Family Alphaflexiviridae

This chapter focuses on the Alphaflexiviridae family, which contains viruses with flexuous filamentous virions that infect plants and a few viruses discovered in plant-infecting fungi. They share a distinct lineage of alphavirus-like replication proteins that is unusual in lacking any recognized protease domain. The virions are flexuous filaments, usually 12–13 nm in diameter and from 470 to about 800 nm in length, depending on the genus. They have helical symmetry with a pitch of about 3.4 nm (range 3.3–3.7 nm) and in some genera there is a clearly visible cross-banding. The virions sediment as a single band or occasionally as two very close bands, and contain a single molecule of linear ssRNA of about 5.9–9.0 kb which is 5–6% by weight of the virion. The viral capsid of all members of the family (except in the genus Lolavirus) is composed of a single polypeptide ranging in size from 18 to 43 kDa. In allexiviruses, a 42 kDa polypeptide was also detected as a minor component of virions. In lolaviruses a shorter (ca. 28 kDa) carboxy coterminal polypeptide forms an equimolar fraction of the virion with the polypeptide originating from the first AUG (ca. 32 kDa). Virions are usually highly immunogenic and within the genera, some viruses are serologically related. Many of the viruses have relatively mild effects on their host and all species can be transmitted by mechanical inoculation. Some of the viruses have no known invertebrate or fungus vectors, however, allexiviruses are thought to be mite-borne.