2-oxoglutarate-dependent dioxygenases and BAHD acyltransferases drive the structural diversification of orobanchol in Fabaceae plants

Strigolactones (SLs), a class of plant apocarotenoids, serve dual roles as rhizosphere-signaling molecules and plant hormones. Orobanchol, a major naturally occurring SL, along with its various derivatives, has been detected in the root exudates of plants of the Fabaceae family. Medicaol, fabacyl acetate, and orobanchyl acetate were identified in the root exudates of barrel medic (Medicago truncatula), pea (Pisum sativum), and cowpea (Vigna unguiculata), respectively. Although the biosynthetic pathway leading to orobanchol production has been elucidated, the biosynthetic pathways of the orobanchol derivatives have not yet been fully elucidated. Here, we report the identification of 2-oxoglutarate-dependent dioxygenases (DOXs) and BAHD acyltransferases responsible for converting orobanchol to these derivatives in Fabaceae plants. First, the metabolic pathways downstream of orobanchol were analyzed using substrate feeding experiments. Prohexadione, an inhibitor of DOX inhibits the conversion of orobanchol to medicaol in barrel medic. The DOX inhibitor also reduced the formation of fabacyl acetate and fabacol, a precursor of fabacyl acetate, in pea. Subsequently, we utilized a dataset based on comparative transcriptome analysis to select a candidate gene encoding DOX for medicaol synthase in barrel medic. Recombinant proteins of the gene converted orobanchol to medicaol. The candidate genes encoding DOX and BAHD acyltransferase for fabacol synthase and fabacol acetyltransferase, respectively, were selected by co-expression analysis in pea. The recombinant proteins of the candidate genes converted orobanchol to fabacol and acetylated fabacol. Furthermore, fabacol acetyltransferase and its homolog in cowpea acetylated orobanchol. The kinetics and substrate specificity analyses revealed high affinity and strict recognition of the substrates of the identified enzymes. These findings shed light on the molecular mechanisms underlying the structural diversity of SLs.

A recently evolved BAHD acetyltransferase, responsible for bitter soyasaponin A production, is indispensable for soybean seed germination

Soyasaponins are major small molecules that accumulate in soybean (Glycine max) seeds. Among them, type-A soyasaponins, fully acetylated at the terminal sugar of their C22 sugar chain, are responsible for the bitter taste of soybean-derived foods. However, the molecular basis for the acetylation of type-A soyasaponins remains unclear. Here, we identify and characterize GmSSAcT1, encoding a BADH-type soyasaponin acetyltransferase that catalyzes three or four consecutive acetylations on type-A soyasaponins in vitro and in planta. Phylogenetic analysis and biochemical assays suggest that GmSSAcT1 likely evolved from acyltransferases present in leguminous plants involved in isoflavonoid acylation. Loss-of-function mutants of GmSSAcT1 exhibited impaired seed germination, which attribute to the excessive accumulation of null-acetylated type-A soyasaponins. We conclude that GmSSAcT1 not only functions as a detoxification gene for high accumulation of type-A soyasaponins in soybean seeds but is also a promising target for breeding new soybean varieties with lower bitter soyasaponin content.

Family characteristics, phylogenetic reconstruction, and potential applications of the plant BAHD acyltransferase family

The BAHD acyltransferase family is a class of proteins in plants that can acylate a variety of primary and specialized secondary metabolites. The typically acylated products have greatly improved stability, lipid solubility, and bioavailability and thus show significant differences in their physicochemical properties and pharmacological activities. Here, we review the protein structure, catalytic mechanism, and phylogenetic reconstruction of plant BAHD acyltransferases to describe their family characteristics, acylation reactions, and the processes of potential functional differentiation. Moreover, the potential applications of the BAHD family in human activities are discussed from the perspectives of improving the quality of economic plants, enhancing the efficacy of medicinal plants, improving plant biomass for use in biofuel, and promoting stress resistance of land plants. This review provides a reference for the research and production of plant BAHD acyltransferases.

A BAHD acyltransferase contributes to the biosynthesis of both ethyl benzoate and methyl benzoate in the flowers of Lilium oriental hybrid 'Siberia'

Lily is a popular flower worldwide due to its elegant appearance and pleasant fragrance. Floral volatiles of lily are predominated by monoterpenes and benzenoids. While a number of genes for monoterpene biosynthesis have been characterized, the molecular mechanism underlying floral benzenoid formation in lily remains unclear. Here, we report on the identification and characterization of a novel BAHD acyltransferase gene that contributes to the biosynthesis of two related floral scent benzoate esters, ethyl benzoate and methyl benzoate, in the scented Lilium oriental hybrid 'Siberia'. The emission of both methyl benzoate and ethyl benzoate in L. 'Siberia' was found to be tepal-specific, floral development-regulated and rhythmic. Through transcriptome profiling and bioinformatic analysis, a BAHD acyltransferase gene designated LoAAT1 was identified as the top candidate gene for the production of ethyl benzoate. In vitro enzyme assays and substrate feeding assays provide substantial evidence that LoAAT1 is responsible for the biosynthesis of ethyl benzoate. It was interesting to note that in in vitro enzyme assay, LoAAT1 can also catalyze the formation of methyl benzoate, which is typically formed by the action of benzoic acid methyltransferase (BAMT). The lack of an expressed putative BAMT gene in the flower transcriptome of L. 'Siberia', together with biochemical and expression evidence, led us to conclude that LoAAT1 is also responsible for, or at least contributes to, the biosynthesis of the floral scent compound methyl benzoate. This is the first report that a member of the plant BAHD acyltransferase family contributes to the production of both ethyl benzoate and methyl benzoate, presenting a new mechanism for the biosynthesis of benzoate esters.

Genome-Wide Analysis of the BAHD Family in Welsh Onion and CER2-LIKEs Involved in Wax Metabolism

BAHD acyltransferases (BAHDs), especially those present in plant epidermal wax metabolism, are crucial for environmental adaptation. Epidermal waxes primarily comprise very-long-chain fatty acids (VLCFAs) and their derivatives, serving as significant components of aboveground plant organs. These waxes play an essential role in resisting biotic and abiotic stresses. In this study, we identified the BAHD family in Welsh onion (Allium fistulosum). Our analysis revealed the presence of AfBAHDs in all chromosomes, with a distinct concentration in Chr3. Furthermore, the cis-acting elements of AfBAHDs were associated with abiotic/biotic stress, hormones, and light. The motif of Welsh onion BAHDs indicated the presence of a specific BAHDs motif. We also established the phylogenetic relationships of AfBAHDs, identifying three homologous genes of CER2. Subsequently, we characterized the expression of AfCER2-LIKEs in a Welsh onion mutant deficient in wax and found that AfCER2-LIKE1 plays a critical role in leaf wax metabolism, while all AfCER2-LIKEs respond to abiotic stress. Our findings provide new insights into the BAHD family and lay a foundation for future studies on the regulation of wax metabolism in Welsh onion.

Field performance of switchgrass plants engineered for reduced recalcitrance

Switchgrass (Panicum virgatum L.) is a promising perennial bioenergy crop that achieves high yields with relatively low nutrient and energy inputs. Modification of cell wall composition for reduced recalcitrance can lower the costs of deconstructing biomass to fermentable sugars and other intermediates. We have engineered overexpression of OsAT10, encoding a rice BAHD acyltransferase and QsuB, encoding dehydroshikimate dehydratase from Corynebacterium glutamicum, to enhance saccharification efficiency in switchgrass. These engineering strategies demonstrated low lignin content, low ferulic acid esters, and increased saccharification yield during greenhouse studies in switchgrass and other plant species. In this work, transgenic switchgrass plants overexpressing either OsAT10 or QsuB were tested in the field in Davis, California, USA for three growing seasons. No significant differences in the content of lignin and cell wall-bound p-coumaric acid or ferulic acid were detected in transgenic OsAT10 lines compared with the untransformed Alamo control variety. However, the transgenic overexpressing QsuB lines had increased biomass yield and slightly increased biomass saccharification properties compared to the control plants. This work demonstrates good performance of engineered plants in the field, and also shows that the cell wall changes in the greenhouse were not replicated in the field, emphasizing the need to validate engineered plants under relevant field conditions.

Illuminating the biosynthesis pathway genes involved in bioactive specific monoterpene glycosides in Paeonia veitchii Lynch by a combination of sequencing platforms

BACKGROUND

Paeonia veitchii Lynch, a well-known herb from the Qinghai-Tibet Plateau south of the Himalayas, can synthesize specific monoterpene glycosides (PMGs) with multiple pharmacological activities, and its rhizome has become an indispensable ingredient in many clinical drugs. However, little is known about the molecular background of P. veitchii, especially the genes involved in the biosynthetic pathway of PMGs.

RESULTS

A corrective full-length transcriptome with 30,827 unigenes was generated by combining next-generation sequencing (NGS) and single-molecule real-time sequencing (SMRT) of six tissues (leaf, stem, petal, ovary, phloem and xylem). The enzymes terpene synthase (TPS), cytochrome P450 (CYP), UDP-glycosyltransferase (UGT), and BAHD acyltransferase, which participate in the biosynthesis of PMGs, were systematically characterized, and their functions related to PMG biosynthesis were analysed. With further insight into TPSs, CYPs, UGTs and BAHDs involved in PMG biosynthesis, the weighted gene coexpression network analysis (WGCNA) method was used to identify the relationships between these genes and PMGs. Finally, 8 TPSs, 22 CYPs, 7 UGTs, and 2 BAHD genes were obtained, and these putative genes were very likely to be involved in the biosynthesis of PMGs. In addition, the expression patterns of the putative genes and the accumulation of PMGs in tissues suggested that all tissues are capable of biosynthesizing PMGs and that aerial plant parts could also be used to extract PMGs.

CONCLUSION

We generated a large-scale transcriptome database across the major tissues in P. veitchii, providing valuable support for further research investigating P. veitchii and understanding the genetic information of plants from the Qinghai-Tibet Plateau. TPSs, CYPs, UGTs and BAHDs further contribute to a better understanding of the biology and complexity of PMGs in P. veitchii. Our study will help reveal the mechanisms underlying the biosynthesis pathway of these specific monoterpene glycosides and aid in the comprehensive utilization of this multifunctional plant.

PoDPBT, a BAHD acyltransferase, catalyses the benzoylation in paeoniflorin biosynthesis in Paeonia ostii

PoDPBT, an O-benzoyltransferase belonging to the BAHD family, can catalyze the benzoylation of 8-debenzoylpaeoniflorin to paeoniflorin. PoDPBT is the first enzyme demonstrated to be involved in the modification stage of paeoniflorin biosynthesis. DFGGG, a new DFGWG-like motif, was revealed in the BAHD family. The transcriptome database provides a resource for further investigation of other enzyme genes involved in paeoniflorin biosynthesis.

BAHD acyltransferase induced by histone deacetylase inhibitor catalyzes 3-O-hydroxycinnamoylquinic acid formation in bamboo cells

Suspension-cultured cells of a bamboo species (Bambusa multiplex; Bm) produce 3-O-feruloylquinic acid (3-FQA) and 3-O-p-coumaroylquinic acid (3-pCQA) by treatment with the histone deacetylase inhibitor suberoyl bis-hydroxamic acid (SBHA). Acyltransferases catalyzing the formation of 5-O-hydroxycinnamoylquinic acid esters by transesterification from hydroxycinnamoyl-CoAs to the C-5 hydroxy group of quinic acid (hydroxycinnamoyl-CoA:quinate hydroxycinnamoyltransferase, HQT) have been identified in the biosynthesis of chlorogenic acids and monolignols; however, an HQT that catalyzes the acylation of the C-3 hydroxy group of quinic acid has not been identified previously. In the present study, we purified a native HQT from SBHA-treated Bm cells. The purified enzyme preferentially accepted feruloyl-/p-coumaroyl-CoAs as acyl-donors and quinic acid as the acyl-acceptor, and the enzyme specifically formed 3-FQA and 3-pCQA but not 5-O-hydroxycinnamoylquinic acid esters or esters with shikimic acid. A cDNA (BmHQT1) encoding this HQT was isolated. Although BmHQT1 is a phylogenetically unique member of the BAHD acyltransferase superfamily that does not cluster with other HQTs, functional characterization of the recombinant enzyme verified that BmHQT1 catalyzes the regiospecific formation of 3-O-hydroxycinnamoylquinic acid esters. Transcript levels of BmHQT1 markedly increased in Bm cells cultured in the presence of SBHA. Moreover, elevated acetylation levels of histone H3 were observed in the coding region of BmHQT1 in the presence of SBHA, indicating that the induced accumulation of 3-FQA/3-pCQA by SBHA is caused by transcriptional activation of BmHQT1 by the action of SBHA as a histone deacetylase inhibitor. The results demonstrate the utility of HDAC inhibitors for discovery of cryptic secondary metabolites and unknown biosynthetic enzymes.

Orthology-based analysis helps map evolutionary diversification and predict substrate class use of BAHD acyltransferases

Large enzyme families catalyze metabolic diversification by virtue of their ability to use diverse chemical scaffolds. How enzyme families attain such functional diversity is not clear. Furthermore, duplication and promiscuity in such enzyme families limits their functional prediction, which has produced a burgeoning set of incompletely annotated genes in plant genomes. Here, we address these challenges using BAHD acyltransferases as a model. This fast-evolving family expanded drastically in land plants, increasing from one to five copies in algae to approximately 100 copies in diploid angiosperm genomes. Compilation of >160 published activities helped visualize the chemical space occupied by this family and define eight different classes based on structural similarities between acceptor substrates. Using orthologous groups (OGs) across 52 sequenced plant genomes, we developed a method to predict BAHD acceptor substrate class utilization as well as origins of individual BAHD OGs in plant evolution. This method was validated using six novel and 28 previously characterized enzymes and helped improve putative substrate class predictions for BAHDs in the tomato genome. Our results also revealed that while cuticular wax and lignin biosynthetic activities were more ancient, anthocyanin acylation activity was fixed in BAHDs later near the origin of angiosperms. The OG-based analysis enabled identification of signature motifs in anthocyanin-acylating BAHDs, whose importance was validated via molecular dynamic simulations, site-directed mutagenesis and kinetic assays. Our results not only describe how BAHDs contributed to evolution of multiple chemical phenotypes in the plant world but also propose a biocuration-enabled approach for improved functional annotation of plant enzyme families.

Identification and characterization of the key enzyme in the biosynthesis of the neurotoxin β-ODAP in grass pea

Grass pea (Lathyrus sativus L.) is a grain legume commonly grown in Asia and Africa for food and forage. It is a highly nutritious and robust crop, capable of surviving both droughts and floods. However, it produces a neurotoxic compound, β-N-oxalyl-L-α,β-diaminopropionic acid (β-ODAP), which can cause a severe neurological disorder when consumed as a primary diet component. While the catalytic activity associated with β-ODAP formation was demonstrated more than 50 years ago, the enzyme responsible for this activity has not been identified. Here, we report on the identity, activity, 3D structure, and phylogenesis of this enzyme—β-ODAP synthase (BOS). We show that BOS belongs to the benzylalcohol O-acetyltransferase, anthocyanin O-hydroxycinnamoyltransferase, anthranilate N-hydroxycinnamoyl/benzoyltransferase, deacetylvindoline 4-O-acetyltransferase superfamily of acyltransferases and is structurally similar to hydroxycinnamoyl transferase. Using molecular docking, we propose a mechanism for its catalytic activity, and using heterologous expression in tobacco leaves (Nicotiana benthamiana), we demonstrate that expression of BOS in the presence of its substrates is sufficient for β-ODAP production in vivo. The identification of BOS may pave the way toward engineering β-ODAP–free grass pea cultivars, which are safe for human and animal consumption.

Pistil Mating Type and Morphology Are Mediated by the Brassinosteroid Inactivating Activity of the S-Locus Gene BAHD in Heterostylous Turnera Species

Heterostyly is a breeding system that promotes outbreeding through a combination of morphological and physiological floral traits. In Turnera these traits are governed by a single, hemizygous S-locus containing just three genes. We report that the S-locus gene, BAHD, is mutated and encodes a severely truncated protein in a self-compatible long homostyle species. Further, a self-compatible long homostyle mutant possesses a T. krapovickasii BAHD allele with a point mutation in a highly conserved domain of BAHD acyl transferases. Wild type and mutant TkBAHD alleles were expressed in Arabidopsis to assay for brassinosteroid (BR) inactivating activity. The wild type but not mutant allele caused dwarfism, consistent with the wild type possessing, but the mutant allele having lost, BR inactivating activity. To investigate whether BRs act directly in self-incompatibility, BRs were added to in vitro pollen cultures of the two mating types. A small morph specific stimulatory effect on pollen tube growth was found with 5 µM brassinolide, but no genotype specific inhibition was observed. These results suggest that BAHD acts pleiotropically to mediate pistil length and physiological mating type through BR inactivation, and that in regard to self-incompatibility, BR acts by differentially regulating gene expression in pistils, rather than directly on pollen.

Stacking AsFMT overexpression with BdPMT loss of function enhances monolignol ferulate production in Brachypodium distachyon

To what degree can the lignin subunits in a monocot be derived from monolignol ferulate (ML-FA) conjugates? This simple question comes with a complex set of variables. Three potential requirements for optimizing ML-FA production are as follows: (1) The presence of an active FERULOYL-CoA MONOLIGNOL TRANSFERASE (FMT) enzyme throughout monolignol production; (2) Suppression or elimination of enzymatic pathways competing for monolignols and intermediates during lignin biosynthesis; and (3) Exclusion of alternative phenolic compounds that participate in lignification. A 16-fold increase in lignin-bound ML-FA incorporation was observed by introducing an AsFMT gene into Brachypodium distachyon. On its own, knocking out the native p-COUMAROYL-CoA MONOLIGNOL TRANSFERASE (BdPMT) pathway that competes for monolignols and the p-coumaroyl-CoA intermediate did not change ML-FA incorporation, nor did partial loss of CINNAMOYL-CoA REDUCTASE1 (CCR1) function, which reduced metabolic flux to monolignols. However, stacking AsFMT into the Bdpmt-1 mutant resulted in a 32-fold increase in ML-FA incorporation into lignin over the wild-type level.

Arabidopsis ECERIFERUM2-LIKEs Are Mediators of Condensing Enzyme Function

Condensing enzymes catalyze the committed reaction of fatty acid elongation and determine the chain length of fatty acids accepted and produced by the elongation complex. While necessary for the elongation of very-long-chain fatty acids (VLCFAs), identified plant condensing enzymes cannot efficiently produce VLCFAs longer than 28 carbons, which are precursors for the most abundant cuticular waxes of most plant species that have been surveyed. The eceriferum2 (cer2) mutant of Arabidopsis thaliana has a severe wax-deficient phenotype and specifically lacks waxes longer than 28 carbons, but the CER2 protein does not share sequence similarity with condensing enzymes. Instead, CER2 is homologous to BAHD acyltransferases. Heterologous expression in yeast previously demonstrated that CER2, and a small clade of BAHD acyltransferases with high sequence identity to CER2, can extend the chain-length specificity of the condensing enzyme CER6. This biochemical function is distinct from that of the broader BAHD acyltransferase family. The product specificity and physiological functions of individual CER2-LIKE proteins are unique. Here, we demonstrate that CER2 physically interacts with the fatty acid elongase. We cloned chimeric CER2-LIKE proteins and expressed these in yeast cells to identify the features that define the substrate specificities of CER2-LIKEs. We generated homology-based structural models to compare CER2-LIKEs and BAHD acyltransferases. In addition, based on the current phylogenetic analysis of the CER2-LIKE clade, we describe two further Arabidopsis CER2-LIKE genes, CER2-LIKE3 and CER2-LIKE4. We used yeast expression and mutant analysis to characterize these genes. Collectively, these results expand our knowledge of the functions of CER2-LIKEs, the BAHD acyltransferase family and cuticular wax metabolism.

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.

The Turnera Style S-Locus Gene TsBAHD Possesses Brassinosteroid-Inactivating Activity When Expressed in Arabidopsis thaliana

Heterostyly distinct hermaphroditic floral morphs enforce outbreeding. Morphs differ structurally, promote cross-pollination, and physiologically block self-fertilization. In Turnera the self-incompatibility (S)-locus controlling heterostyly possesses three genes specific to short-styled morph genomes. Only one gene, TsBAHD, is expressed in pistils and this has been hypothesized to possess brassinosteroid (BR)-inactivating activity. We tested this hypothesis using heterologous expression in Arabidopsis thaliana as a bioassay, thereby assessing growth phenotype, and the impacts on the expression of endogenous genes involved in BR homeostasis and seedling photomorphogenesis. Transgenic A. thaliana expressing TsBAHD displayed phenotypes typical of BR-deficient mutants, with phenotype severity dependent on TsBAHD expression level. BAS1, which encodes an enzyme involved in BR inactivation, was downregulated in TsBAHD-expressing lines. CPD and DWF, which encode enzymes involved in BR biosynthesis, were upregulated. Hypocotyl growth of TsBAHD dwarfs responded to application of brassinolide in light and dark in a manner typical of plants over-expressing genes encoding BR-inactivating activity. These results provide empirical support for the hypothesis that TsBAHD possesses BR-inactivating activity. Further this suggests that style length in Turnera is controlled by the same mechanism (BR inactivation) as that reported for Primula, but using a different class of enzyme. This reveals interesting convergent evolution in a biochemical mechanism to regulate floral form in heterostyly.

Characterization of a sweet basil acyltransferase involved in eugenol biosynthesis

Sweet basil (Ocimum basilicum) plants produce its characteristic phenylpropene-rich essential oil in specialized structures known as peltate glandular trichomes (PGTs). Eugenol and chavicol are the major phenylpropenes produced by sweet basil varieties whose synthetic pathways are not fully elucidated. Eugenol is derived from coniferyl acetate by a reaction catalysed by eugenol synthase. An acyltransferase is proposed to convert coniferyl alcohol to coniferyl acetate which is the first committed step towards eugenol synthesis. Here, we perform a comparative next-generation transcriptome sequencing of different tissues of sweet basil, namely PGT, leaf, leaf stripped of PGTs (leaf-PGT), and roots, to identify differentially expressed transcripts specific to PGT. From these data, we identified a PGT-enriched BAHD acyltransferase gene ObCAAT1 and functionally characterized it. In vitro coupled reaction of ObCAAT1 with eugenol synthase in the presence of coniferyl alcohol resulted in eugenol production. Analysis of ObCAAT1-RNAi transgenic lines showed decreased levels of eugenol and accumulation of coniferyl alcohol and its derivatives. Coniferyl alcohol acts as a common substrate for phenylpropene and lignin biosynthesis. No differences were found in total lignin content of PGTs and leaves of transgenic lines, indicating that phenylpropene biosynthesis is not coupled to lignification in sweet basil.

Functional characterization and catalytic activity improvement of BAHD acyltransferase from Celastrus angulatus Maxim

A BAHD terpene alcohol acyltransferase, CaAT20, was identified from Celastrus angulatus Maxim, expressed in E. coli and functionally characterized. S405A mutant of CaAT20 increased the enzyme activity. Acylation is a diversely physiological process in the biosynthesis of plant secondary metabolites. Plant BAHD acyltransferases play an important role in the modification of volatile esters with biological activities. In this research, a BAHD acyltransferase (CaAT20) was identified from Celastrus angulatus Maxim and the function of this enzyme was characterized. CaAT20 could convert geraniol to geranyl esters by using benzoyl-CoA and acetyl-CoA as the acyl donors respectively. Furthermore, the catalytic activity of CaAT20 for benzoyl-CoA was higher than that of acetyl-CoA. Site-directed mutation of CaAT20 was carried out based on the results of molecular simulation. In vitro site-directed mutant S405A of CaAT20 increased the volume of binding cavity so as to facilitate the entry of geraniol, indicating a more efficient acylation for geraniol and benzoyl-CoA. Our research provides new insight for the catalytic functions of CaAT20.

A BAHD acyltransferase catalyzing 19-O-acetylation of tabersonine derivatives in roots of Catharanthus roseus enables combinatorial synthesis of monoterpene indole alkaloids

While the characterization of the biosynthetic pathway of monoterpene indole alkaloids (MIAs) in leaves of Catharanthus roseus is now reaching completion, only two enzymes from the root counterpart dedicated to tabersonine metabolism have been identified to date, namely tabersonine 19-hydroxylase (T19H) and minovincine 19-O-acetyltransferase (MAT). Albeit the recombinant MAT catalyzes MIA acetylation at low efficiency in vitro, we demonstrated that MAT was inactive when expressed in yeast and in planta, suggesting an alternative function for this enzyme. Therefore, through transcriptomic analysis of periwinkle adventitious roots, several other BAHD acyltransferase candidates were identified based on the correlation of their expression profile with T19H and found to localize in small genomic clusters. Only one, named tabersonine derivative 19-O-acetyltransferase (TAT) was able to acetylate the 19-hydroxytabersonine derivatives from roots, such as minovincinine and hörhammericine, following expression in yeast. Kinetic studies also showed that the recombinant TAT was specific for root MIAs and displayed an up to 200-fold higher catalytic efficiency than MAT. In addition, gene expression analysis, protein subcellular localization and heterologous expression in Nicotiana benthamiana were in agreement with the prominent role of TAT in acetylation of root-specific MIAs, thereby redefining the molecular determinants of the root MIA biosynthetic pathway. Finally, identification of TAT provided a convenient tool for metabolic engineering of MIAs in yeast enabling efficiently mixing different biosynthetic modules spatially separated in the whole plant. This combinatorial synthesis associating several enzymes from Catharanthus roseus resulted in the conversion of tabersonine in tailor-made MIAs bearing both leaf and root-type decorations.

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.

The Unique Role of the ECERIFERUM2-LIKE Clade of the BAHD Acyltransferase Superfamily in Cuticular Wax Metabolism

The elongation of very-long-chain fatty acids is a conserved process used for the production of many metabolites, including plant cuticular waxes. The elongation of precursors of the most abundant cuticular wax components of some plants, however, is unique in requiring ECERIFERUM2-LIKE (CER2-LIKE) proteins. CER2-LIKEs are a clade within the BAHD superfamily of acyltransferases. They are known to be required for cuticular wax production in both Arabidopsis and maize based on mutant studies. Heterologous expression of Arabidopsis and rice CER2-LIKEs in Saccharomyces cerevisiae has demonstrated that they modify the chain-length specificity of elongation when paired with particular condensing enzymes. Despite sequence homology, CER2-LIKEs are distinct from the BAHD superfamily in that they do not appear to use acyl transfer activity to fulfill their biological function. Here, we review the discovery and characterization of CER2-LIKEs, propose several models to explain their function, and explore the importance of CER2-LIKE proteins for the evolution of plant cuticles.

Comparative Biochemistry and In Vitro Pathway Reconstruction as Powerful Partners in Studies of Metabolic Diversity

There are estimated to be >300,000 plant species, producing >200,000 metabolites. Many of these metabolites are restricted to specific plant lineages and are referred to as "specialized" metabolites. These serve varied functions in plants including defense against biotic and abiotic stresses, plant-plant and plant-microbe communication, and pollinator attraction. These compounds also have important applications in agriculture, medicine, skin care, and in diverse aspects of human culture. The specialized metabolic repertoire of plants can vary even within and between closely related species, in terms of the number and classes of specialized metabolites as well as their structural variants. This phenotypic variation can be exploited to discover the underlying variation in the metabolic enzymes. We describe approaches for using the diversity of specialized metabolites and variation in enzyme structure and function to identify novel enzymatic activities and understand the structural basis for these differences. The knowledge obtained from these studies will provide new modules for the synthetic biology toolbox.

p-Coumaroyl-CoA:monolignol transferase (PMT) acts specifically in the lignin biosynthetic pathway in Brachypodium distachyon

Grass lignins contain substantial amounts of p-coumarate (pCA) that acylate the side-chains of the phenylpropanoid polymer backbone. An acyltransferase, named p-coumaroyl-CoA:monolignol transferase (OsPMT), that could acylate monolignols with pCA in vitro was recently identified from rice. In planta, such monolignol-pCA conjugates become incorporated into lignin via oxidative radical coupling, thereby generating the observed pCA appendages; however p-coumarates also acylate arabinoxylans in grasses. To test the authenticity of PMT as a lignin biosynthetic pathway enzyme, we examined Brachypodium distachyon plants with altered BdPMT gene function. Using newly developed cell wall analytical methods, we determined that the transferase was involved specifically in monolignol acylation. A sodium azide-generated Bdpmt-1 missense mutant had no (<0.5%) residual pCA on lignin, and BdPMT RNAi plants had levels as low as 10% of wild-type, whereas the amounts of pCA acylating arabinosyl units on arabinoxylans in these PMT mutant plants remained unchanged. pCA acylation of lignin from BdPMT-overexpressing plants was found to be more than three-fold higher than that of wild-type, but again the level on arabinosyl units remained unchanged. Taken together, these data are consistent with a defined role for grass PMT genes in encoding BAHD (BEAT, AHCT, HCBT, and DAT) acyltransferases that specifically acylate monolignols with pCA and produce monolignol p-coumarate conjugates that are used for lignification in planta.