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.

Natural and induced variations in transcriptional regulator genes result in low-nicotine phenotypes in tobacco.

In tobacco, the homologous ETHYLENE RESPONSE FACTOR (ERF) transcription factors ERF199 and ERF189 coordinate the transcription of multiple metabolic genes involved in nicotine biosynthesis. Natural alleles at the NIC1 and NIC2 loci greatly affect alkaloid accumulation and overlap with ERF199 and ERF189 in the tobacco genome, respectively. In this study, we identified several low-nicotine tobacco varieties lacking ERF199 or ERF189 from a tobacco germplasm collection. We characterized the sequence of these new nic1 and nic2 alleles, as well as the previously defined alleles nic1-1 and nic2-1. Moreover, we examined the influence of different nic alleles on alkaloid contents and expression levels of genes related to nicotine biosynthesis. We also demonstrated that the deletion of a distal genomic region attenuates ERF199 expression, resulting in a moderately negative effect on the alkaloid phenotype. Our study provides new insights into the regulation of nicotine biosynthesis and novel genetic resources to breed low-nicotine tobacco.

Genetic divergence in transcriptional regulators of defense metabolism: insight into plant domestication and improvement.

KEY MESSAGE: A number of mutational changes in transcriptional regulators of defense metabolism have occurred during plant domestication and improvement. Plant domestication and improvement entail genetic changes that underlie divergence in development and metabolism, providing a tremendous model of biological evolution. Plant metabolism produces numerous specialized alkaloids, terpenoids, phenolics, and cyanogenic glucosides with indispensable roles in defense against herbivory and microbial infection. Many compounds toxic or deterrent to predators have been eliminated through domestication and breeding. Series of genes involved in defense metabolism are coordinately regulated by transcription factors that specifically recognize cis-regulatory elements in promoter regions of downstream target genes. Recent developments in DNA sequencing technologies and genomic approaches have facilitated studies of the metabolic and genetic changes in chemical defense that have occurred via human-mediated selection, many of which result from mutations in transcriptional regulators of defense metabolism. In this article, we review such examples in almond (Prunus dulcis), cucumber (Cucumis sativus), pepper (Capsicum spp.), potato (Solanum tuberosum), quinoa (Chenopodium quinoa), sorghum (Sorghum bicolor), and related species and discuss insights into the evolution and regulation of metabolic pathways for specialized defense compounds.

Tomato roots secrete tomatine to modulate the bacterial assemblage of the rhizosphere.

Saponins are the group of plant specialized metabolites which are widely distributed in angiosperm plants and have various biological activities. The present study focused on α-tomatine, a major saponin present in tissues of tomato (Solanum lycopersicum) plants. α-Tomatine is responsible for defense against plant pathogens and herbivores, but its biological function in the rhizosphere remains unknown. Secretion of tomatine was higher at the early growth than the green-fruit stage in hydroponically grown plants, and the concentration of tomatine in the rhizosphere of field-grown plants was higher than that of the bulk soil at all growth stages. The effects of tomatine and its aglycone tomatidine on the bacterial communities in the soil were evaluated in vitro, revealing that both compounds influenced the microbiome in a concentration-dependent manner. Numerous bacterial families were influenced in tomatine/tomatidine-treated soil as well as in the tomato rhizosphere. Sphingomonadaceae species, which are commonly observed and enriched in tomato rhizospheres in the fields, were also enriched in tomatine- and tomatidine-treated soils. Moreover, a jasmonate-responsive ETHYLENE RESPONSE FACTOR 4 mutant associated with low tomatine production caused the root-associated bacterial communities to change with a reduced abundance of Sphingomonadaceae. Taken together, our results highlight the role of tomatine in shaping the bacterial communities of the rhizosphere and suggest additional functions of tomatine in belowground biological communication.

CRISPR/Cas9-mediated disruption of the PYRROLIDINE KETIDE SYNTHASE gene reduces the accumulation of tropane alkaloids in Atropa belladonna hairy roots.

Tropane alkaloids, including clinically important hyoscyamine and scopolamine, are produced in the roots of medicinal plant species, such as Atropa belladonna, from the Solanaceae family. Recent molecular and genomic approaches have advanced our understanding of the metabolic enzymes involved in tropane alkaloid biosynthesis. A noncanonical type III polyketide synthase, pyrrolidine ketide synthase (PYKS) catalyzes a two-step decarboxylative reaction, which involves imine-ketide condensation indispensable to tropane skeleton construction. In this study, we generated pyks mutant A. belladonna hairy roots via CRISPR/Cas9-mediated genome editing and analyzed the metabolic consequences of the loss of PYKS activity on tropane alkaloids, providing insights into a crucial role of the scaffold-forming reaction in the biosynthetic pathway.

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.

JRE4 is a master transcriptional regulator of defense-related steroidal glycoalkaloids in tomato.

Steroidal glycoalkaloids (SGAs) are specialized anti-nutritional metabolites that accumulate in Solanum lycopersicum (tomato) and Solanum tuberosum (potato). A series of SGA biosynthetic genes is known to be upregulated in Solanaceae species by jasmonate-responsive Ethylene Response Factor transcription factors, including JRE4 (otherwise known as GAME9), but the exact regulatory significance in planta of each factor has remained unaddressed. Here, via TILLING-based screening of an EMS-mutagenized tomato population, we isolated a JRE4 loss-of-function line that carries an amino acid residue missense change in a region of the protein important for DNA binding. In this jre4 mutant, we observed downregulated expression of SGA biosynthetic genes and decreased SGA accumulation. Moreover, JRE4 overexpression stimulated SGA production. Further characterization of jre4 plants revealed their increased susceptibility to the generalist herbivore Spodoptera litura larvae. This susceptibility illustrates that herbivory resistance is dependent on JRE4-mediated defense responses, which include SGA accumulation. Ethylene treatment attenuated the jasmonate-mediated JRE4 expression induction and downstream SGA biosynthesis in tomato leaves and hairy roots. Overall, this study indicated that JRE4 functions as a primary master regulator of SGA biosynthesis, and thereby contributes toward plant defense against chewing insects.

Expression of a tobacco nicotine biosynthesis gene depends on the JRE4 transcription factor in heterogenous tomato.

The jasmonate-responsive transcription factor ERF189 in tobacco (Nicotiana tabacum) and its ortholog JRE4 in tomato (Solanum lycopersicum) regulate a series of biosynthetic genes involved in the nicotine and steroidal glycoalkaloid pathways. In tobacco, QUINOLINATE PHOSPHORIBOSYL TRANSFERASE 2 (NtQPT2) is regulated by ERF189; however, we found that the tomato QPT gene is not regulated by JRE4. Here, we explored whether and how NtQPT2 is regulated in a heterogenous tomato host. We used a NtQPT2 promoter-driven reporter gene to examine the cell type-specific and jasmonate-induced expression of this gene in transgenic tomato hairy roots. The downregulation of the reporter in the jre4 loss-of-function tomato mutant and its transactivation by JRE4 in transient expression experiments suggested that JRE4, like its ortholog ERF189 in tobacco, activates the NtQPT2 promoter in tomato. We discuss the evolution of QPT2 in the Nicotiana lineage, which mainly occurred through mutational changes in the promoter that altered the control of the functionally conserved transcription factors.

Genomic Insights into the Evolution of the Nicotine Biosynthesis Pathway in Tobacco.

In tobacco (Nicotiana tabacum), nicotine is the predominant alkaloid. It is produced in the roots and accumulated mainly in the leaves. Jasmonates play a central signaling role in damage-induced nicotine formation. The genome sequence of tobacco provides us an almost complete inventory of structural and regulatory genes involved in nicotine pathway. Phylogenetic and expression analyses revealed a series of structural genes of the nicotine pathway, forming a regulon, under the control of jasmonate-responsive ETHYLENE RESPONSE FACTOR (ERF) transcription factors. The duplication of NAD and polyamine metabolic pathways and the subsequent recruitment of duplicated primary metabolic genes into the nicotine biosynthesis regulon were suggested to be the drivers for pyridine and pyrrolidine ring formation steps early in the pathway. Transcriptional regulation by ERF and cooperatively acting MYC2 transcription factors are corroborated by the frequent occurrence of cognate cis-regulatory elements of the factors in the promoter regions of the downstream structural genes. The allotetraploid tobacco has homologous clusters of ERF genes on different chromosomes, which are possibly derived from two ancestral diploids and include either nicotine-controlling ERF189 or ERF199 A large chromosomal deletion was found within one allele of the nicotine-controlling NICOTINE2 locus, which is part of one of the ERF gene clusters, and which has been used to breed tobacco cultivars with a low-nicotine content.

Jasmonate-induced biosynthesis of steroidal glycoalkaloids depends on COI1 proteins in tomato.

In tomato, perception of jasmonates by a receptor complex, which includes the F-box protein CORONATINE INSENSITIVE 1 (COI1), elicits biosynthesis of defensive steroidal glycoalkaloids (SGAs) via a jasmonate-responsive ERF transcription factor, JRE4/GAME9. Although JRE4 is upregulated by jasmonate and induces the expression of many metabolic genes involved in SGA biosynthesis, it is not known whether JRE4 alone is sufficient for increased SGA biosynthesis upon activation of jasmonate signaling. Here, we show that application of methyl jasmonate induces the expression of JRE4 and SGA biosynthesis genes in leaves and hairy roots of wild-type tomato, but not in jasmonic acid insensitive 1 (jai1), a loss-of-function mutant allele of the tomato COI1 gene. Induced overexpression of JRE4 increased the expression of SGA biosynthesis genes in transgenic hairy roots of both wild-type tomato and the jai1 mutant, suggesting that JRE4 is the primary transcription factor that functions downstream of the jasmonate signaling pathway.

Jasmonate-Responsive ERF Transcription Factors Regulate Steroidal Glycoalkaloid Biosynthesis in Tomato.

Steroidal glycoalkaloids (SGAs) are cholesterol-derived specialized metabolites produced in species of the Solanaceae. Here, we report that a group of jasmonate-responsive transcription factors of the ETHYLENE RESPONSE FACTOR (ERF) family (JREs) are close homologs of alkaloid regulators in Cathranthus roseus and tobacco, and regulate production of SGAs in tomato. In transgenic tomato, overexpression and dominant suppression of JRE genes caused drastic changes in SGA accumulation and in the expression of genes for metabolic enzymes involved in the multistep pathway leading to SGA biosynthesis, including the upstream mevalonate pathway. Transactivation and DNA-protein binding assays demonstrate that JRE4 activates the transcription of SGA biosynthetic genes by binding to GCC box-like elements in their promoters. These JRE-binding elements occur at significantly higher frequencies in proximal promoter regions of the genes regulated by JRE genes, supporting the conclusion that JREs mediate transcriptional co-ordination of a series of metabolic genes involved in SGA biosynthesis.

Tobacco nicotine uptake permease regulates the expression of a key transcription factor gene in the nicotine biosynthesis pathway.

The down-regulation of a tobacco (Nicotiana tabacum) plasma membrane-localized nicotine uptake permease, NUP1, was previously reported to reduce total alkaloid levels in tobacco plants. However, it was unclear how this nicotine transporter affected the biosynthesis of the alkaloid nicotine. When NUP1 expression was suppressed in cultured tobacco cells treated with jasmonate, which induces nicotine biosynthesis, the NICOTINE2-locus transcription factor gene ETHYLENE RESPONSE FACTOR189 (ERF189) and its target structural genes, which function in nicotine biosynthesis and transport, were strongly suppressed, resulting in decreased total alkaloid levels. Conversely, NUP1 overexpression had the opposite effect. In these experiments, the expression levels of the MYC2 transcription factor gene and its jasmonate-inducible target gene were not altered. Inhibiting tobacco alkaloid biosynthesis by suppressing the expression of genes encoding enzymes in the nicotine pathway did not affect the expression of ERF189 and other nicotine pathway genes, indicating that ERF189 is not regulated by cellular alkaloid levels. Suppressing the expression of jasmonate signaling components in cultured tobacco cells showed that NUP1 acts downstream of the CORONATINE INSENSITIVE1 receptor and MYC2, but upstream of ERF189. These results suggest that although jasmonate-activated expression of MYC2 induces the expression of both NUP1 and ERF189, expression of ERF189 may actually be mediated by NUP1. Furthermore, NUP1 overexpression in tobacco plants inhibited the long-range transport of nicotine from the roots to the aerial parts. Thus, NUP1 not only mediates the uptake of tobacco alkaloids into root cells, but also positively controls the expression of ERF189, a key gene in the biosynthesis of these alkaloids.

Molecular evolution of N-methylputrescine oxidase in tobacco.

Biosynthesis of nicotine in tobacco requires N-methylputrescine oxidase (MPO), which belongs to the copper-containing amine oxidase superfamily. Previous studies identified tobacco MPO1 and its close homolog NtDAO1 (formerly called MPO2), of which MPO1 has been shown preferentially to oxidize N-methylated amines. We show here that NtDAO1, as well as a homologous Arabidopsis diamine oxidase (DAO), accept non-N-methylated amines more efficiently than their corresponding N-methylated amines. MPO1 is coordinately regulated with other nicotine biosynthesis genes with regard to COI1-MYC2-dependent jasmonate induction and its dependence on nicotine-specific ERF transcription factors, whereas NtDAO1 is constitutively expressed at low basal levels in tobacco plants. Both MPO1 and NtDAO1 are targeted to peroxisomes by their C-terminal motifs, and the peroxisomal localization of MPO1 is required for it to function in nicotine biosynthesis in jasmonate-elicited cultured tobacco cells. Restricted occurrence of the MPO subfamily in Nicotiana and Solanum indicates that, during the formation of the Solanaceae, MPO has evolved from a DAO, which functions in polyamine catabolism within peroxisomes, by optimizing substrate preference and gene expression patterns to be suitable for alkaloid formation.

Divergent DNA-binding specificities of a group of ETHYLENE RESPONSE FACTOR transcription factors involved in plant defense.

Transcription factors (TFs) recognize target DNA sequences with distinct DNA-binding domains (DBDs). The DBD of Arabidopsis (Arabidopsis thaliana) ETHYLENE RESPONSE FACTOR1 (AtERF1) uses three consecutive ß-strands to recognize a GCC-containing sequence, but tobacco (Nicotiana tabacum) ERF189 and periwinkle (Catharanthus roseus) Octadecanoid-derivative Responsive Catharanthus AP2-domain protein3 (ORCA3) of the same TF subgroup appear to target similar but divergent DNA sequences. Here, we examined how DNA-binding specificities of these TFs have diverged in each plant lineage to regulate distinct defense metabolisms. Extensive mutational analyses of these DBDs suggest that two modes of protein-DNA interactions independently contribute to binding specificity and affinity. Substitution of a conserved arginine to lysine in the first ß-strand of ERF189 relaxes its interaction with the second GC pair of the GCC DNA sequence. By contrast, an increased number of basic amino acids in the first two ß-strands of ORCA3 allows this TF to recognize more than one GCC-related target, presumably via increased electrostatic interactions with the negatively charged phosphate backbone of DNA. Divergent DNA-binding specificities of the ERFs may have arisen through mutational changes of these amino acid residues.

The phosphorylated pathway of serine biosynthesis affects sperm, embryo, and sporophyte development, and metabolism in Marchantia polymorpha.

Serine metabolism is involved in various biological processes. Here we investigate primary functions of the phosphorylated pathway of serine biosynthesis in a non-vascular plant Marchantia polymorpha by analyzing knockout mutants of MpPGDH encoding 3-phosphoglycerate dehydrogenase in this pathway. Growth phenotypes indicate that serine from the phosphorylated pathway in the dark is crucial for thallus growth. Sperm development requires serine from the phosphorylated pathway, while egg formation does not. Functional MpPGDH in the maternal genome is necessary for embryo and sporophyte development. Under high CO2 where the glycolate pathway of serine biosynthesis is inhibited, suppressed thallus growth of the mutants is not fully recovered by exogenously-supplemented serine, suggesting the importance of serine homeostasis involving the phosphorylated and glycolate pathways. Metabolomic phenotypes indicate that the phosphorylated pathway mainly influences the tricarboxylic acid cycle, the amino acid and nucleotide metabolism, and lipid metabolism. These results indicate the importance of the phosphorylated pathway of serine biosynthesis in the dark, in the development of sperm, embryo, and sporophyte, and metabolism in M. polymorpha.

Clustered transcription factor genes regulate nicotine biosynthesis in tobacco.

Tobacco (Nicotiana tabacum) synthesizes nicotine and related pyridine alkaloids in the root, and their synthesis increases upon herbivory on the leaf via a jasmonate-mediated signaling cascade. Regulatory NIC loci that positively regulate nicotine biosynthesis have been genetically identified, and their mutant alleles have been used to breed low-nicotine tobacco varieties. Here, we report that the NIC2 locus, originally called locus B, comprises clustered transcription factor genes of an ethylene response factor (ERF) subfamily; in the nic2 mutant, at least seven ERF genes are deleted altogether. Overexpression, suppression, and dominant repression experiments using transgenic tobacco roots showed both functional redundancy and divergence among the NIC2-locus ERF genes. These transcription factors recognized a GCC-box element in the promoter of a nicotine pathway gene and specifically activated all known structural genes in the pathway. The NIC2-locus ERF genes are expressed in the root and upregulated by jasmonate with kinetics that are distinct among the members. Thus, gene duplication events generated a cluster of highly homologous transcription factor genes with transcriptional and functional diversity. The NIC2-locus ERFs are close homologs of ORCA3, a jasmonate-responsive transcriptional activator of indole alkaloid biosynthesis in Catharanthus roseus, indicating that the NIC2/ORCA3 ERF subfamily was recruited independently to regulate jasmonate-inducible secondary metabolism in distinct plant lineages.

Downregulation of a cluster of genes encoding nitrate transporter 1/peptide transporter family proteins in tomato with a mutated JRE4 transcription factor.

A group of anti-nutritional specialized metabolites called steroidal glycoalkaloids (SGAs) are produced in Solanum species such as tomato, potato, and eggplant. The transcription factor JASMONATE-RESPONSIVE ETHYLENE RESPONSE FACTOR 4 (JRE4) regulates many SGA biosynthesis genes in tomato and potato. Here we report that the expression of a cluster of genes encoding nitrate transporter 1/peptide transporter family (NPF) members is downregulated in the jre4-1 loss-of-function tomato mutant, which has a low-SGA phenotype compared to the wild type. NPFs are a large family of plant membrane transporters that transport a wide range of substrates, including specialized metabolites. We found that the JRE4-regulated NPF genes are induced by the defense-related phytohormone jasmonate. Conversely, jasmonate-mediated induction of gene expression was attenuated by ethylene treatment of the leaves. The co-regulation of the NPF genes with SGA biosynthesis genes by JRE4 suggests that NPF transporters are involved in the SGA pathway.

Induced production of specialized steroids by transcriptional reprogramming in Petunia hybrida.

Plants produce specialized metabolites with defensive properties that are often synthesized through the coordinated regulation of metabolic genes by transcription factors in various biological contexts. In this study, we investigated the regulatory function of the transcription factor PhERF1 from petunia (Petunia hybrida), which belongs to a small group of ETHYLENE RESPONSE FACTOR (ERF) family members that regulate the biosynthesis of bioactive alkaloids and terpenoids in various plant lineages. We examined the effects of transiently overexpressing PhERF1 in petunia leaves on the transcriptome and metabolome, demonstrating the production of a class of specialized steroids, petuniolides, and petuniasterones in these leaves. We also observed the activation of many metabolic genes, including those involved in sterol biosynthesis, as well as clustered genes that encode new metabolic enzymes, such as cytochrome P450 oxidoreductases, 2-oxoglutarate-dependent dioxygenases, and BAHD acyltransferases. Furthermore, we determined that PhERF1 transcriptionally induces downstream metabolic genes by recognizing specific cis-regulatory elements in their promoters. This study highlights the potential of evolutionarily conserved transcriptional regulators to induce the production of specialized products through transcriptional reprogramming.

Recruitment of a duplicated primary metabolism gene into the nicotine biosynthesis regulon in tobacco.

Gene duplication is a powerful source of phenotypic diversity in plants, but the molecular mechanisms that generate new functions in duplicated genes are not fully documented. Here, we analyzed how duplicated genes encoding quinolinate phosphoribosyltransferase (QPT), an enzyme involved in the synthesis of nicotinamide adenine dinucleotide (NAD) and the pyridine moiety of nicotine, are regulated by the jasmonate-responsive transcriptional factor ERF189 that functions critically for nicotine biosynthesis in tobacco (Nicotiana tabacum). The tobacco genome contains duplicated QPT genes; QPT1 is expressed at a constitutive basal level, whereas QPT2 is regulated coordinately with other structural genes involved in nicotine biosynthesis, in terms of tissue specificity, jasmonate induction, and regulation by ERF189. The binding-site specificity of ERF189 was defined as 5'-(A/C)GC(A/C)(A/C)NCC-3' by using a characterized tobacco putrescine N-methyltransferase promoter, and was then used to search for potential binding sites in the QPT promoters. Assays involving in vitro DNA binding, transient transactivation, and transgenic hairy roots revealed that the QPT2 promoter contains three functional ERF189-binding sites, which individually confer incremental ERF189-mediated activation to the promoter. The QPT1 promoter is not bound and regulated by ERF189. These results indicate that one copy of the duplicated QPT genes was recruited to a tobacco alkaloid regulon by evolving multiple target cis-regulatory elements of ERF189 in its promoter, to cope with an increased metabolic demand for pyridine precursors during active alkaloid biosynthesis.

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

In tobacco (Nicotiana tabacum), nicotine and related pyridine alkaloids are produced in the root, and then transported to the aerial parts where these toxic chemicals function as part of chemical defense against insect herbivory. Although a few tobacco transporters have been recently reported to take up nicotine into the vacuole from the cytoplasm or into the cytoplasm from the apoplast, it is not known how the long-range translocation of tobacco alkaloids between organs is controlled. Nicotiana langsdorffii and N. alata are closely related species of diploid Nicotiana section Alatae, but the latter does not accumulate tobacco alkaloids in the leaf. We show here that N. alata does synthesize alkaloids in the root, but lacks the capacity to mobilize the root-borne alkaloids to the aerial parts. Interspecific grafting experiments between N. alata and N. langsdorffii indicate that roots of N. alata are unable to translocate alkaloids to their shoot system. Interestingly, genetic studies involving interspecific hybrids between N. alata and N. langsdorffii and their self-crossed or back-crossed progeny showed that the non-translocation phenotype is dominant over the translocation phenotype. These results indicate that a mechanism to retain tobacco alkaloids within the root organ has evolved in N. alata, which may represent an interesting strategy to control the distribution of secondary products within a whole plant.