NtMYB305a binds to the jasmonate-responsive GAG region of NtPMT1a promoter to regulate nicotine biosynthesis

HY5 and COP1 function antagonistically in the regulation of nicotine biosynthesis in Nicotiana tabacum

Nicotine constitutes approximately 90% of the total alkaloid content in leaves within the Nicotiana species, rendering it the most prevalent alkaloid. While the majority of genes responsible for nicotine biosynthesis express in root tissue, the influence of light on this process through shoot-to-root mobile ELONGATED HYPOCOTYL 5 (HY5) has been recognized. CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), a key regulator of light-associated responses, known for its role in modulating HY5 accumulation, remains largely unexplored in its relationship to light-dependent nicotine accumulation. Here, we identified NtCOP1, a COP1 homolog in Nicotiana tabacum, and demonstrated its ability to complement the cop1-4 mutant in Arabidopsis thaliana at molecular, morphological, and biochemical levels. Through the development of NtCOP1 overexpression (NtCOP1OX) plants, we observed a significant reduction in nicotine and flavonol content, inversely correlated with the down-regulation of nicotine and phenylpropanoid pathway. Conversely, CRISPR/Cas9-based knockout mutant plants (NtCOP1CR) exhibited an increase in nicotine levels. Further investigations, including yeast-two hybrid assays, grafting experiments, and Western blot analyses, revealed that NtCOP1 modulates nicotine biosynthesis by targeting NtHY5, thereby impeding its transport from shoot-to-root. We conclude that the interplay between HY5 and COP1 functions antagonistically in the light-dependent regulation of nicotine biosynthesis in tobacco.

A P450 superfamily member NtCYP82C4 promotes nicotine biosynthesis in Nicotiana tabacum

In plants, cytochrome P450s are monooxygenase that play key roles in the synthesis and degradation of intracellular substances. In tobacco, the majority of studies examining the P450 superfamily have concentrated on the CYP82E subfamily, where multiple family members function as demethylases, facilitating the synthesis of nornicotine. In this study, NtCYP82C4, a tobacco P450 superfamily member, was identified from a gene-edited tobacco mutant that nicotine biosynthesis in tobacco leaves is evidently reduced. Compared to the wild-type plants, the knockout of NtCYP82C4 resulted in a significantly lower nicotine content and biomass in tobacco leaves. Transcriptome and metabolome analyses indicated that the knockout of NtCYP82C4 inhibites secondary metabolic processes in tobacco plants, leading to the accumulation of some important precursors in the nicotine synthesis process, including aspartic acid and nicotinic acid, and increases nitrogen partitioning associated with those processes such as amino acid synthesis and utilization. It is speculated that NtCYP82C4 may function as an important catalase downstream of the nicotine synthesis. Currently, most of the steps and enzymes involved in the nicotine biosynthesis process in tobacco have been elucidated. Here, our study deepens the current understanding of nicotine biosynthesis process and provides new enzyme targets for nicotine synthesis in tobacco plants.

Transcription factors NtNAC028 and NtNAC080 form heterodimers to regulate jasmonic acid biosynthesis during leaf senescence in Nicotiana tabacum

Plant senescence, as a highly integrated developmental stage, involves functional degeneration and nutrient redistribution. NAM/ATAF1/CUC (NAC) transcription factors orchestrate various senescence-related signals and mediate the fine-tuning underlying plant senescence. Previous data revealed that knockout of either NtNAC028 or NtNAC080 leads to delayed leaf senescence in tobacco (Nicotiana tabacum), which implies that NtNAC028 and NtNAC080 play respective roles in the regulation of leaf senescence, although they share 91.87% identity with each other. However, the mechanism underlying NtNAC028- and NtNAC080-regulated leaf senescence remains obscure. Here, we determined that NtNAC028 and NtNAC080 activate a putative jasmonic acid (JA) biosynthetic gene, NtLOX3, and enhance the JA level in vivo. We found that NtNAC028 and NtNAC080 interact with each other and themselves through their NA-terminal region. Remarkably, only the dimerization between NtNAC028 and NtNAC080 stimulated the transcriptional activation activity, but not the DNA binding activity of this heterodimer on NtLOX3. Metabolome analysis indicated that overexpression of either NtNAC028 or NtNAC080 augments both biosynthesis and degradation of nicotine in the senescent stages. Thus, we conclude that NtNAC028 cooperates with NtNAC080 and forms a heterodimer to enhance NtLOX3 expression and JA biosynthesis to trigger the onset of leaf senescence and impact secondary metabolism in tobacco.

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.

Mutation in shoot-to-root mobile transcription factor, ELONGATED HYPOCOTYL 5, leads to low nicotine levels in tobacco

Tobacco remains one of the most commercially important crops due to the parasympathomimetic alkaloid nicotine used in cigarettes. Most genes involved in nicotine biosynthesis are expressed in root tissues; however, their light-dependent regulation has not been studied. Here, we identified the ELONGATED HYPOCOTYL 5 homolog, NtHY5, from Nicotiana tabacum and demonstrated that NtHY5 could complement the Arabidopsis thaliana hy5 mutant at molecular, morphological and biochemical levels. We report the development of CRISPR/Cas9-based knockout mutant plants of tobacco, NtHY5CR, and show down-regulation of the nicotine and phenylpropanoid pathway genes leading to a significant reduction in nicotine and flavonol content, whereas NtHY5 overexpression (NtHY5OX) plants show the opposite effect. Grafting experiments using wild-type, NtHY5CR, and NtHY5OX indicated that NtHY5 moves from shoot-to-root to regulate nicotine biosynthesis in the root tissue. Shoot HY5, directly or through enhancing expression of the root HY5, promotes nicotine biosynthesis by binding to light-responsive G-boxes present in the NtPMT, NtQPT and NtODC promoters. We conclude that the mobility of HY5 from shoot-to-root regulates light-dependent nicotine biosynthesis. The CRISPR/Cas9-based mutants developed, in this study; with low nicotine accumulation in leaves could help people to overcome their nicotine addiction and the risk of death.

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.

Proteomics and Co-expression Network Analysis Reveal the Importance of Hub Proteins and Metabolic Pathways in Nicotine Synthesis and Accumulation in Tobacco (Nicotiana tabacum L.)

Nicotine is a unique alkaloid present in tobacco that is widely used in cigarettes and in the agricultural, chemical, and pharmaceutical industries. However, the research on nicotine is mostly limited to its synthesis pathways, and only a few studies have explored the effects of other metabolic pathways on nicotine precursors. Regulating the nicotine content in tobacco can greatly promoting the application of nicotine in other fields. In this study, we performed global data-independent acquisition proteomics analysis of four tobacco varieties. Of the four varieties, one had high nicotine content and three had a low nicotine content. A total of 31,259 distinct peptides and 6,018 proteins across two samples were identified. A total of 45 differentially expressed proteins (DEPs) co-existed in the three comparison groups and were mainly involved in the transport and metallic processes of the substances. Most DEPs were enriched in the biosynthesis of secondary metals, glutathione metabolism, carbon metabolism, and glycolysis/gluconeogenesis. In addition, the weighted gene co-expression network analysis identified an expression module closely related to the nicotine content (Brown, r = 0.74, P = 0.006). Gene Ontology annotation and Kyoto Encyclopaedia of Genes and Genomes enrichment analysis showed that the module proteins were mainly involved in the synthesis and metabolism of nicotine precursors such as arginine, ornithine aspartate, proline, and glutathione. The increased levels of these precursors lead to the synthesis and accumulation of nicotine in plants. More importantly, these proteins regulate nicotine synthesis by affecting the formation of putrescine, which is the core intermediate product in nicotine anabolism. Our results provide a reference for tobacco variety selection with a suitable nicotine content and regulation of the nicotine content. Additionally, the results highlight the importance of other precursor metabolism in nicotine synthesis.