Dynamic metabolic and transcriptomic profiling reveals the biosynthetic characteristics of hydroxycinnamic acid amides (HCAAs) in sunflower pollen

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.

Industrial chicory genome gives insights into the molecular timetable of anther development and male sterility

Industrial chicory (Cichorium intybus var. sativum) is a biannual crop mostly cultivated for extraction of inulin, a fructose polymer used as a dietary fiber. F1 hybrid breeding is a promising breeding strategy in chicory but relies on stable male sterile lines to prevent self-pollination. Here, we report the assembly and annotation of a new industrial chicory reference genome. Additionally, we performed RNA-Seq on subsequent stages of flower bud development of a fertile line and two cytoplasmic male sterile (CMS) clones. Comparison of fertile and CMS flower bud transcriptomes combined with morphological microscopic analysis of anthers, provided a molecular understanding of anther development and identified key genes in a range of underlying processes, including tapetum development, sink establishment, pollen wall development and anther dehiscence. We also described the role of phytohormones in the regulation of these processes under normal fertile flower bud development. In parallel, we evaluated which processes are disturbed in CMS clones and could contribute to the male sterile phenotype. Taken together, this study provides a state-of-the-art industrial chicory reference genome, an annotated and curated candidate gene set related to anther development and male sterility as well as a detailed molecular timetable of flower bud development in fertile and CMS lines.

Functional characterization of a Flavonol 3-O-rhamnosyltransferase and two UDP-rhamnose synthases from Hypericum monogynum

Rhamnosyltransferase (RT) and rhamnose synthase (Rhs) are the key enzymes that are responsible for the biosynthesis of rhamnosides and UDP-l-rhamnose (UDP-Rha) in plants, respectively. How to discover such enzymes efficiently for use is still a problem to be solved. Here, we identified HmF3RT, HmRhs1, and HmRhs2 from Hypericum monogynum, which is abundant in flavonol rhamnosides, with the help of a full-length and high throughput transcriptome sequencing platform. HmF3RT could regiospecifically transfer the rhamnose moiety of UDP-Rha onto the 3-OH position of flavonols and has weakly catalytic for UDP-xylose (UDP-Xyl) and UDP-glucose (UDP-Glc). HmF3RT showed well quercetin substrate affinity and high catalytic efficiency with K_m of 5.14 μM and k_cat/K_m of 2.21 × 10⁵ S^-1 M^-1, respectively. Docking, dynamic simulation, and mutagenesis studies revealed that V129, D372, and N373 are critical residues for the activity and sugar donor recognition of HmF3RT, mutant V129A, and V129T greatly enhance the conversion rate of catalytic flavonol glucosides. HmRhs1 and HmRhs2 convert UDP-Glc to UDP-Rha, which could be further used by HmF3RT. The HmF3RT and HmRhs1 co-expressed strain RTS1 could produce quercetin 3-O-rhamnoside (quercitrin), kaempferol 3-O-rhamnoside (afzelin), and myricetin 3-O-rhamnoside (myricitrin) at yields of 85.1, 110.7, and 77.6 mg L^-1, respectively. It would provide a valuable reference for establishing a better and more efficient biocatalyst for preparing bioactive flavonol rhamnosides by identifying HmF3RT and HmRhs.