Spray-Induced Gene Silencing (SIGS) has been used against pathogens. The exogenously applied ribonucleic acid (RNA) by spraying directly on to the plant is referred to as SIGS. The RNA uptake ability of many fungal pathogens, the exogenously applied RNAs targeting pathogen/pest genes, results in gene silencing and infection inhibition (Chen et al., 2023). Host-induced gene silencing (HIGS) and SIGS protect plant diseases caused by fungal pathogens. SIGS rely on external application whereas, HIGS is a transgenic technology. HIGS involves host plant expression of double-stranded RNAs (dsRNAs) of a pathogen gene and a derivative (small RNAs) are taken up by the interacting pathogen, triggering silencing of target gene, whereas SIGS involve inhibition of plant pathogen through direct spray of dsRNA or small RNAs (sRNAs) targeting pathogen gene on plant tissue (Sang and Kim 2019).
RNA interference (RNAi) technology is mediated by dsRNA and small interfering RNA (siRNA) molecules that can lead to the degradation of messenger RNA (mRNA) (He et al., 2024). A mechanism of communication between plant and their pathogens, termed cross kingdom RNAi, a phenomenon in which fungi deliver sRNA into plant to silence host plant immune response genes and in return plant also transfers sRNA packaged in extracellular vesicle (especially exosomes) into fungal pathogens to silence virulence-related genes (Cai et al., 2018). The sRNA-containing vesicles accumulate at the infection sites are taken up by the fungal cells (Cai et al., 2018).
HIGS application is limited due lack of efficient transformation technique in many crop plants. The success of SIGS in controlling plant disease is determined by pathogen’s RNA uptake efficiency. Topical application of dsRNAs targeting virulence-related genes in pathogens with high RNA uptake efficiency inhibited infection while the application of dsRNA in pathogen with low RNA uptake efficiency did not suppress infection (Qiao et al., 2021). The uptake efficiency of RNA was different among various fungi. Few fungi cannot take up dsRNA from the environment such as Colletotrichum gloeosporioides which causes anthracnose disease, thus limiting the application of SIGS in control of the plant disease caused by this fungus (Qiao et al., 2021).
The dsRNAs applied for plant protection can be via spraying, root soaking, infiltration, spreading on leaves and injection (Chen et al., 2023). The induction of gene silencing by spraying or otherwise applying RNA avoids need to develop transgenic plants (Wang and Jin 2017). SIGS may confer resistance against fungal pathogen Fusarium graminearum in distal leaf part, suggesting that dsRNAs were translocated into plant cells and tissues and the silencing signals were effectively spread to distal parts (Koch et al., 2016). Wang et al., (2016) show that Botrytis can take up external sRNAs and dsRNAs. Barley cells take up dsRNAs when sprayed on barley leaf surface and transported into other parts of plant through vasculature (Koch et al., 2016). The ds RNA and sRNA sprayed on plant surfaces, have two possible pathways to get into fungal cells, (Wang and Jin 2017):
Pathway one: The external dsRNAs and sRNAs are taken up by plant cells and then transferred into fungal cells (Koch et al., 2016; Wang and Jin 2017), these dsRNAs are cleaved into sRNA by either plant DCL (Dicer-like) proteins or by fungal DCL proteins. At the same time this transferred dsRNA and sRNA in the plant cells spread systemically and are transferred into fungal cells. The systemically spread dsRNAs are processed into sRNA mainly by the fungal DCL protein (Wang and Jin 2017).
Pathway two: The external dsRNA and sRNA are directly taken up by the fungal cells (Wang et al., 2016; Wang and Jin 2017), and the transferred dsRNA are processed into sRNAs by the fungal DCL proteins.
Movement of sRNA from plant to pathogens is by using HIGS, where sRNA are generally made by producing dsRNA in transgenic plant using Arabidopsis or in virusesthat replicate through dsRNA (Bilir et al., 2019). HIGS has potential in controlling plant disease. While the delivery of the inhibitory non-coding dsRNA by transgenic expression is promising but requires generation of transgenic crop plants which may cause substantial delay for application strategies depending on the transformability and genetic stability of the plant species (Koch et al., 2016). Silencing signal in plant is mobile (Koch et al., 2016). Silencing signal with an RNA specificity determinant moves through plasmodesmata and the phloem (Molnar et al., 2010). sRNAs are associated with transcriptional gene silencing by targeting DNA methylation to complementary sequences (Lewsey et al., 2016). The sRNA moves from shoot to root, where they regulate DNA methylation of endogenous transposable elements (Lewsey et al., 2016).
Transfer of dsRNA or siRNA from host plant cells into powdery mildew fungi; these RNAs can disturb the host-pathogen interaction by inducing silencing of fungal housekeeping genes or genes required for development or virulence (Nowara et al., 2010). The inhibitory dsRNA is more effectively absorbed by the fungus through infection hyphae that are in close contact with the plant tissue. Plant long distance trafficking occurs through xylem and phloem conduits. Xylem made up of dead cells distribute water and minerals as well as allow movement of xylem-transmitted viruses. The phloem made up of living enucleated sieve elements assisted by companion cells, distributes photo assimilates, RNA and proteins throughout plants and can transport most plant viruses (Voinnet 2005). Cell to cell communication occurs either via extra-cellular secretion or through membrane-lined pores connecting adjacent cells called plasmodesmata. Therefore, RNA silencing movement occurs through the same phloem and plasmodesmal channels as those used by plant viruses (Voinnet 2005). Exogenously applied dsRNA reaches apoplast including xylem and subsequently translocated into the symplast by an unknown mechanism (Koch et al., 2016). Given accumulation of dsRNA in plant phloem, sucking insects are also SIGS targets, as efficient control of sucking insects by HIGS has been demonstrated (Abdellatef et al., 2015; Koch et al., 2016). Efficiency of RNAi can be influenced by environmental conditions (He et al., 2024).
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