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

Identification of key enzymes participated in the biosynthesis of a rare flavonol patuletin in the medicinal plant Echinacea angustifolia

Echinacea species are widely recognized as medicinal herbs producing top-selling herbal supplements in the United States. Instead of the common flavonoids quercetin and kaempferol and their glycosides, a rutinoside of the rare flavonol patuletin has been identified as the predominant flavonoid in the E. angustifolia, an important resource plant for Echinacea. Patuletin features an additional 6-methoxy group on the A ring compared to quercetin. The specific enzymes responsible for flavone 6-hydroxylation and methylation remain elusive. In this study, we constructed a multi-tissue transcriptome of E. angustifolia and screened flavone 6-hydroxylase (F6H) and methyltransferase (OMT)-encoding genes through phylogenetic analysis. Two CYP450 oxidases, EaF6H1 and EaF6H2, from the CYP706X and CYP82D subfamilies, respectively, were identified as possessing F6H activity, along with two CCoAOMTs (EaCCoAOMT1/2) mediating subsequent 6-O-methylation for the biosynthesis of patuletin. The tissue-specific expression patterns, substrate specificity, and molecular docking simulations of these hydroxylases and OMTs were thoroughly investigated. Finally, we successfully reconstructed patuletin biosynthesis in Nicotiana benthamiana leaves using these identified genes. Our results enhance the understanding of the flavonoid biosynthesis pathway and provide synthetic elements for the production of rare flavonoids from the medicinal plant E. angustifolia.

Hydroxycinnamoyl-coenzyme A: tetrahydroxyhexanedioate hydroxycinnamoyl transferase (HHHT) from Phaseolus vulgaris L.: phylogeny, expression pattern, kinetic parameters, and active site analysis

BAHD acyl-coenzyme A (CoA) transferases comprise a large family of enzymes in plants which transfer an acyl group from a CoA thioester to hydroxyl or amine groups to form esters or amides, respectively. Clade Vb of this family primarily utilizes hydroxycinnamoyl-CoA as the acyl donor. These enzymes are involved in biosynthesis of diverse specialized metabolites with functions such as structure (e.g., lignin formation) and biotic/abiotic stress mitigation. The diversity of these enzymes has arisen from both divergent and convergent evolution, making it difficult to predict substrate specificity or enzyme function based on homology, and relatively few BAHD transferases have been characterized biochemically with respect to substrate specificity. We previously identified a hydroxycinnamoyl-CoA: tetrahydroxyhexanedioate hydroxycinnamoyl transferase (HHHT) from common bean capable of transferring hydroxycinnamic acids to mucic or saccharic acid to form the corresponding esters. Here, to better understand the structure/function relationships of this enzyme, we have further characterized it with respect to expression pattern, kinetic parameters, and predicted three-dimensional (3-D) structure and active site interactions with acceptor substrates. The hhht gene was expressed predominantly in leaves and to a lesser extent flowers and shoots. K M values did not vary greatly among donor or among acceptor substrates (generally less than two-fold), while k cat values were consistently higher for saccharic acid as substrate compared to mucic acid, leading to higher catalytic efficiency (as k cat/K M) for saccharic acid. Both acceptors had similar binding poses when docked into the active site, and the proximity of multiple hydroxyl groups to the catalytic His 150, especially for saccharic acid, might provide some insights into regiospecificity. These findings provide a foundation for better understanding how the 3-D structure of BAHD transferases relates to their substrate specificity, as we explore the chemistry of the active site and interactions with ligands. This could ultimately lead to better prediction of their function and ability to rationally design BAHD transferases to make useful and novel products.

SCPL acyltransferases catalyze the metabolism of chlorogenic acid during purple coneflower seed germination

The metabolism of massively accumulated chlorogenic acid is crucial for the successful germination of purple coneflower (Echinacea purpurea (L.) Menoch). A serine carboxypeptidase-like (SCPL) acyltransferase (chicoric acid synthase, CAS) utilizes chlorogenic acid to produce chicoric acid during germination. However, it seems that the generation of chicoric acid lags behind the decrease in chlorogenic acid, suggesting an earlier route of chlorogenic acid metabolism. We discovered another chlorogenic acid metabolic product, 3,5-dicaffeoylquinic acid, which is produced before chicoric acid, filling the lag phase. Then, we identified two additional typical clade IA SCPL acyltransferases, named chlorogenic acid condensing enzymes (CCEs), that catalyze the biosynthesis of 3,5-dicaffeoylquinic acid from chlorogenic acid with different kinetic characteristics. Chlorogenic acid inhibits radicle elongation in a dose-dependent manner, explaining the potential biological role of SCPL acyltransferases-mediated continuous chlorogenic acid metabolism during germination. Both CCE1 and CCE2 are highly conserved among Echinacea species, supporting the observed metabolism of chlorogenic acid to 3,5-dicaffeoylquinic acid in two Echinacea species without chicoric acid accumulation. The discovery of SCPL acyltransferase involved in the biosynthesis of 3,5-dicaffeoylquinic acid suggests convergent evolution. Our research clarifies the metabolism strategy of chlorogenic acid in Echinacea species and provides more insight into plant metabolism.

EpMYB2 positively regulates chicoric acid biosynthesis by activating both primary and specialized metabolic genes in purple coneflower

Chicoric acid is the major active ingredient of the world-popular medicinal plant purple coneflower (Echinacea purpurea (L.) Menoch). It is recognized as the quality index of commercial hot-selling Echinacea products. While the biosynthetic pathway of chicoric acid in purple coneflower has been elucidated recently, its regulatory network remains elusive. Through co-expression and phylogenetic analysis, we found EpMYB2, a typical R2R3-type MYB transcription factor (TF) responsive to methyl jasmonate (MeJA) simulation, is a positive regulator of chicoric acid biosynthesis. In addition to directly regulating chicoric acid biosynthetic genes, EpMYB2 positively regulates genes of the upstream shikimate pathway. We also found that EpMYC2 could activate the expression of EpMYB2 by binding to its G-box site, and the EpMYC2-EpMYB2 module is involved in the MeJA-induced chicoric acid biosynthesis. Overall, we identified an MYB TF that positively regulates the biosynthesis of chicoric acid by activating both primary and specialized metabolic genes. EpMYB2 links the gap between the JA signaling pathway and chicoric acid biosynthesis. This work opens a new direction toward engineering purple coneflower with higher medicinal qualities.

Deep learning and targeted metabolomics-based monitoring of chewing insects in tea plants and screening defense compounds

Tea is an important cash crop that is often consumed by chewing pests, resulting in reduced yields and economic losses. It is important to establish a method to quickly identify the degree of damage to tea plants caused by leaf-eating insects and screen green control compounds. This study was performed through the combination of deep learning and targeted metabolomics, in vitro feeding experiment, enzymic analysis and transient genetic transformation. A small target damage detection model based on YOLOv5 with Transformer Prediction Head (TPH-YOLOv5) algorithm for the tea canopy level was established. Orthogonal partial least squares (OPLS) was used to analyze the correlation between the degree of damage and the phenolic metabolites. A potential defensive compound, (-)-epicatechin-3-O-caffeoate (EC-CA), was screened. In vitro feeding experiments showed that compared with EC and epicatechin gallate, Ectropis grisescens exhibited more significant antifeeding against EC-CA. In vitro enzymatic experiments showed that the hydroxycinnamoyl transferase (CsHCTs) recombinant protein has substrate promiscuity and can catalyze the synthesis of EC-CA. Transient overexpression of CsHCTs in tea leaves effectively reduced the degree of damage to tea leaves. This study provides important reference values and application prospects for the effective monitoring of pests in tea gardens and screening of green chemical control substances.

Understanding the mechanism of red light-induced melatonin biosynthesis facilitates the engineering of melatonin-enriched tomatoes

Melatonin is a functionally conserved broad-spectrum physiological regulator found in most biological organisms in nature. Enrichment of tomato fruit with melatonin not only enhances its agronomic traits but also provides extra health benefits. In this study, we elucidate the full melatonin biosynthesis pathway in tomato fruit by identifying biosynthesis-related genes that encode caffeic acid O-methyltransferase 2 (SlCOMT2) and N-acetyl-5-hydroxytryptamine-methyltransferases 5/7 (SlASMT5/7). We further reveal that red light supplementation significantly enhances the melatonin content in tomato fruit. This induction relies on the "serotonin-N-acetylserotonin-melatonin" biosynthesis route via the SlphyB2-SlPIF4-SlCOMT2 module. Based on the regulatory mechanism, we design a gene-editing strategy to target the binding motif of SlPIF4 in the promoter of SlCOMT2, which significantly enhances the production of melatonin in tomato fruit. Our study provides a good example of how the understanding of plant metabolic pathways responding to environmental factors can guide the engineering of health-promoting foods.