# Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites
## 3 Discussion  
In this study, we have analyzed the phytohormonal and transcriptomic changes in tomato plants upon single or dual (simultaneous) infestations with naturally competing species of mites that differ in their ability to induce or suppress host defenses. We show that single infestations with specialist, defense-suppressing _A. lycopersici_ or _T. evansi_ both triggered the increased accumulation of JA-Ile and SA, yet yielded very different transcriptomic changes in tomato leaves. The latter is likely the result of distinct suppression mechanisms employed by these mites. Whereas _A. lycopersici_ predominantly suppressed JA-regulated direct and indirect defense responses, _T. evansi_ suppressed both JA and SA-regulated defenses. Moreover, we provide evidence that both specialist mites probably (also) manipulated host processes other than immune responses. The finding that JA-Ile and SA accumulated to similar or higher amounts in dual-infested leaflets as compared to leaflets infested with _T. urticae_ alone (Figure 1) is in agreement with results from previous studies [23; 33]. It implies that suppression of host defenses by _A. lycopersici_ and _T. evansi_ acts downstream from phytohormone accumulation and independently from JA-SA crosstalk. Indeed, there were no indications that these mites actively suppressed JA and/or SA biosynthesis genes (Figures 3 and 4). One exception may be the suppression of two PAL-encoding genes by _T. evansi_, but our evidence for this is weak. In addition, at the transcriptome level there were no indications that suppressor mites exploited crosstalk between hormones that are critical for plant immunity (i.e., JA, SA) versus those that primarily regulate growth and development (i.e., abscisic acid, auxin, brassinosteroids, cytokins, ethylene, gibberellins). If not via the suppression of biosynthetic genes or crosstalk with other hormones, russet mites may nonetheless limit the accumulation of JA-Ile via the induction of a _jasmonate methyltransferase_ (_JMT_: Figure 3). In wild tobacco (_Nicotiana attenuata_) and rice (_Oryza sativa_), elevated _JMT_ expression dampens herbivory-induced JA-Ile accumulation by redirecting the JA flux towards production of methylated JA (MeJA), thereby improving herbivore performance [116; 117; 118]. Clustering and enrichment analyses of the transcriptomic changes in tomato in response to feeding by _T. urticae_ and/or _A. lycopersici_ revealed gene expression activity associated with several core biological processes of plant immunity, that is: (i) perception of herbivore attack; (ii) signaling events immediately downstream from this perception; (iii) production of anti-herbivore compounds, such as defensive proteins and secondary metabolites, and; (iv) inhibition of photosynthesis [2; 119]. In this respect, the enrichment of genes belonging to BP GO category \"recognition of pollen\" among the up-regulated DEGs in mite-infested leaves (except for in Te samples) is noteworthy. Several of the genes of this category encode putative lectin-type PRRs. The recognition of (damaged) self versus non-self is a crucial aspect of plant immunity as well as of reproduction and can be governed by lectin-type PRRs [120; 121; 122]. PRRs required for plant resistance to mites remain to be identified. As ligand binding by a PRR often induces the expression of the corresponding PRR-encoding gene [123], the increased expression of several lectin-type PRR genes further supports our hypothesis that they may be involved in the recognition of mite attack. The highest number of DEGs (5132) was found in leaves dual infested with _T. urticae_ and _A. lycopersici_, i.e., in leaves with the highest density of mites. The plant's response to this dual attack was mostly additive for up- as well as down-regulated genes. The additive response was evident, among others, for the induced SA defenses, while the JA defenses formed a clear exception. The _T.