Benzenoid biosynthesis in the flowers of Eriobotrya japonica: molecular cloning and functional characterization of p-methoxybenzoic acid carboxyl methyltransferase

Chemistry, biosynthesis and biology of floral volatiles: roles in pollination and other functions

Covering: 2010 to 2023Floral volatiles are a chemically diverse group of plant metabolites that serve multiple functions. Their composition is shaped by environmental, ecological and evolutionary factors. This review will summarize recent advances in floral scent research from chemical, molecular and ecological perspectives. It will focus on the major chemical classes of floral volatiles, on notable new structures, and on recent discoveries regarding the biosynthesis and the regulation of volatile emission. Special attention will be devoted to the various functions of floral volatiles, not only as attractants for different types of pollinators, but also as defenses of flowers against enemies. We will also summarize recent findings on how floral volatiles are affected by abiotic stressors, such as increased temperatures and drought, and by other organisms, such as herbivores and flower-dwelling microbes. Finally, this review will indicate current research gaps, such as the very limited knowledge of the isomeric pattern of chiral compounds and its importance in interspecific interactions.

Diverse O-methyltransferases catalyze the biosynthesis of floral benzenoids that repel aphids from the flowers of waterlily Nymphaea prolifera

Nymphaea is a key genus of the ANA grade (Amborellales, Nymphaeales, and Austrobaileyales) of basal flowering plants, which serve as a key model to study the early evolution of floral traits. In this study, we comprehensively investigated the emission, biosynthesis, and biological function of the floral scent in a night-blossoming waterlily Nymphaea prolifera. The headspace volatile collection combined with GC-MS analysis showed that the floral scent of N. prolifera is predominately comprised by methylated benzenoids including anisole, veratrole, guaiacol, and methoxyanisole. Moreover, the emission of these floral benzenoids in N. prolifera exhibited temporal and spatial pattern with circadian rhythm and tissue specificity. By creating and mining transcriptomes of N. prolifera flowers, 12 oxygen methyltransferases (NpOMTs) were functionally identified. By in vitro enzymatic assay, NpOMT3, 6, and 7 could produce anisole and NpOMT5, 7, 9, produce guaiacol, whereas NpOMT3, 6, 9, 11 catalyzed the formation of veratrole. Methoxyanisole was identified as the universal product of all NpOMTs. Expression patterns of NpOMTs provided implication for their roles in the production of the respective benzenoids. Phylogenetic analysis of OMTs suggested a Nymphaea-specific expansion of the OMT family, indicating the evolution of lineage-specific functions. In bioassays, anisole, veratrole, and guaiacol in the floral benzenoids were revealed to play the critical role in repelling waterlily aphids. Overall, this study indicates that the basal flowering plant N. prolifera has evolved a diversity and complexity of OMT genes for the biosynthesis of methylated benzenoids that can repel insects from feeding the flowers. These findings provide new insights into the evolutional mechanism and ecological significance of the floral scent from early-diverged flowering plants.

Identification and characterization of cytochrome P450 CYP77A59 of loquat (Rhaphiolepis bibas) responsible for biosynthesis of phenylacetonitrile, a floral nitrile compound

MAIN CONCLUSION

Cytochrome P450 CYP77A59 is responsible for the biosynthesis of phenylacetonitrile in loquat flowers. Flowers of some plants emit volatile nitrile compounds, but the biosynthesis of these compounds is unclear. Loquat (Rhaphiolepis bibas) flowers emit characteristic N-containing volatiles, such as phenylacetonitrile (PAN), (E/Z)-phenylacetaldoxime (PAOx), and (2-nitroethyl)benzene (NEB). These volatiles likely play a defense role against pathogens and insects. PAN and NEB are commonly biosynthesized from L-phenylalanine via (E/Z)-PAOx. Two cytochrome P450s-CYP79D80 and "promiscuous fatty acid ω-hydroxylase" CYP94A90, which catalyze the formation of (E/Z)-PAOx from L-phenylalanine and NEB from (E/Z)-PAOx, respectively-are involved in NEB biosynthesis. However, the enzymes catalyzing the formation of PAN from (E/Z)-PAOx in loquat have not been identified. In this study, we aimed to identify candidate cytochrome P450s catalyzing PAN formation in loquat flowers. Yeast whole-cell biocatalyst assays showed that among nine candidate cytochrome P450s, CYP77A58 and CYP77A59 produced PAN from (E/Z)-PAOx. CYP77As catalyzed the dehydration of aldoximes, which is atypical of cytochrome P450; the reaction was NADPH-dependent, with an optimum temperature and pH of 40 °C and 8.0, respectively. CYP77As acted on (E/Z)-PAOx, (E/Z)-4-hydroxyphenylacetaldoxime, and (E/Z)-indole-3-acetaldoxime. Previously characterized CYP77As are known to hydroxylate fatty acids; loquat CYP77As did not act on tested fatty acids. We observed higher expression of CYP77A59 in flowers than in buds; expression of CYP77A58 was remarkably reduced in the flowers. Because the flowers, but not buds, emit PAN, CYP77A59 is likely responsible for the biosynthesis of PAN in loquat flowers. This study will help us understand the biosynthesis of floral nitrile compounds.

Biosynthesis of dillapiole/apiole in dill (Anethum graveolens): characterization of regioselective phenylpropene O-methyltransferase

The phenylpropene volatiles dillapiole and apiole impart one of the characteristic aromas of dill (Anethum graveolens) weeds. However, very few studies have been conducted to investigate the chemical composition of volatile compounds from different developmental stages and plant parts of A. graveolens. In this study, we examined the distribution of volatile phenylpropenes, including dillapiole, in dill plants at various developmental stages. We observed that young dill seedlings accumulate high levels of dillapiole and apiole, whereas a negligible proportion was found in the flowering plants and dry seeds. Based on transcriptomics and co-expression approaches with phenylpropene biosynthesis genes, we identified dill cDNA encoding S-adenosyl-L-methionine-dependent O-methyltransferase 1 (AgOMT1), an enzyme that can convert 6- and 2-hydroxymyristicin to dillapiole and apiole, respectively, via the methylation of the ortho-hydroxy group. The AgOMT1 protein shows an apparent K_m value of 3.5 μm for 6-hydroxymyristicin and is 75% identical to the anise (Pimpinella anisum) O-methyltransferase (PaAIMT1) that can convert isoeugenol to methylisoeugenol via methylation of the hydroxy group at the para-position of the benzene ring. AgOMT1 showed a preference for 6-hydroxymyristicin, whereas PaAIMT1 displayed a large preference for isoeugenol. In vitro mutagenesis experiments demonstrated that substituting only a few residues can substantially affect the substrate specificity of these enzymes. Other plants belonging to the Apiaceae family contained homologous O-methyltransferase (OMT) proteins highly similar to AgOMT1, converting 6-hydroxymyristicin to dillapiole. Our results indicate that apiaceous phenylpropene OMTs with ortho-methylating activity evolved independently of phenylpropene OMTs of other plants and the enzymatic function of AgOMT1 and PaAIMT1 diverged recently.

Transcriptomic and metabolomic data reveal key genes that are involved in the phenylpropanoid pathway and regulate the floral fragrance of Rhododendron fortunei

BACKGROUND

To reveal the key genes involved in the phenylpropanoid pathway, which ultimately governs the fragrance of Rhododendron fortunei, we performed a comprehensive transcriptome and metabolomic analysis of the petals of two different varieties of two alpine rhododendrons: the scented R. fortunei and the unscented Rhododendron 'Nova Zembla'.

RESULTS

Our transcriptomic and qRT-PCR data showed that nine candidate genes were highly expressed in R. fortunei but were downregulated in Rhododendron 'Nova Zembla'. Among these genes, EGS expression was significantly positively correlated with various volatile benzene/phenylpropanoid compounds and significantly negatively correlated with the contents of various nonvolatile compounds, whereas CCoAOMT, PAL, C4H, and BALDH expression was significantly negatively correlated with the contents of various volatile benzene/phenylpropanoid compounds and significantly positively correlated with the contents of various nonvolatile compounds. CCR, CAD, 4CL, and SAMT expression was significantly negatively correlated with the contents of various benzene/phenylpropanoid compounds. The validation of RfSAMT showed that the RfSAMT gene regulates the synthesis of aromatic metabolites in R. fortunei.

CONCLUSION

The findings of this study indicated that key candidate genes and metabolites involved in the phenylpropanoid biosynthesis pathway may govern the fragrance of R. fortunei. This lays a foundation for further research on the molecular mechanism underlying fragrance in the genus Rhododendron.

Functional Diversification and Structural Origins of Plant Natural Product Methyltransferases

In plants, methylation is a common step in specialized metabolic pathways, leading to a vast diversity of natural products. The methylation of these small molecules is catalyzed by S-adenosyl-l-methionine (SAM)-dependent methyltransferases, which are categorized based on the methyl-accepting atom (O, N, C, S, or Se). These methyltransferases are responsible for the transformation of metabolites involved in plant defense response, pigments, and cell signaling. Plant natural product methyltransferases are part of the Class I methyltransferase-superfamily containing the canonical Rossmann fold. Recent advances in genomics have accelerated the functional characterization of plant natural product methyltransferases, allowing for the determination of substrate specificities and regioselectivity and further realizing the potential for enzyme engineering. This review compiles known biochemically characterized plant natural product methyltransferases that have contributed to our knowledge in the diversification of small molecules mediated by methylation steps.

Synthesis of deuterium-labeled cinnamic acids: Understanding the volatile benzenoid pathway in the flowers of the Japanese loquat Eriobotrya japonica

Cinnamic acids are widely distributed in plants, including crops for human use, and exhibit a variety of activities that are beneficial to human health. They also occupy a pivotal position in the biosynthesis of phenylpropanoids such as lignins, anthocyanins, flavonoids, and coumarins. In this context, deuterium-labeled cinnamic acids have been used as tracers and internal standards in food and medicinal chemistry as well as plant biochemistry. Therefore, a concise synthesis of deuterium-labeled cinnamic acids would be highly desirable. In this study, we synthesized deuterium-labeled cinnamic acids using readily available deuterium sources. We also investigated a hydrogen-deuterium exchange reaction in an ethanol-d₁ /Et₃ N system. This method can introduce deuterium atoms at the ortho and para positions of the phenolic hydroxy groups as well as at the C-2 position of alkyl cinnamates and is applicable to various phenolic compounds. Using the synthesized labeled compounds, we demonstrated that the benzenoid volatiles, such as 4-methoxybenzaldehyde, in the scent of the flowers of the Japanese loquat Eriobotrya japonica are biosynthesized from phenylalanine via cinnamic and 4-coumaric acids. This study provides easy access to a variety of deuterium-labeled (poly)phenols, as well as to useful tools for studies of the metabolism of cinnamic acids in living systems.

Biochemical characterization of the jasmonic acid methyltransferase gene from wasabi (Eutrema japonicum)

Methyl jasmonate and jasmonic acid play important roles as signaling molecules in regulating plant development and stress-related responses. Previous studies have shown that jasmonic acid carboxyl methyltransferase (JMT), which belongs to the SABATH methyltransferase gene family, catalyzes the transfer of methyl groups from S-adenosyl-L-methionine to the carboxyl groups of jasmonic acid. In the present study, we used RNA-seq analysis to identify a putative JMT gene, EujJMT, in wasabi (Eutrema japonicum). The EujJMT proteins showed the highest similarity (89% identity) to JMT proteins of Brassica rapa. Functional characterization of a recombinant EujJMT protein expressed in Escherichia coli showed the highest level of activity with jasmonic acid among the different carboxylic acids tested. The apparent K_m value of EujJMT using jasmonic acid as substrate was 62.6 µM, which is comparable to the values of known JMTs. Phylogenetic analysis suggested that EujJMT shares a common ancestor with the JMTs of Arabidopsis and Brassica species and that the strict substrate specificity toward jasmonic acid is conserved among Brassicaceae JMTs.

Composition and Biosynthesis of Scent Compounds from Sterile Flowers of an Ornamental Plant Clematis florida cv. 'Kaiser'

Clematisflorida is a popular ornamental vine species known for diverse colors and shapes of its flowers but not for scent. Here we investigated the composition and biosynthesis of floral scent in ‘Kaiser’, a fragrant cultivar of C. florida that has sterile flowers. Volatile profiling revealed that flowers of ‘Kaiser’ emit more than 20 compounds, with monoterpenes being most abundant. Among the three floral organs, namely sepals, transformed-petals, and ovaries, ovaries had the highest rates of total volatile emission. To determine the molecular mechanism underlying floral scent biosynthesis in ‘Kaiser’, we sequenced a flower transcriptome and searched the transcriptome for terpene synthase genes (TPSs), which are key genes for terpene biosynthesis. Among the TPS genes identified, three were putative intact full-length genes and were designated CfTPS1, CfTPS2, and CfTPS3. Phylogenetic analysis placed CfTPS1, CfTPS2, and CfTPS3 to the TPS-g, TPS-b, and TPS-a subfamily, respectively. Through in vitro enzyme assays with Escherichia coli-expressed recombinant proteins, both CfTPS1 and CfTPS2 were demonstrated to catalyze the conversion of geranyl diphosphate to linalool, the most abundant constituent of C. florida floral scent. In addition, CfTPS1 and CfTPS2 produced the sesquiterpene nerolidol from (E,E)-farnesyl diphosphate. CfTPS3 showed sesquiterpene synthase activity and produced multiple products in vitro. All three CfTPS genes showed higher levels of expression in sepals than those in transformed-petals and ovaries. Our results show that despite being sterile, the flowers of ‘Kaiser’ have normal mechanisms for floral scent biosynthesis that make the flowers fragrant.

Biosynthesis of methyl (E)-cinnamate in the liverwort Conocephalum salebrosum and evolution of cinnamic acid methyltransferase

Methyl (E)-cinnamate is a specialized metabolite that occurs in a variety of land plants. In flowering plants, it is synthesized by cinnamic acid methyltransferase (CAMT) that belongs to the SABATH family. While rarely reported in bryophytes, methyl (E)-cinnamate is produced by some liverworts of the Conocephalum conicum complex, including C. salebrosum. In axenically grown thalli of C. salebrosum, methyl (E)-cinnamate was detected as the dominant compound. To characterize its biosynthesis, six full-length SABATH genes, which were designated CsSABATH1-6, were cloned from C. salebrosum. These six genes showed different levels of expression in the thalli of C. salebrosum. Next, CsSABATH1-6 were expressed in Escherichia coli to produce recombinant proteins, which were tested for methyltransferase activity with cinnamic acid and a few related compounds as substrates. Among the six SABATH proteins, CsSABATH6 exhibited the highest level of activity with cinnamic acid. It was renamed CsCAMT. The apparent Km value of CsCAMT using (E)-cinnamic acid as substrate was determined to be 50.5 μM. In contrast, CsSABATH4 was demonstrated to function as salicylic acid methyltransferase and was renamed CsSAMT. Interestingly, the CsCAMT gene from a sabinene-dominant chemotype of C. salebrosum is identical to that of the methyl (E)-cinnamate-dominant chemotype. Structure models for CsCAMT, CsSAMT and one flowering plant CAMT (ObCCMT1) in complex with (E)-cinnamic acid and salicylic acid were built, which provided structural explanations to substrate specificity of these three enzymes. In phylogenetic analysis, CsCAMT and ObCCMT1 were in different clades, implying that methyl (E)-cinnamate biosynthesis in bryophytes and flowering plants originated through convergent evolution.

Selection of the optimal reference genes for expression analyses in different materials of Eriobotrya japonica

BACKGROUND

Loquat (Eriobotrya japonica) is a subtropical tree bearing fruit that ripens during late spring and early summer, which is the off-season for fruit production. The specific flowering habit of loquat, which starts in fall and ends in winter, has attracted an increasing number of researchers who believe that it may represent an ideal model for studying flowering shift adaptations to climate change in Rosaceae. These studies require an understanding of gene expression patterns within the fruit and other tissues of this plant. Although ACTINs (ACTs) have previously been used as reference genes (RGs) for gene expression studies in loquats, a comprehensive analysis of whether these RGs are optimal for normalizing RT-qPCR data has not been performed.

RESULTS

In this study, 11 candidate RGs (RIBOSOMAL-LIKE PROTEIN4 (RPL4), RIBOSOMAL-LIKE PROTEIN18 (RPL18), Histone H3.3 (HIS3), Alpha-tubulin-3 (TUA3), S-Adenosyl Methionine Decarboxylase (SAMDC), TIP41-like Family Protein (TIP41), (UDP)-glucose Pyrophosphorylase (UGPase), 18S ribosomal RNA (18S), Glyceraldehyde-3-phosphate Dehydrogenase (GAPDH), Plasma Intrinsic Protein 2 (PIP2) and ACTIN(ACT)) were assessed to determine their expression stability in 23 samples from different tissues or organs of loquat. Integrated expression stability evaluations using five computational statistical methods (GeNorm, NormFinder, ΔCt, BestKeeper, and RefFinder) suggested that a RG set, including RPL4, RPL18, HIS3 and TUA3, was the most stable one across all of the tested loquat samples. The expression pattern of EjCDKB1;2 in the tested loquat tissues normalized to the selected RG set demonstrated its reliability.

CONCLUSIONS

This study reveals the reliable RGs for accurate normalization of gene expression in loquat. In addition, our findings demonstrate an efficient system for identifying the most effective RGs for different organs, which may be applied to related rosaceous crops.

Molecular cloning and functional characterization of an O-methyltransferase catalyzing 4'-O-methylation of resveratrol in Acorus calamus

Resveratrol and its methyl ethers, which belong to a class of natural polyphenol stilbenes, play important roles as biologically active compounds in plant defense as well as in human health. Although the biosynthetic pathway of resveratrol has been fully elucidated, the characterization of resveratrol-specific O-methyltransferases remains elusive. In this study, we used RNA-seq analysis to identify a putative aromatic O-methyltransferase gene, AcOMT1, in Acorus calamus. Recombinant AcOMT1 expressed in Escherichia coli showed high 4'-O-methylation activity toward resveratrol and its derivative, isorhapontigenin. We purified a reaction product enzymatically formed from resveratrol by AcOMT1 and confirmed it as 4'-O-methylresveratrol (deoxyrhapontigenin). Resveratrol and isorhapontigenin were the most preferred substrates with apparent K_m values of 1.8 μM and 4.2 μM, respectively. Recombinant AcOMT1 exhibited reduced activity toward other resveratrol derivatives, piceatannol, oxyresveratrol, and pinostilbene. In contrast, recombinant AcOMT1 exhibited no activity toward pterostilbene or pinosylvin. These results indicate that AcOMT1 showed high 4'-O-methylation activity toward stilbenes with non-methylated phloroglucinol rings.

Generation of (2-Nitroethyl)benzene and related benzenoids from L-Phenylalanine; flower scents of the Japanese Loquat Eriobotrya japonica [Rosales: Rosaceae]

(2-Nitroethyl)benzene, methyl 4-methoxybenzoate and 4-methoxybenzaldehyde have been known as major scent components in flowers of the Japanese loquat Eriobotrya japonica [Rosales: rosaceae], together with 13 related benzenoids, including Z- and E-2-phenylacetaldoxime and benzyl alcohol. The scents air-trapped from a flowering panicle during 24 h incubation with d8-L-phenylalanine were composed of 15 deuterium labeled compounds {d6-styrene, d5-benzaldehyde, d7-2-phenylacetaldehyde, methyl d5-benzoate, d7 −2-phenylethanol, d7-2-phenylacetonitrile, d4-1,4-dimethoxybenzene, d7-Z-2-phenylacetaldoxime, d4-4-methoxybenzaldehyde, d7-E-2-phenylacetaldoxime, d4-4-methoxybenzyl alcohol, d7-(2-nitroethyl)benzene, methyl d4-4-methoxybenzoate, methyl d6-cinnamate and ethyl d4-4-methoxybenzoate}. On the other hand, hexane extracts of the flower petal incubate with a mixture of d5-Z- and d5-E-2-phenylacetaldoxime after 24 h indicated generation of six d5-labeld components {d5-benzaldehyde, d5-benzyl alcohol, d5-2-phenylacetaldehyde, methyl d5-benzoate, d5-2-phenylethanol, and d5-(2-nitroethyl)benzene}. By comparing those results, (2-nitroethyl)benzene was concluded as a product directly generated from a mixture of Z- and E-2-phenylacetaldoxime together with six minor benzenoids, while two major compounds (4-methoxybenzaldehyde and methyl 4-methoxybenzoate) together with three minors from L-phenylalanine, presumably via L-tyrosine. The other two minor components were derived from L-phenylalanine.

Transcriptome analysis of Petunia axillaris flowers reveals genes involved in morphological differentiation and metabolite transport

The biosynthesis of plant secondary metabolites is associated with morphological and metabolic differentiation. As a consequence, gene expression profiles can change drastically, and primary and secondary metabolites, including intermediate and end-products, move dynamically within and between cells. However, little is known about the molecular mechanisms underlying differentiation and transport mechanisms. In this study, we performed a transcriptome analysis of Petunia axillaris subsp. parodii, which produces various volatiles in its corolla limbs and emits metabolites to attract pollinators. RNA-sequencing from leaves, buds, and limbs identified 53,243 unigenes. Analysis of differentially expressed genes, combined with gene ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses, showed that many biological processes were highly enriched in limbs. These included catabolic processes and signaling pathways of hormones, such as gibberellins, and metabolic pathways, including phenylpropanoids and fatty acids. Moreover, we identified five transporter genes that showed high expression in limbs, and we performed spatiotemporal expression analyses and homology searches to infer their putative functions. Our systematic analysis provides comprehensive transcriptomic information regarding morphological differentiation and metabolite transport in the Petunia flower and lays the foundation for establishing the specific mechanisms that control secondary metabolite biosynthesis in plants.

Functional Characterization of Salicylic Acid Carboxyl Methyltransferase from Camellia sinensis, Providing the Aroma Compound of Methyl Salicylate during the Withering Process of White Tea

Methyl salicylate (MeSA) is one of the volatile organic compounds (VOCs) that releases floral scent and plays an important role in the sweet flowery aroma of tea. During the withering process for white tea producing, MeSA was generated by salicylic acid carboxyl methyltransferase (SAMT) with salicylic acid (SA), and the specific floral scent was formed. In this study, we first cloned a CsSAMT from tea leaves (GenBank accession no. MG459470) and used Escherichia coli and Saccharomyces cerevisiae to express the recombinant CsSAMT. The enzyme activity in prokaryotic and eukaryotic expression systems was identified, and the protein purification, substrate specificity, pH, and temperature optima were investigated. It was shown that CsSAMT located in the chloroplast, and the gene expression profiles were quite different in tea organs. The obtained results might give a new understanding for tea aroma formation, optimization, and regulation and have great significance for improving the specific quality of white tea.

Characterization of an O-methyltransferase specific to guaiacol-type benzenoids from the flowers of loquat (Eriobotrya japonica)

Volatile benzenoids, including methyl p-methoxybenzoate, p-anisaldehyde, and p-anisalcohol, are responsible for the sweet and characteristic fragrance of loquat (Eriobotrya japonica, Rosaceae) flowers. Although the full pathway of volatile benzenoid synthesis has yet to be elucidated, their chemical structures suggest that O-methyltransferases are present in loquat and function in the methylation of the para-OH groups. In the present study, we used RNA-sequencing to identify four loquat genes (EjOMT1, EjOMT2, EjOMT3, and EjOMT4) that encode O-methyltransferases. We found that EjOMT1 was highly expressed in floral tissues, with an expression pattern that coincided with changes in intracellular volatile benzenoids during flower development. Recombinant EjOMT1 protein expressed in Escherichia coli showed the highest activity towards guaiacol with a K_m value of 35 μM. Furthermore, the protein also showed lesser activities towards guaiacol-type benzenoids including eugenol, isoeugenol, vanillin, and ferulic acid, in addition to much weaker activities towards catechol and p-hydroxybenzenoid derivatives. However, no activity was shown towards phenylpropenes without m-methoxy substitution, t-anol and chavicol. Taken together, our findings indicate that EjOMT1 has a broad substrate specificity towards compounds with both para-OH and meta-OCH₃ groups, unlike previously characterized O-methyltransferases for volatile benzenoid/phenylpropanoid biosynthesis.