Root-to-shoot translocation of alkaloids is dominantly suppressed in Nicotiana alata

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.

Evaluation of biotransformation capacity of transplastomic plants and hairy roots of Nicotiana tabacum expressing human cytochrome P450 2D6

Cytochrome P450 monooxygenases (CYPs) are important tools for regio- and stereoselective oxidation of target molecules or engineering of metabolic pathways. Functional heterologous expression of eukaryotic CYPs is often problematic due to their dependency on the specific redox partner and the necessity of correct association with the membranes for displaying enzymatic activity. Plant hosts offer advantages of accessibility of reducing partners and a choice of membranes to insert heterologous CYPs. For the evaluation of plant systems for efficient CYP expression, we established transplastomic plants and hairy root cultures of Nicotiana tabacum carrying the gene encoding human CYP2D6 with broad substrate specificity. The levels of CYP2D6 transcript accumulation and enzymatic activity were estimated and compared with the data of CYP2D6 transient expression in N. benthamiana. The relative level of CYP2D6 transcripts in transplastomic plants was 2-3 orders of magnitude higher of that observed after constitutive or transient expression from the nucleus. CYP2D6 expressed in chloroplasts converted exogenous synthetic substrate loratadine without the need for co-expression of the cognate CYP reductase. The loratadine conversion rate in transplastomic plants was comparable to that in N. benthamiana plants transiently expressing a chloroplast targeted CYP2D6 from the nucleus, but was lower than the value reported for transiently expressed CYP2D6 with the native endoplasmic reticulum signal-anchor sequence. Hairy roots showed the lowest substrate conversion rate, but demonstrated the ability to release the product into the culture medium. The obtained results illustrate the potential of plant-based expression systems for exploiting the enzymatic activities of eukaryotic CYPs with broad substrate specificities.

Biochemistry and Molecular Basis of Intracellular Flavonoid Transport in Plants

Flavonoids are a biochemically diverse group of specialized metabolites in plants that are derived from phenylalanine. While the biosynthesis of the flavonoid aglycone is highly conserved across species and well characterized, numerous species-specific decoration steps and their relevance remained largely unexplored. The flavonoid biosynthesis takes place at the cytosolic side of the endoplasmatic reticulum (ER), but accumulation of various flavonoids was observed in the central vacuole. A universal explanation for the subcellular transport of flavonoids has eluded researchers for decades. Current knowledge suggests that a glutathione S-transferase-like protein (ligandin) protects anthocyanins and potentially proanthocyanidin precursors during the transport to the central vacuole. ABCC transporters and to a lower extend MATE transporters sequester anthocyanins into the vacuole. Glycosides of specific proanthocyanidin precursors are sequestered through MATE transporters. A P-ATPase in the tonoplast and potentially other proteins generate the proton gradient that is required for the MATE-mediated antiport. Vesicle-mediated transport of flavonoids from the ER to the vacuole is considered as an alternative or additional route.

Important Roles of Key Genes and Transcription Factors in Flower Color Differences of Nicotianaalata

Nicotiana alata is an ornamental horticultural plant with a variety of flower colors and a long flowering period. The genes in four different colored N. alata (white, purple, red, and lemon green) were analyzed to explain the differences in flower color using transcriptomes. A total of 32 differential expression genes in the chlorophyll biosynthesis pathway and 41 in the anthocyanin biosynthesis pathway were identified. The enrichment analysis showed that the chlorophyll biosynthesis pathway and anthocyanin biosynthesis pathway play critical roles in the color differences of N. alata. The HEMA of the chlorophyll biosynthesis pathway was up-regulated in lemon green flowers. Compared with white flowers, in the red and purple flowers, F3H, F3'5'H and DFR were significantly up-regulated, while FLS was significantly down-regulated. Seventeen differential expression genes homologous to transcription factor coding genes were obtained, and the homologues of HY5, MYB12, AN1 and AN4 were also involved in flower color differences. The discovery of these candidate genes related to flower color differences is significant for further research on the flower colors formation mechanism and color improvements of N. alata.

Genetic Manipulation of Transcriptional Regulators Alters Nicotine Biosynthesis in Tobacco

The toxic alkaloid nicotine is produced in the roots of Nicotiana species and primarily accumulates in leaves as a specialized metabolite. A series of metabolic and transport genes involved in the nicotine pathway are coordinately upregulated by a pair of jasmonate-responsive AP2/ERF-family transcription factors, NtERF189 and NtERF199, in the roots of Nicotiana tabacum (tobacco). In this study, we explored the potential of manipulating the expression of these transcriptional regulators to alter nicotine biosynthesis in tobacco. The transient overexpression of NtERF189 led to alkaloid production in the leaves of Nicotiana benthamiana and Nicotiana alata. This ectopic production was further enhanced by co-overexpressing a gene encoding a basic helix-loop-helix-family MYC2 transcription factor. Constitutive and leaf-specific overexpression of NtERF189 increased the accumulation of foliar alkaloids in transgenic tobacco plants but negatively affected plant growth. By contrast, in a knockout mutant of NtERF189 and NtERF199 obtained through CRISPR/Cas9-based genome editing, alkaloid levels were drastically reduced without causing major growth defects. Metabolite profiling revealed the impact of manipulating the nicotine pathway on a wide range of nitrogen- and carbon-containing metabolites. Our findings provide insights into the biotechnological applications of engineering metabolic pathways by targeting transcription factors.

The genuine localization of indole alkaloids in Vinca minor and Catharanthus roseus

Based on the occurrence of indole alkaloids in so-called "chloroform leaf surface extracts", it was previously deduced that these alkaloids are present in the cuticle at the leaf surface of Catharanthus roseus and Vinca minor. As no symplastic markers were found in these extracts this deduction seemed to be sound. However, since chloroform is known to destroy biomembranes very rapidly, these data have to be judged with scepticism. We reanalyzed the alleged apoplastic localization of indole alkaloids by employing slightly acidic aqueous surface extracts and comparing the corresponding alkaloid patterns with those of aqueous total leaf extracts. Whereas in the "chloroform leaf surface extracts" all alkaloids are present in the same manner as in the total leaf extracts, no alkaloids occur in the aqueous leaf surface extracts. These results clearly show that chloroform had rapidly destroyed cell integrity, and the related extracts also contain the alkaloids genuinely accumulated within the protoplasm. The related decompartmentation was verified by the massively enhanced concentration of amino acids in aqueous surface extracts of chloroform treated leaves. Furthermore, the chloroform-induced cell disintegration was vividly visualized by confocal laser scanning microscopical analyses, which clearly displayed a strong decrease in the chlorophyll fluorescence in chloroform treated leaves. These findings unequivocally display that the indole alkaloids are not located in the apoplastic space, but exclusively are present symplastically within the cells of V. minor and C. roseus leaves. Accordingly, we have to presume that also other leaf surface extracts employing organic solvents have to be re-investigated.

Localization and Organization of Scopolamine Biosynthesis in Duboisia myoporoides R. Br

Tropane alkaloids (TAs), especially hyoscyamine and scopolamine, are important precursors for anticholinergic and antispasmodic drugs. Hyoscyamine and scopolamine are currently obtained at commercial scale from hybrid crosses of Duboisia myoporoides × Duboisia leichhardtii plants. In this study, we present a global investigation of the localization and organization of TA biosynthesis in a Duboisia myoporoides R. Br. wild-type line. The tissue-specific spatial distribution of TAs within D. myoporoides is presented, including quantification of the TAs littorine, 6-hydroxy hyoscyamine, hyoscyamine, scopolamine and, additionally, hyoscyamine aldehyde as well as scopolamine glucoside. Scopolamine (14.77 ± 5.03 mg g-1), and to a lesser extent hyoscyamine (3.01 ± 1.54 mg g-1) as well as 6-hydroxy hyoscyamine (4.35 ± 1.18 mg g-1), are accumulated in leaves during plant development, with the highest concentration of total TAs detected in 6-month-old plants. Littorine, an early precursor in TA biosynthesis, was present only in the roots (0.46 ± 0.07 mg g-1). During development, the spatial distribution of all investigated alkaloids changed due to secondary growth in the roots. Transcripts of pmt, tr-I and cyp80f1 genes, involved in early stages of TA biosynthesis, were found to be most abundant in the roots. In contrast, the transcript encoding hyoscyamine 6β-hydroxylase (h6h) was highest in the leaves of 3-month-old plants. This investigation presents the spatial distribution of biochemical components as well as gene expression profiles of genetic factors known to participate in TA biosynthesis in D. myoporoides. The results of this investigation may aid in future breeding or genetic enhancement strategies aimed at increasing the yields of TAs in these medicinally valuable plant species.

Quinolizidine Alkaloid Biosynthesis in Lupins and Prospects for Grain Quality Improvement

Quinolizidine alkaloids (QAs) are toxic secondary metabolites found within the genus Lupinus, some species of which are commercially important grain legume crops including Lupinus angustifolius (narrow-leafed lupin, NLL), L. luteus (yellow lupin), L. albus (white lupin), and L. mutabilis (pearl lupin), with NLL grain being the most largely produced of the four species in Australia and worldwide. While QAs offer the plants protection against insect pests, the accumulation of QAs in lupin grain complicates its use for food purposes as QA levels must remain below the industry threshold (0.02%), which is often exceeded. It is not well understood what factors cause grain QA levels to exceed this threshold. Much of the early work on QA biosynthesis began in the 1970-1980s, with many QA chemical structures well-characterized and lupin cell cultures and enzyme assays employed to identify some biosynthetic enzymes and pathway intermediates. More recently, two genes associated with these enzymes have been characterized, however, the QA biosynthetic pathway remains only partially elucidated. Here, we review the research accomplished thus far concerning QAs in lupin and consider some possibilities for further elucidation and manipulation of the QA pathway in lupin crops, drawing on examples from model alkaloid species. One breeding strategy for lupin is to produce plants with high QAs in vegetative tissues while low in the grain in order to confer insect resistance to plants while keeping grain QA levels within industry regulations. With the knowledge achieved on alkaloid biosynthesis in other plant species in recent years, and the recent development of genomic and transcriptomic resources for NLL, there is considerable scope to facilitate advances in our knowledge of QAs, leading to the production of improved lupin crops.

Cellular distribution of alkaloids and their translocation via phloem and xylem: the importance of compartment pH

The physico-chemical background of alkaloid allocation within plants is outlined and discussed exemplarily for pyrrolizidine alkaloids (PAs) and nicotine. The trigger for this discourse is the finding that, for example, PAs, which are taken up from the soil, are translocated in the xylem, whereas - when genuinely present in plants - they are allocated as N-oxides via phloem. Special emphasis is put on the impact of different pH values in certain compartments, as this entails significant changes in the relative lipophilic character of alkaloids: tertiary alkaloids diffuse readily through biomembranes, while the corresponding protonated alkaloids are retained in acidic compartments, i.e. vacuoles or xylem. Therefore, this phenomenon, well known as the 'ion trap mechanism', is also relevant for long-distance transport of alkaloids. Any efficient allocation of typical tertiary alkaloids within the phloem can thus be excluded. In contrast, due to their strongly increased hydrophilic properties, alkaloid-N-oxides or quarternary alkaloids cannot diffuse through biomembranes and, consequently, would be retained in the acidic xylem during translocation. The major aim of this paper is to sharpen the mind for the chemical peculiarities of alkaloids and to consider them adequately in forthcoming investigations on allocation of alkaloids.

(R)-nicotine biosynthesis, metabolism and translocation in tobacco as determined by nicotine demethylase mutants

Nicotine is a chiral compound and consequently exists as two enantiomers. Since (R)-nicotine consists of less than 0.5% of total nicotine pool in tobacco, few investigations relating to (R)-nicotine have been reported. However, previous studies of nicotine demethylases suggested there was substantial amount of (R)-nicotine at synthesis in the tobacco plant. In this study, the accumulation and translocation of (R)-nicotine in tobacco was analyzed. The accumulation of nicotine and its demethylation product the nornicotine enantiomers, were investigated in different tobacco plant parts and at different growth and post-harvest stages. Scion/rootstock grafts were used to separate the contributions of roots (source) from leaves (sink) to the final accumulation of nicotine and nornicotine in leaf tissue. The results indicate that 4% of nicotine is in the (R) form at synthesis in the root. After the majority of (R)-nicotine is selectively demethylated by CYP82E4, CYP82E5v2 and CYP82E10 in the root, nicotine and nornicotine are translocated to leaf, where more nicotine becomes demethylated. Depending on the CYP82E4 activity in senescing leaf, constant low (R)-nicotine remains in the tobacco leaf and variable nornicotine composition is produced. These results confirmed the enantioselectivity of three nicotine demethylases in planta, could be used to predict the changes of nicotine and nornicotine composition, and may facilitate demethylase discovery in the future.

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.

Non-functionalization of two CYP82E nicotine N-demethylase genes abolishes nornicotine formation in Nicotiana langsdorffii

Nornicotine is formed from nicotine by nicotine N-demethylase, a CYP82E family monooxygenase, and accumulates to high levels in some tobacco (Nicotiana tabacum) cultivars and many wild Nicotiana species. Nicotiana langsdorffii does not form nornicotine, whereas the closely related species N. alata accumulates this alkaloid abundantly. We show here that the two nicotine N-demethylase genes in N. langsdorffii have been inactivated by different molecular mechanisms. We identified four N. alata CYP82E genes that encode functional nicotine N-demethylases. In N. langsdorffii, however, one CYP82E gene encoding a functional enzyme was not expressed at all, whereas the other was weakly expressed but contained a one-nucleotide deletion in the first exon, yielding a truncated protein. Expression analysis of interspecific F(1) hybrids between N. alata and N. langsdorffii indicated that cis-acting polymorphisms abolish expression of the otherwise functional CYP82E gene in N. langsdorffii. Segregation analysis of tobacco alkaloids and individual CYP82E alleles in F(2) progeny revealed that duplicated CYP82E genes in both species are genetically linked, and provide genetic evidence that CYP82E genes are solely responsible for nornicotine formation in these wild Nicotiana species.

Variable nornicotine enantiomeric composition caused by nicotine demethylase CYP82E4 in tobacco leaf

Nornicotine is the demethylation product of nicotine and the precursor of tobacco-specific nitrosamine N'-nitrosonornicotine (NNN) in tobacco (Nicotiana tabacum L.). There is an inconsistent enantiomer fraction (EF) of nornicotine reported in the literature. The objective of this study was to explore possible mechanisms to account for the variable EF(nnic) in tobacco. A survey of tobacco with different demethylating capabilities confirmed that there was variable EF(nnic). Experiments of induction and inhibition of the major nicotine demethylase CYP82E4 activity in tobacco demonstrated that CYP82E4 selectively demethylated (S)-nicotine and resulted in different EF(nnic) in tobacco leaves. Results from plants with silenced demethylases by RNAi suggested that other demethylases selectively used (R)-nicotine and resulted in high EF(nnic). In summary, enantioselective demethylation likely plays an important role in contributing to a large and variable EF(nnic) observed in tobacco.