# Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites
## 3 Discussion
lycopersici_ may manipulate ploidy levels (e.g., of underlying mesophyll cells) and redirect the transport of nutrients in infested leaves to meet its metabolic demands, similar to what some plant-feeding microbesdo [127; 128; 129]. This suggests that russet mites may manipulate more than just defense responses to increase the susceptibility of their host and, thus, to promote their own proliferation. Much to our own surprise, we found only 38 tomato genes to be differentially regulated after seven days of infestation with _T. evansi_, as compared to the 3200 and 2460 DEGs upon single infestations with _A. Iyopersici_ or _T. urticae_, respectively. This is extra remarkable given that _T. evansi_ is known to cause ca. 2-fold more feeding damage on tomato than _T. urticae_[33]. This means that the low number of DEGs in the Te samples must be the result of host plant manipulation by _T. evansi_, rather than of limited feeding. As with _A. Iyopersici_, our current analysis did not reveal the exact mechanism(s) by which _T. evansi_ suppresses plant responses. The main reason for this is that the Te-DEGs were not specific; 37 of them were similarly regulated in the Tu samples (Figure 2C), albeit induction of these genes was usually much higher in Tu than in Te samples (Table S4). In leaves dual infested with _T. urticae_ and _T. evansi_, the number of up-regulated DEGs was over 18% lower than in leaves solely infested with _T. urticae_. Furthermore, for numerous up-regulated genes in the Tu samples, the magnitude of induction was considerably lower in Tu+Te samples (Figure 2D). Both findings point to the suppression of _T. urticae_-induced plant responses by _T. evansi_. Indeed, various defense responses were antagonized by _T. evansi_ at the transcriptome level, including genes coding for JA-responsive antinutritive proteins, SA-regulated PR proteins and enzymes associated with secondary metabolism (Figures 3-6; Table 2). In this respect, two things are worthwhile to point out: Firstly, there was very little overlap in the JA defense genes that were suppressed by _A. Iyopersici_ versus by _T. evansi_ (Figure 3). Secondly, the suppression of gene expression by _T. evansi_ in dual-infested plants was evident even though the number of mites on leaflets of these plants was double that of the leaflets solely infested with _T. urticae_ (thus excluding density-mediated effects). Given that (a) plant responses to mites, especially to defense-inducing ones, may vary over time [33; 45] and (b) _T. urticae_ is not immediately--or perhaps not always--facilitated by _T. evansi_ on shared leaves [41; 45], it would be interesting to analyze plant responses to dual attacks at early time points to find out how rapidly _T. urticae_-induced defenses are suppressed. Related to this, as _T. evansi_'s suppression trait appears to be plastic [41], it would be fascinating to explore the extent to which this plasticity is utilized to mitigate the facilitation of competing herbivores, such as _T. urticae_[50]. Finally, our most puzzling finding was that _T. evansi_ not only suppressed _T. urticae_-induced genes, but also counteracted the _T. urticae_-triggered down-regulation of tomato genes (Figures 2D, 4, 5, 6 and 7; Table S9). Thus, _T. evansi_ made dual-infested plants phenotypically more similar to non-infested plants via the transcriptome-wide dampening of _T. urticae_-triggered host responses. How exactly _T. evansi_ achieves this is unknown [50]. We hypothesize that it secretes effector molecules that take control over central regulators of cellular homeostasis [50]. It is also conceivable that its effectors interfere with RNA polymerase activity [130] or silence host gene transcription epigenetically [131; 132]. The ongoing identification and characterization of _in planta_ targets of mite effectors [40; 133; 134] will undoubtedly allow us to better understand how the distinct transcriptomic profiles of suppressor mite-infested tomato plants come about.