Genus: Potexvirus


Genus: Potexvirus

Distinguishing features

Potexviruses have relatively short virions (<700 nm) and their genomes have only five ORFs. Potexviruses infect herbaceous hosts and have no known vectors.

Virion

Morphology

Virions are flexuous filaments, 470–580 nm in length and 13 nm in diameter, with helical symmetry and a pitch of 3.3–3.7 nm (Figure 1.Potexvirus). A central axial canal, about 3 nm in diameter can sometimes be discerned. The number of protein subunits per turn of the primary helix is slightly less than 9.0. The RNA backbone is at a radial position of 3.3 nm (Atabekov et al., 2007).

Figure 1.Potexvirus. Negative-contrast electron micrograph of particles of an isolate of potato virus X. The bar represents 100 nm (Courtesy of D.-E. Lesemann).

Physicochemical and physical properties

The potexvirus virion Mr is about. 3.5×106; S20,w is 115–130S; buoyant density in CsCl is 1.31 g cm−3.

Nucleic acid

Virions of potexviruses contain a single linear molecule of positive-sense RNA of 5.9–7.0 kb, comprising approximately 6% by weight of the virion. The RNA is capped at the 5′ -terminus with m7G and has a polyadenylated tract at the 3′-terminus (Huisman et al., 1988) (Figure 2.Potexvirus). 

Proteins

The only structural protein is the 21–27 kDa capsid protein (CP). 

Carbohydrates

The capsid protein of some strains of the species Potato virus X are reported to be glycosylated (Baratova et al., 2004).

Genome organization and replication

The genomic RNA of potexviruses, as exemplified by potato virus X (PVX), typically has five ORFs (Figure 2.Potexvirus). ORF1, at the 5′-terminus, encodes the replication-associated protein (Rep) and ORF5, located at the 3′-terminus, is the CP gene. Between ORF1 and ORF5 is the triple gene block (TGB) of three overlapping ORFs, the products of which (25, 12 and 8 kDa) are involved in cell-to-cell movement. The 25 kDa protein contains an NTPase-helicase domain, but is not involved in RNA replication. It has been shown to have RNA silencing suppressor activity which is necessary for virus movement. The 12 and 8 kDa proteins contain large blocks of uncharged amino acids and are associated with membrane vesicles derived from the endoplasmic reticulum. The third TGB protein of Alternanthera mosaic virus (AltMV) (but not that of PVX) is targeted to the chloroplast and is required for movement from the epidermis to the mesophyll layer. The CP is also involved in cell-to-cell movement. ORFs 2 to 5 are expressed via the production (and subsequent translation) of subgenomic RNAs (sgRNAs). Two or three 3′-co-terminal sgRNAs can be isolated from plants infected with potexviruses (ca. 2.1, 1.2 and 1.0 kb); the double-stranded counterparts of these sgRNAs have also been detected. The medium-sized sgRNA (1.2 kb) is probably functionally bicistronic, its translation yielding the 12 and 8 kDa proteins.

Virions of PVX contain only genomic RNA, but some other potexviruses also encapsidate the sgRNA for the CP. Genomic RNA is translated as a functionally monocistronic message; only the 5′-proximal Rep gene is translated directly by ribosomes, producing the Rep protein (150–181 kDa). The 5′-UTR leader sequence of PVX RNA consists of 83 nt (excluding the cap-structure) and enhances translation.

Figure 2.Potexvirus. Potato virus X genome organization and expression. Mtr, methyltransferase; Hel, helicase; RdRP, RNA-directed RNA polymerase; TGB, triple gene block; CP, capsid protein;

Biology

Host range

Some potexviruses are moderately pathogenic, causing mosaic or ringspot symptoms in a wide range of mono- and dicotyledonous plant species, but others cause little damage to infected plants. The host range of individual members is usually limited, although AltMV and Plantago asiatica mosaic virus (PlAMV) infect a number of taxonomically diverse crops.

Transmission

Potexviruses are transmitted in nature by mechanical contact. Potato aucuba mosaic virus (PAMV) can be transmitted by aphids when assisted by a potyvirus that provides a helper protein.

Geographical distribution

As a group, potexviruses occur world-wide. The distribution of some species is very wide but others are apparently more geographically restricted.

Cytopathic effects

The cytoplasm of infected cells contains fibrous, banded or irregular aggregates of virus particles, and often membrane accumulation. There is no cytopathology specific to potexviruses, although some viruses induce unique structures such as the beaded sheets found in cells infected by PVX.

Antigenicity

Virions are highly immunogenic; members of some species are antigenically related, but others are serologically distinct.

Derivation of names

Potex: from Potato virus X, the type species of the genus.

Species demarcation criteria

Species are demarcated by:

  • Host range: the natural host range is usually particular to different species.
  • Members of distinct species fail to cross-protect in infected plants.
  • Serology: members of different species (and strains of some members) are readily distinguishable in differential reactions with monoclonal antibodies.
  • Sequence: isolates of different species have less than 72% nt identity (or 80% aa identity) between their CP or Rep genes.

Member species

Exemplar isolate of the species
SpeciesVirus nameIsolateAccession numberRefSeq numberAvailable sequenceVirus Abbrev.
Actinidia virus XActinidia virus XRV-3124KR872420NC_028649Complete genomeAVX
Allium virus Xallium virus XNetherlandsFJ670570NC_012211Complete genomeAVX
Alstroemeria virus Xalstroemeria virus XJapanAB206396NC_007408Complete genomeAlsVX
Alternanthera mosaic virusalternanthera mosaic virusPennsylavaniaAY863024NC_007731Complete genomeAltMV
Asparagus virus 3asparagus virus 3JAB304848NC_010416Complete genomeAV3
Asparagus virus 3scallion virus XAJ316085Complete genomeSVX
Bamboo mosaic virusbamboo mosaic virusOD26017NC_001642Complete genomeBaMV
Cactus virus Xcactus virus XTaiwanAF308158NC_002815Complete genomeCVX
Cassava common mosaic viruscassava common mosaic virusBrazilianU23414NC_001658Complete genomeCsCMV
Cassava virus Xcassava virus XVen164KY288487NC_034375Complete genomeCsVX
Clover yellow mosaic virusclover yellow mosaic virusSitD29630NC_001753Complete genomeClYMV
Cymbidium mosaic viruscymbidium mosaic virusSingaporeU62963NC_001812Complete genomeCymMV
Foxtail mosaic virusfoxtail mosaic virusBancroftM62730NC_001483Complete genomeFoMV
Hosta virus Xhosta virus XKrAJ620114NC_011544Complete genomeHVX
Hydrangea ringspot virushydrangea ringspot virusPD 109AY707100NC_006943Complete genomeHdRSV
Lagenaria mild mosaic viruslagenaria mild mosaic virusAB546335NC_043079Partial genomeLMMV
Lettuce virus Xlettuce virus XKarajAM745758NC_010832Complete genomeLeVX
Lily virus Xlily virus XNetherlandsAJ633822NC_007192Complete genomeLVX
Malva mosaic virusmalva mosaic virusCoteDQ660333NC_008251Complete genomeMalMV
Mint virus Xmint virus XNCGR MEN 454AY789138NC_006948Complete genomeMVX
Narcissus mosaic virusnarcissus mosaic virusZuidemaD13747NC_001441Complete genomeNMV
Nerine virus Xnerine virus XJAB219105NC_007679Complete genomeNVX
Opuntia virus Xopuntia virus XCC10AY366209NC_006060Complete genomeOpVX
Papaya mosaic viruspapaya mosaic virusSitD13957NC_001748Complete genomePapMV
Pepino mosaic viruspepino mosaic virusSp-13AF484251NC_004067Complete genomePepMV
Phaius virus Xphaius virus XJapanAB353071NC_010295Complete genomePhVX
Pitaya virus XPitaya virus XP37JF930327NC_024458Complete genomePiVX
Plantago asiatica mosaic virusplantago asiatica mosaic virusSolovyevZ21647NC_003849Complete genomePlAMV
Plantain virus Xplantain virus XNo entry in GenbankPlVX
Potato aucuba mosaic viruspotato aucuba mosaic virusXuS73580NC_003632Complete genomePAMV
Potato virus Xpotato virus XX3D00344NC_011620Complete genomePVX
Schlumbergera virus Xschlumbergera virus XK11AY366207NC_011659Complete genomeSchVX
Strawberry mild yellow edge virusstrawberry mild yellow edge virusMY-18D12517NC_003794Complete genomeSMYEV
Tamus red mosaic virustamus red mosaic virusITJN389521NC_016003Complete genomeTRMV
Tulip virus Xtulip virus XJAB066288NC_004322Complete genomeTVX
Vanilla virus XVanilla virus XCRV2148POTMF150240NC_035205Complete genomeVaVX
White clover mosaic viruswhite clover mosaic virusForsterX06728NC_003820Complete genomeWClMV
Yam virus Xyam virus XT551KJ711908NC_025252Complete genomeYVX
Zygocactus virus Xzygocactus virus XB1AY366208NC_006059Complete genomeZyVX

Virus names, the choice of exemplar isolates, and virus abbreviations, are not official ICTV designations.

Related, unclassified viruses

Virus name

Accession number

Virus abbreviation

Babaco mosaic virus  

MF978248

Caladium virus X  

AY727533

Cnidium virus X  

LC460456

Euonymus yellow vein virus  

MF078061

Paris polyphylla virus X  

DQ530433

potexvirus 1 LSD-2014  

KJ729597

potexvirus Opuntia  

MH579716

potexvirus ST4  

EU676007

potexvirus ST5  

EU676008

Senna mosaic virus  

KX196173

SenMV

turtle grass virus X  

MH077559

yam potexvirus 1  

KJ815100

yam potexvirus 2  

KJ815103

Virus names and virus abbreviations are not official ICTV designations.