Scopoletin uptake from culture medium and accumulation in the vacuoles after conversion to scopolin in 2,4-D-treated tobacco cells

Phytochemical Profiles and Cytotoxic Activity of Bursera fagaroides (Kunth) Engl. Leaves and Its Callus Culture

Bursera fagaroides, popularly used in México, possesses bioactive lignans. These compounds are low in the bark, and its extraction endangers the life of the trees. The aim of the present investigation was to search for alternative sources of cytotoxic compounds in B. fagaroides prepared as leaves and in vitro callus cultures. The friable callus of B. fagaroides was established using a combination of plant growth regulators: 4 mgL-1 of 2,4-dichlorophenoxyacetic acid (2,4-D), 1 mgL-1 Naphthaleneacetic Acid (NAA) and 1 mgL-1 Zeatin. The maximum cell growth was at day 28 with a specific growth rate of μ = 0.059 days-1 and duplication time td = 11.8 days. HPLC quantification of the dichloromethane callus biomass extract showed that Scopoletin, with a concentration of 10.7 µg g-1 dry weight, was the main compound inducible as a phytoalexin by the addition of high concentrations of 2,4-D, as well as by the absence of nutrients in the culture medium. In this same extract, the compounds γ-sitosterol and stigmasterol were also identified by GC-MS analysis. Open column chromatography was used to separate and identify yatein, acetyl podophyllotoxin and 7',8'-dehydropodophyllotoxin in the leaves of the wild plant. Cytotoxic activity on four cancer cell lines was tested, with PC-3 prostate carcinoma (IC50 of 12.6 ± 4.6 µgmL-1) being the most sensitive to the wild-type plant extract and HeLa cervical carcinoma (IC50 of 72 ± 5 µgmL-1) being the most sensitive to the callus culture extract.

Specialized metabolite modifications in Brassicaceae seeds and plants: diversity, functions and related enzymes

Covering: up to 2023Specialized metabolite (SM) modifications and/or decorations, corresponding to the addition or removal of functional groups (e.g. hydroxyl, methyl, glycosyl or acyl group) to SM structures, contribute to the huge diversity of structures, activities and functions of seed and plant SMs. This review summarizes available knowledge (up to 2023) on SM modifications in Brassicaceae and their contribution to SM plasticity. We give a comprehensive overview on enzymes involved in the addition or removal of these functional groups. Brassicaceae, including model (Arabidopsis thaliana) and crop (Brassica napus, Camelina sativa) plant species, present a large diversity of plant and seed SMs, which makes them valuable models to study SM modifications. In this review, particular attention is given to the environmental plasticity of SM and relative modification and/or decoration enzymes. Furthermore, a spotlight is given to SMs and related modification enzymes in seeds of Brassicaceae species. Seeds constitute a large reservoir of beneficial SMs and are one of the most important dietary sources, providing more than half of the world's intake of dietary proteins, oil and starch. The seed tissue- and stage-specific expressions of A. thaliana genes involved in SM modification are presented and discussed in the context of available literature. Given the major role in plant phytochemistry, biology and ecology, SM modifications constitute a subject of study contributing to the research and development in agroecology, pharmaceutical, cosmetics and food industrial sectors.

Engineered coumarin accumulation reduces mycotoxin-induced oxidative stress and disease susceptibility

Coumarins can fight pathogens and are thus promising for crop protection. Their biosynthesis, however, has not yet been engineered in crops. We tailored the constitutive accumulation of coumarins in transgenic Nicotiana benthamiana, Glycine max and Arabidopsis thaliana plants, as well as in Nicotiana tabacum BY-2 suspension cells. We did so by overexpressing A. thaliana feruloyl-CoA 6-hydroxylase 1 (AtF6'H1), encoding the key enzyme of scopoletin biosynthesis. Besides scopoletin and its glucoside scopolin, esculin at low level was the only other coumarin detected in transgenic cells. Mechanical damage of scopolin-accumulating tissue led to a swift release of scopoletin, presumably from the scopolin pool. High scopolin levels in A. thaliana roots coincided with reduced susceptibility to the root-parasitic nematode Heterodera schachtii. In addition, transgenic soybean plants were more tolerant to the soil-borne pathogenic fungus Fusarium virguliforme. Because mycotoxin-induced accumulation of reactive oxygen species and cell death were reduced in the AtF6'H1-overexpressors, the weaker sensitivity to F. virguliforme may be caused by attenuated oxidative damage of coumarin-hyperaccumulating cells. Together, engineered coumarin accumulation is promising for enhanced disease resilience of crops.

GlPS1 overexpression accumulates coumarin secondary metabolites in transgenic Arabidopsis

The dried root of Glehnia littoralis is a traditional Chinese herbal medicine mainly used to treat lung diseases and plays an important role in fighting coronavirus disease 2019 pneumonia in China. This study focused on the key enzyme gene GlPS1 for furanocoumarin synthesis in G. littoralis. In the 35S:GlPS1 transgenic Arabidopsis study, the Arabidopsis thaliana-overexpressing GlPS1 gene was more salt-tolerant than Arabidopsis in the blank group. Metabolomics analysis showed 30 differential metabolites in Arabidopsis, which overexpressed the GlPS1 gene. Twelve coumarin compounds were significantly upregulated, and six of these coumarin compounds were not detected in the blank group. Among these differential coumarin metabolites, isopimpinellin and aesculetin have been annotated by the Kyoto Encyclopedia of Genes and Genomes and isopimpinellin was not detected in the blank group. Through structural comparison, imperatorin was formed by dehydration and condensation of zanthotoxol and a molecule of isoprenol, and the difference between them was only one isoprene. Results showed that the GlPS1 gene positively regulated the synthesis of coumarin metabolites in A. thaliana and at the same time improved the salt tolerance of A. thaliana.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11240-022-02427-w.

Flavonoid Accumulation Varies in Medicago truncatula in Response to Mercury Stress

Mercury (Hg) contamination is increasing worldwide in both wild ecosystems and agricultural soils due to natural processes, but mostly to anthropic activities. The molecular mechanisms involved in Hg toxicity and tolerance in plants have been extensively studied; however, the role of flavonoids in response to Hg stress remains to be investigated. We conducted a metabolomic study to analyze the changes induced at the secondary metabolite level in three Hg-tolerant and one Hg-sensitive Medicago truncatula cultivars. A total of 46 flavonoid compounds, classified into five different flavonoid families: anthocyanidins, flavones, isoflavones, pterocarpan flavonoids, and flavanones, along with their respective glycoconjugate derivatives, were identified in leaf and root tissues. The synthesis of free isoflavones, followed by monoglycosylation and further malonylation was shown to be characteristic of root samples, whereas higher glycosylation, followed by further acylation with coumaric and ferulic acid was characteristic of leaf tissues. While minor changes were observed in leaves, significant quantitative changes could be observed in roots upon Hg treatment. Some flavonoids were strongly upregulated in roots, including malonylglucosides of biochanin A, formononetin and medicarpin, and aglycones biochanin, daidzein, and irisolidone. Hg tolerance appeared to be mainly associated to the accumulation of formononetin MalGlc, tricin GlcAGlcA, and afrormosin Glc II in leaves, whereas aglycone accumulation was associated with tolerance to Hg stress in roots. The results evidence the alteration of the flavonoid metabolic profile and their glycosylation processes in response to Hg stress. However, notable differences existed between varieties, both in the basal metabolic profile and in the response to treatment with Hg. Overall, we observed an increase in flavonoid production in response to Hg stress, and Hg tolerance appeared to be associated to a characteristic glycosylation pattern in roots, associated with the accumulation of aglycones and monoglycosylated flavonoids. The findings are discussed in the context of the flavonoid biosynthetic pathway to provide a better understanding of the role of these secondary metabolites in the response and tolerance to Hg stress in M. truncatula.

Leaf necrosis resulting from downregulation of poplar glycosyltransferase UGT72A2

Reactive species (RS) causing oxidative stress are unavoidable by-products of various plant metabolic processes, such as photosynthesis, respiration or photorespiration. In leaves, flavonoids scavenge RS produced during photosynthesis and protect plant cells against deleterious oxidative damages. Their biosynthesis and accumulation are therefore under tight regulation at the cellular level. Glycosylation has emerged as an essential biochemical reaction in the homeostasis of various specialized metabolites such as flavonoids. This article provides a functional characterization of the Populus tremula x P. alba (poplar) UGT72A2 coding for a UDP-glycosyltransferase that is localized in the chloroplasts. Compared with the wild type, transgenic poplar lines with decreased expression of UGT72A2 are characterized by reduced growth and oxidative damages in leaves, as evidenced by necrosis, higher content of glutathione and lipid peroxidation products as well as diminished soluble peroxidase activity and NADPH to NADP+ ratio under standard growing conditions. They furthermore display lower pools of phenolics, anthocyanins and total flavonoids but higher proanthocyanidins content. Promoter analysis revealed the presence of cis-elements involved in photomorphogenesis, chloroplast biogenesis and flavonoid biosynthesis. The UGT72A2 is regulated by the poplar MYB119, a transcription factor known to regulate the flavonoid biosynthesis pathway. Phylogenetic analysis and molecular docking suggest that UGT72A2 could glycosylate flavonoids; however, the actual substrate(s) was not consistently evidenced with either in vitro assays nor analyses of glycosylated products in leaves of transgenic poplar overexpressing or downregulated for UGT72A2. This article provides elements highlighting the importance of flavonoid glycosylation regarding protection against oxidative stress in poplar leaves and raises new questions about the link between this biochemical reaction and regulation of the redox homeostasis system.

Retrieving the in vivo Scopoletin Fluorescence Excitation Band Allows the Non-invasive Investigation of the Plant-Pathogen Early Events in Tobacco Leaves

In this study, we developed and applied a new spectroscopic fluorescence method for the in vivo detection of the early events in the interaction between tobacco (Nicotiana tabacum L.) plants and pathogenic bacteria. The leaf disks were infiltrated with a bacterial suspension in sterile physiological solution (SPS), or with SPS alone as control. The virulent Pseudomonas syringae pv. tabaci strain ATCC 11528, its non-pathogenic ΔhrpA mutant, and the avirulent P. syringae pv. tomato strain DC3000 were used. At different post-infiltration time-points, the in vivo fluorescence spectra on leaf disks were acquired by a fiber bundle-spectrofluorimeter. The excitation spectra of the leaf blue emission at 460 nm, which is mainly due to the accumulation of coumarins following a bacterial infiltration, were processed by using a two-bands Gaussian fitting that enabled us to isolate the scopoletin (SCT) contribution. The pH-dependent fluorescence of SCT and scopolin (SCL), as determined by in vitro data and their intracellular localization, as determined by confocal microscopy, suggested the use of the longer wavelength excitation band at 385 nm of 460 nm emission (F385_460) to follow the metabolic evolution of SCT during the plant-bacteria interaction. It was found to be directly correlated (R 2 = 0.84) to the leaf SCT content, but not to that of SCL, determined by HPLC analysis. The technique applied to the time-course monitoring of the bacteria-plant interaction clearly showed that the amount and the timing of SCT accumulation, estimated by F385_460, was correlated with the resistance to the pathogen. As expected, this host defense response was delayed after P. syringae pv. tabaci ATCC 11528 infiltration, in comparison to P. syringae pv. tomato DC3000. Furthermore, no significant increase of F385_460 (SCT) was observed when using the non-pathogenic ΔhrpA mutant of P. syringae pv. tabaci ATCC 11528, which lacks a functional Type Three Secretion System (TTSS). Our study showed the reliability of the developed fluorimetric method for a rapid and non-invasive monitoring of bacteria-induced first events related to the metabolite-based defense response in tobacco leaves. This technique could allow a fast selection of pathogen-resistant cultivars, as well as the on-site early diagnosis of tobacco plant diseases by using suitable fluorescence sensors.

Coumarin Communication Along the Microbiome-Root-Shoot Axis

Plants shape their rhizosphere microbiome by secreting root exudates into the soil environment. Recently, root-exuded coumarins were identified as novel players in plant-microbiome communication. Beneficial members of the root-associated microbiome stimulate coumarin biosynthesis in roots and their excretion into the rhizosphere. The iron-mobilizing activity of coumarins facilitates iron uptake from the soil environment, while their selective antimicrobial activity shapes the root microbiome, resulting in promotion of plant growth and health. Evidence is accumulating that, in analogy to strigolactones and flavonoids, coumarins may act in microbiome-to-root-to-shoot signaling events. Here, we review this multifaceted role of coumarins in bidirectional chemical communication along the microbiome-root-shoot axis.

Fine mapping of the Ca3GT gene controlling anthocyanin biosynthesis in mature unripe fruit of Capsicum annuum L

The anthocyanin biosynthesis gene Ca3GT was fine-mapped in a 110.5-kb region through a map-based cloning strategy. Gene expression and promoter analyses confirmed the strong candidate gene Capana10g001978. Pepper (Capsicum annum L.) fruit can be dark green, green, light green, purple, yellow, or ivory at the juvenile stage. Anthocyanins are responsible for fruit color formation in mature unripe pepper fruit, and transient accumulation of anthocyanins is the main problem in breeding pepper plants with mature purple fruit. Only a few genes controlling this trait have been cloned. The present study aimed to map and identify an anthocyanin biosynthesis gene from pepper using an F₂ population derived from a cross between line '17C3808' (purple mature unripe fruit) and line '17C3807' (green mature unripe fruit). The trait was mapped on a 110.5-kb interval between markers SSR18213 and SSR18228 on chromosome 10. There were three open reading frames in this region; Capana10g001978 was predicted in this region as markers CAPS-78-708 and InDel146 co-segregated with it. Capana10g001978 is a structural gene encoding the GTB transcription factor involved in the biosynthesis of anthocyanins. Comparing parental sequences, two base mutations were identified in the exon of Capana10g001978, at positions + 528 bp and + 708 bp, which resulted in changes in the 176th and 236th amino acid residues, from glutamine (CAA) to histidine (CAC), causing a nonsense mutation (from CAG to CAA). Additionally, Capana10g001978 was highly expressed in the pericarp of mature, unripe pepper fruit. There were four single nucleotide polymorphisms, three sequence deletions, and one sequence insertion in the promoter region of purple, mature, and unripe pepper fruit, leading to the formation of a W-box and a GT1-motif. Thus, Capana10g001978 is a strong candidate gene of Ca3GT involved in anthocyanin biosynthesis in mature unripe pepper fruit. These results provide important information regarding the isolation and characterization of Ca3GT, and they are the starting point for studying the regulatory pathway responsible for anthocyanin biosynthesis in pepper.

Endoplasmic reticulum-derived bodies enable a single-cell chemical defense in Brassicaceae plants

Brassicaceae plants have a dual-cell type of chemical defense against herbivory. Here, we show a novel single-cell defense involving endoplasmic reticulum (ER)-derived organelles (ER bodies) and the vacuoles. We identify various glucosinolates as endogenous substrates of the ER-body β-glucosidases BGLU23 and BGLU21. Woodlice strongly prefer to eat seedlings of bglu23 bglu21 or a glucosinolate-deficient mutant over wild-type seedlings, confirming that the β-glucosidases have a role in chemical defense: production of toxic compounds upon organellar damage. Deficiency of the Brassicaceae-specific protein NAI2 prevents ER-body formation, which results in a loss of BGLU23 and a loss of resistance to woodlice. Hence, NAI2 that interacts with BGLU23 is essential for sequestering BGLU23 in ER bodies and preventing its degradation. Artificial expression of NAI2 and BGLU23 in non-Brassicaceae plants results in the formation of ER bodies, indicating that acquisition of NAI2 by Brassicaceae plants is a key step in developing their single-cell defense system.

Kenji Yamada et al. describe a single-cell chemical defense strategy in Brassicaceae plants that requires formation of endoplasmic reticulum-derived organelles for the accumulation of β-glucosidases. They find that seedlings lacking a specific β-glucosidase lose their resistance to predation by woodlice.

You Want it Sweeter: How Glycosylation Affects Plant Response to Oxidative Stress

Oxidative stress is a cellular threat which puts at risk the productivity of most of crops valorized by humankind in terms of food, feed, biomaterial, or bioenergy. It is therefore of crucial importance to understand the mechanisms by which plants mitigate the deleterious effects of oxidizing agents. Glycosylation of antioxidant molecules and phytohormones modifies their chemical properties as well as their cellular and histological repartition. This review emphasizes the mechanisms and the outcomes of this conjugation reaction on plant ability to face growing conditions favoring oxidative stress, in mirror with the activity of deglycosylating enzymes. Pioneer evidence bridging flavonoid, glycosylation, and redox homeostasis paved the way for numerous functional analyses of UDP-glycosyltransferases (UGTs), such as the identification of their substrates and their role to circumvent oxidative stress resulting from various environmental challenges. (De)glycosylation appears as a simple chemical reaction regulating the biosynthesis and/or the activity of a myriad of specialized metabolites partaking in response to pathogen and abiotic stresses. This outcome underlies the possibility to valorize UGTs potential to upgrade plant adaptation and fitness in a rising context of sub-optimal growing conditions subsequent to climate change.

The Age of Coumarins in Plant-Microbe Interactions

Coumarins are a family of plant-derived secondary metabolites that are produced via the phenylpropanoid pathway. In the past decade, coumarins have emerged as iron-mobilizing compounds that are secreted by plant roots and aid in iron uptake from iron-deprived soils. Members of the coumarin family are found in many plant species. Besides their role in iron uptake, coumarins have been extensively studied for their potential to fight infections in both plants and animals. Coumarin activities range from antimicrobial and antiviral to anticoagulant and anticancer. In recent years, studies in the model plant species tobacco and Arabidopsis have significantly increased our understanding of coumarin biosynthesis, accumulation, secretion, chemical modification and their modes of action against plant pathogens. Here, we review current knowledge on coumarins in different plant species. We focus on simple coumarins and provide an overview on their biosynthesis and role in environmental stress responses, with special attention for the recently discovered semiochemical role of coumarins in aboveground and belowground plant-microbe interactions and the assembly of the root microbiome.

Mechanistic differences in the uptake of salicylic acid glucose conjugates by vacuolar membrane-enriched vesicles isolated from Arabidopsis thaliana

Salicylic acid (SA) is a plant hormone involved in a number of physiological responses including both local and systemic resistance of plants to pathogens. In Arabidopsis, SA is glucosylated to form either SA 2-O-β-d-glucose (SAG) or SA glucose ester (SGE). In this study, we show that SAG accumulates in the vacuole of Arabidopsis, while the majority of SGE was located outside the vacuole. The uptake of SAG by vacuolar membrane-enriched vesicles isolated from Arabidopsis was stimulated by the addition of MgATP and was inhibited by both vanadate (ABC transporter inhibitor) and bafilomycin A₁ (vacuolar H⁺ -ATPase inhibitor), suggesting that SAG uptake involves both an ABC transporter and H⁺ -antiporter. Despite its absence in the vacuole, we observed the MgATP-dependent uptake of SGE by Arabidopsis vacuolar membrane-enriched vesicles. SGE uptake was not inhibited by vanadate but was inhibited by bafilomycin A₁ and gramicidin D providing evidence that uptake was dependent on an H⁺ -antiporter. The uptake of both SAG and SGE was also inhibited by quercetin and verapamil (two known inhibitors of multidrug efflux pumps) and salicin and arbutin. MgATP-dependent SAG and SGE uptake exhibited Michaelis-Menten-type saturation kinetics. The vacuolar enriched-membrane vesicles had a 46-fold greater affinity and a 10-fold greater transport activity with SGE than with SAG. We propose that in Arabidopsis, SAG is transported into the vacuole to serve as a long-term storage form of SA while SGE, although also transported into the vacuole, is easily hydrolyzed to release the active hormone which can then be remobilized to other cellular locations.

Auxin-like effects of the natural coumarin scopoletin on Arabidopsis cell structure and morphology

The mode of action and phytotoxic potential of scopoletin, a natural compound belonging to the group of coumarins, has been evaluated in detail. Analysis conducted by light and electron transmission microscopy showed strong cell and tissue abnormalities on treated roots, such as cell wall malformations, multi-nucleated cells, abnormal nuclei and tissue disorganization. Scopoletin compromised root development by inducing wrong microtubule assembling, mitochondrial membrane depolarization and ultimate cell death, in a way similar to auxin herbicides. The structural similarities of the natural compound scopoletin and the auxin herbicide 2,4-D, as well as the ability of scopoletin to fit into the auxin-binding site TIR1, were analyzed, suggesting that the phytotoxic activity of scopoletin matches with that exhibited by auxinic herbicides.

Identification of QTLs affecting scopolin and scopoletin biosynthesis in Arabidopsis thaliana

BACKGROUND

Scopoletin and its glucoside scopolin are important secondary metabolites synthesized in plants as a defense mechanism against various environmental stresses. They belong to coumarins, a class of phytochemicals with significant biological activities that is widely used in medical application and cosmetics industry. Although numerous studies showed that a variety of coumarins occurs naturally in several plant species, the details of coumarins biosynthesis and its regulation is not well understood. It was shown previously that coumarins (predominantly scopolin and scopoletin) occur in Arabidopsis thaliana (Arabidopsis) roots, but until now nothing is known about natural variation of their accumulation in this model plant. Therefore, the genetic architecture of coumarins biosynthesis in Arabidopsis has not been studied before.

RESULTS

Here, the variation in scopolin and scopoletin content was assessed by comparing seven Arabidopsis accessions. Subsequently, a quantitative trait locus (QTL) mapping was performed with an Advanced Intercross Recombinant Inbred Lines (AI-RILs) mapping population EstC (Est-1 × Col). In order to reveal the genetic basis of both scopolin and scopoletin biosynthesis, two sets of methanol extracts were made from Arabidopsis roots and one set was additionally subjected to enzymatic hydrolysis prior to quantification done by high-performance liquid chromatography (HPLC). We identified one QTL for scopolin and five QTLs for scopoletin accumulation. The identified QTLs explained 13.86% and 37.60% of the observed phenotypic variation in scopolin and scopoletin content, respectively. In silico analysis of genes located in the associated QTL intervals identified a number of possible candidate genes involved in coumarins biosynthesis.

CONCLUSIONS

Together, our results demonstrate for the first time that Arabidopsis is an excellent model for studying the genetic and molecular basis of natural variation in coumarins biosynthesis in plants. It additionally provides a basis for fine mapping and cloning of the genes involved in scopolin and scopoletin biosynthesis. Importantly, we have identified new loci for this biosynthetic process.

2-Oxoglutarate-dependent dioxygenases in the biosynthesis of simple coumarins

Coumarins are natural plant products that have been the subject of extensive phytochemical and pharmacological research studies in the past few decades. The core structure of coumarins is derived from the respective cinnamates via ortho-hydroxylation of the aromatic ring, trans/cis isomerization, and lactonization. Various substitution patterns of coumarins have been reported, whereas the biosynthesis of coumarins remains elusive. Ortho-hydroxylation is a key step in simple coumarin biosynthesis as a branch point from the lignin biosynthetic pathway. 2-Oxoglutarate-dependent dioxygenases (2OGDs) from plants convert cinnamate derivatives into simple coumarins through the process of ortho-hydroxylation. This review describes the 2OGDs involved in coumarin biosynthesis and their substrate specificities.

Involvement of phospholipases C and D in the defence responses of riboflavin-treated tobacco cells

Riboflavin is an activator of defence responses in plants that increases resistance against diseases caused by fungal, oomycete, bacterial and viral pathogens. However, the mechanisms driving defence activation by riboflavin are poorly understood. We investigated the signal transduction pathways of phospholipase C (PLC) and phospholipase D (PLD) in tobacco (Nicotiana tabacum) suspension cells using a pharmacological approach to confirm whether riboflavin-mediated activation of the defence response is dependent on both PLC and PLD. The expression patterns analysed by quantitative reverse transcription-polymerase chain reaction demonstrated that the tobacco PLC and PLD gene families were differentially expressed in riboflavin-treated tobacco cells. PLC and PLD expression accompanied defence responses including the expression of defence response genes (PAL, PR-1a and PR-1b), the production of hydrogen peroxide and the accumulation of the phytoalexin scopoletin in tobacco cells treated with riboflavin. These defence responses were significantly inhibited in the presence of the PLC inhibitor U73122 and the PLD inhibitor 1-butanol; however, inhibitor analogues had no effect. Moreover, treating tobacco cells with phosphatidic acid, a signalling molecule produced by phospholipase catalysis, induced the accumulation of the phytoalexin scopoletin and compensated for the suppressive effects of U73122 and 1-butanol on riboflavin-induced accumulation of the phytoalexin. These results offer pharmacological evidence that PLC and PLD play a role in riboflavin-induced defences of tobacco.

Interspecific somatic hybrids between Cyclamen persicum and C. coum, two sexually incompatible species

By applying polyethylene glycol (PEG)-mediated protoplast fusion, the first somatic hybrids were obtained between Cyclamen persicum (2n = 2x = 48) and C. coum (2n = 2x = 30)-two species that cannot be combined by cross breeding. Heterofusion was detected by double fluorescent staining with fluorescein diacetate and scopoletin. The highest heterofusion frequencies (of about 5%) resulted from a protocol using a protoplast density of 1 × 10(6)/mL and 40% PEG. The DNA content of C. coum was estimated for the first time by propidium iodide staining to be 14.7 pg/2C and was 4.6 times higher than that of C. persicum. Among 200 in vitro plantlets regenerated from fusion experiments, most resembled the C. coum parent, whereas only 5 plants showed typical C. persicum phenotypes and 46 had a deviating morphology. By flow cytometry, six putative somatic hybrids were identified. A species-specific DNA marker was developed based on the sequence of the 5.8S gene in the ribosomal nuclear DNA and its flanking internal transcribed spacers ITS1 and ITS2. The hybrid status of only one plant could be verified by the species-specific DNA marker as well as sequencing of the amplification product. RAPD markers turned out to be less informative and applicable for hybrid identification, as no clear additivity of the parental marker bands was observed. Chromosome counting in root tips of four hybrids revealed the presence of the 30 C. coum chromosomes and 2-41 additional ones indicating elimination of C. persicum chromosomes.

Identification and functional characterization of cDNAs coding for hydroxybenzoate/hydroxycinnamate glucosyltransferases co-expressed with genes related to proanthocyanidin biosynthesis

Grape proanthocyanidins (PAs) play a major role in the organoleptic properties of wine. They are accumulated mainly in grape skin and seeds during the early stages of berry development. Despite the recent progress in the identification of genes involved in PA biosynthesis, the mechanisms involved in subunit condensation, galloylation, or fine regulation of the spatio-temporal composition of grape berries in PAs are still not elucidated. Two Myb transcription factors, VvMybPA1 and VvMybPA2, controlling the PA pathway have recently been identified and ectopically over-expressed in an homologous system. In addition to already known PA genes, three genes coding for glucosyltransferases were significantly differentially expressed between hairy roots over-expressing VvMybPA1 or VvMybPA2 and control lines. The involvement of these genes in PA biosynthesis metabolism is unclear. The three glucosyltransferases display high sequence similarities with other plant glucosyltransferases able to catalyse the formation of glucose esters, which are important intermediate actors for the synthesis of different phenolic compounds. Studies of the in vitro properties of these three enzymes (K(m), V(max), substrate specificity, pH sensitivity) were performed through production of recombinant proteins in E. coli and demonstrated that they are able to catalyse the formation of 1-O-acyl-Glc esters of phenolic acids but are not active on flavonoids and stilbenes. The transcripts are expressed in the early stages of grape berry development, mainly in the berry skins and seeds. The results presented here suggest that these enzymes could be involved in vivo in PA galloylation or in the synthesis of hydroxycinnamic esters.

Unique defense strategy by the endoplasmic reticulum body in plants

The endoplasmic reticulum (ER) is a site for the production of secretory proteins. Plants have developed ER subdomains for protein storage. The ER body is one such structure, which is observed in Brassicaceae plants. ER bodies accumulate in seedlings and roots or in wounded leaves in Arabidopsis. ER bodies contain high amounts of the β-glucosidases PYK10/BGLU23 in seedlings and roots or BGLU18 in wounded tissues. These results suggest that ER bodies are involved in the metabolism of glycoside molecules, presumably to produce repellents against pests and fungi. When Arabidopsis roots are homogenized, PYK10 formed large protein aggregates that include other β-glucosidases (BGLU21 and BGLU22), GDSL lipase-like proteins (GLL22) and cytosolic jacalin-related lectins (PBP1/JAL30, JAL31, JAL33, JAL34 and JAL35). Glucosidase activity increases by the aggregate formation. NAI1, a basic helix-loop-helix transcription factor, regulates the expression of the ER body proteins PYK10 and NAI2. Reduced expression of NAI2, PYK10 and BGLU21 resulted in abnormal ER body formation, indicating that these components regulate ER body formation. PYK10, BGLU21 and BGLU22 possess hydrolytic activity for scopolin, a coumaroyl glucoside that accumulates in the roots of Arabidopsis, and nai1 and pyk10 mutants are more susceptible to the symbiotic fungus Piriformospora indica. Therefore, it appears that the ER body is a unique organelle of Brassicaceae plants that is important for defense against pests and fungi.

Downregulation of putative UDP-glucose: flavonoid 3-O-glucosyltransferase gene alters flower coloring in Phalaenopsis

Anthocyanin is the primary pigment contributing to red, violet, and blue flower color formation. The solubility of anthocyanins is enhanced by UDP glucose: flavonoid 3-O-glucosyltransferase (UFGT) through transfer of the glucosyl moiety from UDP-glucose to 3-hydroxyl group to produce the first stable pigments. To assess the possibility that UFGT is involved in the flower color formation in Phalaenopsis, the transcriptional activities of PeUFGT3, and other flower color-related genes in developing red or white flower buds were examined using RT-PCR analysis. In contrast with chalcone synthase, chalcone isomerase, and anthocyanidin synthase genes, PeUFGT3 transcriptional activity was higher expressed in the red color of Phalaenopsis cultivars. In the red labellum of Phalaenopsis 'Luchia Lady', PeUFGT3 also showed higher expression levels than that in the white perianth. PeUFGT3 was predominantly expressed in the red region of flower among various Phalaenopsis cultivars. To investigate the role of PeUFGT3 in red flower color formation, PeUFGT3 was specifically knocked down using RNA interference technology via virus inducing gene silencing in Phalaenopsis. The PeUFGT3-suppressed Phalaenopsis exhibited various levels of flower color fading that was well correlated with the extent of reduced level of PeUFGT3 transcriptional activity. Furthermore, there was a significant decrease in anthocyanin content in the PeUFGT3-suppressed Phalaenopsis flowers. The decrease of anthocyanin content due to PeUFGT3 gene silencing possibly caused the faded flower color in PeUFGT3-suppressed Phalaenopsis. Consequently, these results suggested that the glycosylation-related gene PeUFGT3 plays a critical role in red color formation in Phalaenopsis.

Malonylation is a key reaction in the metabolism of xenobiotic phenolic glucosides in Arabidopsis and tobacco

Tobacco cells (Nicotiana tabacum L.) accumulate harmful naphthols in the form of malonylated glucosides (Taguchi et al., 2005). Here, we showed that the malonylation of glucosides is a system to metabolize xenobiotics and is common to higher plants. Moreover, some plantlets including Arabidopsis thaliana excreted some of the incorporated naphthols into the culture media as their glucosides. In order to analyze the function of malonylation in the metabolism of these xenobiotics, we identified a malonyltransferase gene (At5g39050) responsible for the malonylation of these compounds in A. thaliana. The recombinant enzyme had malonyltransferase activity toward several phenolic glucosides including naphthol glucosides. A knockout mutant of At5g39050 (pmat1) exposed to naphthols accumulated only a few malonylglucosides in the cell, and released larger amounts of simple glucosides into the culture medium. In contrast, forced expression of At5g39050 in the pmat1 mutant resulted in increased malonylglucoside accumulation and decreased glucoside excretion to the media. The results provided clear evidence of whether the release of glucosides or the storage of malonylglucosides was determined by the At5g39050 expression level. A similar event in naphthol metabolism was observed in the tobacco mutant with a suppressed malonyltransferase gene (NtMaT1). These results suggested that malonylation could be a key reaction to separate the way of xenobiotics disposition, that is, release from cell surface or storage in vacuoles.

Constitutive expression of bergaptol O-methyltransferase in Glehnia littoralis cell cultures

We investigated whether exogenously supplied precursors of bergapten, namely umbelliferone, psoralen and bergaptol, could be utilized to produce bergapten without elicitation in Glehnia littoralis cell suspension cultures. The levels of added psoralen and bergaptol in the medium soon decreased, and this was followed by the detection of bergapten in both culture fluid and cells. Umbelliferone was also incorporated but in this case no bergapten was produced; instead, skimmin, umbelliferone monoglucoside, was detected. To determine whether conversion of psoralen to bergapten was due to enzyme induction by precursor feeding, the transcript accumulations and enzyme activities of bergaptol O-methyltransferase (BMT, EC 2.1.1.69), which catalyzes the last step of bergapten synthesis, and of phenylalanine ammonia-lyase (PAL, EC 4.3.1.5), which catalyzes the initial step of the phenylpropanoid biosynthetic pathway and is known as a marker enzyme of elicitation, were examined. The results showed that both the expression and the activity of BMT were always detected in all cells, including control cells. Since PAL was slightly induced in the cells supplied with/without precursors, phenylethyl alcohol (PEA, a competitive inhibitor of PAL) was applied to suspension cells prior to the addition of psoralen. PAL activity was effectively inhibited by PEA at 1-5 mM concentrations. Under these conditions, PEA did not affect bergapten production by cell cultures fed with psoralen at all. These results demonstrate that BMT is constitutively expressed in G. littoralis cell cultures.

Molecular cloning, characterization, and downregulation of an acyltransferase that catalyzes the malonylation of flavonoid and naphthol glucosides in tobacco cells

Tobacco cells (Nicotiana tabacum L. Bright Yellow T-13) exposed to harmful naphthols accumulate them as glucosylated and further modified compounds [Taguchi et al. (2003a) Plant Sci. 164, 231-240]. In this study, we identified the accumulated compounds to be 6'-O-malonylated glucosides of naphthols. Cells treated with various phenolic compounds accumulated the flavonoids mainly as malonylglucosides. To clarify the function of this malonylation in tobacco, we isolated the cDNA encoding a malonyltransferase (NtMaT1) from a cDNA library derived from tobacco cells. The heterologous expression of the gene in Escherichia coli revealed that the recombinant enzyme had malonyltransferase activity against several phenolic glucosides such as flavonoid 7-O-glucosides, flavonoid 3-O-glucosides and naphthol glucosides. The substrate preference of the enzyme was similar to that of the tobacco cell extract. Malonylation activity in the transgenic cells markedly decreased with the suppression of the expression of NtMaT1 mRNA in tobacco BY-2 cells by RNA interference. The compounds administered to the transgenic cells were accumulated in the cells as glucosides or other modified compounds in place of malonylglucosides. These results show that NtMaT1 is the main catalyst of malonylation on glucosides of xenobiotic flavonoids and naphthols in tobacco plants.

The formation, vacuolar localization, and tonoplast transport of salicylic acid glucose conjugates in tobacco cell suspension cultures

The metabolism of salicylic acid (SA) in tobacco (Nicotiana tabacum L. cv. KY 14) cell suspension cultures was examined by adding [7-14C]SA to the cell cultures for 24 h and identifying the metabolites through high performance liquid chromatography analysis. The three major metabolites of SA were SA 2-O-beta-D: -glucose (SAG), methylsalicylate 2-O-beta-D: -glucose (MeSAG) and methylsalicylate. Studies on the intracellular localization of the metabolites revealed that all of the SAG associated with tobacco protoplasts was localized in the vacuole. However, the majority of the MeSAG was located outside the vacuole. The tobacco cells contained an SA inducible SA glucosyltransferase (SAGT) enzyme that formed SAG. The SAGT enzyme was not associated with the vacuole and appeared to be a cytoplasmic enzyme. The vacuolar transport of SAG was characterized by measuring the uptake of [14C]SAG into tonoplast vesicles isolated from tobacco cell cultures. SAG uptake was stimulated eightfold by the addition of MgATP. The ATP-dependent uptake of SAG was inhibited by bafilomycin A1 (a specific inhibitor of the vacuolar H(+)-ATPase) and dissipation of the transtonoplast H(+)-electrochemical gradient. Vanadate was not an inhibitor of SAG uptake. Several beta-glucose conjugates were strong inhibitors of SAG uptake, whereas glutathione and glucuronide conjugates were only marginally inhibitory. The SAG uptake exhibited Michaelis-Menten type saturation kinetics with a K(m) and V(max) value of 11 microM and 205 pmol min-1 mg-1, respectively, for SAG. Based on the transport characteristics it appears as if the vacuolar uptake of SAG in tobacco cells occurs through an H(+)-antiport-type mechanism.

Umbelliferone released from hairy root cultures of Pharbitis nil treated with copper sulfate and its subsequent glucosylation

Hairy root cultures of Pharbitis nil treated with CuSO4 and methyl jasmonate (MeJA) produced umbelliferone (1) and scopoletin (2) in the culture medium, and skimmin (3), a beta-D-glucopyranoside of 1, was isolated from the hairy roots. While 1 in the medium increased and reached a maximal level 16 h after the treatment with CuSO4, the amount of 3 in the hairy roots decreased, reaching a minimal level after 8 h, before recovering to a level higher than the basal level after 24 h and then continuously increasing. These observations suggest that 1 was released by the hydrolysis of 3. Umbelliferone (1) inhibited hairy root growth, while skimmin (3) did not. This result suggests that, after the release of 1 as a phytoalexin, the hairy roots glycosylated 1 for the detoxification and re-use of 3 as a source of phytoalexin.

Cloning and characterization of a glucosyltransferase that reacts on 7-hydroxyl group of flavonol and 3-hydroxyl group of coumarin from tobacco cells

In higher plants, secondary metabolites are often converted to their glycoconjugates by glycosyltransferases (GTases). We cloned a cDNA encoding GTase (NtGT2) from tobacco (Nicotiana tabacum L.). The recombinant enzyme expressed in Escherichia coli (rNTGT2) showed glucosylation activity against several kinds of phenolic compounds, particularly the 7-hydroxyl group of flavonoids and 3-hydroxycoumarin. The K(m) values of kaempferol and 3-hydroxycoumarin with rNTGT2 are 6.5 microM and 23.6 microM, respectively. The deduced amino acid sequence of NTGT2 shows 60-70% identity to that of anthocyanin 5-O-glucosyltransferase (A5GT); rNTGT2 did not show activity against the anthocyanins tested. NtGT2 gene expression was induced by treating tobacco cells with plant hormones such as salicylic acid. We consider that NtGT2 gene might have evolved from the same ancestral gene as the A5GT genes to the stress-inducible GTases that react on several phenolic compounds.

Molecular cloning and heterologous expression of novel glucosyltransferases from tobacco cultured cells that have broad substrate specificity and are induced by salicylic acid and auxin

Scopoletin is one of the phytoalexins in tobacco. Cells of the T-13 cell line (Nicotiana tabacum L. Bright Yellow) accumulate a large amount of scopoletin, also known as 7-hydroxy-6-methoxycoumarin, as a glucoconjugate, scopolin, in vacuoles. We report here the molecular cloning of glucosyltransferases that can catalyze the glucosylation of many kinds of secondary metabolites including scopoletin. Two cDNAs encoding glucosyltransferase (NtGT1a and NtGT1b) were isolated from a cDNA library derived from the tobacco T-13 cell line by screening with heterologous cDNAs as a probe. The deduced amino-acid sequences of NtGT1a and NtGT1b exhibited 92% identity with each other, approximately 20-50% identities with other reported glucosyltransferases. Heterologous expression of these genes in Escherichia coli showed that the recombinant enzymes had glucosylation activity against both flavonoids and coumarins. They also strongly reacted with 2-naphthol as a substrate. These recombinant enzymes can utilize UDP-glucose as the sugar donor, but they can also utilize UDP-xylose as a weak donor. RNA blot analysis showed that these genes are induced by salicylic acid and auxin, but the time course of the expression was different. This result is similar to the changes in scopoletin glucosylation activity in these tobacco cells after addition of these plant growth regulators. These results might suggest that one of the roles of the products of these genes is scopoletin glucosylation, in response to salicylic acid and/or auxin, together with the other glucosyltransferases in tobacco cells.

Plant hormone regulation on scopoletin metabolism from culture medium into tobacco cells

Tobacco (Nicotiana tabacum L. Bright Yellow) T-13 cell line has an ability for production of scopoletin. In this cell culture, scopoletin is taken up from culture medium and accumulated in vacuoles after conversion to scopolin when cells are treated with 2,4-dichlorophenoxyacetic acid (2,4-D) (Taguchi et al. (2000)). To clarify the effect of 2,4-D on tobacco cells, its interaction with several other plant hormones was investigated. Other auxins also stimulated the uptake in the same manner as 2,4-D did, although higher concentrations were required than that of 2,4-D. When p-chlorophenoxyisobutyric acid (PCIB), an antiauxin, was added to the cell culture before 2,4-D, it inhibited 2,4-D-stimulated scopoletin uptake. This result suggests that the stimulation of scopoletin uptake was one of the auxin effects on tobacco cells. Among other classes of plant hormones that were tested, only salicylic acid stimulated the uptake. When these hormones were added to the cell cultures before 2,4-D, methyl jasmonate and kinetin reduced scopoletin uptake. These results suggest that this scopoletin uptake by tobacco cells is regulated by the interaction between different plant hormones.

Purification and characterization of UDP-glucose: hydroxycoumarin 7-O-glucosyltransferase, with broad substrate specificity from tobacco cultured cells

The enzyme UDP-glucose: hydroxycoumarin 7-O-glucosyltransferase (CGTase), which catalyzes the formation of scopolin from scopoletin, was purified approximately 1200-fold from a culture of 2,4-D-treated tobacco cells (Nicotiana tabacum L. cv. Bright Yellow T-13) with a yield of 7%. Purification to apparent homogeneity, as judged by SDS-PAGE, was achieved by sequential anion-exchange chromatography, hydroxyapatite chromatography, gel filtration, a second round of anion-exchange chromatography, and affinity chromatography on UDP-glucuronic acid agarose. The purified enzyme had a pH optimum of 7.5, an isoelectric point (pI) of 5.0, and a molecular mass of 49 kDa. The enzyme did not require metal cofactors for activity. Its activity was inhibited by Zn(2+), Co(2+) and Cu(2+) ions, as well as by SH-blocking reagents. The K(m) values for UDP-glucose, scopoletin and esculetin were 43, 150 and 25 µM, respectively. A study of the initial rate of the reaction suggested that the reaction proceeded via a sequential mechanism. The purified enzyme preferred hydroxycoumarins as substrates but also exhibited significant activity with flavonoids. A database search using the amino terminus amino acid sequence of CGTase revealed strong homology to the amino acid sequences of other glucosyltransferases in plants.