Consequences of russet mite-induced tomato defenses for community interactions

Plants make use of inducible defenses to resist herbivores. Two hormone signaling pathways play a major role in the regulation of these defense responses: the JA pathway and the SA pathway, and these two are well-known to antagonize each other’s action. Some herbivores evolved traits to interfere with induced defenses but the extent to which such traits promote the success of competitors as well is unclear. The research described in this thesis shows that defense suppression is favourable for herbivores but can backfire within natural communities. I found that russet mites (Aculops lycopersici), a major pest of tomato, facilitate their natural spider mite competitors and suppress jasmonate (JA) defenses, which would otherwise hinder their growth. Although russet mites also induce salicylate (SA), suppression of JA-defenses appeared SA-independent. However, induction of SA did account for the facilitation of spider mites, thereby inhibiting the russet mite’s population growth. This means that whether or not host-defense manipulation improves a herbivore’s fitness depends on interactions with other attackers via induced-host defenses.
Predatory mites (A. limonicus) were able to establish themselves on russet mite-infested plants because of the degradation of trichomes that is associated with russet mite herbivory, However, biological control was still insufficient, likely due to the fact that too many russet mites still receive protection from predation from trichomes that have not yet degraded. Sequencing of the russet mite genome allowed for the discovery of candidate effector proteins, which might be involved in russet mite-induced defense suppression as well as trichome degradation.