RNAi-mediated down-regulation of ornithine decarboxylase (ODC) leads to reduced nicotine and increased anatabine levels in transgenic Nicotiana tabacum L

Genetic regulation and manipulation of nicotine biosynthesis in tobacco: strategies to eliminate addictive alkaloids

Tobacco (Nicotiana tabacum L.) is a widely cultivated crop of the genus Nicotiana. Due to the highly addictive nature of tobacco products, tobacco smoking remains the leading cause of preventable death and disease. There is therefore a critical need to develop tobacco varieties with reduced or non-addictive nicotine levels. Nicotine and related pyridine alkaloids biosynthesized in the roots of tobacco plants are transported to the leaves, where they are stored in vacuoles as a defense against predators. Jasmonate, a defense-related plant hormone, plays a crucial signaling role in activating transcriptional regulators that coordinate the expression of downstream metabolic and transport genes involved in nicotine production. In recent years, substantial progress has been made in molecular and genomics research, revealing many metabolic and regulatory genes involved in nicotine biosynthesis. These advances have enabled us to develop tobacco plants with low or ultra-low nicotine levels through various methodologies, such as mutational breeding, genetic engineering, and genome editing. We review the recent progress on genetic manipulation of nicotine production in tobacco, which serves as an excellent example of plant metabolic engineering with profound social implications.

Advances in regulatory mechanism(s) and biotechnological approaches to modulate nicotine content in tobacco

More than 8 million deaths are caused by tobacco-related diseases every year. A staggering 1.2 million of those fatalities occur due to second-hand smoke exposure among non-smokers, but more than 7 million are due to direct tobacco use among smokers. Nicotine acts as the key ingredient triggering the addiction. The United States Food and Drug Administration (FDA) has classified more than 90 chemical components of tobacco and related smoke as hazardous or potentially hazardous leading to cancer, cardiovascular, respiratory, and reproductive disorders. Hence, reducing nicotine content has been the foremost objective to reduce health and death risks. Therefore, various biotechnological approaches for developing tobacco varieties with low nicotine concentrations are urgently required for the welfare of humankind. In recent years, numerous advancements have been made in nicotine-based tobacco research, suggesting regulatory components involved in nicotine biosynthesis and developing nicotine-less tobacco varieties through biotechnological approaches. This review highlights the various regulatory components and major approaches used to modulate nicotine content in tobacco cultivars.

Suppression of pyrrolidine ring biosynthesis and its effects on gene expression and subsequent accumulation of anatabine in leaves of tobacco (N. tabacum L.)

BACKGROUND

Anatabine, although being one of four major tobacco alkaloids, is never accumulated in high quantity in any of the naturally occurring species from the Nicotiana genus. Previous studies therefore focused on transgenic approaches to synthetize anatabine, most notably by generating transgenic lines with suppressed putrescine methyltransferase (PMT) activity. This led to promising results, but the global gene expression of plants with such distinct metabolism has not been analyzed. In the current study, we describe how these plants respond to topping and the downstream effects on alkaloid biosynthesis.

RESULTS

The surge in anatabine accumulation in PMT transgenic lines after topping treatment and its effects on gene expression changes were analyzed. The results revealed increases in expression of isoflavone reductase-like (A622) and berberine bridge-like enzymes (BBLs) oxidoreductase genes, previously shown to be crucial for the final steps of nicotine biosynthesis. We also observed significantly higher methylputrescine oxidase (MPO) expression in all plants subjected to topping treatment. In order to investigate if MPO suppression would have the same effects as that of PMT, we generated transgenic plants. These plants with suppressed MPO expression showed an almost complete drop in leaf nicotine content, whereas leaf anatabine was observed to increase by a factor of ~ 1.6X.

CONCLUSION

Our results are the first concrete evidence that suppression of MPO leads to decreased nicotine in favor of anatabine in tobacco roots and that this anatabine is successfully transported to tobacco leaves. Alkaloid transport in plants remains to be investigated to higher detail due to high variation of its efficiency among Nicotiana species and varieties of tobacco. Our research adds important step to better understand pyrrolidine ring biosynthesis and its effects on gene expression and subsequent accumulation of anatabine.

Hairy root culture: a potent method for improved secondary metabolite production of Solanaceous plants

Secondary metabolites synthesized by the Solanaceous plants are of major therapeutic and pharmaceutical importance, many of which are commonly obtained from the roots of these plants. 'Hairy roots', mirroring the same phytochemical pattern of the corresponding root of the parent plant with higher growth rate and productivity, are therefore extensively studied as an effective alternative for the in vitro production of these metabolites. Hairy roots are the transformed roots, generated from the infection site of the wounded plants with Agrobacterium rhizogenes. With their fast growth, being free from pathogen and herbicide contamination, genetic stability, and autotrophic nature for plant hormones, hairy roots are considered as useful bioproduction systems for specialized metabolites. Lately, several elicitation methods have been employed to enhance the accumulation of these compounds in the hairy root cultures for both small and large-scale production. Nevertheless, in the latter case, the cultivation of hairy roots in bioreactors should still be optimized. Hairy roots can also be utilized for metabolic engineering of the regulatory genes in the metabolic pathways leading to enhanced production of metabolites. The present study summarizes the updated and modern biotechnological aspects for enhanced production of secondary metabolites in the hairy root cultures of the plants of Solanaceae and their respective importance.

Update on the sources, pharmacokinetics, pharmacological action, and clinical application of anisodamine

Anisodamine is an anticholinergic drug extracted and isolated from the Anisodus tanguticus (Maxim.) Pascher of the Solanaceae family which is also a muscarinic receptor antagonist. Owing to the lack of natural sources of anisodamine, synthetic products are now used. Using ornithine and arginine as precursor compounds, putrescine is catalyzed by different enzymes and then undergoes a series of reactions to produce anisodamine. It has been used clinically to protect cardiac function and treat septic shock, acute pancreatitis, calculous renal colic, bronchial asthma, blood circulation disturbances, jaundice, analgesia, vertigo, acute poisoning, and other conditions.This review describes the relevant pharmacokinetic parameters. Anisodamine is poorly absorbed in the gastrointestinal tract, and it is not as effective as intravenous administration. For clinical medication, intravenous infusion should be used rather than rapid intravenous injection. With the advancement of research in recent years, the application scope of anisodamine has expanded, with significant developments and application values surging.This review systematically describes the sources, pharmacokinetics, pharmacological effects and clinical application of anisodamine, in order to provide a basis for clinical use.

Fractionation and Extraction Optimization of Potentially Valuable Compounds and Their Profiling in Six Varieties of Two Nicotiana Species

There is an increasingly urgent call to shift industrial processes from fossil fuel feedstock to sustainable bio-based resources. This change becomes of high importance considering new budget requirements for a carbon-neutral economy. Such a transformation can be driven by traditionally used plants that are able to produce large amounts of valuable biologically relevant secondary metabolites. Tobacco plants can play a leading role in providing value-added products in remote areas of the world. In this study, we propose a non-exhaustive list of compounds with potential economic interest that can be sourced from the tobacco plant. In order to optimize extraction methodologies, we first analyzed their physico-chemical properties using rapid solubility tests and high-resolution microfractionation techniques. Next, to identify an optimal extraction for a selected list of compounds, we compared 13 different extraction method-solvent combinations. We proceeded with profiling some of these compounds in a total of six varieties from Nicotiana tabacum and Nicotiana rustica species, identifying the optimal variety for each. The estimated expected yields for each of these compounds demonstrate that tobacco plants can be a superior source of valuable compounds with diverse applications beyond nicotine. Among the most interesting results, we found high variability of anatabine content between species and varieties, ranging from 287 to 1699 µg/g. In addition, we found that CGA (1305 µg/g) and rutin (7910 µg/g) content are orders of magnitude lower in the Burley variety as compared to all others.

A de novo regulation design shows an effectiveness in altering plant secondary metabolism

Introduction

Transcription factors (TFs) and cis-regulatory elements (CREs) control gene transcripts involved in various biological processes. We hypothesize that TFs and CREs can be effective molecular tools for De Novo regulation designs to engineer plants.

Objectives

We selected two Arabidopsis TF types and two tobacco CRE types to design a De Novo regulation and evaluated its effectiveness in plant engineering.

Methods

G-box and MYB recognition elements (MREs) were identified in four Nicotiana tabacum JAZs (NtJAZs) promoters. MRE-like and G-box like elements were identified in one nicotine pathway gene promoter. TF screening led to select Arabidopsis Production of Anthocyanin Pigment 1 (PAP1/MYB) and Transparent Testa 8 (TT8/bHLH). Two NtJAZ and two nicotine pathway gene promoters were cloned from commercial Narrow Leaf Madole (NL) and KY171 (KY) tobacco cultivars. Electrophoretic mobility shift assay (EMSA), cross-linked chromatin immunoprecipitation (ChIP), and dual-luciferase assays were performed to test the promoter binding and activation by PAP1 (P), TT8 (T), PAP1/TT8 together, and the PAP1/TT8/Transparent Testa Glabra 1 (TTG1) complex. A DNA cassette was designed and then synthesized for stacking and expressing PAP1 and TT8 together. Three years of field trials were performed by following industrial and GMO protocols. Gene expression and metabolic profiling were completed to characterize plant secondary metabolism.

Results

PAP1, TT8, PAP1/TT8, and the PAP1/TT8/TTG1 complex bound to and activated NtJAZ promoters but did not bind to nicotine pathway gene promoters. The engineered red P + T plants significantly upregulated four NtJAZs but downregulated the tobacco alkaloid biosynthesis. Field trials showed significant reduction of five tobacco alkaloids and four carcinogenic tobacco specific nitrosamines in most or all cured leaves of engineered P + T and PAP1 genotypes.

Conclusion

G-boxes, MREs, and two TF types are appropriate molecular tools for a De Novo regulation design to create a novel distant-pathway cross regulation for altering plant secondary metabolism.

Putrescine biosynthetic pathways modulate root growth differently in tomato seedlings grown under different N sources

The biosynthesis of putrescine is mainly driven by arginine decarboxylase (ADC) and ornithine decarboxylase (ODC). Hence, in this study, we generated independent ADC and ODC transgenic silenced tomato lines (SilADC and SilODC, respectively) to test the effect of defective ADC and ODC gene expression on root development under nitrate (NN) or ammonium (NA) conditions. The results showed that SilODC seedlings displayed an increase in ADC expression that led to polyamine accumulation, suggesting a compensatory effect of ADC. However, this effect was not observed in SilADC seedlings. These pathways are involved in different growth processes. The SilADC seedlings showed an increase in fresh weight, shoot length, lateral root number and shoot:root ratio under the NN source and an enhancement in fresh weight, and shoot and root length under NA conditions. However, SilODC seedlings displayed greater weight and shoot length under the NN source, whereas a decrease in lateral root density was found under NA conditions. Moreover, two overexpressed ODC lines were generated to check the relevance of the compensatory effect of the ADC pathway when ODC was silenced. These overexpressed lines showed not only an enhancement of almost all the studied growth parameters under both N sources but also an amelioration of ammonium syndrome under NA conditions. Together, these results reflect the importance of both pathways in plant growth, particularly ODC silencing, which requires compensation by ADC induction.

CRISPR/Cas9-Mediated Genome Editing in Comfrey (Symphytum officinale) Hairy Roots Results in the Complete Eradication of Pyrrolizidine Alkaloids

Comfrey (Symphytum officinale) is a medicinal plant with anti-inflammatory, analgesic, and proliferative properties. However, its pharmaceutical application is hampered by the co-occurrence of toxic pyrrolizidine alkaloids (PAs) in its tissues. Using a CRISPR/Cas9-based approach, we introduced detrimental mutations into the hss gene encoding homospermidine synthase (HSS), the first pathway-specific enzyme of PA biosynthesis. The resulting hairy root (HR) lines were analyzed for the type of gene-editing effect that they exhibited and for their homospermidine and PA content. Inactivation of only one of the two hss alleles resulted in HRs with significantly reduced levels of homospermidine and PAs, whereas no alkaloids were detectable in HRs with two inactivated hss alleles. PAs were detectable once again after the HSS-deficient HRs were fed homospermidine confirming that the inability of these roots to produce PAs was only attributable to the inactivated HSS and not to any unidentified off-target effect of the CRISPR/Cas9 approach. Further analyses showed that PA-free HRs possessed, at least in traces, detectable amounts of homospermidine, and that the PA patterns of manipulated HRs were different from those of control lines. These observations are discussed with regard to the potential use of such a CRISPR/Cas9-mediated approach for the economical exploitation of in vitro systems in a medicinal plant and for further studies of PA biosynthesis in non-model plants.

Genetic attenuation of alkaloids and nicotine content in tobacco (Nicotiana tabacum)

MAIN CONCLUSION

The role of six alkaloid biosynthesis genes in the process of nicotine accumulation in tobacco was investigated. Downregulation of ornithine decarboxylase, arginine decarboxylase, and aspartate oxidase resulted in viable plants with a significantly lower nicotine content. Attenuation of nicotine accumulation in Nicotiana tabacum was addressed upon the application of RNAi technologies. The approach entailed a downregulation in the expression of six different alkaloid biosynthesis genes encoding upstream enzymes that are thought to function in the pathway of alkaloid and nicotine biosynthesis. Nine different RNAi constructs were designed to lower the expression level of the genes that encode the enzymes arginine decarboxylase, agmatine deiminase, aspartate oxidase, arginase, ornithine decarboxylase, and SAM synthase. Agrobacterium-based transformation of tobacco leaves was applied, and upon kanamycin selection, T0 and subsequently T1 generation seeds were produced. Mature T1 plants in the greenhouse were topped to prevent flowering and leaf nos. 3 and 4 below the topping point were tested for transcript levels and product accumulation. Down-regulation in arginine decarboxylase, aspartate oxidase, and ornithine decarboxylase consistently resulted in lower levels of nicotine in the leaves of the corresponding plants. Transformants with the aspartate oxidase RNAi construct showed the lowest nicotine level in the leaves, which varied from below the limit of quantification (20 μg per g dry leaf weight) to 1.3 mg per g dry leaf weight. The amount of putrescine, the main polyamine related to nicotine biosynthesis, showed a qualitative correlation with the nicotine content in the arginine decarboxylase and ornithine decarboxylase RNAi-expressing transformants. A putative early senescence phenotype and lower viability of the older leaves was observed in some of the transformant lines. The results are discussed in terms of the role of the above-mentioned genes in the alkaloid biosynthetic pathway and may serve to guide efforts to attenuate nicotine content in tobacco leaves.

Transcriptome analysis reveals key genes involved in the regulation of nicotine biosynthesis at early time points after topping in tobacco (Nicotiana tabacum L.)

BACKGROUND

Nicotiana tabacum is an important economic crop. Topping, a common agricultural practice employed with flue-cured tobacco, is designed to increase leaf nicotine contents by increasing nicotine biosynthesis in roots. Many genes are found to be differentially expressed in response to topping, particularly genes involved in nicotine biosynthesis, but comprehensive analyses of early transcriptional responses induced by topping are not yet available. To develop a detailed understanding of the mechanisms regulating nicotine biosynthesis after topping, we have sequenced the transcriptomes of Nicotiana tabacum roots at seven time points following topping.

RESULTS

Differential expression analysis revealed that 4830 genes responded to topping across all time points. Amongst these, nine gene families involved in nicotine biosynthesis and two gene families involved in nicotine transport showed significant changes during the immediate 24 h period following topping. No obvious preference to the parental species was detected in the differentially expressed genes (DEGs). Significant changes in transcript levels of nine genes involved in nicotine biosynthesis and phytohormone signal transduction were validated by qRT-PCR assays. 549 genes encoding transcription factors (TFs), found to exhibit significant changes in gene expression after topping, formed 15 clusters based on similarities of their transcript level time-course profiles. 336 DEGs involved in phytohormone signal transduction, including genes functionally related to the phytohormones jasmonic acid, abscisic acid, auxin, ethylene, and gibberellin, were identified at the earliest time point after topping.

CONCLUSIONS

Our research provides the first detailed analysis of the early transcriptional responses to topping in N. tabacum, and identifies excellent candidates for further detailed studies concerning the regulation of nicotine biosynthesis in tobacco roots.

Increased Leaf Nicotine Content by Targeting Transcription Factor Gene Expression in Commercial Flue-Cured Tobacco (Nicotiana tabacum L.)

Nicotine, the most abundant pyridine alkaloid in cultivated tobacco (Nicotiana tabacum L.), is a potent inhibitor of insect and animal herbivory and a neurostimulator of human brain function. Nicotine biosynthesis is controlled developmentally and can be induced by abiotic and biotic stressors via a jasmonic acid (JA)-mediated signal transduction mechanism involving members of the APETALA 2/ethylene-responsive factor (AP2/ERF) and basic helix-loop-helix (bHLH) transcription factor (TF) families. AP2/ERF and bHLH TFs work combinatorically to control nicotine biosynthesis and its subsequent accumulation in tobacco leaves. Here, we demonstrate that overexpression of the tobacco NtERF32, NtERF221/ORC1, and NtMYC2a TFs leads to significant increases in nicotine accumulation in T2 transgenic K326 tobacco plants before topping. Up to 9-fold higher nicotine production was achieved in transgenics overexpressing NtERF221/ORC1 under the control of a constitutive GmUBI3 gene promoter compared to wild-type plants. The constitutive 2XCaMV35S promoter and a novel JA-inducible 4XGAG promoter were less effective in driving high-level nicotine formation. Methyljasmonic acid (MeJA) treatment further elevated nicotine production in all transgenic lines. Our results show that targeted manipulation of NtERF221/ORC1 is an effective strategy for elevating leaf nicotine levels in commercial tobacco for use in the preparation of reduced risk tobacco products for smoking replacement therapeutics.

Improvement of phytochemical production by plant cells and organ culture and by genetic engineering

Plants display an amazing ability to synthesize a vast array of secondary metabolites that are an inexhaustible source of phytochemicals, bioactive molecules some of which impact the human health. Phytochemicals present in medicinal herbs and spices have long been used as natural remedies against illness. Plant tissue culture represents an alternative to whole plants as a source of phytochemicals. This approach spares agricultural land that can be used for producing food and other raw materials, thus favoring standardized phytochemical production regardless of climatic adversities and political events. Over the past 20 years, different strategies have been developed to increase the synthesis and the extraction of phytochemicals from tissue culture often obtaining remarkable results. Moreover, the availability of genomics and metabolomics tools, along with improved recombinant methods related to the ability to overexpress, silence or disrupt one or more genes of the pathway of interest promise to open new exciting possibilities of metabolic engineering. This review provides a general framework of the cellular and molecular tools developed so far to enhance the yield of phytochemicals. Additionally, some emerging topics such as the culture of cambial meristemoid cells, the selection of plant cell following the expression of genes encoding human target proteins, and the bioextraction of phytochemicals from plant material have been addressed. Altogether, the herein described techniques and results are expected to improve metabolic engineering tools aiming at improving the production of phytochemicals of pharmaceutical and nutraceutical interest.

Nicotine Biosynthesis inNicotiana: A Metabolic Overview

Alkaloids are important compounds found in Nicotiana plants, essential in plant defense against herbivores. The main alkaloid of Nicotiana tabacum, nicotine, is produced in roots and translocated to the leaves. Nicotine is formed by a pyrrolidine and a pyridine ring in a process involving several enzymes. The pyridine ring of nicotine is derived from nicotinic acid, whereas the pyrrolidine ring originates from polyamine putrescine metabolism. After synthesis in root cortical cells, a set of transporters is known to transport nicotine upward to the aerial part and store it in leaf vacuoles. Moreover, nicotine can be metabolized in leaves, giving rise to nornicotine through the N-demethylation process. Some Nicotiana wild species produce acyltransferase enzymes, which allow the plant to make N-acyl-nornicotine, an alkaloid with more potent insecticidal properties than nicotine. However, although we can find a wealth of information about the alkaloid production in Nicotiana spp., our understanding about nicotine biosynthesis, transport, and metabolism is still incomplete. This review will summarize these pathways on the basis on recent literature, as well as highlighting questions that need further investigation.

Polyamines delay leaf maturation in low-alkaloid tobacco varieties

The development of low-alkaloid (LA) tobacco varieties is an important target in the tobacco breeding industry. However, LA Burley 21 plants, in which the Nic1 and Nic2 loci controlling nicotine biosynthesis are deleted, are characterized by impaired leaf maturation that leads to poor leaf quality before and after curing. Polyamines are involved in key developmental, physiological, and metabolic processes in plants, and act as anti-senescence and anti-ripening regulators. We investigated the role of polyamines in tobacco leaf maturation by analyzing the free and conjugated polyamine fractions in the leaves and roots of four Burley 21 varieties: NA (normal alkaloid levels, wild-type control), HI (high intermediates, nic2 -), LI (low intermediates, nic1 -), and LA (nic1 - nic2 -). The pool of conjugated polyamines increased with plant age in the roots and leaves of all four varieties, but the levels of free and conjugated putrescine and spermidine were higher in the LI and LA plants than NA controls. The increase in the polyamine content correlated with delayed maturation and senescence, i.e., LA plants with the highest polyamine levels showed the most severe impaired leaf maturation phenotype, characterized by higher chlorophyll content and more mesophyll cells per unit leaf area. Treatment of LA plants with inhibitors of polyamine biosynthesis and/or the growth regulator Ethephon® reduced accumulation of polyamines, achieving a partial amelioration of the LA phenotype. Our data show that the regulation of polyamine homeostasis is strongly disrupted in LA plants, and that free and conjugated polyamines contribute to the observed impairment of leaf maturation.

Transcriptomic analysis reveals overdominance playing a critical role in nicotine heterosis in Nicotiana tabacum L

BACKGROUND

As a unique biological phenomenon, heterosis has been concerned with the superior performance of the heterosis than either parents. Despite several F1 hybrids, containing supernal nicotine content, had been discovered and applied to heterosis utilization in Nicotiana tabacum L., nevertheless, the potential molecular mechanism revealing nicotine heterosis has not been illustrated clearly.

RESULT

Phenotypically, the F1 hybrids (Vall6 × Basma) show prominent heterosis in nicotine content by 3 years of field experiments. Transcriptome analysis revealed that genes participating in nicotine anabolism (ADC, PMT, MPO, QPT, AO, QS, QPT, A622, BBLs) and nicotine transport (JAT2, MATE1 and 2, NUP1 and 2) showed an upregulated expression in the hybrid, a majority of which demonstrated an overdominant performance. RT-PCR confirmed that nicotine anabolism was induced in the hybrid.

CONCLUSIONS

These findings strongly suggest that nicotine synthesis and transport efficiency improved in hybrid and overdominance at gene-expression level played a critical role in heterosis of nicotine metabolism.

Transcriptome-wide analysis of jasmonate-treated BY-2 cells reveals new transcriptional regulators associated with alkaloid formation in tobacco

Jasmonates (JAs) are well-known regulators of stress, defence, and secondary metabolism in plants, with JA perception triggering extensive transcriptional reprogramming, including both activation and/or repression of entire metabolic pathways. We performed RNA sequencing based transcriptomic profiling of tobacco BY-2 cells before and after treatment with methyl jasmonate (MeJA) to identify novel transcriptional regulators associated with alkaloid formation. A total of 107,140 unigenes were obtained through de novo assembly, and at least 33,213 transcripts (31%) encode proteins, in which 3419 transcription factors (TFs) were identified, representing 72 gene families, as well as 840 transcriptional regulators (TRs) distributed among 19 gene families. After MeJA treatment BY-2 cells, 7260 differentially expressed transcripts were characterised, which include 4443 MeJA-upregulated and 2817 MeJA-downregulated genes. Of these, 227 TFs/TRs in 36 families were specifically upregulated, and 102 TFs/TRs in 38 families were downregulated in MeJA-treated BY-2 cells. We further showed that the expression of 12 ethylene response factors and four basic helix-loop-helix factors increased at the transcriptional level after MeJA treatment in BY-2 cells and displayed specific expression patterns in nic mutants with or without MeJA treatments. Our data provide a catalogue of transcripts of tobacco BY-2 cells and benefit future study of JA-modulated regulation of secondary metabolism in tobacco.

Transcriptome response and developmental implications of RNAi-mediated ODC knockdown in tobacco

Polyamines (PAs) are ubiquitously present polycationic compounds that play a critical role in various growth and developmental processes including stress responses in plants. Yet, their specific functions and mode of action remain largely unknown. In the present study, we have targeted tobacco ornithine decarboxylase gene (ODC) by RNA interference to modulate cellular PA levels and study the effects at different developmental time points. Down-regulation of ODC resulted in significant physiological and morphological anomalies including reduced leaf size, reduced chlorophyll and carotene content, decreased abiotic stress tolerance, early onset of senescence, delayed flowering, partial male and female sterility, reduced seed setting, delayed seed germination, reduced seed viability, and poor in vitro regeneration response from leaf explants. Also, for the first time, microarray analysis has been attempted to study genome-wide gene expression changes in response to lowered PA titers in an ODC knockdown line. A number of transcription factors, auxin- and ethylene-responsive genes, stress-induced genes, lignin-biosynthesis genes, photosynthesis-related genes, senescence-associated genes, membrane proteins, and protein kinases were found to be affected, suggesting a probable list of PA-responsive genes. Transcriptome analysis has also indicated many genes, which could directly or indirectly be responsible for regulating the PA metabolic pathway. Various phenotypic changes observed upon ODC knockdown along with the identification of a number of gene targets means it is a step forward in envisaging possible mechanisms of PA action and for assigning them with specific roles in various developmental processes they are known to be a part of.

Effects of down-regulating ornithine decarboxylase upon putrescine-associated metabolism and growth in Nicotiana tabacum L

Transgenic plants of Nicotiana tabacum L. homozygous for an RNAi construct designed to silence ornithine decarboxylase (ODC) had significantly lower concentrations of nicotine and nornicotine, but significantly higher concentrations of anatabine, compared with vector-only controls. Silencing of ODC also led to significantly reduced concentrations of polyamines (putrescine, spermidine and spermine), tyramine and phenolamides (caffeoylputrescine and dicaffeoylspermidine) with concomitant increases in concentrations of amino acids ornithine, arginine, aspartate, glutamate and glutamine. Root transcript levels of S-adenosyl methionine decarboxylase, S-adenosyl methionine synthase and spermidine synthase (polyamine synthesis enzymes) were reduced compared with vector controls, whilst transcript levels of arginine decarboxylase (putrescine synthesis), putrescine methyltransferase (nicotine production) and multi-drug and toxic compound extrusion (alkaloid transport) proteins were elevated. In contrast, expression of two other key proteins required for alkaloid synthesis, quinolinic acid phosphoribosyltransferase (nicotinic acid production) and a PIP-family oxidoreductase (nicotinic acid condensation reactions), were diminished in roots of odc-RNAi plants relative to vector-only controls. Transcriptional and biochemical differences associated with polyamine and alkaloid metabolism were exacerbated in odc-RNAi plants in response to different forms of shoot damage. In general, apex removal had a greater effect than leaf wounding alone, with a combination of these injury treatments producing synergistic responses in some cases. Reduced expression of ODC appeared to have negative effects upon plant growth and vigour with some leaves of odc-RNAi lines being brittle and bleached compared with vector-only controls. Together, results of this study demonstrate that ornithine decarboxylase has important roles in facilitating both primary and secondary metabolism in Nicotiana.

Transgenic and mutation-based suppression of a berberine bridge enzyme-like (BBL) gene family reduces alkaloid content in field-grown tobacco

Motivation exists to develop tobacco cultivars with reduced nicotine content for the purpose of facilitating compliance with expected tobacco product regulations that could mandate the lowering of nicotine levels per se, or the reduction of carcinogenic alkaloid-derived tobacco specific nitrosamines (TSNAs). A berberine bridge enzyme-like (BBL) gene family was recently characterized for N. tabacum and found to catalyze one of the final steps in pyridine alkaloid synthesis for this species. Because this gene family acts downstream in the nicotine biosynthetic pathway, it may represent an attractive target for genetic strategies with the objective of reducing alkaloid content in field-grown tobacco. In this research, we produced transgenic doubled haploid lines of tobacco cultivar K326 carrying an RNAi construct designed to reduce expression of the BBL gene family. Field-grown transgenic lines carrying functional RNAi constructs exhibited average cured leaf nicotine levels of 0.684%, in comparison to 2.454% for the untransformed control. Since numerous barriers would need to be overcome to commercialize transgenic tobacco cultivars, we subsequently pursued a mutation breeding approach to identify EMS-induced mutations in the three most highly expressed isoforms of the BBL gene family. Field evaluation of individuals possessing different homozygous combinations of truncation mutations in BBLa, BBLb, and BBLc indicated that a range of alkaloid phenotypes could be produced, with the triple homozygous knockout genotype exhibiting greater than a 13-fold reduction in percent total alkaloids. The novel source of genetic variability described here may be useful in future tobacco breeding for varied alkaloid levels.

Transcriptomic analysis reveals that reactive oxygen species and genes encoding lipid transfer protein are associated with tobacco hairy root growth and branch development

The hairy root, a specialized plant tissue that emerges from a cell transformed with transfer DNA (T-DNA) from Agrobacterium rhizogenes, can be used to study root biology and utilized in biotechnological applications. The rol genes are known to participate in the generation of hairy roots; however, the means by which the rol genes contribute to the initiation and the maintenance of hairy roots remains largely unknown. We demonstrated that tobacco hairy roots lacking either rolB or rolC exhibited fewer branch roots and lost their growth ability in long-term subculture. Additionally, a microarray analysis revealed that the expression of several genes encoding lipid transfer proteins (LTP) and reactive oxygen species (ROS)-related genes was significantly suppressed in rolB- or rolC-deficient hairy roots. We found that hairy root clones that exhibited greater branching expressed higher levels of RolB or RolC and the genes encoding LTP identified from the microarray. When hairy roots were compared with intact roots, the expression levels of LTP-encoding genes were dramatically different. In addition, ROS were present at lower levels in rolB- and rolC-deficient hairy roots. We therefore suggest that upregulating LTP and increasing the level of ROS is important for hairy root growth.

NtERF32: a non-NIC2 locus AP2/ERF transcription factor required in jasmonate-inducible nicotine biosynthesis in tobacco

Nicotine biosynthesis in tobacco (Nicotiana tabacum L.) is highly regulated by jasmonic acid (JA). Two nuclear loci, A and B (renamed NIC1 and NIC2) have been identified that mediate JA-inducible nicotine formation and total alkaloid accumulation. NIC2 was recently shown to be a cluster of seven genes encoding Apetala2/Ethylene-Response Factor (AP2/ERF)-domain transcription factors (TFs) in Group IX of the tobacco AP2/ERF family. Here we report the characterization of several NtERF TF genes that are not within the NIC2 locus, but required for methyl JA (MeJA)-induced nicotine biosynthesis. Expression of NtERF1, NtERF32, and NtERF121 is rapidly induced (<30 min) by MeJA treatment. All three of these TFs specifically bind the GCC box-like element of the GAG motif required for MeJA-induced transcription of NtPMT1a, a gene encoding putrescine N-methyltransferase, the first committed step in the synthesis of the nicotine pyrrolidine ring. Ectopic overexpression of NtERF32 increases expression of NtPMT1a in vivo and elevates total alkaloid contents, whereas RNAi-mediated knockdown of NtERF32 reduces the mRNA levels of multiple genes in the nicotine biosynthetic pathway including NtPMT1a and quinolinate phosphoribosyltransferase (NtQPT2), and lowers nicotine and total alkaloid levels. We conclude that NtERF32 and related ERF genes are important non-NIC2 locus associated transcriptional regulators of nicotine and total alkaloid formation.

Molecular genetics of alkaloid biosynthesis in Nicotiana tabacum

Alkaloids represent an extensive group of nitrogen-containing secondary metabolites that are widely distributed throughout the plant kingdom. The pyridine alkaloids of tobacco (Nicotiana tabacum L.) have been the subject of particularly intensive investigation, driven largely due to the widespread use of tobacco products by society and the role that nicotine (16) (see Fig. 1) plays as the primary compound responsible for making the consumption of these products both pleasurable and addictive. In a typical commercial tobacco plant, nicotine (16) comprises about 90% of the total alkaloid pool, with the alkaloids nornicotine (17) (a demethylated derivative of nicotine), anatabine (15) and anabasine (5) making up most of the remainder. Advances in molecular biology have led to the characterization of the majority of the genes encoding the enzymes directly responsible the biosynthesis of nicotine (16) and nornicotine (17), while notable gaps remain within the anatabine (15) and anabasine (5) biosynthetic pathways. Several of the genes involved in the transcriptional regulation and transport of nicotine (16) have also been elucidated. Investigations of the molecular genetics of tobacco alkaloids have not only provided plant biologists with insights into the mechanisms underlying the synthesis and accumulation of this important class of plant alkaloids, they have also yielded tools and strategies for modifying the tobacco alkaloid composition in a manner that can result in changing the levels of nicotine (16) within the leaf, or reducing the levels of a potent carcinogenic tobacco-specific nitrosamine (TSNA). This review summarizes recent advances in our understanding of the molecular genetics of alkaloid biosynthesis in tobacco, and discusses the potential for applying information accrued from these studies toward efforts designed to help mitigate some of the negative health consequences associated with the use of tobacco products.

Fifty years of alkaloid biosynthesis in Phytochemistry

An overview is presented of the studies related to the biosynthesis of alkaloids published in Phytochemistry in the past 50 years.

RNAi-mediated down-regulation of ornithine decarboxylase (ODC) impedes wound-stress stimulation of anabasine synthesis in Nicotiana glauca

Unlike most Nicotiana species, leaf tissues of the globally significant weed Nicotiana glauca Grah. (Argentinian tree tobacco) contains anabasine as the main component of its alkaloid pool, with concentrations typically increasing several fold in response to wounding of plants. The Δ(1)-piperidinium ring of anabasine is synthesised from cadaverine, via the decarboxylation of lysine, however the identity of the protein catalysing this reaction remains unknown. Recent studies indicate that ornithine decarboxylase (ODC), an enzyme involved in the synthesis of the diamine putrescine, may also possess LDC activity. Previously we found that ODC transcript is markedly up-regulated in leaves of N. glauca in response to wounding. In order to examine the role of ODC in the synthesis of anabasine in N. glauca, transcript levels were constitutively down-regulated in hairy root cultures and transgenic plants via the introduction of a CaMV35S driven ODC-RNAi construct. In addition to the anticipated marked reduction in nicotine concentrations, demonstrating that the ODC-RNAi construct was functioning in vivo, we observed that N. glauca ODC-RNAi hairy root cultures had a significantly diminished capacity to elevate anabasine synthesis in response to treatment with the wound-associated hormone methyl jasmonate, when compared to vector-only controls. We observed also that ODC-RNAi transgenic plants had significantly reduced ability to increase anabasine concentrations following removal of the plant apex. We conclude that ODC does have an important role in enabling N. glauca to elevate levels of anabasine in response to wound-associated stress.

Structure and expression of the quinolinate phosphoribosyltransferase (QPT) gene family in Nicotiana

Synthesis of wound-inducible pyridine alkaloids is characteristic of species in the genus Nicotiana. The enzyme quinolinate phosphoribosyltransferase (QPT) plays a key role in facilitating the availability of precursors for alkaloid synthesis, in addition to its ubiquitous role in enabling NAD(P)(H) synthesis. In a previous study, we reported that Nicotiana tabacum L. var. NC 95 possesses a QPT RFLP pattern similar to its model paternal progenitor species, Nicotiana tomentosiformis Goodsp. Here we show that although some varieties of N. tabacum (e.g. NC 95 and LAFC 53) possess QPT genomic contributions from only its paternal progenitor species, this is not the case for many other N. tabacum varieties (e.g. Xanthi, Samsun, Petite Havana SR1 and SC 58) where genomic QPT sequences from both diploid progenitor species have been retained. We also report that QPT is encoded by duplicate genes (designated QPT1 and QPT2) not only in N. tabacum, but also its model progenitor species Nicotiana sylvestris Speg. and Comes and N. tomentosiformis as well as in the diploid species Nicotiana glauca Graham. Previous studies have demonstrated that the N. tabacum QPT2 gene encodes a functional enzyme via complementation of a nadC(-)Escherichia coli mutant. Using a similar experimental approach here, we demonstrate that the N. tabacum QPT1 gene also encodes a functional QPT protein. We observe too that QPT2 is the predominate transcript present in both alkaloid and non-alkaloid synthesising tissues in N. tabacum and that promoter regions of both QPT1 and QPT2 are able to produce GUS activity in reproductive tissues. In N. tabacum and in several other Nicotiana species tested, QPT2 transcript levels increase following wounding or methyl jasmonate treatment whilst QPT1 transcript levels remain largely unaltered by these treatments. Together with conclusions from recently published studies involving functional interaction of MYC2-bHLH and specific ERF-type and transcription factors with QPT2-promoter sequences from N. tabacum, our results suggest that whilst both members of the QPT gene family can contribute to the transcript pool in both alkaloid producing and non-producing tissues, it is QPT2 that is regulated in association with inducible defensive pyridine alkaloid synthesis in species across the genus Nicotiana.

Use of the wound-inducible NtQPT2 promoter from Nicotiana tabacum for production of a plant-made vaccine

The wound-inducible quinolinate phosphoribosyl transferase promoter from Nicotiana tabacum (NtQPT2) was assessed for its capacity to produce B-subunit of the heat-labile toxin (LTB) from enterotoxigenic Escherichia coli in transgenic plant tissues. Comparisons were made with the widely used and constitutive Cauliflower Mosaic Virus 35S (CaMV35S) promoter. The NtQPT2 promoter produced somewhat lower average concentrations of LTB protein per unit weight of hairy root tissue but allowed better growth thereby producing similar or higher overall average yields of LTB per culture batch. Transgenic tobacco plants containing the NtQPT2-LTB construct contained LTB protein in roots but not leaves. Moreover, wounding NtQPT2-LTB transgenic plants, by removal of apices, resulted in an approximate 500% increase in LTB levels in roots when analysed several days later. CaMV35S-LTB transgenic plants contained LTB protein in leaves and roots but wounding made no difference to their LTB content.

Plant ornithine decarboxylase is not post-transcriptionally feedback regulated by polyamines but can interact with a cytosolic ribosomal protein S15 polypeptide

The formation of putrescine by ornithine decarboxylase (ODC) is a key regulatory step in polyamine biosynthesis in metazoa and fungi. Excess polyamines post-transcriptionally induce the synthesis of a unique non-competitive protein inhibitor of ODC, termed antizyme. Binding of antizyme to an ODC monomer subunit results in enzymatic inhibition, rapid ubiquitin-independent degradation of ODC by the 26S proteasome and recycling of antizyme. Plants possess an additional route for synthesizing putrescine via arginine decarboxylase (ADC). No homologue of ODC antizyme has been detected in plant genomes but several biochemical studies have reported plant ODC antizyme proteins of 9 and 16 kDa. Here we show that plant cells grown in liquid culture do not exhibit any substantial post-transcriptional, polyamine-responsive feedback regulation of ODC or ADC. However, using the yeast two hybrid system, a plant ODC-binding polypeptide was detected: the C-terminal 84-87 amino acids of cytosolic ribosomal protein (rp) S15. The Arabidopsis rpS15 polypeptide interacted specifically with plant ODC but not with human or Saccharomyces cerevisiae ODCs. Co-expression of either the full length or C-terminal rpS15 polypeptides with a plant ODC in yeast did not reduce ODC enzymatic activity. Only the full length mRNA encoding rpS15 was detected in Arabidopsis cells, suggesting that the C-terminal rpS15 polypeptide is encoded by a low abundance mRNA or the polypeptide is not physiologically relevant in plants. These results confirm the primacy of S-adenosylmethionine decarboxylase as the key regulatory enzyme in plant polyamine biosynthesis.