Plant Defenses Against Tetranychus urticae: Mind the Gaps

Abstract

The molecular interactions between a pest and its host plant are the consequence of an evolutionary arms race based on the perception of the phytophagous arthropod by the plant and the different strategies adopted by the pest to overcome plant triggered defenses. The complexity and the different levels of these interactions make it difficult to get a wide knowledge of the whole process. Extensive research in model species is an accurate way to progressively move forward in this direction. The two-spotted spider mite, Tetranychus urticae Koch has become a model species for phytophagous mites due to the development of a great number of genetic tools and a high-quality genome sequence. This review is an update of the current state of the art in the molecular interactions between the generalist pest T. urticae and its host plants. The knowledge of the physical and chemical constitutive defenses of the plant and the mechanisms involved in the induction of plant defenses are summarized. The molecular events produced from plant perception to the synthesis of defense compounds are detailed, with a special focus on the key steps that are little or totally uncovered by previous research.

Keywords: Tetranychus urticae; constitutive and inducible defenses; mite counter-defenses; plant defense; signalling events.

Figure 1

Phenotypic features of the plant–spider mite interaction. T. urticae life cycle (a), symptoms of mite-infested bean plants (b,c), and spider mite silk web (d), molts (e) and feces (f). Bar scales are indicated: 100 µM (a), 500 µM (c), 200 µM (d) and 250 µM (e,f).

Figure 2

Plant event in responses to T. urticae infestation. Specific plant receptors (PRRs) recognize elicitors/effectors (HAMPs) derived from either the plant or the spider mite that induce alterations in the membrane potential (Vm), cytosolic Ca²⁺ influxes and ROS/RNS burst. Ca²⁺-sensing proteins, MPKs and phosphatases (APC21) participate in the defense transduction pathway. H₂O₂ content is highly regulated by ROS-related enzymes (BB22, AO, GSTU and CPX7). Besides, genes such as MATI, PP2A5 and others still unknown participate in the tight regulation of the hormonal crosstalk, mainly in the Jasmonic Acid/Salicylic Acid balance. All together plus some transcription factors (ABI4 and other unknown TFs) regulate the induction of the synthesis of a battery of defense molecules. Unknown genes, molecules, pathways and responses are indicated in red.