Molecular Pathogens 2024, Vol.15, No.4, 189-199 http://microbescipublisher.com/index.php/mp 193 Figure 2 The discovery of RTIV in Asian wild rice colonies (Adopted from Yan et al., 2022) Image caption: (A) Assembled RTIV contigs from vdSAR. (B) Schematic representation of the RTIV genome and subgenomic RNA. (C) Small RNA-Seq of Wild-rice individual, the single-nucleotide resolution maps of the positive- and negative-strand 20-24nt vsiRNAs in the viral genome from the sequenced small RNA pool. (D) The RTIV genome maps of RNA-seq reads sequenced from colony no. 9 wild rice plants. (E) Size distribution of the positive- and negative-strand small RNA reads derived from RTIV. (F) Northern blot detection of the viral genomic/subgenomic (sg) RNAs (top 2 panels) and vsiRNAs (bottom 2 panels) in two individual wild rice plants from colony no. 9. 25S rRNA was stained and U6 was probed to show equal loading. (G) Phylogenetic relationship of RTIV with known members of Polerovirus and other genera in the family Solemoviridae based on the RdRP (Adopted from Yan et al., 2022) 4 Mycoviruses in Rice-Associated Fungi 4.1 Overview of mycoviruses and their hosts Mycoviruses, or fungal viruses, are widespread across all major fungal taxa and are currently classified into 23 viral families and the genus botybirnavirus by the International Committee on the Taxonomy of Viruses (ICTV) (Hough et al., 2023). These viruses are characterized by the absence of an extracellular phase and rely on intracellular transmission mechanisms such as cell division, sporogenesis, and hyphal anastomosis. Mycoviruses have been identified in a variety of fungal hosts, including plant pathogenic fungi, endophytic fungi, and fungi involved in human and animal diseases (Kotta-Loizou, 2021). Recent advances in nucleic acid sequencing technologies have significantly increased the number of identified mycoviruses, with a notable diversity in genome types, including dsRNA, ssRNA, and ssDNA. 4.2 Transmission dynamics between fungi and plants The transmission of mycoviruses occurs primarily through intracellular routes, such as hyphal anastomosis and vertical transmission via conidia (García-Pedrajas et al., 2019; Tonka et al., 2022). This mode of transmission limits the spread of mycoviruses to individuals within the same or closely related vegetative compatibility groups (Ghabrial et al., 2015). However, artificial transfection methods have shown promise in expanding the host range of mycoviruses, potentially allowing them to infect a variety of fungal species. This expanded host range could be leveraged to control a broader spectrum of fungal pathogens in agricultural settings. 4.3 Influence of mycoviruses on fungal pathogenicity in rice Mycoviruses can significantly influence the pathogenicity of their fungal hosts, often inducing hypovirulence, which reduces the virulence of the fungus and its ability to cause disease. For instance, mycoviruses have been shown to alter the pathogenicity of Rhizoctonia solani, a major pathogen responsible for rice sheath blight, by inducing hypovirulence (Abdoulaye et al., 2019; Umer et al., 2023). This hypovirulence is associated with changes in the expression of key pathogenicity factor genes in the fungal host, thereby reducing its ability to infect and damage rice plants (Zhang et al., 2020). 4.4 Role of mycoviruses in controlling fungal infections The potential of mycoviruses as biocontrol agents has garnered significant interest due to their ability to reduce the virulence of plant pathogenic fungi. Mycoviruses that induce hypovirulence in their fungal hosts can be used to manage fungal diseases in crops, including rice. For example, the application of hypovirulent mycovirus-infected fungal strains has been shown to reduce the severity of diseases such as rice sheath blight and improve crop yields. The diversity of mycoviruses identified in major rice pathogens, such as Pyricularia oryzae, Ustilaginoidea virens, and Rhizoctonia solani, highlights the potential for developing targeted biocontrol strategies using specific mycoviruses (He et al., 2022). 5 Mechanisms of Virus and Mycovirus-Plant Interactions 5.1 Molecular and cellular mechanisms of rice-virus interactions Rice plants have evolved sophisticated mechanisms to counteract viral infections. These mechanisms include the activation of resistance (R) genes, RNA silencing, and autophagy. R genes play a crucial role in recognizing specific viral proteins and triggering defense responses. RNA silencing, mediated by virus-derived small interfering RNAs (vsiRNAs), targets viral RNA for degradation, thereby limiting viral replication. Autophagy, a cellular degradation process, also contributes to antiviral defense by degrading viral components and enhancing immune responses (Jin et al., 2020).
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