Enzyme Complexes of Ptr4CL and PtrHCT Modulate Co-enzyme A Ligation of Hydroxycinnamic Acids for Monolignol Biosynthesis in Populus trichocarpa

Dual feedback inhibition of ATP-dependent caffeate activation economizes ATP in caffeate-dependent electron bifurcation

The acetogen Acetobacterium woodii couples caffeate reduction with ferredoxin reduction and NADH oxidation via electron bifurcation, providing additional reduced ferredoxin for energy conservation and cell synthesis. Caffeate is first activated by an acyl-CoA synthetase (CarB), which ligates CoA to caffeate at the expense of ATP. After caffeoyl-CoA is reduced to hydrocaffeoyl-CoA, the CoA moiety in hydrocaffeoyl-CoA could be recycled for caffeoyl-CoA synthesis by an ATP-independent CoA transferase (CarA) to save energy. However, given that CarA and CarB are co-expressed, it was not well understood how ATP could be saved when both two competitive pathways of caffeate activation are present. Here, we reported a dual feedback inhibition of the CarB-mediated caffeate activation by the intermediate hydrocaffeoyl-CoA and the end-product hydrocaffeate. As the product of CarA, hydrocaffeate inhibited CarB-mediated caffeate activation by serving as another substrate of CarB with hydrocaffeoyl-CoA produced. It effectively competed with caffeate even at a concentration much lower than caffeate. Hydrocaffeoyl-CoA formed in this process can also inhibit CarB-mediated caffeate activation. Thus, the dual feedback inhibition of CarB, together with the faster kinetics of CarA, makes the ATP-independent CarA-mediated CoA loop the major route for caffeoyl-CoA synthesis, further saving ATP in the caffeate-dependent electron-bifurcating pathway. A genetic architecture similar to carABC has been found in other anaerobic bacteria, suggesting that the feedback inhibition of acyl-CoA ligases could be a widely employed strategy for ATP conservation in those pathways requiring substrate activation by CoA.

IMPORTANCE

This study reports a dual feedback inhibition of caffeoyl-CoA synthetase by two downstream products, hydrocaffeate and hydrocaffeoyl-CoA. It elucidates how such dual feedback inhibition suppresses ATP-dependent caffeoyl-CoA synthesis, hence making the ATP-independent route the main pathway of caffeate activation. This newly discovered mechanism contributes to our current understanding of ATP conservation during the caffeate-dependent electron-bifurcating pathway in the ecologically important acetogen Acetobacterium woodii. Bioinformatic mining of microbial genomes revealed contiguous genes homologous to carABC within the genomes of other anaerobes from various environments, suggesting this mechanism may be widely used in other CoA-dependent electron-bifurcating pathways.

Identification of the function of a key gene NnHCT1 in lignin synthesis in petioles of Nelumbo nucifera

Leaf petiole or stem strength is an important agronomic trait affecting the growth of underground organs as a channel for material exchange and plays a vital role in the quality and yield of crops and vegetables. There are two different types of petioles in lotus, floating leaf petioles and vertical leaf petioles; however, the internal difference mechanism between these petioles is unclear. In this study, we investigated the differences between the initial vertical leaf petioles and the initial floating leaf petioles based on RNA sequencing (RNA-seq), and >2858 differentially expressed genes were annotated. These genes were chiefly enriched in phenylpropanoid biosynthesis, which is the source of the lignin and cellulose in petioles and stems. Lignin biology-related gene NnHCT1 was identified, and subsequent biological function validation demonstrated that the transient overexpression of NnHCT1 significantly increased the lignin and cellulose contents in lotus petioles and tobacco leaves. In contrast, silencing NnHCT1 through virus-induced gene silencing significantly reduced petiole lignin synthesis. Additionally, differentially up-regulated MYB family transcription factors were identified using RNA-seq. Yeast-one-hybrid and dual-luciferase reporter assays demonstrated that MYB4 could bind to the NnHCT1 promoter and up-regulate NnHCT1 expression. These findings demonstrate the significant potential of NnHCT1 to enhance lignin synthesis, thereby improving stem or petiole resistance to stunting and explaining the need for the study of differential petiole relationships in plants.

Molecular cloning and functional analysis of 4-coumarate: CoA ligases from Marchantia paleacea and their roles in lignin and flavanone biosynthesis

Phenylpropanoids play important roles in plant physiology and the enzyme 4-coumarate: coenzyme A ligase (4CL) catalyzes the formation of thioesters. Despite extensive characterization in various plants, the functions of 4CLs in the liverwort Marchantia paleacea remain unknown. Here, four 4CLs from M. paleacea were isolated and functionally analyzed. Heterologous expression in Escherichia coli indicated the presence of different enzymatic activities in the four enzymes. Mp4CL1 and Mp4CL2 were able to convert caffeic, p-coumaric, cinnamic, ferulic, dihydro-p-coumaric, and 5-hydroxyferulic acids to their corresponding CoA esters, while Mp4CL3 and Mp4CL4 catalyzed none. Mp4CL1 transcription was induced when M. paleacea thalli were treated with methyl jasmonate (MeJA). The overexpression of Mp4CL1 increased the levels of lignin in transgenic Arabidopsis. In addition, we reconstructed the flavanone biosynthetic pathway in E. coli. The pathway comprised Mp4CL1, co-expressed with chalcone synthase (CHS) from different plant species, and the efficiency of biosynthesis was optimal when both the 4CL and CHS were obtained from the same species M. paleacea.

Pour some sugar on me: The diverse functions of phenylpropanoid glycosylation

The phenylpropanoid metabolism is the source of a vast array of specialized metabolites that play diverse functions in plant growth and development and contribute to all aspects of plant interactions with their surrounding environment. These compounds protect plants from damaging ultraviolet radiation and reactive oxygen species, provide mechanical support for the plants to stand upright, and mediate plant-plant and plant-microorganism communications. The enormous metabolic diversity of phenylpropanoids is further expanded by chemical modifications known as "decorative reactions", including hydroxylation, methylation, glycosylation, and acylation. Among these modifications, glycosylation is the major driving force of phenylpropanoid structural diversification, also contributing to the expansion of their properties. Phenylpropanoid glycosylation is catalyzed by regioselective uridine diphosphate (UDP)-dependent glycosyltransferases (UGTs), whereas glycosyl hydrolases known as β-glucosidases are the major players in deglycosylation. In this article, we review how the glycosylation process affects key physicochemical properties of phenylpropanoids, such as molecular stability and solubility, as well as metabolite compartmentalization/storage and biological activity/toxicity. We also summarize the recent knowledge on the functional implications of glycosylation of different classes of phenylpropanoid compounds. A balance of glycosylation/deglycosylation might represent an essential molecular mechanism to regulate phenylpropanoid homeostasis, allowing plants to dynamically respond to diverse environmental signals.

Class I and II NADPH-cytochrome P450 reductases exhibit different roles in triterpenoid biosynthesis in Lotus japonicus

Cytochrome P450 monooxygenases (CYPs) are enzymes that play critical roles in the structural diversification of triterpenoids. To perform site-specific oxidations of the triterpene scaffold, CYPs require electrons transferred by NADPH-cytochrome P450 reductase (CPR), which is classified into two main classes, class I and class II, based on their structural difference. Lotus japonicus is a triterpenoids-producing model legume with one CPR class I gene (LjCPR1) and a minimum of two CPR class II genes (LjCPR2-1 and LjCPR2-2). CPR classes I and II from different plants have been reported to be involved in different metabolic pathways. By performing gene expression analyses of L. japonicus hairy root culture treated with methyl jasmonate (MeJA), this study revealed that LjCPR1, CYP716A51, and LUS were down-regulated which resulted in no change in betulinic acid and lupeol content. In contrast, LjCPR2s, bAS, CYP93E1, and CYP72A61 were significantly upregulated by MeJA treatment, followed by a significant increase of the precursors for soyasaponins, i.e. β-amyrin, 24-OH β-amyrin, and sophoradiol content. Triterpenoids profile analysis of LORE1 insertion and hairy root mutants showed that the loss of the Ljcpr2-1 gene significantly reduced soyasaponins precursors but not in Ljcpr1 mutants. However, Ljcpr1 and Ljcpr2-1 mutants showed a significant reduction in lupeol and oleanolic, ursolic, and betulinic acid contents. Furthermore, LjCPR1, but not LjCPR2, was crucial for seed development, supporting the previous notion that CPR class I might support plant basal metabolism. This study suggests that CPR classes I and II play different roles in L. japonicus triterpenoid biosynthesis.

Genome-edited rice deficient in two 4-COUMARATE:COENZYME A LIGASE genes displays diverse lignin alterations

The 4-coumarate:coenzyme A ligase (4CL) is a key enzyme that contributes to channeling metabolic flux in the cinnamate/monolignol pathway, leading to the production of monolignols, p-hydroxycinnamates, and a flavonoid tricin, the major building blocks of lignin polymer in grass cell walls. Vascular plants often contain multiple 4CL genes; however, the contribution of each 4CL isoform to lignin biosynthesis remains unclear, especially in grasses. In this study, we characterized the functions of two rice (Oryza sativa L.) 4CL isoforms (Os4CL3 and Os4CL4) primarily by analyzing the cell wall chemical structures of rice mutants generated by CRISPR/Cas9-mediated targeted mutagenesis. A series of chemical and nuclear magnetic resonance analyses revealed that loss-of-function of Os4CL3 and Os4CL4 differently altered the composition of lignin polymer units. Loss of function of Os4CL3 induced marked reductions in the major guaiacyl and syringyl lignin units derived from both the conserved non-γ-p-coumaroylated and the grass-specific γ-p-coumaroylated monolignols, with more prominent reductions in guaiacyl units than in syringyl units. In contrast, the loss-of-function mutation to Os4CL4 primarily decreased the abundance of the non-γ-p-coumaroylated guaiacyl units. Loss-of-function of Os4CL4, but not of Os4CL3, reduced the grass-specific lignin-bound tricin units, indicating that Os4CL4 plays a key role not only in monolignol biosynthesis but also in the biosynthesis of tricin used for lignification. Further, the loss-of-function of Os4CL3 and Os4CL4 notably reduced cell-wall-bound ferulates, indicating their roles in cell wall feruloylation. Overall, this study demonstrates the overlapping but divergent roles of 4CL isoforms during the coordinated production of various lignin monomers.