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Pawpaw (Asimina triloba) trees exhibiting stunting and foliar mosaic, chlorosis, or distortions were observed in New York. In 2021, leaf samples from two symptomatic trees and a sapling, as well as two asymptomatic trees, were tested for the presence of viruses and viroids by high-throughput sequencing (HTS) using total RNA after ribosomal RNA depletion. HTS sequence information revealed tobacco ringspot virus (TRSV) and tomato ringspot virus (ToRSV) in symptomatic but not in asymptomatic leaves. HTS reads and de novo-assembled contigs covering the genomes of both viruses were obtained, with a higher average read depth for RNA2 than RNA1. The occurrence of TRSV and ToRSV was confirmed in the original leaf samples used for HTS and 12 additional trees and saplings from New York and Maryland in 2022 by RT-PCR combined with Sanger sequencing, and DAS-ELISA. Single infections by TRSV in 11 of 14 trees and dual infections by TRSV and ToRSV in 3 of 14 trees were identified. The nucleotide sequence identity of partial gene fragments of TRSV and ToRSV was high among pawpaw isolates (94.9-100% and 91.8-100%, respectively) and between pawpaw isolates and isolates from other horticultural crops (93.6-100% and 71.3-99.3%, respectively). This study is the first to determine the virome of pawpaw.

Viral suppressors of RNA silencing (VSRs) encoded by grapevine fanleaf virus (GFLV), one of the most economically consequential viruses of grapevine ( Vitis spp.), were recently identified. GFLV VSRs include the RNA1-encoded protein 1A and the putative helicase protein 1B Hel , as well as their fused form (1AB Hel ). Key characteristics underlying the suppression function of the GFLV VSRs are unknown. In this study, we explored the role of the conserved tryptophan-glycine (WG) motif in protein 1A and glycine-tryptophan (GW) motif in protein 1B Hel in their systemic RNA silencing suppression ability by co-infiltrating Nicotiana benthamiana 16c line plants with a GFP silencing construct and a wildtype or a mutant GFLV VSR. We analyzed and compared wildtype and mutant GFLV VSRs for their (i) efficiency at suppressing RNA silencing, (ii) ability to limit siRNA accumulation, (iii) modulation of the expression of six host genes involved in RNA silencing, (iv) impact on virus infectivity in planta , and (v) variations in predicted protein structures using molecular and biochemical assays, as well as bioinformatics tools such as AlphaFold2. Mutating W to alanine (A) in WG of proteins 1A and 1AB Hel abolished their ability to induce systemic RNA silencing suppression, limit siRNA accumulation, and downregulate NbAGO2 expression by 1AB Hel . This mutation in the GFLV genome resulted in a non-infectious virus. Mutating W to A in GW of proteins 1BHel and 1ABHel reduced their ability to suppress systemic RNA silencing and abolished the downregulation of NbDCL2 , NbDCL4, , and NbRDR6 expression by 1B Hel . This mutation in the GFLV genome delayed infection at the local level and inhibited systemic infection in planta . Double mutations of W to A in WG and GW of protein 1ABHel abolished its ability to induce RNA silencing suppression, limit siRNA accumulation, and downregulate NbDCL2 and NbRDR6 expression. Finally, in silico protein structure prediction indicated that a W to A substitution potentially modifies the structure and physicochemical properties of the three GFLV VSRs. Together, this study provided insights into the specific roles of WG/GW not only in GFLV VSR functions but also in GFLV biology., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Choi, Browning, Schmitt-Keichinger and Fuchs.)

Grapevine fanleaf virus (GFLV) (genus Nepovirus , family Secoviridae ) causes fanleaf degeneration, one of the most damaging viral diseases of grapevines. Despite substantial advances at deciphering GFLV-host interactions, how this virus overcomes the host antiviral pathways of RNA silencing is poorly understood. In this study, we identified viral suppressors of RNA silencing (VSRs) encoded by GFLV, using fluorescence assays, and tested their capacity at modifying host gene expression in transgenic Nicotiana benthamiana expressing the enhanced green fluorescent protein gene ( EGFP ). Results revealed that GFLV RNA1-encoded protein 1A, for which a function had yet to be assigned, and protein 1B Hel , a putative helicase, reverse systemic RNA silencing either individually or as a fused form (1AB Hel ) predicted as an intermediary product of RNA1 polyprotein proteolytic processing. The GFLV VSRs differentially altered the expression of plant host genes involved in RNA silencing, as shown by reverse transcription-quantitative PCR. In a co-infiltration assay with an EGFP hairpin construct, protein 1A upregulated NbDCL2 , NbDCL4 , and NbRDR6 , and proteins 1B Hel and 1A+1B Hel upregulated NbDCL2 , NbDCL4 , NbAGO1 , NbAGO2 , and NbRDR6 , while protein 1AB Hel upregulated NbAGO1 and NbRDR6 . In a reversal of systemic silencing assay, protein 1A upregulated NbDCL2 and NbAGO2 and protein 1AB Hel upregulated NbDCL2 , NbDCL4 , and NbAGO1 . This is the first report of VSRs encoded by a nepovirus RNA1 and of two VSRs that act either individually or as a predicted fused form to counteract the systemic antiviral host defense, suggesting that GFLV might devise a unique counterdefense strategy to interfere with various steps of the plant antiviral RNA silencing pathways during infection. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license., Competing Interests: The author(s) declare no conflict of interest.