The roles of membranes and associated cytoskeleton in plant virus replication and cell-to-cell movement.

The infection of plants by viruses depends on cellular mechanisms that support the replication of the viral genomes, and the cell-to-cell and systemic movement of the virus via plasmodesmata (PD) and the connected phloem. While the propagation of some viruses requires the conventional endoplasmic reticulum (ER)-Golgi pathway, others replicate and spread between cells in association with the ER and are independent of this pathway. Using selected viruses as examples, this review re-examines the involvement of membranes and the cytoskeleton during virus infection and proposes potential roles of class VIII myosins and membrane-tethering proteins in controlling viral functions at specific ER subdomains, such as cortical microtubule-associated ER sites, ER-plasma membrane contact sites, and PD.

Turnip mosaic virus in oilseed rape activates networks of sRNA-mediated interactions between viral and host genomes.

Virus-induced plant diseases in cultivated plants cause important damages in yield. Although the mechanisms of virus infection are intensely studied at the cell biology level, only little is known about the molecular dialog between the invading virus and the host genome. Here we describe a combinatorial genome-wide approach to identify networks of sRNAs-guided post-transcriptional regulation within local Turnip mosaic virus (TuMV) infection sites in Brassica napus leaves. We show that the induction of host-encoded, virus-activated small interfering RNAs (vasiRNAs) observed in virus-infected tissues is accompanied by site-specific cleavage events on both viral and host RNAs that recalls the activity of small RNA-induced silencing complexes (RISC). Cleavage events also involve virus-derived siRNA (vsiRNA)-directed cleavage of target host transcripts as well as cleavage of viral RNA by both host vasiRNAs and vsiRNAs. Furthermore, certain coding genes act as virus-activated regulatory hubs to produce vasiRNAs for the targeting of other host genes. The observations draw an advanced model of plant-virus interactions and provide insights into the complex regulatory networking at the plant-virus interface within cells undergoing early stages of infection.

m5C Methylation Guides Systemic Transport of Messenger RNA over Graft Junctions in Plants.

In plants, transcripts move to distant body parts to potentially act as systemic signals regulating development and growth. Thousands of messenger RNAs (mRNAs) are transported across graft junctions via the phloem to distinct plant parts. Little is known regarding features, structural motifs, and potential base modifications of transported transcripts and how these may affect their mobility. We identified Arabidopsis thaliana mRNAs harboring the modified base 5-methylcytosine (m5C) and found that these are significantly enriched in mRNAs previously described as mobile, moving over graft junctions to distinct plant parts. We confirm this finding with graft-mobile methylated mRNAs TRANSLATIONALLY CONTROLLED TUMOR PROTEIN 1 (TCTP1) and HEAT SHOCK COGNATE PROTEIN 70.1 (HSC70.1), whose mRNA transport is diminished in mutants deficient in m5C mRNA methylation. Together, our results point toward an essential role of cytosine methylation in systemic mRNA mobility in plants and that TCTP1 mRNA mobility is required for its signaling function.

Crosstalk between PTGS and TGS pathways in natural antiviral immunity and disease recovery.

Virus-induced diseases cause severe damage to cultivated plants, resulting in crop losses. Certain plant-virus interactions allow disease recovery at later stages of infection and have the potential to reveal important molecular targets for achieving disease control. Although recovery is known to involve antiviral RNA silencing1,2, the specific components of the many plant RNA silencing pathways 3 required for recovery are not known. We found that Arabidopsis thaliana plants infected with oilseed rape mosaic virus (ORMV) undergo symptom recovery. The recovered leaves contain infectious, replicating virus, but exhibit a loss of viral suppressor of RNA silencing (VSR) protein activity. We demonstrate that recovery depends on the 21-22 nt siRNA-mediated post-transcriptional gene silencing (PTGS) pathway and on components of a transcriptional gene silencing (TGS) pathway that is known to facilitate non-cell-autonomous silencing signalling. Collectively, our observations indicate that recovery reflects the establishment of a tolerant state in infected tissues and occurs following robust delivery of antiviral secondary siRNAs from source to sink tissues, and establishment of a dosage able to block the VSR activity involved in the formation of disease symptoms.