Figure 2.

Model of plant defence strategies and their adaptive values under different biotic stress conditions. Plants in environments with relatively high disease pressure from a wide array of different attackers benefit from constitutive priming of basal resistance mechanisms, which provide broad‐spectrum protection against pests and diseases (strategy A). However, this defence strategy may affect the plant's ability to associate with plant‐beneficial microbes, such as mycorrhizae or nitrogen‐fixing bacteria. In this situation, plants would benefit more from an increased ability to attract and associate with disease‐suppressing plant‐beneficial microbes (strategy B). Plants in environments with a constant pressure from one dominant biotrophic pathogen benefit from effector‐triggered immunity (ETI; strategy C). ETI can be broken and give rise to an ongoing ‘zigzag’ evolution, as described by Jones and Dangl (2006). Inducible defence priming on perception of stress‐indicating signals provides a cost‐efficient adaptation to environments with variable degrees of disease pressure (strategy D). Because priming of defence and induction of defence are both associated with costs on plant growth and reproduction, a relatively unresponsive immune system would be beneficial in environments with relatively low disease pressure (strategy E).