# Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites
## 1 Introduction
Exactly which plant processes or responses underlie these indirect interactions between mites is not known, but we hypothesize they largely result from plant defense suppression by _A. Lycopersic_ and _T. evansi_, respectively, in combination with defense induction by _T. urticae_[23,33]. This hypothesis is supported by several observations: Firstly, when feeding from tomato, _T. urticae_ induces JA as well as SA-regulated defense responses that significantly reduce its performance (i.e., when not adapted to these defenses), with JA defenses being most important for resistance to _T. urticae_[34, 35, 36, 37, 38, 39, 40, 41]. Accordingly, relative to wild-type (WT) plants, _T. urticae_ performs much better on the JA biosynthesis mutant _defenseless-1 (def-1)_[34,38]. In fact, the reproductive performance of this mite on _def-1_ is as high as on WT leaflets that are simultaneously infested with _A. Lycopersic_ and, moreover, cannot be enhancedfurther via a co-infestation with russet mites [23]. This suggests that on WT plants _T. urticae_ benefits from the suppression of JA defenses by _A. lycopersici_. Secondly, transcript levels of several JA-regulated, defense-associated marker genes (_Polyphenol Oxidase F_, _Threonine-Deaminase II_, _Jasmonate-Inducible Protein 21_, _Wound-Induced Proteinase Inhibitor II_) were significantly lower in tomato leaflets dual infested with _T. urticae_ and _A. lycopersici_ than in leaflets solely infested with _T. urticae_, but were still higher than or equal to the levels found in _A. lycopersici_-infested leaflets or in non-infested leaflets of control plants [23]. Note that expression of these genes was not induced upon single infestation with _A. lycopersici_[23]. The SA response showed the opposite pattern, i.e., an additive response, as the transcript level of the SA-regulated, defense-associated marker gene _Pathogenesis-Related Protein 6_ was highest in dual-infested leaflets [23]. The concentrations of JA, JA-Ile (the main biologically active jasmonate) and SA were also highest in dual-infested leaflets [23]. In tomato plants carrying the _355::mahG_ transgene, which are unable to accumulate SA [42], the suppression of JA defense marker gene expression in dual-infested leaflets was no longer detected, whereas these genes were still not induced upon single infestation with russet mites. Together, this indicates that russet mites suppress JA defenses downstream from hormone accumulation and independently from JA-SA crosstalk [23]. Yet, the plant-mediated facilitation of _T. urticae_ by _A. lycopersici_ does depend on JA-SA crosstalk [23]. Thirdly, transcript levels of the JA defense marker gene _Proteinase Inhibitor IIc_ and the SA defense marker gene _Pathogenesis-Related Protein 1a_ were significantly lower in tomato leaflets dual infested with _T. urticae_ and _T. evansi_ than in leaflets solely infested with _T. urticae_[33]. Here too, expression of the marker genes was higher in the dual-infested leaflets than in leaflets solely infested with _T. evansi_ or in non-infested leaflets of control plants [33]. This means that _T. evansi_ suppresses both JA and SA defenses [33]. Like with russet mites, this suppression acts downstream from hormone accumulation and independently from JA-SA crosstalk [33]. It is important to point out that the suppression of gene expression in leaflets dual infested with inducer (_T. urticae_) and suppressor (_A. lycopersici_ or _T. evansi_) mites was detected despite the presence of a higher number of mites on these leaflets, i.e., relative to the inducer mite-only treatment. Although hormonal and especially transcriptomic changes in plants upon infestation with defense-inducing _T. urticae_ have been well characterized in various species [33; 37; 39; 41; 43; 44; 45; 46; 47; 48], our understanding of plant responses to suppressor mites is relatively limited [23; 30; 33; 41; 49; 50; 51; 52].