# Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites
## 2 Results
### Trepenoids  
Overall, transcriptional changes in the terpenoid biosynthetic pathway of our mite-infested tomato plants were relatively minor in terms of both the number of DEGs as well as their FCs (Figure 6). Most terpenoid-DEGs and highest FCs were observed in the Tu+Al samples, while no significant changes were found in Te. Here, we highlight four main findings of our transcriptional analysis of the terpenoid pathway. Figure 6: Gene-expression heat map depicting the relative transcript levels of tomato (_Solanum lycopersicum_) genes that encode proteins with a (predicted) function in terpenoid biosynthesis and that were differentially expressed in leaves after seven days of infestation with herbivorous mites. Tomato leaves were infested with either a single mite species (_Tetranychus urticae_ (Tu), _Tetranychus evansi_ (Te),  
or _Aculops lycopersic_ (Al)) or two species simultaneously (_T. urticae_ plus _T. evansi_ (Tu+Te), or _T. urticae_ plus _A. lycopersic_ (Tu+Al)). Non-infested plants served as controls and were used as a common reference in the transcriptional comparisons. Presented genes were differentially expressed (Benjamini and Hochberg false discovery rate adjusted \(p\leq 0.05\); Log\({}_{2}\) fold change (FC) \(\geq 0.585\)) in at least one of the mite-infestation treatments. The different (sub)sections of the pathway are specified on the left. Dark green squares in the leftmost column denote that transcription of the respective gene was found to be significantly induced in the tomato jasmonic acid (JA)-biosynthesis mutant _def-1_, 24 h after exogenous application of JA (for details see [39]). The light green square denotes JA-inducibility of the respective gene according to [97]. Firstly, we noted the strong, simultaneous down-regulation of _CPT1/NDPS1_ and _TPS20/PHS1_ in tomato leaves (dual) infested with _A. lycopersici_ (Figure 6). These genes appeared to be down-regulated in Tu samples as well, but this was not statistically significant. _CPT1/NDPS1_ is one of the most strongly down-regulated genes in the Al samples (Table S6; #3) and encodes a neryl diphosphate (NPP)-producing cis-prenyltransferase [98]. The terpene synthase (TPS) encoded by _TPS20/PHS1_ catalyzes the conversion of NPP into several monoterenes (C\({}_{10}\)), including \(\alpha\)- and \(\beta\)-phellandrene [98]. The highly reduced expression of _CPT1/NDPS1_ and _TPS20/PHS1_ in _A. lycopersici_-infested leaves suggests that phellandrene may be emitted in lower amounts by these plants as compared to non-infested controls. Although the magnitude of FCs was smaller than for _CPT1/NDPS1_ and _TPS20/PHS1_, we found similar expression patterns for _TPS17_ and _TPS21_ (Figure 6). _TPS17_ encodes a sesquiterpene (C\({}_{15}\)) synthase [99] and _TPS21_ a diterpene (C\({}_{20}\)) synthase [100]. The main reaction products of these enzymes are valencene and lycosantalene, respectively [99; 100]. Secondly, consistent with the increased emission of the monoterene lanalool, the sesquiterpene (_E_)-nerolidol and the homoterpene (C\({}_{16}\)) (_E_, _E_)-4,8,12-trimethyltrideca-1,3,7,11-tetrare (TMTT) in response to _T. urticae_ feeding [37], expression of two TPS genes involved in the biosynthesis of these compounds, i.e., _TPS5/MTS1_ and _TPS46/GLS_, was significantly up-regulated in Tu samples. The trichome-localized TPSS/MTS1 preferentially produces linalool from geranyl diphosphate (GPP), yet can also generate (_E_)-nerolidol from farnesyl diphosphate (FPP) [101]. _TPS5/MTS1_ was induced in Tu+Te as well, but not in leaflets (dual) infested with _A. lycopersici_. TPS46/GLS, uses geranylgeranyl diphosphate (GGPP) as a substrate to produce the diterpene geranyllinalool [97], which is the precursor of TMTT. The enzyme(s) responsible for the conversion of geranyllinalool into TMTT has/have not been identified in tomato. Although enzymatically less efficient, TPS46/GLS can synthesize (_E_)-nerolidol from FPP [97]. _TPS46/GLS_ expression was up-regulated upon all mite infestation treatments, except for the single infestation with _T.