The sobemoviruses constitute a homogeneous group based on their physicochemical parameters, including stability characteristics, and genomic expression strategies. These viruses possess relatively simple organizations, and their genomes are the smallest among the plant viruses. Furthermore, sobemoviruses represent a class of genetically stable viruses because few naturally occurring variants or strains have been identified. Also, the markedly divergent and non-overlapping host ranges underscore a high level of biological specificity and host plant adaptability of sobemoviruses.

Though the primary structures of the coat proteins of SBMV and tobacco necrosis virus are largely similar, there is only a limited resemblance in their polymerase sequences around the GDD motif. However, considerable similarities exist in the amino acid sequence motifs of the putative polymerase and nucleic acid helicase proteins between sobemoviruses and members of luteovirus subgroup II. Moreover, presence of the ACAAAA element at the 5' end of genomic and sgRNAs is a property which sobemoviruses share with members of the dianthovirus group and luteovirus subgroup II. LTSV, RYMV, SCMoV, SNMV and VTMoV can be distinguished from other sobemoviruses because they encapsidate satRNAs. That different sobemoviruses can support replication of a given satRNA is a reflection of the nonspecific nature of such an association. In this regard, LTSV satRNA seems most versatile because it interacts with a suitable helper virus in divergent dicotyledonous and monocotyledonous species. Likewise, LTSV, more than any other sobemovirus, is effective in supporting replication and encapsidation of sobemo-viral satRNAs. It is rather interesting that satRNA of RYMV, a virus with host range restricted to the monocotyledons, exhibits structural homology with satRNA of LTSV which affects dicotyledonous species. Finally, RYMV satRNA, the smallest viroid-like RNA associated with a plant virus, possesses retroviroid and viroid-like structures making it a probable candidate as an evolutionary bridge between these classes of subviral plant pathogens. Thus, sobemoviral satRNAs offer attractive possibilities for indepth study of molecular interactions between two distinctive biological entities and on the nature and mode of origin of satRNAs associated with plant viruses.

See also: Dianthoviruses (Tombusviridae); Luteovirus; Necroviruses (Tombusviridae); Plant virus disease - economic aspects; Vectors: Plant viruses; Virus structure: Principles of virus structure.

Further Reading

Brunt A, Crabtree A and Gibbs A (1990) Viruses of Tropical Plants. Wallingford: CAB International.

Francki RIB, Milne RG and Hatta T (1985) Sobemovirus group. In: Atlas of Plant Viruses, vol 1. Boca Raton: CRC Press.

Hull, R (1988) The Sobemovirus group. In: Koenig R (ed.) The Plant Viruses, vol. 3, Polyhedral virions with mono-partite RNA genomes. New York: Plenum.

Sehgal, OP (1995) Sobemoviruses. In: Singh RP, Singh US and Kohmoto K (eds) Pathogenesis and host specificity in plant diseases; histopathological, biochemical, genetic and molecular bases, vol. 3, Viruses and Viroids. Oxford: Pergamon.

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