Nanoviruses

Abstract

In the late 1980s, a new type of plant virus with a single-stranded DNA (ssDNA) genome was discovered. It differed from geminiviruses, the only other known group of ssDNA viruses of plants, in particle morphology, genome size, number and size of DNA components, genomic organization, mode of transcription, and vector species. Meanwhile, four of these viruses, namely banana bunchy top virus, faba bean necrotic yellows virus, milk vetch dwarf virus, and subterranean clover stunt virus, have been formally described from several Old World countries. They are not known to occur in the New World. They infect bananas ( Musa spp.) and a range of legume crops and are not transmitted by sap and through seeds due to their phloem limitation. Under natural conditions, all these viruses are transmitted by certain aphid species in a persistent manner and do not replicate in their vectors. Because of the disease symptoms (dwarfing) and the small size of the virions and genome components these viruses have been referred to as nanoviruses. Based on their distinctive features, these viruses have recently been assigned to the family Nanoviridae . Differences in biology (host range, aphid vectors) and both genome size and organization formed the basis for subdividing these viruses (‘nanovirids’) into the genera Nanovirus and Babuvirus. Viruses of the genus Nanovirus infect dicots (legumes) whereas those of the genus Babuvirus occur on monocots only. Available information suggests that the babuvirus genome consists of six ssDNA components each of about 1.1 kb whereas eight ssDNA components each of 0.9–1.0 kb form the nanovirus genome. The understanding of the genomic organization of the nanovirids is complicated by the fact that up to four additional replication initiator protein (Rep)-encoding components have been found in plants infected with nanovirids. Since these DNA components occur only erratically in nanovirid infections, they are regarded as nonintegral genome segments, which behave like satellite DNAs.