Potexviruses and Carlaviruses – Short Filamentous Viruses

Abstract

Potexviruses and carlaviruses are plant‐infecting viruses belonging to the family Aphaflexiviridae and Betaflexiviridae, respectively. Both have slightly flexuous filamentous particles measuring 470–580×13 nm (potexviruses), and 610–700×12–15 nm (carlaviruses). Virions contain 94–95% protein and 5–6% positive sense, single‐stranded ribonucleic acidc. 5800–7000 nucleotides (nt) (potexviruses) and 7900–9000 nt (carlaviruses) in size. Both virus types have a genome with a similar structural organisation, comprising five (potexviruses) or six (carlaviruses) open reading frames, three of which, known as the ‘triple gene block’, encode polypeptides that facilitate virus movement within the host. Natural hosts of both virus types are herbaceous (weeds, ornamentals, vegetables) rather than woody plants. Many potexviruses are agents of severe diseases, whereas a high percentage of carlaviruses induces symptomless infections. Transmission of potexviruses in nature is, by and large, by mechanical contact, whereas carlaviruses are transmitted primarily by aphids with a nonpersistent modality.

Key Concepts:

  • Potexviruses and carlaviruses infect primarily herbaceous hosts after mechanical or vector‐driven transmission, respectively.

  • Particles of both genera are filamentous and slightly flexuous and accumulate in aggregates in the cytoplasm of parenchyma cells.

  • Demarcation of species within the genera takes into account molecular and biological criteria.

  • Both genera are characterised by the presence of the so‐called triple gene block that provides the support for virus movement in infected hosts.

  • Potexviruses and carlaviruses constitute models for virus replication, movement and encapsidation studies.

  • Control is based on preventive measures, such as sanitary selection, production of virus‐free seed lots and resistance through conventional breeding and genetic engineering.

Keywords: taxonomy; plant viruses; genus Potexvirus; genus Carlavirus; mechanical transmission; insect‐transmitted pathogens; nonpersistent transmission

Figure 1.

A particle of Potato virus X. Bar, 100 nm. Courtesy of D‐L Lesemann.

Figure 2.

Folding of the coat polypeptide of Potato virus X. α‐Helices are shown as cylinders and >β‐strands as arrows. The N‐ and C‐termini of the polypeptide chain are indicated and the positions of some residues are as numbered. Reproduced from Baratova et al..

Figure 3.

Organisation of the genomic RNA of a potexvirus. Met, methyltransferase; Hel, helicase; RdRp, RNA‐dependent RNA polymerase; TGB, triple gene block; CP, coat protein.

Figure 4.

A particle of Carnation latent virus. Bar, 100 nm. Courtesy of RG Milne.

Figure 5.

Organisation of the genomic RNA of a carlavirus. Met, methyltransferase; A, AlkB domain; O, OTu‐like peptidase; P, papain‐like protease; Hel, helicase; RdRp, RNA‐dependent RNA polymerase; TGB, triple gene block; CP, coat protein.

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Purcifull DE and Edwardson JR (1981) Potexviruses. In: Kurstak E (ed.) Handbook of Plant Virus Infections and Comparative Diagnosis, pp. 627–693. Amsterdam: Elsevier/North‐Holland.

Wetter C and Milne RG (1981) Carlaviruses. In: Kurstak E (ed.) Handbook of Plant Virus Infections and Comparative Diagnosis, pp. 695–730. Amsterdam: Elsevier/North‐Holland.

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Martelli, Giovanni P, and Rubino, Luisa(May 2012) Potexviruses and Carlaviruses – Short Filamentous Viruses. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000754.pub3]