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

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.

Structured plant metabolomics for the simultaneous exploration of multiple factors

Multiple factors act simultaneously on plants to establish complex interaction networks involving nutrients, elicitors and metabolites. Metabolomics offers a better understanding of complex biological systems, but evaluating the simultaneous impact of different parameters on metabolic pathways that have many components is a challenging task. We therefore developed a novel approach that combines experimental design, untargeted metabolic profiling based on multiple chromatography systems and ionization modes, and multiblock data analysis, facilitating the systematic analysis of metabolic changes in plants caused by different factors acting at the same time. Using this method, target geraniol compounds produced in transgenic tobacco cell cultures were grouped into clusters based on their response to different factors. We hypothesized that our novel approach may provide more robust data for process optimization in plant cell cultures producing any target secondary metabolite, based on the simultaneous exploration of multiple factors rather than varying one factor each time. The suitability of our approach was verified by confirming several previously reported examples of elicitor-metabolite crosstalk. However, unravelling all factor-metabolite networks remains challenging because it requires the identification of all biochemically significant metabolites in the metabolomics dataset.

Toward better understanding of postharvest deterioration: biochemical changes in stored cassava (Manihot esculenta Crantz) roots

Food losses can occur during production, postharvest, and processing stages in the supply chain. With the onset of worldwide food shortages, interest in reducing postharvest losses in cassava has been increasing. In this research, the main goal was to evaluate biochemical changes and identify the metabolites involved in the deterioration of cassava roots. We found that high levels of ascorbic acid (AsA), polyphenol oxidase (PPO), dry matter, and proteins are correlated with overall lower rates of deterioration. On the other hand, soluble sugars such as glucose and fructose, as well as organic acids, mainly, succinic acid, seem to be upregulated during storage and may play a role in the deterioration of cassava roots. Cultivar Branco (BRA) was most resilient to postharvest physiological deterioration (PPD), while Oriental (ORI) was the most susceptible. Our findings suggest that PPO, AsA, and proteins may play a distinct role in PPD delay.

Galanthamine, an anticholinesterase drug, effects plant growth and development in Artemisia tridentate Nutt. via modulation of auxin and neutrotransmitter signaling

Galanthamine is a naturally occurring acetylcholinesterase (AchE) inhibitor that has been well established as a drug for treatment of mild to moderate Alzheimer disease, but the role of the compound in plant metabolism is not known. The current study was designed to investigate whether galanthamine could redirect morphogenesis of Artemisia tridentata Nutt. cultures by altering concentration of endogenous neurosignaling molecules acetylcholine (Ach), auxin (IAA), melatonin (Mel), and serotonin (5HT). Exposure of axenic A. tridentata cultures to 10 µM galanthamine decreased the concentration of endogenous Ach, IAA, MEL, and AchE, and altered plant growth in a manner reminiscent of 2-4D toxicity. Galanthamine itself demonstrated IAA activity in an oat coleotile elongation bioassay, 20 µM galanthamine showed no significant difference compared with 5 μM IAA or 5 μM 1-Naphthaleneacetic acid (NAA). Metabolomic analysis detected between 20,921 to 27,891 compounds in A. tridentata plantlets and showed greater commonality between control and 5 µM treatments. Furthermore, metabolomic analysis putatively identified coumarins scopoletin/isoscopoletin, and scopolin in A. tridentata leaf extracts and these metabolites linearly increased in response to galanthamine treatments. Overall, these data indicate that galanthamine is an allelopathic phytochemical and support the hypothesis that neurologically active compounds in plants help ensure plant survival and adaptation to environmental challenges.

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.

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.