Introduction

Taxonomic Structure of Family

Family	Caulimoviridae
Genus	Caulimovirus
Genus	“Petunia Vein Clearing-like Viruses”
Genus	“Soybean Chlorotic Mottle-like Viruses”
Genus	“Cassava Vein Mosaic-like Viruses”
Genus	Badnavirus
Genus	“Rice Tungro Bacilliform-like Viruses”

Virion Properties

Morphology

Virions are either bacilliform or isometric depending on the genus. There is no envelope.

Physicochemical and Physical Properties

Virions have buoyant densities in CsCl of 1.37  g/cm³ (genera Caulimovirus, “Soybean chlorotic mottle-like viruses”, “Petunia vein clearing-like viruses”, “Cassava vein mosaic-like viruses,”) or in Cs₂SO₄ of 1.31  g/cm³ (genera “Rice tungro bacilliform-like viruses” and Badnavirus). S_20w is in the range of 200S to 220S. Particles are very stable between pH4 and pH9 and in high salt concentrations.

Nucleic Acid

Virions contain a single molecule of circular dsDNA of 7.2-8.1  kbp. Each strand of the genome has discontinuities at specific places, one strand has one discontinuity and the other has between one and three discontinuities.

Proteins

Genomes contain between two and seven ORFs, depending on the genus. The functions of virus-encoded proteins common to all genera are the capsid protein (CP), an aspartate protease, a reverse transcriptase and a ribonuclease H.

Lipids

None reported.

Carbohydrates

The CP of Cauliflower mosaic virus (CaMV) is glycosylated. No carbohydrates have been reported for the virions of other species.

Genome Organization and Replication

One strand of DNA is the coding sequence. The genome organization is dependent upon genus and is one of the main characteristics that distinguishes the genera from each other.

Following entry into the cell, the discontinuities in the genomes are sealed to give supercoiled DNA which forms minichromosomes in the nucleus. These are transcribed asymmetrically by host DNA-dependent RNA polymerases to give a more-than-genome length transcript (35S or 34S RNA) which has a terminal redundancy of about 35 to 270  nts, dependent upon species. This transcript serves both as a template for reverse transcription to give the negative DNA strand and for expression of at least some of the ORFs. Species in genus Caulimovirus produce a specific mRNA (19S RNA) for ORF6; no subgenomic mRNAs have been reported for genera “Petunia vein clearing-like viruses”, “Soybean chlorotic mottle-like viruses” and “Cassava vein mosaic-like viruses”. ORF4 of Rice tungro bacilliform virus (RTBV) is expressed from an RNA spliced from the 35S RNA. The replication cycle, in contrast to that of retroviruses, is episomal, and does not involve an integration phase. Negative strand DNA synthesis is primed by host cytosolic tRNA^met and synthesis of both strands is performed by the viral reverse transcriptase and ribonuclease H. The site-specific discontinuities are at the priming sites for both negative- and positive-strand DNA synthesis and are made by the oncoming strand displacing the existing strand for a short distance and not ligating to form a closed circle.

Antigenic Properties

Virions range from moderate to efficient immunogens. There is pronounced antigenic variability in species in the genus Badnavirus. There are some serological cross reactions among species in different genera.

Biological Properties

The host ranges of most species are narrow. Those of species in the genera “Petunia vein clearing-like viruses”, Caulimovirus, “Soybean chlorotic mottle-like viruses” and “Cassava vein mosaic-like viruses”, are restricted to dicotyledenous plant species; species in the genus “Rice tungro bacilliform-like viruses” infect monocotyledonous plants species and those of the genus Badnavirus infect either dicotyledonous or monocotyledonous plant species. Many virus species are spread by vegetative propagation.

The geographic range of many species is wide; most species in the genera “Rice tungro bacilliform-like viruses” and Badnavirus are primarily tropical or subtropical whereas most of the species in the genera “Petunia vein clearing-like viruses”, Caulimovirus, “Soybean chlorotic mottle-like viruses” and “Cassava vein mosaic-like viruses”, are found in temperate regions.

The symptoms caused by these viruses are variable dependent on virus species, host and climatic conditions. Mosaic symptoms predominate amongst members of the genera “Petunia vein clearing-like viruses”, Caulimovirus, “Soybean chlorotic mottle-like viruses” and “Cassava vein mosaic-like viruses”, whereas interveinal chlorotic mottling is the most frequent symptom of species in the genera “Rice tungro bacilliform-like viruses” and Badnavirus.

Most species infect most cell types of their hosts although some in the genera “Rice tungro bacilliform-like viruses” and Badnavirus are restricted to the vascular system. Virions occur in the cytoplasm and those of species in the genera “Petunia vein clearing-like viruses”, Caulimovirus, “Soybean chlorotic mottle-like viruses” and “Cassava vein mosaic-like viruses”, are associated with proteinaceous inclusion bodies.