A plant virus movement protein regulates the Gcn2p kinase in budding yeast

A New Ilarvirus Found in French Hydrangea

In this study, a new virus was identified in French hydrangea plants, exhibiting chlorotic vein banding and necrotic ring spots on older leaves. The virus was mechanically transmitted to herbaceous hosts, in which it induced local and systemic or only local symptoms. The genome of the new virus was characterized and consisted of three RNA sequences that were 3422 (RNA 1), 2905 (RNA 2) and 2299 (RNA 3) nucleotides long, with five predicted open reading frames; RNA2 was bicistronic and contained conserved domains and motifs typical of ilarviruses. The phylogenetic analysis of the predicted proteins—p1, p2a, p3a and p3b—revealed its close relationship to recognized members of subgroup 2 within the genus Ilarvirus. Homologous antiserum was effective in the detection of the virus in plant extracts and no cross reactions with two other distinct members of subgroup 2 were observed. Overall, the biological features, phylogenetic relationships and serological data suggest that this virus is a new member of the genus, for which we propose the name hydrangea vein banding virus (HdVBV).

Patellins 3 and 6, two members of the Plant Patellin family, interact with the movement protein of Alfalfa mosaic virus and interfere with viral movement

Movement proteins (MPs) encoded by plant viruses interact with host proteins to facilitate or interfere with intra- and/or intercellular viral movement. Using yeast two-hybrid and bimolecular fluorescence complementation assays, we herein present in vivo evidence for the interaction between Alfalfa mosaic virus (AMV) MP and Arabidopsis Patellin 3 (atPATL3) and Patellin 6 (atPATL6), two proteins containing a Sec14 domain. Proteins with Sec14 domains are implicated in membrane trafficking, cytoskeleton dynamics, lipid metabolism and lipid-mediated regulatory functions. Interestingly, the overexpression of atPATL3 and/or atPATL6 interfered with the plasmodesmata targeting of AMV MP and correlated with reduced infection foci size. Consistently, the viral RNA levels increased in the single and double Arabidopsis knockout mutants for atPATL3 and atPATL6. Our results indicate that, in general, MP-PATL interactions interfere with the correct subcellular targeting of MP, thus rendering the intracellular transport of viral MP-containing complexes less efficient and diminishing cell-to-cell movement.

Phosphorylation of alfalfa mosaic virus movement protein in vivo

The 32-kDa movement protein, P3, of alfalfa mosaic virus (AMV) is essential for cell-to-cell spread of the virus in plants. P3 shares many properties with other virus movement proteins (MPs); however, it is not known if P3 is posttranslationally modified by phosphorylation, which is important for the function of other MPs. When expressed in Nicotiana tabacum, P3 accumulated primarily in the cell walls of older leaves or in the cytosol of younger leaves. When expressed in Pischia pastoris, P3 accumulated primarily in a soluble form. Metabolic labeling indicated that a portion of P3 was phosphorylated in both tobacco and yeast, suggesting that phosphorylation regulates the function of this protein as it does for other virus MPs.

The molecular biology of ilarviruses

Ilarviruses were among the first 16 groups of plant viruses approved by ICTV. Like Alfalfa mosaic virus (AMV), bromoviruses, and cucumoviruses they are isometric viruses and possess a single-stranded, tripartite RNA genome. However, unlike these other three groups, ilarviruses were recognized as being recalcitrant subjects for research (their ready lability is reflected in the sigla used to create the group name) and were renowned as unpromising subjects for the production of antisera. However, it was recognized that they shared properties with AMV when the phenomenon of genome activation, in which the coat protein (CP) of the virus is required to be present to initiate infection, was demonstrated to cross group boundaries. The CP of AMV could activate the genome of an ilarvirus and vice versa. Development of the molecular information for ilarviruses lagged behind the knowledge available for the more extensively studied AMV, bromoviruses, and cucumoviruses. In the past 20 years, genomic data for most known ilarviruses have been developed facilitating their detection and allowing the factors involved in the molecular biology of the genus to be investigated. Much information has been obtained using Prunus necrotic ringspot virus and the more extensively studied AMV. A relationship between some ilarviruses and the cucumoviruses has been defined with the recognition that members of both genera encode a 2b protein involved in RNA silencing and long distance viral movement. Here, we present a review of the current knowledge of both the taxonomy and the molecular biology of this genus of agronomically and horticulturally important viruses.

Ilarviruses of Prunus spp.: a continued concern for fruit trees

Prunus spp. are affected by a large number of viruses, causing significant economic losses through either direct or indirect damage, which results in reduced yield and fruit quality. Among these viruses, members of the genus Ilarvirus (isometric labile ringspot viruses) occupy a significant position due to their distribution worldwide. Although symptoms caused by these types of viruses were reported early in the last century, their molecular characterization was not achieved until the 1990s, much later than for other agronomically relevant viruses. This was mainly due to the characteristic liability of virus particles in tissue extracts. In addition, ilarviruses, together with Alfalfa mosaic virus, are unique among plant viruses in that they require a few molecules of the coat protein in the inoculum in order to be infectious, a phenomenon known as genome activation. Another factor that has made the study of this group of viruses difficult is that infectious clones have been obtained only for the type member of the genus, Tobacco streak virus. Four ilarviruses, Prunus necrotic ringspot virus, Prune dwarf virus, Apple mosaic virus, and American plum line pattern virus, are pathogens of the main cultivated fruit trees. As stated in the 9th Report of the International Committee on Taxonomy of Viruses, virions of this genus are "unpromising subjects for the raising of good antisera." With the advent of molecular approaches for their detection and characterization, it has been possible to get a more precise view of their prevalence and genome organization. This review updates our knowledge on the incidence, genome organization and expression, genetic diversity, modes of transmission, and diagnosis, as well as control of this peculiar group of viruses affecting fruit trees.

A critical review of translation initiation factor eIF2α kinases in plants - regulating protein synthesis during stress

Eukaryotic cells must cope with environmental stress. One type of general stress response is the downregulation of protein synthesis in order to conserve cellular resources. Protein synthesis is mainly regulated at the level of mRNA translation initiation and when the α subunit of eukaryotic translation initiation factor 2 (eIF2) is phosphorylated, protein synthesis is downregulated. Although eIF2 has the same translation initiation function in all eukaryotes, it is not known whether plants downregulate protein synthesis via eIF2α phosphorylation. Similarly, although there is evidence that plants possess eIF2α kinases, it is not known whether they operate in a similar manner to the well characterised mammalian and yeast eIF2α kinases. Two types of eIF2α kinases have been reported in plants, yet the full understanding of the plant eIF2α phosphorylation mechanism is still lacking. Here we review the current knowledge of the eIF2α phosphorylation mechanism within plants and discuss plant eIF2α, plant eIF2α kinase GCN2 and the data supporting and contradicting the hypothesis that a functional orthologue for the eIF2α kinase PKR, is present and functional in plants.