase-mediated digestibility of RNAi-RGP1 Brachypodium CWs resulted in a near twofold increase of released total carbohydrate. However, cellulolytic hydrolysis of CW material was inhibited in leaves of RNAi-RGP1 mutants. Our results indicate that targeted manipulation of UDP-sugar biosynthesis can result in biomass with substantially altered compositions and highlights the complex effect CW composition has on digestibility.

Identification and suppression of the p-coumaroyl CoA:hydroxycinnamyl alcohol transferase in Zea mays L

Grasses, such as Zea mays L. (maize), contain relatively high levels of p-coumarates (pCA) within their cell walls. Incorporation of pCA into cell walls is believed to be due to a hydroxycinnamyl transferase that couples pCA to monolignols. To understand the role of pCA in maize development, the p-coumaroyl CoA:hydroxycinnamyl alcohol transferase (pCAT) was isolated and purified from maize stems. Purified pCAT was subjected to partial trypsin digestion, and peptides were sequenced by tandem mass spectrometry. TBLASTN analysis of the acquired peptide sequences identified a single full-length maize cDNA clone encoding all the peptide sequences obtained from the purified enzyme. The cDNA clone was obtained and used to generate an RNAi construct for suppressing pCAT expression in maize. Here we describe the effects of suppression of pCAT in maize. Primary screening of transgenic maize seedling leaves using a new rapid analytical platform was used to identify plants with decreased amounts of pCA. Using this screening method, mature leaves from fully developed plants were analyzed, confirming reduced pCA levels throughout plant development. Complete analysis of isolated cell walls from mature transgenic stems and leaves revealed that lignin levels did not change, but pCA levels decreased and the lignin composition was altered. Transgenic plants with the lowest levels of pCA had decreased levels of syringyl units in the lignin. Thus, altering the levels of pCAT expression in maize leads to altered lignin composition, but does not appear to alter the total amount of lignin present in the cell walls.

Perennial peanut (Arachis glabrata Benth.) leaves contain hydroxycinnamoyl-CoA:tartaric acid hydroxycinnamoyl transferase activity and accumulate hydroxycinnamoyl-tartaric acid esters

Many plants accumulate hydroxycinnamoyl esters to protect against abiotic and biotic stresses. Caffeoyl esters in particular can be substrates for endogenous polyphenol oxidases (PPOs). Recently, we showed that perennial peanut (Arachis glabrata Benth.) leaves contain PPO and identified one PPO substrate, caftaric acid (trans-caffeoyl-tartaric acid). Additional compounds were believed to be cis- and trans-p-coumaroyl tartaric acid and cis- and trans-feruloyl-tartaric acid, but lack of standards prevented definitive identifications. Here we characterize enzymatic activities in peanut leaves to understand how caftaric acid and related hydroxycinnamoyl esters are made in this species. We show that peanut leaves contain a hydroxycinnamoyl-CoA:tartaric acid hydroxycinnamoyl transferase (HTT) activity capable of transferring p-coumaroyl, caffeoyl, and feruloyl moieties from CoA to tartaric acid (specific activities of 11 ± 2.8, 8 ± 1.8, 4 ± 0.8 pkat mg(-1) crude protein, respectively). The HTT activity was used to make cis- and trans-p-coumaroyl- and -feruloyl-tartaric acid in vitro. These products allowed definitive identification of the corresponding cis- and trans-hydroxycinnamoyl esters extracted from leaves. We tentatively identified sinapoyl-tartaric acid as another major phenolic compound in peanut leaves that likely participates in secondary reactions with PPO-generated quinones. These results suggest hydroxycinnamoyl-tartaric acid esters are made by an acyltransferase, possibly a BAHD family member, in perennial peanut. Identification of a gene encoding HTT and further characterization of the enzyme will aid in identifying determinants of donor and acceptor substrate specificity for this important class of biosynthetic enzymes. An HTT gene could also provide a means by genetic engineering for producing caffeoyl- and other hydroxycinnamoyl-tartaric acid esters in forage crops that lack them.

Both cyclic-AMP and cyclic-GMP can act as regulators of the phenylpropanoid pathway in Arabidopsis thaliana seedlings

Cyclic nucleotides (cAMP and cGMP) are important signaling molecules that control a range of cellular functions and modulate different reactions. It is known that under abiotic or biotic stress plant cells synthesize these nucleotides and that they also enhance the activity of the phenylpropanoid pathway. Wondering what is the relation between these two facts, we investigated how the exogenously applied membrane-permeable derivatives, 8-Br-cAMP or 8-Br-cGMP, which are believed to act as the original cyclic nucleotides, affect the expression of the genes for and the specific activity of three enzymes of the phenylpropanoid pathway in Arabidopsis thaliana seedlings. We found that the expression of the genes of phenylalanine ammonia-lyase (PAL2), 4-coumarate:coenzyme A ligase (4CL1) and chalcone synthase (CHS), and the specific activities of PAL (EC 4.3.1.5), 4CL (EC 6.2.1.12) and CHS (EC 2.3.1.74) were induced in the same way by either of these cyclic nucleotides used at 5 μM concentration. None of the possible cAMP and cGMP degradation products (AMP, GMP, adenosine or guanosine) evoked such effects. Expression of PAL1, 4CL2 and 4CL3 were practically not affected. Although the investigated nucleotides induced rapid expression of the aforementioned enzymes, they did not affect the level of anthocyanins within the same period. We discuss the effects exerted by the exogenously administered cyclic nucleotides, their relation with stress and the role which the phenylpropanoid pathways the cyclic nucleotides may play in plants.

Gene expression patterns, localization, and substrates of polyphenol oxidase in red clover ( Trifolium pratense L.)

Polyphenol oxidase (PPO) genes and their corresponding enzyme activities occur in many plants; natural PPO substrates and enzyme/substrate localization are less well characterized. Leaf and root PPO activities in Arabidopsis and five legumes were compared with those of high-PPO red clover ( Trifolium pratense L.). Red clover PPO enzyme activity decreased leaves > stem > nodules > peduncle = petiole > embryo; PPO1 and PPO4 genes were expressed early in leaf emergence, whereas PPO4 and PPO5 predominated in mature leaves. PPO1 was expressed in embryos and nodules. PPO substrates, phaselic acid and clovamide, were detected in leaves, and clovamide was detected in nodules. Phaselic acid and clovamide, along with caffeic and chlorogenic acids, were suitable substrates for PPO1, PPO4, and PPO5 genes expressed in alfalfa ( Medicago sativa L.) leaves. PPO enzyme presence and activity were colocalized in leaves and nodules by cytochemistry. Substrates and PPO activity were localized in developing squashed cell layer of nodules, suggesting PPO may have a developmental role in nodules.

Efficacy of various naturally occurring caffeic acid derivatives in preventing post-harvest protein losses in forages

BACKGROUND

In red clover, oxidation of endogenous o-diphenols by polyphenol oxidase (PPO) inhibits post-harvest proteolyis. This system is transferable to alfalfa by providing PPO (via a transgene) and o-diphenol PPO substrates (via exogenous application). To exploit the PPO system for protein protection, it would be advantageous to produce PPO substrates in alfalfa, which lacks them. We assessed the extent of PPO-mediated proteolytic inhibition by phenolic compounds, especially those whose biosynthesis could be engineered into alfalfa.

RESULTS

Tested compounds included o-diphenols (caffeic acid, phaselic acid, chlorogenic acid, clovamide) and monophenols (p-coumaric acid, p-coumaroyl-malic acid). In the presence of PPO, 2 mmol o-diphenol g⁻¹ protein reduced 24 h proteolysis 68-87% (P < 0.001) and as little as 0.25 mmol g⁻¹ protein still decreased 24 h proteolysis 43-60% (P < 0.001). At high concentrations, clovamide inhibited 24 h proteolysis 50% (P < 0.001) in the absence of PPO, likely due to non-PPO oxidation. Monophenol p-coumaric acid did not inhibit 24 h proteolyis, although high levels of its malate ester did exhibit PPO- and oxygen-independent inhibition (37%, P < 0.001).

CONCLUSIONS

For PPO-mediated proteolytic inhibition, pathways for both phaselic acid and chlorogenic acid may be good targets for engineering into alfalfa. Clovamide may be useful for inhibiting proteolysis without PPO.

A critique of widely used normalization software tools and an alternative method to identify reliable reference genes in red clover (Trifolium pratense L.)

Determination of appropriate reference genes is crucial to normalization of gene expression data and prevention of biased results in qRT-PCR studies. This study is the first attempt to systematically compare potential reference genes to detect the most constitutively expressed reference genes for accurate normalization in red clover tissues including leaves, stems and roots. To identify the best-suited reference gene(s) for normalization, several statistical algorithms such as geNorm, BestKeeper and NormFinder have been developed. All these algorithms are based on the key assumption that none of the investigated candidate reference genes show systematic variation in their expression profile across the samples being considered. However, this assumption is likely to be violated in practice. The authors therefore suggest a simple and novel stability index based on the analysis of variance model which is free from the assumption made by the algorithms. We assessed the expression stability of eight candidate reference genes including actin (ACT), glyceraldehyde-3-phosphate-dehydrogenase (GADPH), elongation factor-1alpha (EF-1α), translation initiation factor (EIF-4a), ubiquitin-conjugating enzyme E2 (UBC2), polyubiquitin (UBQ10), sand family protein (SAND) and yellow-leaf-specific protein 8 (YLS8). Our results indicated that UBC2 and UBQ10 ranked as the two most stably expressed genes in leaf tissue. UBC2 and YLS8 were defined as optimal control genes for stem tissue. EIF-4a and UBC2 were found to be the most stable reference gene for root tissue. GAPDH and SAND showed relatively low stability in expression study of red clover. When all tested tissues were considered, we observed that YLS8 and UBC2 showed remarkable stability in their expression level across tissues.

A structural basis for the biosynthesis of the major chlorogenic acids found in coffee

Chlorogenic acids (CGAs) are a group of phenolic secondary metabolites produced by certain plant species and an important component of coffee (Coffea spp.). The CGAs have been implicated in biotic and abiotic stress responses, while the related shikimate esters are key intermediates for lignin biosynthesis. Here, two hydroxycinnamoyl-coenzyme A shikimate/quinate hydroxycinnamoyl transferases (HCT/HQT) from coffee were biochemically characterized. We show, to our knowledge for the first time, that in vitro, HCT is capable of synthesizing the 3,5-O-dicaffeoylquinic acid diester, a major constituent of the immature coffee grain. In order to further understand the substrate specificity and catalytic mechanism of the HCT/HQT, we performed structural and mutagenesis studies of HCT. The three-dimensional structure of a native HCT and a proteolytically stable lysine mutant enabled the identification of important residues involved in substrate specificity and catalysis. Site-directed mutagenesis confirmed the role of residues leucine-400 and phenylalanine-402 in substrate specificity and of histidine-153 and the valine-31 to proline-37 loop in catalysis. In addition, the histidine-154-asparagine mutant was observed to produce 4-fold more dichlorogenic acids compared with the native protein. These data provide, to our knowledge, the first structural characterization of a HCT and, in conjunction with the biochemical and mutagenesis studies presented here, delineate the underlying molecular-level determinants for substrate specificity and catalysis. This work has potential applications in fine-tuning the levels of shikimate and quinate esters (CGAs including dichlorogenic acids) in different plant species in order to generate reduced or elevated levels of the desired target compounds.

Pigment accumulation and transcription of LhMYB12 and anthocyanin biosynthesis genes during flower development in the Asiatic hybrid lily (Lilium spp.)

Anthocyanin biosynthesis is often regulated by MYB transcription factors that are classified into AN2 and C1 subgroups. The AN2 subgroup regulates the late genes in the anthocyanin biosynthesis pathway of eudicots, whereas the C1 subgroup controls both early and late genes in monocots. Anthocyanin is a major pigment in Asiatic hybrid lilies (Lilium spp.), with LhMYB12 being the first AN2 subgroup in monocots. In this study, the accumulation of pigments and gene transcripts during flower bud development was evaluated to determine the genes regulated by LhMYB12. LhMYB12 and anthocyanin biosynthesis genes showed the same transcription profiles, with LhMYB12 directly activating the promoters of chalcone synthase and dihydroflavonol 4-reductase. This indicates that LhMYB12 regulates both early and late genes, despite belonging to the AN2 subgroup. The cultivar Landini accumulated anthocyanin and flavonol. The contents of these pigments increased during the late stages of flower bud development; this might result from the coordinated expression of early and late genes. During the early stages of flower bud development, the tepals contained no flavonoids but accumulated cinnamic acid derivatives. These results indicate that the profiles of pigment accumulation and gene transcription in lily tepals are unique among angiosperm flowers.

Investigations into the phenolic constituents of dog's mercury (Mercurialis perennis L.) by LC-MS/MS and GC-MS analyses

INTRODUCTION

Dog's mercury (Mercurialis perennis L.) is a traditional European medicinal plant considered as a rich source of bioactive natural products. Yet phytochemical data of the plant are scant.

OBJECTIVE

This study aimed to identify the hydrophilic phenolic constituents from M. perennis by aqueous and hydroalcoholic extraction.

METHODOLOGY

Extracts of herbal parts were investigated in-depth by HPLC(DAD)-MS/MS and GC/MS analyses. In addition, a novel compound was isolated and fully characterised by 1- and 2D-NMR experiments.

RESULTS

Several conjugates of caffeic, p-coumaric and ferulic acids together with glucaric or 2-hydroxyglutaric acids (depsides) were detected in the aqueous extracts from aerial plant parts by use of LC-MS/MS techniques as well UV-spectral data. By implementation of preparative chromatography on polyamide pretreated with formic acid followed by vacuum liquid chromatography on reversed-phase C(18) -silica, one of the predominant depsides was isolated as a pure compound. The NMR spectra ((1) H and (13) C NMR) together with 2D-hetereonuclear multiple bond correlation NMR experiments (gHMBC and gHSQC) and chiral GC investigation, allowed identification of this compound as (-)-(E)-caffeoyl-2-(R)-oxoglutarate. This structure was additionally supported by GC/MS data after silylation and methylation reactions. The hydroalcoholic extract from aerial parts was separated by solvent partition between ethyl acetate and n-butanol. The latter fraction (n-butanol) yielded a mixture of mono- and oligo-glycosides of kaempferol and quercetin, all of them being assigned by LC-MS/MS.

CONCLUSIONS

The present investigation constitutes the first comprehensive report on the hydrophilic constituents of the rarely studied plant Mercurialis and thus completes the phytochemical knowledge on M. perennis.

Diadenosine polyphosphates (Ap3A and Ap4A) behave as alarmones triggering the synthesis of enzymes of the phenylpropanoid pathway in Arabidopsis thaliana

It is known that cells under stress accumulate various dinucleoside polyphosphates, compounds suggested to function as alarmones. In plants, the phenylpropanoid pathways yield metabolites protecting these organisms against various types of stress. Observations reported in this communication link these two phenomena and provide an example of a metabolic "addressee" for an "alarm" signaled by diadenosine triphosphate (Ap3A) or diadenosine tetraphosphate (Ap4A). In response to added Ap3A or Ap4A, seedlings of Arabidopsis thaliana incubated in full nutrition medium increased both the expression of the genes for and the specific activity of phenylalanine ammonia-lyase and 4-coumarate:coenzyme A ligase, enzymes that control the beginning of the phenylpropanoid pathway. Neither adenine mononucleotides (AMP, ADP or ATP) nor adenosine evoked such effects. Reactions catalyzed in vitro by these enzymes were not affected by Ap3A or Ap4A.

Molecular and biochemical basis for stress-induced accumulation of free and bound p-coumaraldehyde in cucumber

To elucidate the genetic and biochemical regulation of elicitor-induced p-coumaraldehyde accumulation in plants, we undertook a multifaceted approach to characterize the metabolic flux through the phenylpropanoid pathway via the characterization and chemical analysis of the metabolites in the p-coumaryl, coniferyl, and sinapyl alcohol branches of this pathway. Here, we report the identification and characterization of four cinnamyl alcohol dehydrogenases (CADs) from cucumber (Cucumis sativus) with low activity toward p-coumaraldehyde yet exhibiting significant activity toward other phenylpropanoid hydroxycinnamaldehydes. As part of this analysis, we identified and characterized the activity of a hydroxycinnamoyl-coenzyme A:shikimate hydroxycinnamoyl transferase (HCT) capable of utilizing shikimate and p-coumaroyl-coenzyme A to generate p-coumaroyl shikimate. Following pectinase treatment of cucumber, we observed the rapid accumulation of p-coumaraldehyde, likely the result of low aldehyde reductase activity (i.e. alcohol dehydrogenase in the reverse reaction) of CsCAD enzymes on p-coumaraldehyde. In parallel, we noted a concomitant reduction in the activity of CsHCT. Taken together, our findings support the hypothesis that the up-regulation of the phenylpropanoid pathway upon abiotic stress greatly enhances the overall p-coumaryl alcohol branch of the pathway. The data presented here point to a role for CsHCT (as well as, presumably, p-coumarate 3-hydroxylase) as a control point in the regulation of the coniferyl and sinapyl alcohol branches of this pathway. This mechanism represents a potentially evolutionarily conserved process to efficiently and quickly respond to biotic and abiotic stresses in cucurbit plants, resulting in the rapid lignification of affected tissues.

Hydroxycinnamoylmalic acids and their methyl esters from pear (Pyrus pyrifolia Nakai) fruit peel

Two novel caffeoylmalic acid methyl esters, 2-O-(trans-caffeoyl)malic acid 1-methyl ester (6) and 2-O-(trans-caffeoyl)malic acid 4-methyl ester (7), were isolated from pear (Pyrus pyrifolia Nakai cv. Chuhwangbae) fruit peels. In addition, 5 known hydroxycinnamoylmalic acids and their methyl esters were identified: 2-O-(trans-coumaroyl)malic acid (1), 2-O-(cis-coumaroyl)malic acid (2), 2-O-(cis-coumaroyl)malic acid 1-methyl ester (3), 2-O-(trans-coumaroyl)malic acid 1-methyl ester (4), and 2-O-(trans-caffeoyl)malic acid (phaselic acid, 5). The chemical structures of these compounds were determined by spectroscopic data from ESI MS and NMR. Of all the isolated compounds, five hydroxycinnamoylmalic acids and their methyl esters (2-4, 6, 7) were identified in the pear for the first time.

Amides and Esters of Phenylpropenoic Acids from the Aerial Parts of Trifolium pallidum

Two new derivatives of phenylpropenoic acids, N-trans-feruloyl-L-DOPA and O-trans-caffeoylmalic acid dimethyl ester, along with four known N-trans-caffeoyl-L-DOPA (clovamide), N-trans-caffeoyl-L-DOPA-methyl ester, O-trans-caffeoylmalic acid, O-trans-feruloyl-malic acid and quercetin 3- O-β-D-glucopyranoside were isolated from the aerial parts of Trifolium pallidum. Their structures were elucidated by extensive spectroscopic methods including 1D- (1H, 13C) and 2D-NMR (DQF-COSY, HSQC, HMBC) experiments as well as mass spectrometry analysis.

Red clover HCT2, a hydroxycinnamoyl-coenzyme A:malate hydroxycinnamoyl transferase, plays a crucial role in biosynthesis of phaselic acid and other hydroxycinnamoyl-malate esters in vivo

In red clover (Trifolium pratense) leaves, phaselic acid (2-O-caffeoyl-L-malate) accumulates to several mmol kg(-1) fresh weight and is a crucial component of a natural system that prevents protein breakdown during harvest and storage of this forage crop. Previously, we identified HCT2, a red clover gene encoding a hydroxycinnamoyl-Coenzyme A (CoA) hydroxycinnamoyl transferase capable of transferring p-coumaroyl and caffeoyl moieties from their CoA derivatives to malic acid to form the corresponding hydroxycinnamoyl-malate esters in vitro. Here, we carried out a detailed kinetic analysis of the enzyme and examined its in vivo function in red clover via reverse genetics. The kinetic analysis indicates that in vitro, despite similar Km values for the tested hydroxycinnamoyl-CoA derivatives, HCT2 favors transfer to malate of p-coumaroyl and feruloyl moieties over caffeoyl moieties by greater than 5-fold. Reverse reaction (transfer of hydroxycinnamoyl moieties from malate to CoA) by HCT2 was observed with p-coumaroyl-malate but not phaselic acid. Analysis of red clover plants down-regulated for HCT2 expression via RNA interference showed a significant and substantial correlation between HCT2 mRNA levels and phaselic acid accumulation (P<0.005). In several of the HCT2-silenced plants, phaselic acid and p-coumaroyl-malate levels were reduced to <5% that of wild-type controls. These reductions resulted in easily observable phenotypes including reduced polyphenol oxidase-mediated browning and a reduction in blue epidermal fluorescence under ultraviolet light. These results demonstrate a crucial role for HCT2 in phaselic acid accumulation in red clover and define a previously undescribed pathway for the biosynthesis of hydroxycinnamoyl-malate esters in plants.

Novel hydroxycinnamoyl-coenzyme A quinate transferase genes from artichoke are involved in the synthesis of chlorogenic acid

Artichoke (Cynara cardunculus subsp. scolymus) extracts have high antioxidant capacity, due primarily to flavonoids and phenolic acids, particularly chlorogenic acid (5-caffeoylquinic acid [CGA]), dicaffeoylquinic acids, and caffeic acid, which are abundant in flower bracts and bioavailable to humans in the diet. The synthesis of CGA can occur following different routes in plant species, and hydroxycinnamoyl-coenzyme A transferases are important enzymes in these pathways. Here, we report on the isolation and characterization of two novel genes both encoding hydroxycinnamoyl-coenzyme A quinate transferases (HQT) from artichoke. The recombinant proteins (HQT1 and HQT2) were assayed after expression in Escherichia coli, and both showed higher affinity for quinate over shikimate. Their preferences for acyl donors, caffeoyl-coenzyme A or p-coumaroyl-coenzyme A, were examined. Modeling and docking analyses were used to propose possible pockets and residues involved in determining substrate specificities in the HQT enzyme family. Quantitative real-time polymerase chain reaction analysis of gene expression indicated that HQT1 might be more directly associated with CGA content. Transient and stable expression of HQT1 in Nicotiana resulted in a higher production of CGA and cynarin (1,3-dicaffeoylquinic acid). These findings suggest that several isoforms of HQT contribute to the synthesis of CGA in artichoke according to physiological needs and possibly following various metabolic routes.

Antimicrobial effect of red clover (Trifolium pratense) phenolic extract on the ruminal hyper ammonia-producing bacterium, Clostridium sticklandii

Ruminal proteolysis and subsequent amino acid degradation represent considerable economic loss in ruminant production. The hyper ammonia-producing bacteria (HAB) are largely responsible for amino acid deamination in the rumen. HAB can be controlled with ionophores, but they are also susceptible to antimicrobial plant secondary metabolites. Red clover (Trifolium pratense) is rich in soluble phenolics, and it is also more resistant to proteolysis than other legumes. The goal of this study was to identify phenolic compounds from Trifolium pratense cultivar Kenland, and determine if any of the compounds possessed antimicrobial activity against the bovine HAB, Clostridium sticklandii SR. HPLC analysis revealed that clover tissues were rich in the isoflavonoids formononetin and biochanin A, particularly in plants left to wilt for 24 h. Biochanin A inhibited C. sticklandii in bioassays that employed thin-layer chromatography (TLC). Both clover extracts and biochanin A inhibited the growth of C. sticklandii in broth culture, but formononetin had no effect. These results indicate that clover phenolic compounds may have a role in preventing amino acid fermentation.

Red clover coumarate 3'-hydroxylase (CYP98A44) is capable of hydroxylating p-coumaroyl-shikimate but not p-coumaroyl-malate: implications for the biosynthesis of phaselic acid

Red clover (Trifolium pratense) leaves accumulate several mumol of phaselic acid [2-O-caffeoyl-L-malate] per gram fresh weight. Post-harvest oxidation of such o-diphenols to o-quinones by endogenous polyphenol oxidases (PPO) prevents breakdown of forage protein during storage. Forages like alfalfa (Medicago sativa) lack both foliar PPO activity and o-diphenols. Consequently, breakdown of their protein upon harvest and storage results in economic losses and release of excess nitrogen into the environment. Understanding how red clover synthesizes o-diphenols such as phaselic acid will help in the development of forages utilizing this natural system of protein protection. We have proposed biosynthetic pathways in red clover for phaselic acid that involve a specific hydroxycinnamoyl-CoA:malate hydroxycinnamoyl transferase. It is unclear whether the transfer reaction to malate to form phaselic acid involves caffeic acid or p-coumaric acid and subsequent hydroxylation of the resulting p-coumaroyl-malate. The latter would require a coumarate 3'-hydroxylase (C3'H) capable of hydroxylating p-coumaroyl-malate, an activity not previously described. Here, a cytochrome P450 C3'H (CYP98A44) was identified and its gene cloned from red clover. CYP98A44 shares 96 and 79% amino acid identity with Medicago truncatula and Arabidopsis thaliana C3'H proteins that are capable of hydroxylating p-coumaroyl-shikimate and have been implicated in monolignol biosynthesis. CYP98A44 mRNA is expressed in stems and flowers and to a lesser extent in leaves. Immune serum raised against CYP98A44 recognizes a membrane-associated protein in red clover stems and leaves and cross-reacts with C3'H proteins from other species. CYP98A44 expressed in Saccharomyces cerevisiae is capable of hydroxylating p-coumaroyl-shikimate, but not p-coumaroyl-malate. This finding indicates that in red clover, phaselic acid is likely formed by transfer of a caffeoyl moiety to malic acid, although the existence of a second C3'H capable of hydroxylating p-coumaroyl-malate cannot be definitively ruled out.