Abstract
Plant secondary metabolites play a key role in plant-insect interactions, whether constitutive or induced, C- or N-based. Anti-herbivore defences against insects can act as repellents, deterrents, growth inhibitors or cause direct mortality. In turn, insects have evolved a variety of strategies to act against plant toxins, e.g., avoidance, excretion, sequestration and degradation of the toxin, eventually leading to a co-evolutionary arms race between insects and plants and to co-diversification. Anti-herbivore defences also negatively impact mutualistic partners, possibly leading to an ecological cost of toxin production. However, in other cases toxins can also be used by plants involved in mutualistic interactions to exclude inadequate partners and to modify the cost/benefit ratio of mutualism to their advantage. When considering the whole community, toxins have an effect at many trophic levels. Aposematic insects sequester toxins to defend themselves against predators. Depending on the ecological context, toxins can either increase insects’ vulnerability to parasitoids and entomopathogens or protect them, eventually leading to self-medication. We conclude that studying the community-level impacts of plant toxins can provide new insights into the synthesis between community and evolutionary ecology.
Highlights
Chemical defence products may range from low molecular weight compounds, called secondary metabolites, to peptides and proteins that are active against insects (Table 1)
Toxins linked with anti-herbivore defence sometimes impact on mutualistic interactions in a positive way for the plant, as in the two examples above, but they can have a negative impact, as it has been hypothesised for toxic nectars, which might be a by-product of the presence of toxins in other parts of the plant [48]
The fitness consequences of toxin production on both plants and insects depend on its community-level consequences
Summary
Plants have evolved a whole arsenal of defence strategies against herbivores, including the synthesis of a tremendous variety of chemical compounds. Chemical defence products may range from low molecular weight compounds, called secondary metabolites, to peptides and proteins that are active against insects (Table 1). Secondary metabolites are organic compounds that are not directly involved in the normal growth, development, or reproduction of plants [1]. We have limited our review to secondary metabolites, excluding large proteins such as lectins which can sometimes act as toxins against insects, but which are involved in the growth and development of the plant
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