Nicotine is a naturally occurring organic compound primarily recognized for its presence in certain plants. This molecule belongs to a class of compounds known as alkaloids, which are nitrogen-containing substances often found in living organisms. While commonly associated with tobacco, nicotine is also present in smaller quantities in other plants. This article explores the biological processes by which this compound is naturally created within plants.
Natural Origin and Location
Nicotine is synthesized predominantly in the roots of certain plant species, particularly those belonging to the Nicotiana genus, which includes tobacco (Nicotiana tabacum). Other plants in the Solanaceae family, such as tomatoes and eggplants, also contain trace amounts of nicotine. After its production in the root cells, nicotine is transported throughout the plant, primarily moving upwards to the leaves via the xylem, where it is stored. This storage often occurs within vacuoles, which are compartments inside plant cells.
The concentration of nicotine can vary significantly depending on the plant species and even within different parts of the same plant. For instance, in Nicotiana tabacum, nicotine levels in leaves can range from 0.6% to 9.0%, while in Nicotiana rustica, it can reach up to 18.76%. The plant hormone jasmonate plays a role in signaling and activating the genes involved in nicotine biosynthesis, often in response to physical damage or herbivory.
Key Precursors and Enzymes
The creation of nicotine within plants relies on specific basic building blocks, known as precursor molecules, and specialized biological catalysts called enzymes. Enzymes are proteins that accelerate chemical reactions in living systems without being consumed in the process. Nicotine is composed of two distinct ring structures: a pyrrolidine ring and a pyridine ring. Each of these rings is formed from different precursor molecules through separate pathways.
The pyrrolidine ring originates from amino acids such as ornithine or arginine. These amino acids are common in plant metabolism and are converted into an intermediate compound called putrescine. For the pyridine ring, the primary precursor is nicotinic acid. This molecule is also a primary metabolite involved in the production of nicotinamide adenine dinucleotide (NAD), an important cofactor in various cellular reactions. Specific enzymes, like ornithine decarboxylase (ODC) and putrescine N-methyltransferase (PMT), are necessary to transform these precursors into the building blocks that ultimately form nicotine.
The Biosynthetic Pathway
This complex process can be broadly divided into the formation of the two constituent rings and their subsequent joining. PMT is recognized as a key enzyme, initiating the specific pathway towards nicotine. Subsequently, N-methylputrescine is oxidized by N-methylputrescine oxidase (MPO) to create the N-methylpyrrolinium cation, which is the direct precursor to the pyrrolidine ring.
In this pathway, aspartate is transformed into quinolinate through the action of aspartate oxidase (AO) and quinolinate synthase (QS). Quinolinate is then converted to nicotinic acid mononucleotide by quinolinate phosphoribosyltransferase (QPT), another important enzyme in this branch of the pathway. QPT activity is higher in roots, the primary site of nicotine synthesis, and its transcript levels increase following damage to the plant, indicating its role in defense-induced production.
Finally, the N-methylpyrrolinium cation from the pyrrolidine pathway and nicotinic acid (or a derivative) from the pyridine pathway condense to form the complete nicotine molecule. While the specific enzyme responsible for this final condensation step has been less clear, an oxidoreductase called A622 is involved in a later step of nicotine biosynthesis.
Ecological Role
Plants synthesize nicotine primarily as a defense mechanism against various threats in their environment. This compound acts as a natural insecticide, deterring herbivores like insects from consuming plant tissues. Nicotine achieves this by interfering with the nervous systems of pests, specifically by binding to acetylcholine receptors, which can lead to paralysis and even death in insects at relatively low concentrations.
The production of nicotine can be increased in response to herbivore attack or physical damage to the plant. Studies have shown that plants with reduced nicotine levels due to genetic modifications experience more damage from native herbivores compared to their wild-type counterparts. This demonstrates the effectiveness of nicotine as a resistance trait under natural conditions. Beyond insects, nicotine deters a variety of other organisms.