200
August 1978
Family: Alphaflexiviridae
Genus: Potexvirus
Species:
Acronym:


Potexvirus group

Renate Koenig
Biologische Bundesanstalt fur Land- und Forstwirtschaft, Institut für Viruskrankheiten der Pflanzen, Braunschweig, Germany

D.-E. Lesemann
Biologische Bundesanstalt fur Land- und Forstwirtschaft, Institut für Viruskrankheiten der Pflanzen, Braunschweig, Germany

Contents

Type Member
Main Characteristics
Members
Geographical Distribution etc
Association with Vectors
Ecology
Relations with Cells and Tissues
Particle Properties
Genome Properties
Replication
Satellites
Defective-Interfering RNA
Relationships within the Taxon
Notes on Tentative Members
Affinities with Other Groups
Notes
References
Acknowledgements
Figures

Type Member

Type member: potato virus X

Main Characteristics

Flexuous filamentous particles normally 470-580 nm long and 13 nm in diameter, sedimenting at 114-130 S. Each particle is constructed of c. 1000- 1500 protein subunits of a single protein species (M. Wt 1.8-2.7 x 104) arranged as a helix (pitch 3.3-3.7 nm) enclosing the genome which is a single molecule of single-stranded RNA (M. Wt c. 2.0 x 106) and constitutes 5-7% of the particle weight. The protein of several potexviruses can be partially degraded in situ. Thermal inactivation point 60-80°C, longevity in sap usually several weeks to months, dilution end-point usually 10-5-10-6. Potexviruses cause mosaic or ringspot symptoms in a wide range of monocotyledonous and dicotyledonous plants; the natural host range of individual viruses, however, is rather limited. Virus particles, frequently in large aggregates, occur in the cytoplasm and occasionally in the nuclei. Transmissible by inoculation of sap, but usually not by vectors or seed. Some potexviruses infecting legumes are transmitted to a small proportion of seed.

Members

Table 1 lists definitive and possible members with some of their properties.

Desc.
no.
or
ref.
Reported
particle
length
(nm)
Properties of possible members as compared
with properties of definitive members
Similar properties Different properties
A. DEFINITIVE MEMBERS
Cactus virus X (CaVX) 58 520
Clover yellow mosaic virus (ClYMV) 111 540
Cymbidium mosaic virus (CybMV) 27 475
Hydrangea ringspot virus (HyRSV) 114 490
Narcissus mosaic virus (NaMV) 45 550
Nerine virus X (NeVX) 336, (a) 530-545
Papaya mosaic virus (PaMV) 56 530
Pepino mosaic virus (PepMV) 350, (b) 508
Potato virus X (PVX) 4, 354 515
Viola mottle virus (VioMV) 247, (c) 480
White clover mosaic virus (WClMV) 41 480
 
B. POSSIBLE MEMBERS
Artichoke curly dwarf virus (d) 580 DEP** 10-3-10-4, TIP*** 55°-60°C
Bamboo mosaic virus (e) 490 TIP 65°-70°C, DEP 10-5-10-7 No serological relationships found to PVX, PaMV or ClYMV
Barley virus B-1 (f) 512
Boletus virus (g) 500 Central canal clearly visible
Cassava common mosaic virus (CCMV) 90 495 TIP 65°-70°C, DEP 10-5-10-6 No serological relationship found to PVX
Centrosema mosaic virus (h) 580 Transmitted by aphids, DEP 10-3, TIP 55°-58°C
Daphne virus X 195 499 TIP 80-85°C, DEP 10-5-10-6
No aphid transmission
No serological relationship found to ClYMV, NaMV, PVX or WClMV
Dioscorea latent virus (DLV) 335, (i) 395-445 No aphid transmission, high concentration in host plants TIP 55°-60°C
Hippeastrum latent virus (j) 596 TIP 70°-80°C DEP 10-3-10-4
Lily virus (k) 550 No serological relationships found to PVX, NaMV or WClMV
Malva veinal necrosis virus (l) 525 No aphid transmission, TIP 70°C
Negro coffee mosaic virus (m) 550-580 Transmitted by aphids, particle width 21 nm
Parsley virus 5 (PalV 5) (n) 479 Serological relationship to PVX Transmitted by aphids in persistent manner possibly only in the presence of a helper virus
Parsnip virus 3 (PanV 3) (o) as PVX Serological relationship to PVX
Potato aucuba mosaic virus (PoAMV) 98 580 Base composition of RNA similar to PVX No serological relationships found to other potexviruses; transmitted by aphids in the presence of a helper virus
Rhododendron necrotic ringspot virus (p) 460-550 So far no mechanical transmission achieved
Rhubarb virus I and II (q) 478 TIP 60°-65°C and 70°-75°C DEP 10-3-10-4
Wineberry latent virus 304, (r) 510 TIP 70°C DEP 10-4, no serological relationships found to PVX, HyRSV, NaMV or WClMV
Zygocactus virus (ZyV) (s) 580 TIP 72°-74°C Low concentration in host plants, no serological relationships found to PVX or CaVX

* From Lesemann & Koenig, 1977; updated.
** DEP = Infectivity dilution end-point in plant sap.
*** TIP = Thermal inactivation point.

(a) Maat, 1976; (b) R.A. C. Jones & R. Koenig, unpublished data; (c) Lisa & Dellavalle, 1977; (d) Morton, 1961; (e) Lin et al., 1977; (f) Pop & Prodan, 1967; (g) Huttinga et al., 1975; (h) Varma et al., 1968; (i) Waterworth et al., 1974; (j) Brölman-Hupkes, 1975; (k) Stone, 1977; (l) Costa & Kitajima, 1970; (m) Verma & Niazi, 1974; (n) Frowd & Tomlinson, 1972; (o) Garrett & Tomlinson, 1966; (p) Coyier et al., 1977; (q) MacLachlan, 1960; (r) Jones, 1977; (s) Casper & Brandes, 1969.

Geographical Distribution etc

Most potexviruses are found wherever their hosts are grown. PaMV and CCMV may be restricted to the American continent (Desc. 56 & Desc. 90).

Association with Vectors

Most of the definitive potexviruses are not transmitted by vectors. Transmission of PVX by grasshoppers, probably mechanically on the insect’s mouthparts (Schmutterer, 1961), and by the fungus Synchytrium endobioticum (Nienhaus & Stille, 1965), has been reported. Goth (1962) found a low proportion of aphids transmitting WC1MV. Some possible members are transmitted by aphids, and some of these only in the presence of a helper virus.

Ecology

Potexviruses are usually highly infectious and are spread by mechanical contact, and especially with horticultural or agricultural equipment. Dissemination over wide distances occurs in infected planting material; it is aided by the vegetative propagation of the majority of the natural host plants that are grown commercially. Virus overwintering in weeds may serve as inoculum in some instances.

Relations with Cells and Tissues

The definitive potexviruses induce in host cells relatively large inclusions in the form of fibrous masses composed of virus particles. These inclusions are distributed in many tissues of the host plants and occur also in apical dome cells (Pennazio & Appiano, 1975). With many potexviruses the shape of the inclusions is rather irregular, but in some cases (CaVX and NaMV) specific spindle- or loop-shaped or circular bodies are formed. The inclusions are not separated from the cytoplasm by a membrane, and may contain strands or layers of cytoplasmic material. In well preserved tissues the inclusions often appear banded; the width of the bands corresponds to one or several particle lengths and results from a parallel arrangement of virus particles with the ends aligned more or less in one plane. The pattern of arrangement of virus particles in the inclusions seems to be rather susceptible to disturbing influences during preparation for microscopy (unsuitable suspending medium, unsuitable fixatives, distortion of tissues) (Christie & Edwardson, 1977) so that the banded structure is very often destroyed. The result is that the inclusions appear to consist of large masses of virus particles arranged in a roughly parallel fashion with whorl-like areas where the orientation of particles is changing gradually. Paracrystalline particle arrangements may represent a secondary type of aggregation (Christie & Edwardson, 1977).

Membranous inclusions are induced by several potexviruses. In Vicia faba, C1YMV induces amorphous inclusions which contain viral antigen and are located in the cytoplasm and in the vacuole (Schlegel & Delisle, 1971). PVX induces, in addition to virus particle masses, a unique type of inclusion containing proteinaceous sheets termed ‘laminated inclusion components’ (LIC) (Shalla & Shepard, 1972). The LIC are antigenically unrelated to the viral coat protein and may or may not bear bead-like structures on both surfaces. The beads are not ribosomes. The LIC seem not to be correlated with virus synthesis because they are detected later than the first synthesized virus (Honda et al., 1975; Allison & Shalla, 1974).

A more detailed review of the relations of potexviruses with cells and tissues is given by Lesemann & Koenig (1977).

Particle Properties

Rapidly purified particles normally contain a single protein species but in some potexviruses this may easily become partially degraded. In PVX the in situ degradation may occur at the N- as well as the C-terminus of the protein (Koenig et al., 1978). Virus preparations may thus yield several proteins. The M. Wt of undegraded protein is 2.76 x 104 for CybMV (Frowd & Tremaine, 1977), c. 2.6 x 104 for PVX and PoAMV, and between 1.8 and 2.3 x 104 for most other potexviruses. It is not known whether the lower protein M. Wts are the result of a very rapid partial degradation or whether the cistrons for the protein subunits differ in size in different potexviruses.

The protein subunits form a helix with a basic pitch between 3.3 and 3.7 nm. In NaMV and PVX the layer-line spacings determined by X-ray diffraction depend on the water content indicating that the interactions between successive turns of the helix are less strong than in TMV. This may explain why the particles are more flexuous than those of TMV (Tollin et al., 1967).

With PVX the structural integrity of the particles probably depends on ionic bonds between the RNA and the protein subunits because reassembly of the latter into helical arrays has been achieved only in the presence of the RNA (Goodman et al., 1975; Kaftanova et al., 1975). The protein of PaMV, however, will reassemble into helical rods in the absence of its RNA which suggests that protein-protein interactions play a more important role in the stability of this virus (Erickson, Bancroft & Home, 1976). Reassembly products of PVX protein without RNA have a stacked disk structure (Kaftanova et al., 1975; Goodman et al., 1975), whereas NaMV protein in the absence of RNA assembles into rod-like structures with a double helical arrangement (Robinson et al., 1975).

In the native particles the reported number of protein subunits per turn of the helix ranges from 6.8 for NaMV (Tollin et al., 1975) to 10 for PVX (Wilson & Tollin, 1969) and 11 for WC1MV (Varma et al., 1968). With PVX, the helix may have an integral number of subunits (Wilson & Tollin, 1969) or may be repeated every eight turns (Goodman et al., 1975). With NaMV it is repeated every five turns (Tollin et al., 1975).

A central canal has been seen only occasionally in particles of potexviruses (Francki, 1966; Varma et al., 1968). With PVX its diameter has been calculated to be about 3.1 nm (Wilson & Tollin, 1969). In particles of the filamentous Boletus virus a canal is clearly visible (Huttinga et al., 1975).

Genome Properties

The genome of potexviruses consists of a single molecule of a single-stranded infective RNA. The RNA of PVX has a base composition of G22; A32; C24; U22. Adenine is also the most common base in all other potexvirus RNA preparations studied so far (Frowd & Tremaine, 1977; Desc. 4, Desv. 41, Desc. 56, Desc. 98). HyRSV RNA, the only one which has been studied after formaldehyde denaturation, has a M. Wt of 2.1 x 106, a Tm of 55.2°C in 0.1 M-sodium phosphate pH 7.0, and a hyperchromicity of 22.4% (Hill et al., 1977). A higher M. Wt is found for the native RNA, presumably because it retains considerable secondary structure. This may explain the higher values found with some of the other potexviruses.

Replication

Little is known about the site and the mechanism of the replication of potexviruses. With C1YMV there is an indication that the synthesis of virus RNA is initiated in the nucleus (De Zoeten & Schlegel, 1967).

Relationships within the Taxon

Definitive potexviruses are serologically interrelated (Fig.1). Most of these relationships are rather distant. The difference in coat protein M. Wt and the ability of PVX to induce the formation of unique inclusions may be an indication that the group is less homogeneous than originally thought.

Affinities with Other Groups

No close affinities of potexviruses with viruses in other groups are known. It is not known whether the reactivity of antisera to PVX with a potyvirus from Gloriosa reflects a true relationship or a convergence (Koenig & Lesemann, 1974).

References

  1. Allison & Shalla, Phytopathology 64: 784, 1974.
  2. Bercks & Brandes, Phytopath. Z. 42: 45, 1961.
  3. Bercks & Brandes, Phytopath. Z. 47: 381, 1963.
  4. Brandes, Mitt. biol. BundAnst. Ld- u. Forstw. 110, 130 pp., 1964.
  5. Brandes & Bercks, Phytopath. Z. 46: 291, 1962/63.
  6. Brölman-Hupkes, Neth. J. Pl. Path. 81: 226, 1975.
  7. Casper & Brandes, J. gen. Virol. 5: 155, 1969.
  8. Christie & Edwardson, Monograph Ser. Fla agric. Exp. Stn 9, 150 pp., 1977.
  9. Costa & Kitajima, Bragantia 29: LI, 1970.
  10. Coyier, Stace-Smith, Allen & Leung, Phytopathology 67: 1090, 1977.
  11. De Bokx, Pl. Dis. Reptr 49: 742, 1965.
  12. De Zoeten & Schlegel, Virology 32: 416, 1967.
  13. Erickson, Bancroft & Home, Virology 72: 514, 1976.
  14. Francki, Aust. J. biol. Sci. 19: 555, 1966.
  15. Frowd & Tomlinson, Ann. appl. Biol. 72: 177, 1972.
  16. Frowd & Tremaine, Phytopathology 67: 43, 1977.
  17. Garrett & Tomlinson, Rep. natn. Veg. Res. Stn for 1965: 74, 1966.
  18. Goodman, Horne & Hobart, Virology 68: 299, 1975.
  19. Goth, Phytopathology 52: 1228, 1962.
  20. Hill, Benner & Zeyen, J. gen. Virol. 34: 115, 1977.
  21. Honda, Kajita, Matsui, Otsuki & Takebe, Phytopath. Z. 84: 66, 1975.
  22. Huttinga, Wichers & Dieleman-van-Zaayen, Neth. J. Pl. Path. 81: 102, 1975.
  23. Jones, Ann. appl. Biol. 86: 199, 1977.
  24. Kaftanova, Kiselev, Novikov & Atabekov, Virology 65: 283, 1975.
  25. Kitajima & Costa, Bragantia 25: XXIII, 1966.
  26. Koenig, Phytopath. Z. 84: 193, 1975.
  27. Koenig & Bercks, Phytopath. Z. 61: 382, 1968.
  28. Koenig & Lesemann, Phytopath. Z. 80: 136, 1974.
  29. Koenig, Tremaine & Shepard, J. gen. Virol. 38: 329, 1978.
  30. Lesemann & Koenig, in The Atlas of Insect and Plant Viruses, p. 331. Ed.K. Maramorosch. New York: Academic Press, 1977.
  31. Lin, Kitajima, Cupertino & Costa, Phytopathology 67: 1439, 1977.
  32. Lisa & Dellavalle, Phytopath. Z. 89: 82, 1977.
  33. Maat, Neth. J. Pl. Path. 82: 95, 1976.
  34. MacLachlan, Can. J. Pl. Sci. 40: 104, 1960.
  35. Morton, Phytopathology 51: 731, 1961.
  36. Nienhaus & Stille, Phytopath. Z. 54: 335, 1965.
  37. Pennazio & Appiano, Phytopath. medit. 14: 12, 1975.
  38. Pop & Prodan, Anal. Inst. cercet. Prot. Plant 4: 47, 1967.
  39. Robinson, Hutcheson, Tollin & Wilson, J. gen. Virol. 29: 325, 1975.
  40. Schlegel & Delisle, Virology 45: 747, 1971.
  41. Schmutterer, Z. angew. Ent. 47: 277, 1961.
  42. Shalla & Shepard, Virology 49: 654, 1972.
  43. Stone, Rep. Glasshouse Crops Res. Inst. for 1976: 123, 1977.
  44. Tollin, Wilson, Young, Cathro & Mowat, J. molec. Biol. 26: 352, 1967.
  45. Tollin, Wilson & Mowat, J. gen. Virol. 29: 331, 1975.
  46. Varma, Gibbs, Woods & Finch, J. gen. Virol. 2: 107, 1968.
  47. Varma, Gibbs & Woods, J. gen. Virol. 8: 21, 1970.
  48. Verma & Niazi, Z. PflKrankh. PflPath. PflSchutz: 608, 1974.
  49. Waterworth, Lawson & Kahn, J. agric. Univ. Puerto Rico 58: 351, 1974.
  50. Wilson & Tollin, J. gen. Virol. 5: 151, 1969.


Figure 1

Serological relationships in the potexvirus group (updated from Lesemann & Koenig, 1977). Numbers refer to the references listed below. Blank squares indicate that no results of serological tests have been published. Red numbers (on pink background) indicate that serological tests gave positive results at least with some antisera. Grey numbers (on pale blue background) indicate that tests gave negative results; however, because most relationships among potexviruses are rather distant, further tests with different antisera could perhaps yield positive results.

References: (1) Waterworth et al., 1974; (2) Frowd & Tremaine, 1977; (3) Bercks & Brandes, 1961; (4) Koenig & Bercks, 1968; (5) Koenig, 1975; (6) Bercks & Brandes, 1963; (7) Varma et al., 1970; (8) Brandes & Bercks, 1962/63; (9) Maat, 1976; (10) Brandes, 1964; (11) Frowd & Tomlinson, 1972; (12) Garrett & Tomlinson, 1966; (13) De Bokx, 1965; (14) Kitajima & Costa, 1966; (15) Casper & Brandes, 1969; (16) R. Koenig, unpublished data; (17) Lisa & Dellavalle, 1977; (18) Jones & Koenig, unpublished data.