Morphology

Virions are flexuous filaments with no envelope and are 11 - 15 nm in diameter, with a helical pitch of about 3.4 nm (Fig. 1). Particle lengths of members of some of the six genera differ. Members of the genera Potyvirus, Ipomovirus, Macluravirus, Rymovirus, Tritimovirus and the unassigned viruses are monopartite with particle modal lengths of 650 - 900 nm; members of the genus Bymovirus are bipartite with particles of two modal lengths of 250 - 300 and 500 - 600 nm.

 

Figure 1: (Left) Schematic diagram of a potyvirus particle. The N-terminal ~ 30 amino acids (large rectangle) and C-terminal ~ 19 amino acids (small rectangle) of the capsid protein molecules are exposed on the surface of the intact virus particle (from Shukla and Ward, 1989). (Right) Negative contrast electron micrograph of particles of Plum pox virus (PPV) stained with 1% PTA, pH 6.0. The bar represents 200 nm (from Scottish Crop Research Institute).

Physicochemical and Physical Properties

Member viruses of the genera Potyvirus and Rymovirus have a density in CsCl of about 1.31 g/cm³ and an S_20w of 150 - 160S. Member viruses of the genus Bymovirus have a density in CsCl of about 1.29 g/cm³.

Nucleic Acid

Member viruses of the genera Potyvirus and Rymovirus have a single molecule of positive sense, ssRNA, 8.5 - 10.0 kb in size (Mr 3.0 x 10⁶). Virions are infectious. A protein (VPg Mr about 24 x 10³) is covalently linked to the 5'-terminal nucleotide. A polyadenylate tract (20 to 160 adenosines) is present at the 3' terminus. The complete nucleotide sequence is known for nearly 30 members of the genus Potyvirus, for two members of the genus Bymovirus, and one member of the genus Ipomovirus. Member viruses of the bipartite genus Bymovirus have two positive sense, ssRNA molecules; RNA1 is 7.5 - 8.0 kb in size (Mr 2.6 x 10⁶) and RNA2 is 3.5 - 4.0 kb in size (Mr 1.5 x 10⁶). Both RNAs have 3' terminal polyadenylate tracts and a VPg is probably present at the 5'-termini. The complete nucleotide sequence of Barley yellow mosaic virus (BaYMV) RNAs has been determined, and about 70% of Wheat streak mosaic virus (WSMV) has been sequenced.

Proteins

The genome-derived polyprotein is cleaved into several proteins, some of which form inclusion bodies in the cell (see genus descriptions). Virions contain one capsid protein (CP), Mr of 28.5 - 47 x 10³. N- and C-terminal residues are positioned on the exterior of the virion. Mild trypsin treatment removes N- and C-terminal segments, leaving a trypsin-resistant core of about Mr 24 x 10³. Plant proteases may degrade the CP in vivo as happens in vitro during purification using some procedures or from certain hosts. All potyvirus CPs display significant amino acid sequence homology in the trypsin-resistant core, but little homology in their N and C-terminal segments.

Lipids

None reported.

Carbohydrates

None reported.

Genetic information encoded by the RNA genome is organized as a single ORF. Genetic maps for the genus Potyvirus, and BaYMV, a member of the genus Bymovirus, are presented in genera descriptions. For members of the genus Potyvirus, the genome is expressed initially as a polyprotein which undergoes co- and post-translational proteolytic processing by three viral-encoded proteinases to form individual gene products. Genomic RNA replicates via the production of a full-length negative sense RNA.

The viral proteins are moderately immunogenic; there are serological relationships between members. A conserved internal trypsin-resistant core CP epitope has been identified, that is similar in most members of the family.

Inclusion Body Formation

All members of the family Potyviridae form cytoplasmic cylindrical inclusion (CI) bodies during infection. The CI is an array of a viral protein of an Mr ˜ 70 x 10³ which possesses ATPase and helicase activities. The viruses encode and express the following proteins, but inclusion bodies composed of these proteins are not formed in all instances (some potyviruses induce nuclear inclusion bodies which are co-crystals of two viral-encoded proteins present in equimolar amounts): The small nuclear inclusion (NIa) protein (Mr 49 x 10³) is a polyprotein consisting of the VPg and proteinase. The large nuclear inclusion (NIb) protein has amino acid motifs of RNA-dependent RNA-polymerases. NIa and NIb are also found in the cytoplasm. Amorphous inclusion bodies are also evident in the cytoplasm during certain potyvirus infections and represent aggregations of the protein HC-Pro. This protein has a helper component activity and a proteolytic activity associated with it. Bymoviruses do not encode a protein analogous to the helper component in length, but a protein with Mr 28 x 10³ from RNA2 of BaYMV has amino acid domains with sequence similarities to the potyvirus protein HC-Pro.

Host Range

Some members have a narrow host range, most members infect an intermediate number of plants, and a few members infect species in up to 30 families. Transmission to most hosts is readily accomplished by mechanical inoculation. Many viruses are widely distributed. Distribution may be aided by seed transmission in some cases.

Transmission

Potyviruses are vectored by a variety of organisms. Members of the genus Potyvirus and Macluravirus have aphid vectors that transmit in a non-persistent, non-circulative manner. A helper component and a particular CP amino acid triplet (i.e., DAG for some potyviruses) are required for aphid transmission. Rymoviruses and tritimoviruses are transmitted by mites, possibly in a persistent-circulative manner. Bymoviruses are transmitted by a fungus vector. Ipomoviruses appear to be transmitted by a whitefly.

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