The nonstructural protein NSs encoded by tomato zonate spot virus suppresses RNA silencing by interacting with NbSGS3

Citrus tristeza virus p20 suppresses antiviral RNA silencing by co-opting autophagy-related protein 8 to mediate the autophagic degradation of SGS3

Viruses exploit autophagy to degrade host immune components for their successful infection. However, how viral factors sequester the autophagic substrates into autophagosomes remains largely unknown. In this study, we showed that p20 protein, a viral suppressor of RNA silencing (VSR) encoded by citrus tristeza virus (CTV), mediated autophagic degradation of SUPPRESSOR OF GENE SILENCING 3 (SGS3), a plant-specific RNA-binding protein that is pivotal in antiviral RNA silencing. CTV infection activated autophagy, and the overexpression of p20 was sufficient to induce autophagy. Silencing of autophagy-related genes NbATG5 and NbATG7 attenuated CTV infection in Nicotiana benthamiana plants. In contrast, knockdown of the autophagy negative-regulated genes NbGAPCs led to virus accumulation, indicating the proviral role of autophagy in CTV infection. Further investigation found that p20 interacted with autophagy-related protein ATG8 through two ATG8-interacting motifs (AIMs) and sequestered SGS3 into autophagosomes by forming the ATG8-p20-SGS3 ternary complex. The mutations of the two AIMs in p20 (p20mAIM1 and p20mAIM5) abolished the interaction of p20 with ATG8, resulting in the deficiency of autophagy induction, SGS3 degradation, and VSR activity. Consistently, N. benthamiana plants infected with mutated CTVmAIM1 and CTVmAIM5 showed milder symptoms and decreased viral accumulation. Taken together, this study uncovers the molecular mechanism underlying how a VSR mediates the interplay between RNA silencing and autophagy to enhance the infection of a closterovirus.

A novel RAV transcription factor from pear interacts with viral RNA-silencing suppressors to inhibit viral infection

In plants, RNA silencing constitutes a strong defense against viral infection, which viruses counteract with RNA-silencing suppressors (RSSs). Understanding the interactions between viral RSSs and host factors is crucial for elucidating the molecular arms race between viruses and host plants. We report that the helicase motif (Hel) of the replicase encoded by apple stem grooving virus (ASGV)-the main virus affecting pear trees in China-is an RSS that can inhibit both local and systemic RNA silencing, possibly by binding double-stranded (ds) siRNA. The transcription factor related to ABSCISIC ACID INSENSITIVE3/VIVIPAROUS1 from pear (PbRAV1) enters the cytoplasm and binds Hel through its C terminus, thereby attenuating its RSS activity by reducing its binding affinity to 21- and 24-nt ds siRNA, and suppressing ASGV infection. PbRAV1 can also target p24, an RSS encoded by grapevine leafroll-associated virus 2 (GLRaV-2), with similar negative effects on p24's suppressive function and inhibition of GLRaV-2 infection. Moreover, like the positive role of the PbRAV1 homolog from grapevine (VvRAV1) in p24's previously reported RSS activity, ASGV Hel can also hijack VvRAV1 and employ the protein to sequester 21-nt ds siRNA, thereby enhancing its own RSS activity and promoting ASGV infection. Furthermore, PbRAV1 neither interacts with CP, an RSS encoded by grapevine inner necrosis virus, nor has any obvious effect on CP's RSS activity. Our results identify an RSS encoded by ASGV and demonstrate that PbRAV1, representing a novel type of RAV transcription factor, plays a defensive role against viral infection by targeting viral RSSs.

The functions of triple gene block proteins and coat protein of apple stem pitting virus in viral cell-to-cell movement

Apple stem pitting virus is a species in the genus Foveavirus in the family Betaflexiviridae. Apple stem pitting virus (ASPV) commonly infects apple and pear plants grown worldwide. In this study, by integrating bimolecular fluorescence complementation, split-ubiquitin-based membrane yeast two-hybrid, and Agrobacterium-mediated expression assays, the interaction relationships and the subcellular locations of ASPV proteins TGBp1-3 and CP in Nicotiana benthamiana leaf cells were determined. Proteins CP, TGBp1, TGBp2, and TGBp3 were self-interactable, and TGBp2 played a role in the formation of perinuclear viroplasm and enhanced the colocalization of TGBp3 with CP and TGBp1. We found that the plant microfilament and endoplasmic reticulum structures were involved in the production of TGBp3 and TGBp2 vesicles, and their disruption decreased the virus accumulation level in the systemic leaves. The TGBp3 motile vesicles functioned in delivering the viral ribonucleoprotein complexes to the plasma membrane. Two cysteine residues at sites 35 and 49 of the TGBp3 sorting signal were necessary for the diffusion of TGBp3-marked vesicles. Furthermore, our results revealed that TGBp1, TGBp2, and CP could increase plasmodesmal permeability and move to the adjacent cells. This study demonstrates an interaction network and a subcellular location map of four ASPV proteins and for the first time provides insight into the functions of these proteins in the movement of a foveavirus.

The Single Distinct Leader Protease Encoded by Alpinia oxyphylla Mosaic Virus (Genus Macluravirus) Suppresses RNA Silencing Through Interfering with Double-Stranded RNA Synthesis

The genomic 5'-terminal regions of viruses in the family Potyviridae (potyvirids) encode two types of leader proteases: serine-protease (P1) and cysteine-protease (HCPro), which differ greatly in the arrangement and sequence composition among inter-genus viruses. Most potyvirids have the same tandemly arranged P1 and HCPro, whereas viruses in the genus Macluravirus encode a single distinct leader protease, a truncated version of HCPro with yet-unknown functions. We investigated the RNA silencing suppression (RSS) activity and its underpinning mechanism of the distinct HCPro from alpinia oxyphylla mosaic macluravirus (aHCPro). Sequence analysis revealed that macluraviral HCPros have obvious truncations in the N-terminal and middle regions when aligned to their counterparts in potyviruses (well-characterized viral suppressors of RNA silencing). Nearly all defined elements essential for the RSS activity of potyviral counterparts are not distinguished in macluraviral HCPros. Here, we demonstrated that aHCPro exhibits a similar anti-silencing activity with the potyviral counterpart. However, aHCPro fails to block both the local and systemic spreading of RNA silencing. In line, aHCPro interferes with the dsRNA synthesis, an upstream step in the RNA silencing pathway. Affinity-purification and NanoLC-MS/MS analysis revealed that aHCPro has no association with core components or their potential interactors involving in dsRNA synthesis from the protein layer. Instead, the ectopic expression of aHCPro significantly reduces the transcript abundance of RDR2, RDR6, SGS3, and SDE5. This study represents the first report on the anti-silencing function of Macluravirus-encoded HCPro and the underlying molecular mechanism.

Turnip crinkle virus-encoded suppressor of RNA silencing interacts with Arabidopsis SGS3 to enhance virus infection

Most plant viruses encode suppressors of RNA silencing (VSRs) to protect themselves from antiviral RNA silencing in host plants. The capsid protein (CP) of Turnip crinkle virus (TCV) is a well-characterized VSR, whereas SUPPRESSOR OF GENE SILENCING 3 (SGS3) is an important plant-encoded component of the RNA silencing pathways. Whether the VSR activity of TCV CP requires it to engage SGS3 in plant cells has yet to be investigated. Here, we report that TCV CP interacts with SGS3 of Arabidopsis in both yeast and plant cells. The interaction was identified with the yeast two-hybrid system, and corroborated with bimolecular fluorescence complementation and intracellular co-localization assays in Nicotiana benthamiana cells. While multiple partial TCV CP fragments could independently interact with SGS3, its hinge domain connecting the surface and protruding domains appears to be essential for this interaction. Conversely, SGS3 enlists its N-terminal domain and the XS rice gene X and SGS3 (XS) domain as the primary CP-interacting sites. Interestingly, SGS3 appears to stimulate TCV accumulation because viral RNA levels of a TCV mutant with low VSR activities decreased in the sgs3 knockout mutants, but increased in the SGS3-overexpressing transgenic plants. Transgenic Arabidopsis plants overexpressing TCV CP exhibited developmental abnormalities that resembled sgs3 knockout mutants and caused similar defects in the biogenesis of trans-acting small interfering RNAs. Our data suggest that TCV CP interacts with multiple RNA silencing pathway components that include SGS3, as well as previously reported DRB4 (dsRNA-binding protein 4) and AGO2 (ARGONAUTE protein 2), to achieve efficient suppression of RNA silencing-mediated antiviral defence.

P3/P3N-PIPO of PVY interacting with BI-1 inhibits the degradation of NIb by ATG6 to facilitate virus replication in N. benthamiana

Introduction

Autophagy not only plays an antiviral role but also can be utilized by viruses to facilitate virus infection. However, the underlying mechanism of potato virus Y (PVY) infection against plant autophagy remains unclear. BI-1, localizing to the endoplasmic reticulum (ER), is a multifunctional protein and may affect the virus infection.

Methods

In this study, Y2H, BiFC, qRT-PCR, RNA-Seq, WB and so on were used for research.

Results

P3 and P3N-PIPO of PVY can interact with the Bax inhibitor 1 (BI-1) of N. benthamiana. However, BI-1 knockout mutant showed better growth and development ability. In addition, when the BI-1 gene was knocked out or knocked down in N. benthamiana, the PVY-infected mutant showed milder symptoms and lower virus accumulation. Analysis of transcriptome data showed that the deletion of NbBI-1 weakened the gene expression regulation induced by PVY infection and NbBI-1 may reduce the mRNA level of NbATG6 by regulated IRE1-dependent decay (RIDD) in PVY-infected N. benthamiana. The expression level of the ATG6 gene of PVY-infected WT was significantly down-regulated, relative to the PVY-infected mutant. Further results showed that ATG6 of N. benthamiana can degrade NIb, the RNA-dependent RNA polymerase (RdRp) of PVY. NbATG6 has a higher mRNA level in PVY-infected BI-1 knockout mutants than in PVY-infected WT.

Conclussion

The interaction of P3 and/or P3N-PIPO of PVY with BI-1 decrease the expression of the ATG6 gene might be mediated by RIDD, which inhibits the degradation of viral NIb and enhances viral replication.