It's also one of the most-studied and best understood of all plants. 3,000 chemicals have been identified in the plant itself and 4,000 have been identified in its smoke!
Nicotiana tabacum is the main commercially-grown species and is thought to be descended from some combination of wild species such as N. sylvestris (the gardener's woodland/night-scented tobacco). Many of these wild Nicotiana species, which are found around the world, are also able to accumulate nicotine and related alkaloids just like N. tabacum.
While nicotine is a major component of tobacco, it can also been found in much lower concentrations in other plants, including horsetails, zinnias, acacias, cherry trees, walnut trees, stinging nettles and hops, It's also present in many other Solanaceous plants at trace levels (including tomato leaves!). It's present at sufficient levels in a Solanaceous shrub, Duboisia hopwoodii, that the indigenous Australians chewed it and used it to poison small watering holes; intoxicating thirsty emus and kangaroos, making them much easier to catch.
N. tabacum doesn't actually synthesize nicotine in its leaves - but in its roots, where it's then translocated to its leaves in the sap. Like many plants, tobacco produces more of this (anti-herbivory) secondary metabolite when it's grown outside in the field rather than a greenhouse. Nicotine mimics the neurotransmitter acetylcholine and is a potent insecticide, though some pests, like the notorious tobacco hornworm, are resistant to it. Its possible that hornworms are able to co-opt this poison to protect themselves from parasitic wasps.
Some people have tried using nicotine as an "all-natural, organic pesticide." Back when I managed a small greenhouse as an undergrad, I used a slurried extract of tobacco and chili peppers to kill back the aphids and was pretty satisfied with the results. Unfortunately, it's not a great choice in agriculture as it's very toxic to mammals and other non-target organisms. Similarly, from what I've heard it's pretty common for people working in tobacco fields to get sick thanks to constant contact between tobacco sap and their permeable skin. I've also heard some fellow scientists complain that they feel sick after working with tomato plants for long periods of time, though I don't know if this is related...
In humans, the acutely fatal oral dose of nicotine is the same amount contained in 5 cigarettes or a single cigar! You're all still alive to read this because relatively little of the alkaloid both survives the flames and is actually absorbed into your body.
Nicotine isn't the only important chemical in tobacco leaves. A whole set of related alkaloids in addition to other chemicals, such as sugars, play important roles in the quality of tobacco produced for consumption. Major changes in the plant's chemical profile occur as green leaves are dried and cured to produce a final product. Varieties and curing processes differ based on the specific qualities desired in the end product. For example, "converter" varieties of tobacco are able to change up to 95% of their nicotine content to the related "nornicotine" during senescence and air curing. Tobacco chemical profiles are changed a final time when the cured leaves catch fire and transform into smoke.
Wikipedia has some pretty fascinating descriptions of the different market classes of tobacco and all the different ways their leaves are processed and cured.* Other sites describe the fermentation process in more depth and discuss how the microbial community of a curing leaf changes.
Some tobacco enthusiasts even grow and cure their own. It may not be as politically correct as winemaking or homebrewing but I draw no distinction. Have any of you ever tried it? Maybe with ornamental varieties?
*As someone who doesn't smoke, I don't think about this crop very much, but all this writing makes me nostalgic for the time and place I grew up in - when fall harvest included the familiar sight of slat-stripped barns hung with tobacco.
Now buried under concrete, one farm at a time...
much from: Eckart, E. 2008. Solanaceae and Convolvulaceae: Secondary Metabolites