Plants are challenged by pests and pathogens. Host-induced gene silencing (HIGS) is RNA interference (RNAi)-based approach in which small interfering RNAs (siRNAs) are produced in the host plant which subsequently move into pathogen to silence the pathogen genes (Govindarajulu et al., 2015). Not every pest or pathogen can efficiently uptake double-stranded RNAs (dsRNAs) (Qiao et al., 2021). The close contact between plant and fungal cell not only facilitate the exchange of proteins but also RNA as a carrier of biological information affecting the outcome of interaction (Nowara et al., 2010). The plant endogenous small RNAs (sRNAs) including microRNAs (miRNAs) and siRNAs are integral regulatory components of plant defense machinery against bacterial and fungal pathogens (Katiyar-Agarwal and Jin 2010).
The dsRNAs and siRNAs can trigger homologous RNA degradation or inhibit mRNA translation. This process is known as RNA silencing and plays a role in innate immunity (Bilir et al., 2019). Plant protection measure relies on RNAi, which mediates sRNA-directed post-transcriptional gene silencing (PTGS). PTGS is initiated by dsRNA molecules which are processed by Dicer-like proteins into siRNAs (21 – 24 nucleotides in length). Subsequently siRNAs bind to Argonaute proteins (AGOs) to form RNA induced silencing complexes (RISCs) that target complementary RNA degradation or translational inhibition (Hammond et al., 2001; Koch and Wassenegger 2021). The destruction of mRNA is accomplished by a multicomponent nuclease called RISC (Hammond et al., 2001). In plants, the RNA silencing signal move short distances from cell to cell (via intercellular plasmodesmata) or long distances (systemically via phloem vasculature) (Mermigka et al., 2016; Kehr and Kragler 2018, Dubrovina and Kiselev 2019). HIGS approach silence genes of interest in target organisms. RNAi-based technologies have been applied in form of transgenic plant generation and HIGS (Das and Sherif 2020).
HIGS is found to be effective against obligate biotrophic, hemibiotrophic fungi and oomycetes where a direct contact is established with the living cell (Govindarajulu et al., 2015; Hu et al., 2015; Sanju et al., 2015), HIGS is also found to be effective against necrotrophic fungal pathogen; (Zhou et al., 2016). HIGS efficacy, depends on uptake of the RNAi molecule (long unprocessed dsRNAs vs siRNAs) which may affect the outcome of each HIGS (Koch and Wassenegger 2021).
Recent studies suggest that plant secrete RNA into leaf apoplast (Zand Karimi et al., 2022) raises the possibility that RNA may be deposited onto the leaf surface (Karimi and Innes 2022).HIGS in some fungal pathogen may be mediated by the direct uptake of naked dsRNA as opposed to siRNA produced by the plant or dsRNAs packaged by the plant (Karimi and Innes 2022).
The transfer of dsRNA or siRNA from plant into the fungal pathogen is via an exosomal pathway; these RNA can disturb the host-pathogen interaction by inducing silencing of fungal genes required for virulence (Nowara et al., 2010). RNAs move systemically in plants, including movement of viruses via plasmodesmata and phloem (Voinnet 2005). For Phytophthora infestans, RNA uptake was limited and varied in different cell types and development stage (Qiao et al., 2021). RNA silencing signal can be transferred from host plant to the fungal pathogen and that HIGS has the potential to develop rice blast disease resistant plant (Wang and Dean 2022). During infection, fungus may be able to take up siRNA from plant cells resulting in decreased Magnaporthe oryzae transcription factor gene(MoAP1) expression via RNAi mechanism, as a result virulence is compromised because of low expression of MoAP1 (Guo et al., 2019). MoAP1 encodes a transcription factor essential for conidia production and is highly expressed in conidia as well as in invasive hyphal growth (Guo et al., 2011; Karimi and Innes 2022). Similar observations were made for fungal pathogen Puccinia triticina and Puccinia striiformis f. sp. tritici in which genes highly expressed in early stages of infection were targeted by HIGS resulting in robust resistance (Karimi and Innes 2022). Suggesting siRNA are transferred into fungi prior to the formation of haustoria possibly from the leaf surface during the growth of germ tubes (Karimi and Innes 2022).
During plant-microbe interaction, plant and microbe can exchange RNA molecules, which then integrate into RNA silencing machinery. Fungal pathogen Botrytis cinerea transports its sRNA into host plant cells, hijacks the host plant RNAi machinery by binding to the Arabidopsis AGOs and selectively silencing the host plant immunity genes (Weiberg et al., 2013). HIGS is used as a strategy to improve plant resistance by silencing the virulence genes essential for the fungal pathogens Verticillium dahlia causingwilt in tomato and Arabidopsis (Song and Thomma 2018). Suppression of fungal pathogen Phakopsora pachyrhizi causing Asian soybean rust is through HIGS and spray-induced gene silencing (Hu et al., 2020).
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