Scientists Learn How Tobacco Plants Make Nicotine to Enable Low-Nicotine Tobacco

Biochemists at the University of York, working with international collaborators, have completed the enzymatic pathway tobacco plants use to assemble nicotine from simple chemical building blocks. The discovery resolves a biological puzzle that has stood since chemists first extracted nicotine from tobacco foliage in 1828.

The findings clarify the genetic and biochemical mechanisms responsible for producing the addictive alkaloid. Beyond its implications for agricultural tobacco cultivation, the research provides molecular tools to remove nicotine from tobacco species utilized as production platforms for pharmaceuticals, vaccines, and diagnostic proteins.

“It is a big moment in plant science and biochemistry that we now have the answer we have been chasing for more than 200 years!” said Dr. Benjamin Lichman from the Centre for Novel Agricultural Products at the University of York.

The Two-Century Search for Nicotine Biosynthesis

Nicotine functions primarily as an alkaloid defense mechanism in the wild. Tobacco plants produce the compound in their roots and transport it to their leaves to deter herbivorous insects and grazing animals. When consumed by humans, the molecule binds directly to nicotinic acetylcholine receptors in the nervous system, functioning as a stimulant and driving physical dependency.

For decades, researchers understood the basic chemical precursors required for nicotine formation. Structural assays confirmed that the molecule consists of two distinct heterocyclic rings joined by a single carbon-carbon bond:

• A Pyridine Ring: Derived from nicotinic acid (a form of vitamin B3).
• A Pyrrolidine Ring: Formed from the amino acid-derived cation N-methylpyrrolinium.

Despite knowing the source materials, scientists could not identify the catalytic machinery responsible for bonding the two rings. Attempts to observe the direct condensation of these molecules in laboratory environments repeatedly stalled because nicotinic acid remains chemically stable under standard biological conditions. Without an activation step, the energy barrier to bind the rings remained too high for known plant enzymes to execute.

The Mechanism: Cryptic Activating Glucosylation

The University of York team discovered that the tobacco plant bypasses this chemical barrier by employing an unexpected intermediate: glucose. The plant temporarily binds a glucose sugar molecule to nicotinic acid, destabilizing the compound and lowering the activation energy required for subsequent enzymatic reactions.

This biological strategy, termed cryptic activating glucosylation, acts as a temporary molecular handle. Once the glucose activates the precursor, other enzymes couple the ring to N-methylpyrrolinium. As soon as the structural bond forms, another enzyme cleaves the sugar from the complex, leaving behind a finished nicotine molecule.

Because the glucose molecule detaches before the final product exits the enzymatic cluster, it left no chemical footprint in the finished alkaloid. This transient nature explained why generations of analytical chemists failed to isolate the intermediate stages of the pathway.

“The puzzle of how tobacco plants produce nicotine, however, has been around since the late 1820s, when nicotine was first extracted from the plants,” Lichman said. “With this new knowledge we can remove or repurpose the nicotine that is produced naturally by the plant and create better biotechnology tools, and there is also exciting potential for the future to adapt tobacco’s nicotine forming system to make useful pharmaceutical compounds.”

The Four-Enzyme Cascade

To verify the reaction sequence, the research team isolated and purified four distinct enzymes from tobacco tissues, reconstituting the entire metabolic pathway outside the living cell.

Using high-resolution X-ray crystallography, first author Benjamin Schwabe resolved the three-dimensional structures of NaGR and NicGS at atomic resolution. The structural models revealed that NicGS physically orients the reaction intermediates within its binding pocket to direct the spatial configuration of the final bond.

This structural control is biologically decisive. Biological molecules often exist as chiral enantiomers—mirror-image versions of the same chemical formula. Tobacco plants produce almost exclusively (S)-nicotine, the optical isomer that binds effectively to mammalian neurological receptors. When the researchers tested the pathway without NicGS, the system produced an uncontrolled racemic mixture of both left- and right-handed isomers, proving that NicGS dictates the stereochemical precision of natural tobacco.

Verification in Living Plant Systems

Following cell-free laboratory synthesis, the researchers validated the enzymatic pathway within living plant tissue. They introduced the genes encoding the four enzymes into Nicotiana benthamiana, a wild Australian tobacco relative widely utilized in agricultural and pharmaceutical research.

The engineered plants produced labeled nicotine molecules that mirrored natural plant profiles. Furthermore, by knocking out individual genes within the cascade, the researchers verified each step:

• Suppressing NicGH caused the plants to accumulate stable, glucose-bound nicotine precursors without producing free nicotine.
• Omitting NicGS disrupted the stereospecificity of the pathway, verifying its regulatory role in living cellular environments.

Improving Tobacco Platforms for Molecular Farming

The identification of these four enzymes carries direct utility for modern biomanufacturing. While commercial tobacco remains synonymous with cigarettes, plant biologists frequently use tobacco species for molecular farming—using rapidly growing plant tissues as bioreactors to synthesize medical proteins, monoclonal antibodies, and vaccine antigens.

Nicotiana benthamiana is favored in molecular farming because it yields high volumes of biomass within weeks and readily accepts transient expression vectors. However, native nicotine production presents a persistent manufacturing hurdle. The alkaloid is toxic to humans at pharmaceutical concentrations and co-purifies with target proteins, requiring costly, multi-stage filtration processes to isolate therapeutic molecules.

“Tobacco plants can be used in biotechnology as platforms for producing vaccines or other pharmaceutical products, but it is plagued by the presence of nicotine which contaminates the products and requires processing to remove it,” Lichman explained.

By using targeted gene-editing tools like CRISPR to deactivate the genes encoding NaGT or NicGS, agricultural scientists can breed non-nicotinic tobacco lines. These plants retain their rapid vegetative growth and protein-synthesis capacity while completely omitting alkaloid contamination, lowering manufacturing costs for plant-derived therapeutics.

Engineering Novel Pharmaceutical Alkaloids

The discovery also establishes an enzymatic framework for synthesizing non-addictive medicinal compounds. The modular nature of the four-enzyme system permits researchers to feed alternative precursor molecules into the cascade.

In proof-of-concept trials, the research team demonstrated that substituting precursor analogs enabled the enzymes to produce related alkaloids, including nornicotine and anabasine. Because many plant-derived alkaloids serve as structural backbones for central nervous system drugs, pain therapies, and insecticides, adapting this enzymatic mechanism offers a clean method to produce complex chemical compounds without high-temperature industrial synthesis.

“It opens up new ways to use tobacco plants for good: not in cigarettes, but for medicines and other valuable products,” Lichman said.

• Research findings are available online in the journal Nature Communications.