Cucurbit chlorotic yellows virus p22 suppressor of RNA silencing binds single-, double-stranded long and short interfering RNA molecules in vitro

Leucine 127 of Cucurbit Chlorotic Yellows Virus P22 Is Crucial for Its RNA Silencing Suppression Activity and Pathogenicity

Plant viruses produce particular suppressors to antagonize the host defense response of RNA silencing to establish infection. Cucurbit chlorotic yellows virus (CCYV), a member of the genus Crinivirus of the family Closteroviridae, severely damages the production of economically essential cucurbits worldwide. Here, we used the attenuated zucchini yellow mosaic virus (ZYMV) vector ZAC to express individual coding sequences, including CP, CPm, P25, and P22, of a Taiwan CCYV isolate (CCYV-TW) to identify their possible roles as pathogenicity determinants. ZAC is an HC-Pro function mutant that lacks the ability of local lesion induction on Chenopodium quinoa leaves and induces mild mottling followed by recovery on its natural host zucchini squash plants. Only the recombinant expressing CCYV-TW P22 complemented the effect of ZAC HC-Pro dysfunction, causing more severe symptoms on zucchini squash plants and restoring lesion formation on C. quinoa leaves, with lesions forming faster than those generated by the wild-type ZYMV. This suggests that CCYV-TW P22 is a virulence enhancer. Sequence analysis of criniviral P22s revealed the presence of four conserved leucine residues (L10, L17, L84, and L127) and one conserved lysine residue (K185). The five P22 residues conserved among the CCYV isolates and the P22 orthologs of two other criniviruses were each substituted with alanine in CCYV-TW P22 to investigate its ability to suppress RNA silencing and pathogenicity. The results provide new insights into CCYV-P22, showing that the L127 residue of P22 is indispensable for maintaining its stability in RNA silencing suppression and essential for virulence enhancement.

Impact of viral silencing suppressors on plant viral synergism: a global agro-economic concern

Plant viruses are known for their devastating impact on global agriculture. These intracellular biotrophic pathogens can infect a wide variety of plant hosts all over the world. The synergistic association of plant viruses makes the situation more alarming. It usually promotes the replication, movement, and transmission of either or both the coexisting synergistic viral partners. Although plants elicit a robust antiviral immune reaction, including gene silencing, to limit these infamous invaders, viruses counter it by encoding viral suppressors of RNA silencing (VSRs). Growing evidence also suggests that VSRs play a driving role in mediating the plant viral synergism. This review briefly discusses the evil impacts of mixed infections, especially synergism, and then comprehensively describes the emerging roles of VSRs in mediating the synergistic association of plant viruses. KEY POINTS: • Synergistic associations of plant viruses have devastating impacts on global agriculture. • Viral suppressors of RNA silencing (VSRs) play key roles in driving plant viral synergism.

Mapping of Functional Subdomains in the atALKBH9B m6A-Demethylase Required for Its Binding to the Viral RNA and to the Coat Protein of Alfalfa Mosaic Virus

N ⁶-methyladenosine (m⁶A) modification is a dynamically regulated RNA modification that impacts many cellular processes and pathways. This epitranscriptomic methylation relies on the participation of RNA methyltransferases (referred to as "writers") and demethylases (referred to as "erasers"), respectively. We previously demonstrated that the Arabidopsis thaliana protein atALKBH9B showed m⁶A-demethylase activity and interacted with the coat protein (CP) of alfalfa mosaic virus (AMV), causing a profound impact on the viral infection cycle. To dissect the functional activity of atALKBH9B in AMV infection, we performed a protein-mapping analysis to identify the putative domains required for regulating this process. In this context, the mutational analysis of the protein revealed that the residues between 427 and 467 positions are critical for in vitro binding to the AMV RNA. The atALKBH9B amino acid sequence showed intrinsically disordered regions (IDRs) located at the N-terminal part delimiting the internal AlkB-like domain and at the C-terminal part. We identified an RNA binding domain containing an RGxxxRGG motif that overlaps with the C-terminal IDR. Moreover, bimolecular fluorescent experiments allowed us to determine that residues located between 387 and 427 are critical for the interaction with the AMV CP, which should be critical for modulating the viral infection process. Finally, we observed that atALKBH9B deletions of either N-terminal 20 residues or the C-terminal's last 40 amino acids impede their accumulation in siRNA bodies. The involvement of the regions responsible for RNA and viral CP binding and those required for its localization in stress granules in the viral cycle is discussed.

Cucumber Ribosomal Protein CsRPS21 Interacts With P22 Protein of Cucurbit Chlorotic Yellows Virus

Cucurbit chlorotic yellows virus (CCYV) is a cucurbit-infecting crinivirus. RNA silencing can be initiated as a plant defense against viruses. Viruses encode various RNA silencing suppressors to counteract antiviral silencing. P22 protein encoded by RNA1 of CCYV is a silencing suppressor, but its mechanism of action remains unclear. In this study, the cucumber ribosomal-like protein CsRPS21 was found to interact with P22 protein in vitro and in vivo. A conserved CsRPS21 domain was indispensable for its nuclear localization and interaction with P22. Transient expression of CsRPS21 in Nicotiana benthamiana leaves interfered with P22 accumulation and inhibited P22 silencing suppressor activity. CsRPS21 expression in N. benthamiana protoplasts inhibited CCYV accumulation. Increasing numbers of ribosomal proteins are being found to be involved in viral infections of plants. We identified a P22-interacting ribosomal protein, CsRPS21, and uncovered its role in early viral replication and silencing suppressor activity. Our study increases knowledge of the function of ribosomal proteins during viral infection.