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Plant–bacteria and plant–fungus associations both involve reciprocal signaling between the two species (see Figure 35.6). Parasitic plants also need to detect signals from nearby plants so they can grow toward them and obtain their nutrients, but obviously this is to the disadvantage of the potential host plant. In one interesting case, a parasitic plant has evolved the ability to recognize the chemical signals produced by plants to attract beneficial fungi.

The holoparasite Striga (witchweed) is a serious pest of cereal crops in Africa. Key Concept 35.4 described how arbuscular fungi are attracted to plant roots by compounds called strigolactones. One of these same molecules induces the seed germination of some parasitic plants, including Striga. Scientists strongly suspect that this is no coincidence. The mycorrhizal interaction is ancient (more than 400 million years old) and predates the evolution of parasitic plants. For this reason scientists hypothesize that a mechanism evolved in the ancestors of modern Striga to recognize a compound that was already produced by plants to attract soil microbes.

In Striga we thus find an example of “opportunistic evolution”—that is, the repurposing of preexisting processes rather than the development of new processes. This is not the first time you have encountered this phenomenon in this chapter. Recall that the formation of nodules by rhizobia uses some of the same mechanisms used by arbuscular fungi to establish residence inside plant cells (see Figure 35.6). This implies an evolutionary connection between the two symbioses.