Closteroviridae

Plant viruses grouped within this family have remarkable properties, prominent among which is their genomic size: Citrus tristeza virus (CTV) has the largest (19.3 kb) genome reported for a plant monopartite single-stranded RNA (+) virus. Virions are filamentous and typically flexuous particles, approximately 12 nm in diameter and 650 to 2000 nm in length, with a unique bipolar (“rattlesnake”) morphology: the major coat protein (CP) encapsidates most of the genomic RNA, with a minor CP (CPm) coating a small 5’-terminal fragment (virion tail) and other viral-encoded proteins being also incorporated to this tail. The genome is monopartite (genus Closterovirus, type member Beet yellows virus, and genus Ampelovirus, type member Grapevine leafroll-associated virus 3) or bipartite (genus Crinivirus, type member Lettuce infectious yellows virus, with at least one example of tripartite genome). The genomic RNA (or RNA1 in criniviruses) directs translation of the two 5’-proximal ORFs (via a peculiar ribosomal frameshift mechanism and proteolytic processing) that encode replication-related components, with the 3’-proximal ORFs encoding proteins expressed from 3’-coterminal subgenomic RNAs. A genomic signature of members of the family Closteroviridae is the presence of a five-gene block of proteins involved in virion assembly and movement that, in addition to the CP and CPm, include a small transmembrane protein, a homologue of the HSP70 class of heat-shock proteins and a diverged CP. Members of this family encode suppressors of RNA silencing differing in number (up to three in CTV), and in mode of action: intracellular, intercellular, or both. In this same context Sweet potato chlorotic stunt virus codes for a singular suppressor: an RNase III that catalyzes cleavage of the small interfering RNAs mediating RNA silencing. Host range is usually narrow and, in order to expand it, some member(s) of the family, illustrated by the case of CTV, have evolved by acquiring multiple non-conserved genes. Virion accumulation is restricted to the phloem, with aphids, mealybugs and whiteflies (depending on the genus) operating as natural vectors. Disease symptoms may be expressed in leaves, fruits and trunk of the woody hosts. Natural host include plants like beet, lettuce, tomato, citrus and grapevine, and damages can be economically very relevant: CTV has changed the course of the citrus industry. Altogether these properties make the family Closteroviridae particularly attractive from both a basic and applied point of view.