# Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites
## 3 Discussion
urticae_-induced up-regulation of canonical JA-responsive genes that code for antinutritive proteins (e.g., proteinase inhibitors, aminopeptides, polyphenol oxidases) was dramatically reduced or fully compromised in Tu+Al samples (Figure 3 and Figure S1; Table 1). We observed the same expression pattern for the JA-responsive _TPSS/MTS1_, which encodes the terpene synthase responsible for linalool production [101]. These results provide clear evidence for the suppression of JA defenses by russet mites. Note that this suppression is detected despite the much higher number of mites on the dual-infested plants, i.e., compared to the single infestation with _T. urticae_. Accordingly, expression of most of the JA-responsive defense-associated genes was not induced upon single infestation with _A. lycopersici_. We additionally found that the expression levels of several genes associated with other (potential) defense responses were down-regulated in plants (dual) infested with _A. lycopersici_, i.e., well below levels found in non-infested control plants. This concerns the genes coding for: (a) CHS1, CHS2 and CHII of the phenylpropanol pathway (Figure 5), i.e., enzymes that fulfill gatekeeper functions in flavonoid and anthocyanin biosynthesis [79; 88; 89; 90]; (b) CPT1/NDPS1 and TPS20/PHS1 (Figure 6), which are responsible for the production of the volatile monoterpene phellandrene in glandular trichomes [98], and; (c) GAME enzymes as well as the main JA-regulated GAME transcription factor (Figure 7), which together are required for the production of steroidal (glycol)alkaloids [108; 113]. Hence, the production of various classes of secondary metabolites (flavonoids, anthocyanins, terpen, steroidal glycoalkaloids) may be attenuated in leaves (dual) infested with _A. lycopersici_. This is reminiscent of a recent report on whitefly (_Bemisia tabaci_)-infested tomato plants, whose leaves contained lower amounts of various flavonoids and anthocyanins, while also the emission of \(\alpha\)-phellandrene from these plants was reduced [124]. Whiteflies have a higher fitness on tomato leaves that have previously been infested by conspecifics than on leaves of non-infested control plants and strongly prefer to feed and oviposit on such conspecific-infested leaves [124]. The lower flavonoid levels in infested leaves were found to be causally linked with _B. tabaci_'s oviposition preference, whereas the reduced \(\alpha\)-phellandrene emission was causally linked with _B. tabaci_'s foraging preference for conspecific-infested plants [124]. Interestingly, the decreased flavonoid and \(\alpha\)-phellandrene production coincided with reduced transcript levels of tomato _CHS1_, _CHII_, _FLS_, _DFR_ and _TPS20/PHS1_[124]. It was therefore suggested that whiteflies suppress the accumulation of specific flavonoids, anthocyanins and terpenoids to alter the behavior of conspecifics, which may ultimately enhance their performance [124]. Whether the here detected down-regulation of _CHS1_, _CHS2_, _CHII_, _CPT1/NDPS1_ and _TPS20/PHS1_ (as well as the _GAME_ genes) has a similar function in russet mite infestations and, hence, is herbivore-adaptive, remains unclear. For instance, because russet mites are less mobile than whitefly adults and because the down-regulation of trichome-localized terpenoid biosynthesis genes may (also) be associated with the deterioration of glandular trichomes, a phenotype commonly observed on russet mite-attacked plants [125]. Finally, although the analysis of Al (and Tu+Al)-specific DEGs did not provide clear information on how _A. lycopersici_ suppresses plant defenses, we did note the specific up-regulation of genes involved in cell cycle control or in the transport of various metabolites. Whereas spider mites feed from mesophyll cells whilst avoiding to pierce the epidermis [126], russet mites are much smaller and therefore restricted to feed from epidermal cells. Since the content of normal epidermal cells is presumably not very nutritious for mites, _A.