Viral and Cellular Factors Involved in Phloem Transport of Plant Viruses

Abstract

Phloem transport of plant viruses is an essential step in the setting-up of a complete infection of a host plant. After an initial replication step in the first cells, viruses spread from cell-to-cell through mesophyll cells, until they reach the vasculature where they rapidly move to distant sites in order to establish the infection of the whole plant. This last step is referred to as systemic transport, or long-distance movement, and involves virus crossings through several cellular barriers: bundle sheath, vascular parenchyma, and companion cells for virus loading into sieve elements (SE). Viruses are then passively transported within the source-to-sink flow of photoassimilates and are unloaded from SE into sink tissues. However, the molecular mechanisms governing virus long-distance movement are far from being understood. While most viruses seem to move systemically as virus particles, some viruses are transported in SE as viral ribonucleoprotein complexes (RNP). The nature of the cellular and viral factors constituting these RNPs is still poorly known. The topic of this review will mainly focus on the host and viral factors that facilitate or restrict virus long-distance movement.

Comparative Proteomics Analysis of Arabidopsis Phloem Exudates Collected During the Induction of Systemic Acquired Resistance

“…Not surprisingly, some pathogens have coopted the plant vasculature to better exploit their hosts. Classic examples of this strategy include the systemic movement of plant viruses through the phloem (Hipper et al, 2013), vasculature-infecting microbes (Yadeta and Thomma, 2013), and phloem-feeding herbivores (Kaloshian and Walling, 2005;Howe and Jander, 2008). In response, plants have developed sophisticated interorgan resistance responses to limit the spread of infecting pathogens as well as to prevent and/or limit the effectiveness of future infection(s).…”

The Nucleolar Fibrillarin Protein Is Required for Helper Virus-Independent Long-Distance Trafficking of a Subviral Satellite RNA in Plants

“…For example, phloem RBP50 binds to the polypyrimidinetract binding motif of GA-INSENSITIVE PHLOEM RNA ) for phloem-mediated trafficking. In addition, phloem RBPs can selectively bind to small RNAs, as well as mRNAs or viral RNAs, to mediate their trafficking (Aoki et al, 2005;Kehr and Buhtz, 2008;Ham et al, 2009;Hipper et al, 2013). Thus, phloem RBPs are translocated with plant RNAs and are likely to be important determinants of plant RNA vascular trafficking (Lucas et al, 2001;Aoki et al, 2005;Kehr and Buhtz, 2008;Ham et al, 2009;Turgeon and Wolf, 2009;Pallas and Gómez, 2013).…”

“…Viral spread from infected cells to neighboring cells requires that the PD size exclusion limits be increased through the action of viral movement protein (MP) (Gopinath and Kao, 2007;Canetta et al, 2008;Harries et al, 2009;Hipper et al, 2013). Changes in PD permeability are also thought to enable movement into the vascular system during systemic phloem-mediated trafficking.…”

“…Changes in PD permeability are also thought to enable movement into the vascular system during systemic phloem-mediated trafficking. Thus, viruses must have evolved various strategies to interact with cellular factors to be loaded into and unloaded from the vascular system (Chen et al, 2000;Kim et al, 2007;Harries et al, 2009;Raffaele et al, 2009;Taliansky et al, 2010;Semashko et al, 2012;Hipper et al, 2013). Host factors such as myosin are required for MP targeting to and virus movement through the PD (Amari et al, 2014;Harries et al, 2009); intercellular and long-distance trafficking of Tobacco mosaic virus (TMV) (Chen et al, 2000) and Groundnut rosette virus (GRV) (Kim et al, 2007;Taliansky et al, 2010;Semashko et al, 2012) were substantially delayed in plants silenced for PECTIN METHYLESTERASE and fibrillarin.…”

Membrane-associated virus replication complexes locate to plant conducting tubes

“…Most plant viruses move systemically through the phloem along the sourceto-sink flow of photoassimilates for long-distance movement (reviewed in 4 ). The viral entity loads into phloem sieve elements through pore-plasmodesmata units (PPUs) that connect the sieve elements and companion cells in all vein classes of source leaves.…”