METIs acting at site I (METI-Is) were first launched as excellent miticides in the early 90s' (Hollingworth and Ahammadsahib, 1995, Hollingworth et al., 1994, Obata et al., 2006). In addition to electron transport inhibitors, a number of acaricides directly interfere with ATP synthesis at complex IV (IRAC group 12), such as organotin compounds (cyhexatin, fenbutatin oxide), propargite and diafenthiuron (Carbonaro et al., 1986, Kadir and Knowles, 1991, Miyasono et al., 1992, Ruder and Kayser, 1994). Most recently, a number of compounds such as the beta-ketonitriles have been uncovered and commercialized as specific inhibitors of complex II (here referred to as METI site II, IRAC group 25) (Hayashi et al., 2013, Nakahira, 2011). Acequinocyl and the carbazate bifenazate have been shown to act as inhibitors of cytochrome b at the Q 0-site in complex III (here referred to as METI site III, IRAC group 20) (Van Leeuwen et al., 2008, Van Nieuwenhuyse et al., 2009).
A number of commercial acaricides target these processes at different sites: classical Mitochondrial Electron Transport Inhibitors (METIs) such as quinazolines, pyrimidinamines, pyrazoles and pyridazinones act at complex I (here referred to as METI site I, IRAC group 21, Fig. 1) (Hollingworth and Ahammadsahib, 1995, Hollingworth et al., 1994). The proton gradient created during this process is the driving force for ATP synthesis by the F 0F 1 ATPase (complex V) (Karp, 2008). In the mitochondrial inner membrane, four large transmembrane complexes (complex I-IV) mediate electron transport via several redox reactions from NADPH and FADH 2 to oxygen as a final electron acceptor. Inhibition of electron transport at the mitochondrial respiratory chain has been a successful mode of action especially in the case of acaricides (Lummen, 2007, Van Leeuwen and Dermauw, 2016). Last, we used CRISPR-Cas9 genome editing tools to introduce the mutation in the Drosophila PSST homologue. Additionally, an independent genetic mapping approach QTL analysis identified the genomic region of pyridaben resistance, which included the PSST gene. Selection experiments with a strain segregating for the mutant allele, together with marker-assisted back-crossing of the mutation in a susceptible background, confirmed the involvement of the mutation in METI-I resistance. The position of the mutation was studied using the high-resolution crystal structure of Thermus thermophilus, and was located in a stretch of amino acids previously photo-affinity labeled by fenpyroximate. Here, we report the discovery of a mutation (H92R) in the PSST homologue of complex I in METI-I resistant T. urticae strains. Increased activity of P450 monooxygenases has been often associated with resistance, but target-site based resistance mechanisms were never reported. Because of their very frequent use, resistance evolved fast more than 20 years ago, and is currently wide-spread. The acaricidal compounds pyridaben, tebufenpyrad and fenpyroximate are frequently used in the control of phytophagous mites such as Tetranychus urticae, and are referred to as Mitochondrial Electron Transport Inhibitors, acting at the quinone binding pocket of complex I (METI-I acaricides).
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