Closteroviridae

Abstract: Citrus tristeza virus (CTV) (genus Closterovirus, family Closteroviridae) is the causal agent of devastating epidemics that changed the course of the citrus industry. Adapted to replicate in phloem cells of a few species within the family Rutaceae and to transmission by a few aphid species, CTV and citrus probably coevolved for centuries at the site of origin of citrus plants. CTV dispersal to other regions and its interaction with new scion varieties and rootstock combinations resulted in three distinct syndromes named tristeza, stem pitting and seedling yellows. The first, inciting decline of varieties propagated on sour orange, has forced the rebuilding of many citrus industries using tristeza-tolerant rootstocks. The second, inducing stunting, stem pitting and low bearing of some varieties, causes economic losses in an increasing number of countries. The third is usually observed by biological indexing, but rarely in the field. CTV polar virions are composed of two capsid proteins and a single-stranded, positive-sense genomic RNA (gRNA) of approximately 20 kb, containing 12 open reading frames (ORFs) and two untranslated regions (UTRs). ORFs 1a and 1b, encoding proteins of the replicase complex, are directly translated from the gRNA, and together with the 5' and 3'UTRs are the only regions required for RNA replication. The remaining ORFs, expressed via 3'-coterminal subgenomic RNAs, encode proteins required for virion assembly and movement (p6, p65, p61, p27 and p25), asymmetrical accumulation of positive and negative strands during RNA replication (p23), or suppression of post-transcriptional gene silencing (p25, p20 and p23), with the role of proteins p33, p18 and p13 as yet unknown. Analysis of genetic variation in CTV isolates revealed (1) conservation of genomes in distant geographical regions, with a limited repertoire of genotypes, (2) uneven distribution of variation along the gRNA, (3) frequent recombination events and (4) different selection pressures shaping CTV populations. Measures to control CTV damage include quarantine and budwood certification programmes, elimination of infected trees, use of tristeza-tolerant rootstocks, or cross protection with mild isolates, depending on CTV incidence and on the virus strains and host varieties predominant in each region. Incorporating resistance genes into commercial varieties by conventional breeding is presently unfeasible, whereas incorporation of pathogen-derived resistance by plant transformation has yielded variable results, indicating that the CTV-citrus interaction may be more specific and complex than initially thought. A deep understanding of the interactions between viral proteins and host and vector factors will be necessary to develop reliable and sound control measures.

Abstract: The family Closteroviridae comprises more than 30 plant viruses with flexuous, filamentous virions and includes representatives with either mono- or bipartite positive-strand ssRNA genomes. Closteroviruses are transmitted semipersistently by insects from three families of Homoptera, in infected plants are associated with phloem tissue, and demonstrate an astonishing genetic diversity that suggests extensive, on-going evolution. Phylogenetic analyses of their replicative genes as well as the conserved HSP70 demonstrate that closteroviruses co-evolved with their insect vectors, resulting in three major lineages, i.e. aphid-, mealybug-, and whitefly-transmitted viruses. Closteroviruses apparently represent an ancient and diverse virus family that may pose threats to agriculture and needs serious attention.

Abstract: Grapevine leafroll-associated virus 2 (GLRaV-2) was recognized in 1984, but its nomenclature was settled only in 1995. It is one of the five viruses involved in grapevine leafroll disease. It is also associated with graft incompatibility disorders. Because of peculiar genome structure and evolutionary history, GLRaV-2 belongs to the genus Closterovirus in the family Closteroviridae. The GLRaV-2 genome is a single-stranded, positive-sense RNA of ca. 16,500 nucleotides that is presumably capped at the 5′ terminus but lacks a poly(A) tail at the 3′ end. The nine ORFs encoded by GLRaV-2 comprise three functional modules, including the replication module, the quintuple gene block module, and the module for RNA silencing suppressors. The translation product of ORF1a lacks an AlkB domain, unlike all members of the genus Ampelovirus that infect the grapevine. The genetic diversity of GLRaV-2 is very high, with six lineages that are characterized by up to 25% nucleotide divergence and different biological behavior on plant hosts. All Vitis vinifera, rootstocks, and interspecific hybrids are susceptible to GLRaV-2. The identification of GLRaV-2 in wild Vitis spp. native to the American continent supports the hypothesis that the virus originated from North America. The virus is restricted to the phloem tissue, and infected cells are characterized by a massive presence of membranous vesicles that are typical of infection by members of the genus Closterovirus. No arthropod vector is known for GLRaV-2, but the virus is graft transmissible and mechanically transmissible to some herbaceous hosts, unlike other GLRaVs. Management of GLRaV-2 is through the use of virus-tested, clean propagation materials. The recent development of infectious GLRaV-2 clones sets the foundation for studies on gene function, virus replication, pathological properties, as well as virus-host interactions.

Abstract: Since 1997, a yellowing disease has been observed in greenhouse tomato (Lycopersicon escu-lentum). By 2001, the disease was widespread, including open field tomato crops, and in most cases its incidence was 80 to 90% or even 100%. Epidemics in glasshouses were mainly associated with high populations of the whitefly Trialeurodes vaporariorum and Bemisia tabaci, the major whitefly pests in vegetable crops in Greece. The main leaf symptoms were severe yellowing, rolling, and brittleness. Samples from symptomatic plants were analyzed by polymerase chain reaction (PCR) and shown to be infected with Tomato infectious chlorosis virus (TICV) and Tomato chlorosis virus (ToCV) (family Closteroviridae, genus Crinivirus). TICV was found in 164 of 183 symptomatic samples, while ToCV was less representative (25/183). Sequence comparisons of the amplified 229-bp and 466-bp products revealed 99 and 100% identity with the reported sequences of TICV and ToCV, respectively. A reverse transcription (RT) multiplex PCR assay using a simple sample preparation procedure was developed to allow rapid, specific, and simultaneous detection of both ToCV and TICV sequences in two steps. The method involves a one-tube RT-PCR step in which the combination of primers amplifies part of the heat shock protein to homologue gene of both ToCV and TICV, followed by a multiplex nested PCR amplification. This is the first report of TICV and ToCV in Greece and, as far as we know, the first report of TICV in Europe.