Gardeners and farmers know that nicotine has its uses: a quick squirt of a nicotine solution keeps pests at bay. But do the plants that make this compound use it to deter invaders? This isn't as obvious as one might think; evolution might have rendered the insects that eat these particular plants completely resistant to nicotine. Anke Steppuhn et al. aimed to find out (PLoS Biol. 2, e217; 2004).
They produced transgenic tobacco plants (Nicotiana attenuata) in which a key enzyme in nicotine synthesis was silenced. The nicotine concentration in these plants dropped to 3–4% of normal levels. Experiments with greenhouse-grown tobacco showed that the larvae of a key pest — the tobacco hornworm Manduca sexta — much preferred leaves from the transgenic plants than from wild-type plants. The larvae also grew much more quickly when reared on the transgenic plants, implying that although these bugs can tolerate nicotine, they fare better without it.
Experiments with field-grown tobacco gave similar results, and showed that the transgenic plants also lost significantly more leaf area to pests than did wild-type plants. The inescapable conclusion is that secondary metabolites such as nicotine, although not essential for normal plant growth and reproduction, nonetheless make significant contributions to ecology.
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Tromans, A. The benefits of nicotine. Nature 430, 980 (2004). https://doi.org/10.1038/430980a