# Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites
## 1 Introduction  
Plants are usually attacked by multiple microbial and arthropod species, who attempt to consume them. To resists these attacks, plants have evolved a diverse set of constitutive and inducible defense traits [1,2]. The establishment and regulation of plant defenses critically depends on the action of two hormones: jasmonic acid (JA) and salicylic acid (SA). Whereas effective defenses against herbivores and necrotrophic pathogens generally require JA to accumulate, those against biotrophic pathogens depend on SA signaling [3,4]. The JA and SA signaling pathways interact, i.e., \"crosstalk\", with each other as well as with signaling pathways of hormones that primarily regulate plant growth and development, presumably to fine-tune defense responses and to minimize growth-defense tradeoffs [5, 6, 7]. In turn, both herbivores and pathogens have evolved various traits that enable them to overcome plant defenses [8], for instance by suppressing them through exploitation of the host's hormonal crosstalk mechanisms or through sabotage of the host's molecular machinery [9,10]. By inducing or suppressing defenses, an attacking organism can indirectly influence the performance and behavior of other species that utilize the same host plant, e.g., species that either simultaneously or sequentially feed from a shared host, as well as their natural enemies [11, 12, 13]. Indirect plant-mediated interactions between phytophagous organisms are omnipresent and are major determinants of the plant-associated microbial and arthropod community composition (e.g., [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]). With respect to herbivous arthropods, most plant-mediated interactions appear to be highly unidirectional and result in interference [18,26]. Although less frequent, plant-mediated interactions between herbivous arthropods can also result in facilitation [18], for instance when one herbivore suppresses plant defenses or induces changes in the host's resource availability which (also) benefit a second herbivore [27], potentially leading to reduced plant fitness and increased crop losses. Despite the ecological and agricultural relevance of plant-mediated interactions between herbivores, the underlying molecular mechanisms remain poorly understood [18,26, 27, 28, 29]. Among the common pests of cultivated tomato (_Solanum Lycopersicum_) are several species of mites (which are piercing-sucking, cell-content feeders), i.e., the generalist two-spotted spider mite (_Tetranychus urticae_) as well as the specialized tomato red spider mite (_Tetranychus evansi_) and the Eriophyid tomato russet mite (_Aculops lycopersic_) [23,30,31]. Unless one of these mite species manages to monopolize its host (e.g., via encapsulation with silken web [32]), they may indirectly interact with one another. This can have major consequences for population growth dynamics and, hence, may determine which species can develop into a pest and which not. For example, field-grown tomato plants that already suffered from a natural _A. Lycopersic_ infestation were frequently found to be invaded by _T. urticae_ as well, while infestations in the reverse order hardly occurred [23]. Spider mite populations grew larger on plants with russet mites than on plants without [23]. Laboratory experiments demonstrated that _A. Lycopersic_ facilitated _T. urticae_ on shared leaflets of intact tomato plants [23]. Additionally, _T. urticae_ appeared to interfere with _A. Lycopersic_ population growth on such dual-infested leaflets, providing an explanation for the observed species-succession pattern [23]. Similarly, _T. urticae_ can be facilitated by _T. evansi_ when these mites share a tomato leaflet, i.e., either simultaneously [33] or sequentially [32], while _T. evansi_'s developmental rate, survival and reproductive performance all decrease dramatically when feeding on leaf material previously attacked by _T. urticae_ as compared to on non-infested leaf material from control plants [30,32].