The Salivary Protein Repertoire of the Polyphagous Spider Mite Tetranychus urticae: A Quest for Effectors

Abstract

The two-spotted spider mite Tetranychus urticae is an extremely polyphagous crop pest. Alongside an unparalleled detoxification potential for plant secondary metabolites, it has recently been shown that spider mites can attenuate or even suppress plant defenses. Salivary constituents, notably effectors, have been proposed to play an important role in manipulating plant defenses and might determine the outcome of plant-mite interactions.Here, the proteomic composition of saliva from T. urticae lines adapted to various host plants-bean, maize, soy, and tomato-was analyzed using a custom-developed feeding assay coupled with nano-LC tandem mass spectrometry. About 90 putative T. urticae salivary proteins were identified. Many are of unknown function, and in numerous cases belonging to multimembered gene families. RNAseq expression analysis revealed that many genes coding for these salivary proteins were highly expressed in the proterosoma, the mite body region that includes the salivary glands. A subset of genes encoding putative salivary proteins was selected for whole-mount in situ hybridization, and were found to be expressed in the anterior and dorsal podocephalic glands. Strikingly, host plant dependent expression was evident for putative salivary proteins, and was further studied in detail by micro-array based genome-wide expression profiling. This meta-analysis revealed for the first time the salivary protein repertoire of a phytophagous chelicerate. The availability of this salivary proteome will assist in unraveling the molecular interface between phytophagous mites and their host plants, and may ultimately facilitate the development of mite-resistant crops. Furthermore, the technique used in this study is a time-and resourceefficient method to examine the salivary protein composition of other small arthropods for which saliva or salivary glands cannot be isolated easily. The family of spider mites (Chelicerata: Acari: Tetranychidae) comprises well over 1000 species, including several that are important pests on crops, and about 0.9 billion euro is being spent annually for their control worldwide (1, 2). These minute herbivores-about 0.5 mm in size-use their stylets to pierce leaf mesophyll cells and to inject saliva, after which they suck out the cytoplasm. This results in cell death visible as chlorotic spots sometimes accompanied by necrosis, and ultimately in leaf abscission (3,4). Among the spider mites, the From the ‡Laboratory

Overcompensation of herbivore reproduction through hyper‐suppression of plant defenses in response to competition

“…Quantitative differences in the expression levels of effector‐encoding genes have also been observed between virulent and avirulent isolates of filamentous phytopathogens when infecting plants and, hence, are thought to determine their degree of virulence (Cooke et al ., ; Hacquard et al ., ). Consistently, when feeding from tomato, the transcript abundances of these ( Te28 , Tu28 , Te84 and Tu84 ) and (putative) other (Jonckheere et al ., ) spider mite effector‐encoding genes probably affect the magnitude of key defenses and concomitantly the performance of the mites. Given that the magnitude of JA and SA defenses reported here (Figs 1–5) correlates well with the transcript levels of mite effector‐encoding genes (Fig.…”

Tetranins: new putative spider mite elicitors of host plant defense

“…Although previous proteome studies on spider mites identified about 90 and 177 putative SGPs from T. urticae and Tetranychus truncates (Jonckheere et al ., ; Zhu et al ., ), neither Tet1 nor Tet2 was listed among them. However, we observed that Tet1 is expressed predominantly in the anterior podocephalic salivary glands, possibly together with the silk glands, of T. urticae , implying that Tet1 is secreted outside of the body.…”

Rapid specialization of counter defenses enables two-spotted spider mite to adapt to novel plant hosts

“…Suppression of induced plant defenses was proposed to be the common and most efficient 183 mechanism of TSSM adaptation to new host plants (Jonckheere et al, 2016;Villarroel et al, 184 2016; Blaazer et al, 2018;Jonckheere et al, 2018). Indeed, interference with either JA-185 biosynthesis or its signalling appears to be an effective way to attenuate a whole range of 186…”

“…Arabidopsis-adapted mites are responsive to Arabidopsis xenobiotics 213 Metabolic resistance is another mechanism of herbivore adaptation to xenobiotic challenges 214 previously implicated in the evolution of TSSM resistance to pesticides (Van Leeuwen and 215 Dermauw, 2016) On the ancestral bean plants, all marker transcripts were at similar low levels in both bean-and 230…”