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RNA interference (RNAi) regulates gene expression in eukaryotes, and it is an emerging tool in crop protection by exogenous applications of double-stranded RNAs (dsRNAs) to silence the expression of essential pest genes. Nevertheless, delivery of dsRNAs to sap-sucking insects is a major challenge for RNAi applications. The present work aimed at verifying whether in leafhopper species, RNAi can be triggered by plant-mediated delivery, and providing a proof of concept towards field applications. Two phytoplasma vectors species, Euscelidius variegatus and Scaphoideus titanus (Hemiptera: Cicadellidae), were used as case study. Gene silencing can be achieved efficiently in both species through microinjection of dsRNAs, despite the technique being time consuming and inapplicable on large scale. This protocol was set as gold standard for the development of a higher throughput approach. Soaking of nymphs in a solution with co-adjuvant and dsRNAs as well as insect feeding on whole plants or detached leaves immersed in a dsRNA solution were assayed as alternative delivery strategies. Nymph soaking did not induce specific gene silencing, while plant absorption proved to be suitable to deliver both a coloured solution and control dsRNAs targeting green fluorescent protein gene. Insect feeding on detached leaves immersed in dsRNA solution was selected to test silencing of two gut-specific (legumain and natterin) and one ubiquitous (ATP synthase β) genes. The expression of the three genes significantly decreased in E. variegatus insects fed on dsRNA-treated plants. Similarly, a significant reduction of ATP synthase β transcript was measured in S. titanus fed on dsRNA-treated plants. [ABSTRACT FROM AUTHOR]

The leafhopper Euscelidius variegatus Kirschbaum (Hemiptera: Cicadellidae) is a natural vector of the 'Candidatus Phytoplasma asteris' and a laboratory vector of the phytoplasma associated with grapevine Flavescence dorée. Previous studies indicated that RNA interference (RNAi) by dsRNA injection efficiently works in E. variegatus and that silencing of ATP synthase β increases mortality and impairs phytoplasma multiplication. Here, a nearly complete female sterility was observed when ATP synthase β was silenced. The sterility was associated with the absence of mature eggs in the ovaries. On the contrary, male genitalia morphology and sperm motility were similar to those observed in E. variegatus control specimens, treated with dsRNAs targeting green fluorescent protein (dsGFP). An over-expression of hexamerin (amino acid storage protein) and cathepsin L (lysosome proteinase) was observed at transcript and protein level in dsATP-injected females in comparison with dsGFP-injected insects, in both whole body and dissected ovary samples. Conversely, the expression of other genes known to be involved in oocyte development, namely vitellogenin, perilipin and digestive cystein protein, was not altered in dsATP-injected E. variegatus insects in comparison with dsGFP ones. Possible roles of ATP synthase β, hexamerin and cathepsin L in oocyte and egg development are discussed. Insecticide treatments against vectors are the main strategies to counteract phytoplasma diseases, with negative impact on environment and public health. RNAi is a promising sustainable approach against insect vectors and ATP synthase β could be a valid target gene, as its silencing, besides increasing mortality and reducing phytoplasma multiplication, induces female sterility. [ABSTRACT FROM AUTHOR]

The leafhopper Euscelidius variegatus is a natural vector of chrysanthemum yellows phytoplasma (CY) and an efficient vector of flavescence dorée phytoplasma (FD) under laboratory conditions. During a transcriptome sequencing (RNA-seq) project aimed at investigating the interactions between the insect and the two phytoplasmas, a 10,616-nucleotide-long contig with high sequence similarity to known picorna-like viruses was identified among the assembled insect transcripts. The discovery came totally unexpected, because insects from the laboratory colony did not show any evident symptom that could be related to the presence of a virus. The amino acid sequence, the shape and size of viral particles, and the results of phylogenetic analysis suggest that this virus, named Euscelidius variegatus virus 1 (EVV-1), can be considered a new member of a new species in the genus Iflavirus. EVV-1 was detected in all of the tested insects from the laboratory colony used for RNA-seq, both in phytoplasma-exposed and in non-exposed insects, but the viral load measured in FD-exposed samples was significantly lower than that in non-exposed insects. This result suggests the possible existence of an intriguing cross-talk among insects, endogenous bacteria, and viruses. The identification of two other E. variegatus laboratory colonies that were free of EVV-1 could represent the key to addressing some basic virological issues, e.g., viral replication and transmission mechanisms, and offer the opportunity to use infectious clones to express heterologous genes in the leafhopper and manipulate the expression of endogenous genes by promoting virus-induced gene silencing. [ABSTRACT FROM AUTHOR]

Scaphoideus titanus (Hemiptera: Cicadellidae) is the natural vector of Flavescence dorée phytoplasma, a quarantine pest of grapevine with severe impact on European viticulture. RNA interference (RNAi) machinery components are present in S. titanus transcriptome and injection of ATP synthase β dsRNAs into adults caused gene silencing, starting three days post injection (dpi) up to 20 dpi, leading to decrease cognate protein. Silencing of this gene in the closely related leafhopper Euscelidiusvariegatus previously showed female sterility and lack of mature eggs in ovaries. Here, alteration of developing egg morphology in S. titanus ovaries as well as overexpression of hexamerin transcript (amino acid storage protein) and cathepsin L protein (lysosome proteinase) were observed in dsATP-injected females. To evaluate RNAi-specificity, E.variegatus was used as dsRNA-receiving model-species. Different doses of two sets of dsRNA-constructs targeting distinct portions of ATP synthase β gene of both species induced silencing, lack of egg development, and female sterility in E. variegatus, indicating that off-target effects must be evaluated case by case. The effectiveness of RNAi in S. titanus provides a powerful tool for functional genomics of this non-model species and paves the way toward RNAi-based strategies to limit vector population, despite several technical and regulatory constraints that still need to be overcome to allow open field application. [ABSTRACT FROM AUTHOR]

• RNAi in the phytoplasma vector Euscelidius variegatus is long-lasting. • Silencing of ATP synthase β transcript reduces expression of the cognate protein. • dsRNA injection generates siRNAs mapping only on the fragment used to design dsRNAs. • Silenced insects acquire chrysanthemum yellows and Flavescence dorée phytoplasmas. • Silencing of ATP synthase β reduces the multiplication of both phytoplasmas. The leafhopper Euscelidius variegatus is a natural vector of the chrysanthemum yellows phytoplasma (CYp) and a laboratory vector of the Flavescence dorée phytoplasma (FDp). Previous studies indicated a crucial role for insect ATP synthase α and β subunits during phytoplasma infection of the vector species. Gene silencing of ATP synthase β was obtained by injection of specific dsRNAs in E. variegatus. Here we present the long-lasting nature of such silencing, its effects on the small RNA profile, the significant reduction of the corresponding protein expression, and the impact on phytoplasma acquisition capability. The specific transcript expression was silenced at least up to 37 days post injection with an average reduction of 100 times in insects injected with dsRNAs targeting ATP synthase β (dsATP) compared with those injected with dsRNAs targeting green fluorescent protein (dsGFP), used as negative controls. Specific silencing of this gene was also confirmed at protein level at 15 days after the injection. Total sRNA reads mapping to dsATP and dsGFP sequences in analysed libraries showed in both cases a peak of 21 nt, a length consistent with the generation of dsRNA-derived siRNAs by RNAi pathway. Reads mapped exclusively to the fragment corresponding to the injected dsATPs, probably indicating the absence of a secondary machinery for siRNA synthesis. Insects injected either with dsATP or dsGFP successfully acquired CYp and FDp during feeding on infected plants. However, the average phytoplasma amount in dsATP insects was significantly lower than that measured in dsGFP specimens, indicating a probable reduction of the pathogen multiplication when ATP synthase β was silenced. The role of the insect ATP synthase β during phytoplasma infection process is discussed. [ABSTRACT FROM AUTHOR]