The majority of plant-infecting viruses are transmitted to their host plants by vectors. The interactions between viruses and vector vary in duration and specificity but some common themes in vector transmission have emerged: 1) plant viruses encode structural proteins on the surface of the virion that are essential for transmission, and in some cases additional non-structural helper proteins that act to bridge the virion to the vector binding site; 2) viruses bind to specific sites in or on vectors and are retained there until they are transmitted to their plant hosts; and 3) viral determinants of vector transmission are promising candidates for translational research aimed at disrupting transmission or decreasing vector populations. In this review, we focus on well-characterized insect vector-transmitted viruses in the following genera: Caulimovirus, Crinivirus, Luteovirus, Geminiviridae, Reovirus, Tospovirus, and Tenuivirus. New discoveries regarding these genera have increased our understanding of the basic mechanisms of virus transmission by arthropods, which in turn have enabled the development of innovative strategies for breaking the transmission cycle.

Most plant viruses are absolutely dependent on a vector for plant-to-plant spread. Although a number of different types of organisms are vectors for different plant viruses, phloem-feeding Hemipterans are the most common and transmit the great majority of plant viruses. The complex and specific interactions between Hemipteran vectors and the viruses they transmit have been studied intensely, and two general strategies, the capsid and helper strategies, are recognized. Both strategies are found for plant viruses that are transmitted by aphids in a nonpersistent manner. Evidence suggests that these strategies are found also for viruses transmitted in a semipersistent manner. Recent applications of molecular and cell biology techniques have helped to elucidate the mechanisms underlying the vector transmission of several plant viruses. This review examines the fundamental contributions and recent developments in this area.

Viral infection in both plant and invertebrate hosts requires a virus-encoded function to block the RNA silencing antiviral defense. Here, we report the identification and characterization of three distinct suppressors of RNA silencing encoded by the Ϸ20-kb plus-strand RNA genome of citrus tristeza virus (CTV). When introduced by genetic crosses into plants carrying a silencing transgene, both p20 and p23, but not coat protein (CP), restored expression of the transgene. Although none of the CTV proteins prevented DNA methylation of the transgene, export of the silencing signal (capable of mediating intercellular silencing spread) was detected only from the F1 plants expressing p23 and not from the CP-or p20-expressing F1 plants, demonstrating suppression of intercellular silencing by CP and p20 but not by p23. Thus, intracellular and intercellular silencing are each targeted by a CTV protein, whereas the third, p20, inhibits silencing at both levels. Notably, CP suppresses intercellular silencing without interfering with intracellular silencing. The novel property of CP suggests a mechanism distinct to p20 and all of the other viral suppressors known to interfere with intercellular silencing and that this class of viral suppressors may not be consistently identified by Agrobacterium coinfiltration because it also induces RNA silencing against the infiltrated suppressor transgene. Our analyses reveal a sophisticated viral counter-defense strategy that targets the silencing antiviral pathway at multiple steps and may be essential for protecting CTV with such a large RNA genome from antiviral silencing in the perennial tree host.RNA interference ͉ citrus tristeza virus ͉ virus synergy ͉ antiviral immunity T he discoveries of viral suppressors of RNA silencing play an important role in establishing RNA silencing as a natural antiviral response in both plant and invertebrate hosts (1). Many plant viral proteins have been identified as suppressors of RNA silencing since the initial reports in late 1998 (2-4). RNA silencing suppressors from different virus taxons are structurally diverse. However, they are typically required for long-distance virus spread and influence virulence and accumulation levels in infected plants (5-7). Progress is being made toward understanding the molecular mechanisms involved in viral suppression of RNA silencing (8-11). In RNA silencing assays, plant viral suppressors differ by their ability to suppress intracellular and͞or intercellular silencing (12-16). Intercellular silencing is mediated by the non-cell-autonomous silencing signal that can spread to destroy homologous RNAs in neighboring or distant tissues that do not contain the initial trigger, such as a silencing transgene or a replicating virus (17-19). The differential silencing suppression is best illustrated by the potyviral helper component proteinase (HC-Pro) and the cucumber mosaic virus (CMV) 2b protein. When introduced by genetic crosses into the Nicotiana tabacum line 6b5 carrying a silencing ␤-glucuronidase (GUS) transgen...

Viruses in the genus Tenuivirus (Tenuiviruses) cause a number of important diseases in economically important crop plants including rice and maize. Tenuiviruses are transmitted from plant to plant by specific planthopper vectors, and their transmission relationship is circulative-propagative. Thus, Tenuiviruses have host ranges including plants and animals (planthoppers). Four or five characteristic, circular ribonucleoprotein particles (RNPs), each containing a single Tenuivirus genomic RNA, can be isolated from Tenuivirus-infected plants. The genomic RNAs range in size from ca 9.0 kb to 1.3 kb and together give a total genome size of ca 18-19 kb. The genomic RNAs are either negative-sense or ambisense, and expression of the ambisense RNAs utilizes cap-snatching during mRNA transcription. The combination of characteristics exhibited by Tenuiviruses are quite different than those found for most plant viruses and are more similar to vertebrate-infecting viruses in the genus Phlebovirus of the Bunyaviridae.

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We examined the population structure and genetic variation of four genomic regions within and between 30 Citrus tristeza virus (CTV) isolates from Spain and California. Our analyses showed that most isolates contained a population of sequence variants, with one being predominant. Four isolates showed two major sequence variants in some genomic regions. The two major variants of three of these isolates showed very low nucleotide identity to each other but were very similar to those of other isolates, suggesting the possibility of mixed infections with two divergent isolates. Incongruencies of phylogenetic relationships in the different genomic regions and statistical analyses suggested that the genomes of some CTV sequence variants originated by recombination events between diverged sequence variants. No correlation was observed between geographic origin and nucleotide distance, and thus from a genetic view, the Spanish and Californian isolates analyzed here could be considered members of the same population.Citrus tristeza virus (CTV) is distributed worldwide and is the causal agent of one of the most economically important diseases of citrus. CTV, a member of the genus Closterovirus within the family Closteroviridae, is phloem limited and is transmitted by aphids in a semipersistent manner. CTV virions are filamentous flexuous particles about 2,000 nm long, with two coat proteins (CP and CPm) covering 95 and 5% of the particle length, respectively (8). The CTV genome is a single-stranded, positive-sense RNA of 19,226 to 19,296 nucleotides (nt) (18,27,48,51) organized in 12 open reading frames encoding at least 19 proteins. These include two papain-like proteases, replication-associated proteins (RNA polymerase, helicase, and methyltransferase), a homologue of the HSP70 protein, two coat proteins (CP and CPm), RNA-binding protein p23 (23), a p20 protein that accumulates in the amorphous inclusion bodies (14), and other proteins of so far unknown function (p61, p13, and p18) (Fig. 1). CTV-infected plants contain, in addition to the genomic RNA, 3Ј-coterminal subgenomic RNAs (15) and defective RNAs (D RNAs), the latter resulting from extensive internal deletions of the genomic RNA (2,26,28,50).CTV isolates differing in the type and intensity of symptoms induced in different citrus species and cultivars and in their aphid transmissibility have been reported worldwide (38). In the last two decades, efforts have been taken to develop molecular techniques for rapid differentiation of CTV isolates and identification of molecular markers related to CTV-induced symptoms. Variation in serological reactivity, peptide maps of the coat protein, double-stranded RNA (dsRNA) patterns, hybridization with cDNA probes, restriction fragment length polymorphism, and single-strand conformation polymorphism (SSCP) have been described in attempts to differentiate CTV isolates (29).Nucleotide sequence analysis is the most accurate procedure for CTV differentiation and estimation of molecular or genetic variation. To date, the complete g...

In this study, we completed the whole genome sequence of a new tobamovirus isolated from tomato plants grown in greenhouses in Jordan during the spring of 2015. The 6393-nt single-stranded RNA (ssRNA) genome encodes four proteins, as do other tobamoviruses: two replication-related proteins of 126 kDa and 183 kDa, a 30-kDa movement protein (MP) and a 17.5-kDa coat protein (CP). Phylogenetic analysis showed that this virus does not group with either the tomato mosaic virus (ToMV) or the tobacco mosaic virus (TMV) clades. Instead, it stems from a branch leading to the TMV clade. Analysis of possible recombination events between this virus and representative isolates of closely related tomato-infecting tobamoviruses showed that at least one region originated by recombination. We provide evidence that we have identified a new tobamovirus, for which we propose the name "tomato brown rugose fruit virus".

The potato/tomato psyllid, Bactericerca cockerelli (B. cockerelli), and the Asian citrus psyllid, Diaphorina citri (D. citri), are very important plant pests, but they are also vectors of phloem-limited bacteria that are associated with two devastating plant diseases. B. cockerelli is the vector of Candidatus Liberibacter psyllaurous (solanacearum), which is associated with zebra chip disease of potatoes, and D. citri is the vector of Ca. Liberibacter asiaticus, which is associated with the Huanglongbing (citrus greening) disease that currently threatens the entire Florida citrus industry. Here we used EST sequence information from D. citri to identify potential targets for RNA interference in B. cockerelli. We targeted ubiquitously expressed and gut-abundant mRNAs via injection and oral acquisition of double-stranded RNAs and siRNAs and were able to induce mortality in recipient psyllids. We also showed knockdown of target mRNAs, and that oral acquisition resulted primarily in mRNA knockdown in the psyllid gut. Concurrent with gene knockdown was the accumulation of target specific ∼ 21 nucleotide siRNAs for an abundant mRNA for BC-Actin. These results showed that RNAi can be a powerful tool for gene function studies in psyllids, and give support for continued efforts for investigating RNAi approaches as possible tools for psyllid and plant disease control.