How does nicotine protect tobacco plants from insects and herbivores?

Tobacco plants (Nicotiana tabacum and related species) synthesize nicotine in their roots and transport it to their leaves, where it functions as a natural chemical defense against insects and other herbivores.12

Nicotine acts as a potent insecticide because it is highly toxic to many insects. When an herbivorous insect consumes the plant, the nicotine binds to nicotinic acetylcholine receptors within the insect's nervous system. This interaction disrupts normal nervous system signaling, which can lead to paralysis and death. By accumulating this toxic alkaloid in its tissues, the tobacco plant effectively deters or eliminates organisms that attempt to feed on it.132