 184 September 1977 | Family: Virgaviridae Genus: Tobamovirus Species: Acronym: |
A. J. Gibbs
Research School of Biological Sciences, Australian National University, Canberra, Australia
Other tobamoviruses and tobamo-like viruses with particles of similar morphology have less stable particles of two or more modal lengths. These viruses have more limited host ranges, and some are transmitted efficiently by root-infecting plasmodiophoromycete fungi in which they persist for long periods.
Table 1 Definitive and tentative members of the tobamovirus group
| Description No. or ref. |
Particle length(s) (nm) |
RNA M.Wt (x 10-6) |
Coat protein M.Wt (x 10-4) |
Amino acid residues/ subunit |
Stability* | Natural vector | ||
| A. DEFINITIVE MEMBERS | ||||||||
| Tobacco mosaic virus (TMV) | 151 | 300 | 2.05 | 1.75 | 158 | + + + | unknown | |
| Cucumber green mottle mosaic virus (CGMMV) | 154 | 300 | - | 1.71 | 160 | + + + | unknown | |
| Cucumber virus 4 (CV4) | 154 | 300 | - | 1.70 | 158 | + + + | unknown | |
| Frangipani virus (FV) | 196 (a) | c. 300 | - | - | - | + + + | unknown | |
| Odontoglossum ringspot virus (ORSV) | 155 | 300 | - | 1.76 | 157 | + + + | unknown | |
| Ribgrass mosaic virus (HRV) | 152 | 300 | c. 2.0 | 1.75 | 156 | + + + | unknown | |
| Sammons' opuntia virus (SOV) | (b) | 300 | c. 2.0 | c. 1.75 | - | + + + | unknown | |
| Sunn-hemp mosaic virus (SHMV) | 153 | 300; 40 | 2.0; 0.3 | 1.81 | 161 | + + + | unknown | |
| U2 - tobacco mosaic virus (T2MV) | 351 (c) | 300 | 2.0 | 1.75 | 158 | + + + | unknown | |
| Tomato mosaic virus (ToMV) | 156 | 300 | c. 2.0 | 1.76 | 158 | + + + | unknown | |
| B. TENTATIVE MEMBERS | ||||||||
| Beet necrotic yellow vein virus (BNYVV) | 144 (d) | 390; 265; c.100 | 2.3; 1.8; 0.7 | 2.1 | 197 | ++ | Polymyxa betae | |
| Chara corallina virus (CCV) | (e) | 532 | 3.6 | 1.75 | c. 170 | + | unknown | |
| Nicotiana velutina mosaic virus (NVMV) | 189 (f) | 100-175; c.290 | - | 2.14 | - | + | unknown | |
| Peanut clump virus (PCV) | 235 (g) | 245; 190 | - | - | - | + | unknown | |
| Potato mop-top virus (PMTV) | 138 | 250-300; 100-150 | - | 1.98 | - | + + | Spongospora subterranea | |
| Soil-borne wheat mosaic virus (SBWMV) | 77 | 300; 110-160 |
2.0; 1.0 | - | - | + | Polymyxa graminis | |
(a) Francki, Zaitlin & Grivell, 1971,
(b) Sammons & Chessin, 1961,
(c) Siegel & Wildman, 1954,
(d) Putz, 1977,
(e) Skotnicki et al., 1976b,
(f) Randles et al., 1976,
(g) Thouvenel et al., 1976
| *Stability: | T.I.P. | L.I.V. |
| + + + | > 85° | decades |
| ++ | 70° -85° | days |
| + | < 70° | hours |
The fungus-transmitted tobamoviruses often occur in patches in crops, reflecting the distribution of the vector; there is no evidence of spread by contact. These viruses cause obvious symptoms in the aerial parts of infected plants in cool weather.
PCV is soil-borne and probably has a fungus vector (Thouvenel et al., 1976). The well developed propensity of NVMV for seed transmission may be an adaptation to its host’s semi-arid environment (Randles et al., 1976).
Tobamovirus particles are found mainly in the cytoplasm, but also in cell organelles. The particles often aggregate to form inclusions. Most commonly these are crystalline plates, often hexagonal, with the particles aligned perpendicularly to the plane of the plate; sometimes they are long paracrystalline fibres of aligned particles. Cell constituents may also aggregate with or without virus particles to form amoeboid inclusions or X-bodies.
Probably the uncoated genome RNA of TMV can spread from cell to cell through plasmodesmata; intact particles can spread through the phloem but whether they can also spread through plasmodesmata is unknown (Gibbs, 1976). The pattern of systemic invasion of test plants by PMTV suggests that it may only spread through plasmodesmata (Desc. 138).
Virus-like particles have been seen in Polymyxa betae zoospores carrying BNYVV (Desc. 144).
The particles may be disassembled and reassembled into infective particles or stacked discs of protein subunits (Durham et al., 1971); the double disc of protein seems an important intermediate in assembly, which starts at a specific site on the genome near, but not at, the 3' terminus (Butler et al., 1977).
Preparations of definitive tobamoviruses contain some particles less than 300 nm long. These particles contain viral messenger RNAs, notably the coat protein messenger, which in SHMV preparations is in 40 nm long particles (Higgins et al., 1976; Beachy & Zaitlin, 1977).
Most of the tentative tobamoviruses have particles of more than one modal length, but it is not known whether these viruses have a divided genome. The particles have a morphology that closely resembles that of the definitive tobamoviruses, but their basic pitch is greater (2.5-2.9 nm) perhaps because their proteins are larger (up to 2.1 x 104 M. Wt), and they are much less stable.
TMV genome RNA terminates at the 5' end in a guanosine residue and the 5' position of this residue is linked by a triphosphate group to the 5' position of 7-methyl-guanosine; this structure, which is found in most eukaryote mRNAs, is required for infectivity (Ohno et al., 1976). The 3' end of the TMV genome has the sequence Cp Cp Cp A-OH; it will accept and bind histidine in the presence of amino acyl-tRNA ligase (Litvak et al., 1973). The ‘origin’ for particle reassembly is c. 925 nucleotides, and the start of the coat protein gene c. 750 nucleotides, from the 3' end of the genome (Zimmern & Wilson, 1976; Hunter et al., 1976).
Table 2. Number of amino acid residues per subunit of coat protein*
No. of
isolates
in clusterTMV†
CGMMV
CV4
FV
ORSV
HRV
SOV
SHMV
T2MV
ToMV
BNYVV
CCV
19
2
7
1
3
2
1
1
7
32
1
1
ala
14,13-15
21
20,19
14
11
18,17
12
12
18,17
11,12
19
14
arg
11,10-12
8
10,9
11
10
10,11
8
12
8
9,8-10
10
8
asp
18,17-19
20,21
17,18-20
17
20
17,16
12
18
22
18,17-19
24
25
cys
1
0
0
1
1
1
1
0
1
1,0
0
N.D.
glu
16,15-18
10
10
16
15
22,21
12
16
16
19,18-21
14
15
gly
6,5-8
9
5,6
9
7
4,3
5
4
4,5
6,7
14
12
his
0,1
1
0
1
0
1
0
1
0
0
2
1
ile
8,9
7
6-7
11
8,9
8,7
7
10
8
7,6-8
5
6
leu
12,12-14
18
12-14
13
14
11,12
12
15
11
13,12-14
19
12
lys
2,3
4
4,3-5
4
1
2
4
1
1
2
12
10
met
0,1
0
0
0
3
3,4
1
0
2
1
7
3
phe
8,9
9
11
7
7
6,5
6
6
8
8,7-10
6
14
pro
8,9
6
9,8-10
4
9
8,9
6
8
10
8,9
10
9
ser
16,13-17
24
21-24
14
12
13,16
9
18
10
15,13-16
18
15
thr
16,14-17
10
12,10-11
13
21
13
11
19
19
16,14-17
15
14
trp
3
2
1
5
3
2,3
N.D.
1
2
3
4
N.D.
tyr
4,5
4
4
5
6,5
7
5
8
6
5,4-6
4
4
val
14,13-15
7
14,12
13
9,10
10
9
12
12
15,14-18
14
8
Total
Residues
in protein158
160,161
c. 158
158
157,158
156,158
N.D.
161
158
158
197
N.D.
* The most common number; other numbers, if found, are given in italics
N.D. Not determined
† Viruses are arranged in the same order as in Table 1
The computed relationships of these groups are shown in Fig.1, and the relatedness of individual viruses within and between groups correlates well with their serological specificity (Fig.2), coat protein primary structure, cross-protection behaviour, and the host from which they were first isolated (Gibbs & Harrison, 1976). Fig.1 and Fig.2 show that the three clusters of tobamoviruses from solanaceous plants (TMV, ToMV and T2MV) are closely related to, but distinct from, one another. They are close to ORSV and more distantly related to SHMV and HRV. The other definitive tobamoviruses are even more distantly related to one another, with the cucurbit tobamoviruses most distant both from the other viruses and from each other.
BNYVV, PMTV, SBWMV, and probably also PCV and NVMV are closely related to one another both in the properties of their particles, and also by their pathology, ecology and vector relations. They probably form a coherent subgroup of the tobamoviruses; indeed the particles of PMTV and SBWMV are serologically related (Randles et al., 1976). Particles of PMTV and SBWMV also are distantly serologically related to TMV particles (Kassanis et al., 1972; Powell, 1976).
Diagram illustrating a principal co-ordinates analysis (Gower, 1966; 1967) of the definitive tobamoviruses computed from the amino acid compositions of their coat proteins. Analyses for 75 isolates were classified (MULTCLAS program using Euclidean metric; Lance & Williams, 1967a, b) and fell into 10 clusters or individuals. The mean compositions of these clusters or individuals were then used to compute similarity measures whose principal co-ordinates were calculated. The cluster called TMV contained 19 isolates (type, 01, 03, 04, 05, 06, 0M, M, J14D1, YA, GA, Mosch 6 & 19, Mutant 470 & 483, PM1D, PM21B, PM4A, PM4C), the CGMMV cluster 2 isolates (CGMMV, CV3 Jap), the CV4 cluster 7 isolates (4 of CV4, CV3 Berk, CV4 Czech, CV4 Berk), the ORSV cluster 3 isolates (ORSV, 02, 07), the HRV cluster 2 isolates (HRV, Lychnis), the T2MV cluster 7 isolates (U2, G-TAMV, HNO2 262 & 328, P0249, NBSI 206 & 223), and the ToMV cluster 32 isolates (Y-TAMV, dahlemense, NBSI331, Mosch 1-5, 7-10 & 12-18, VC6O, VC61, AC9, HD, SJ, VEN, Aus-11, Dut-1, YLGP, K-1, PTA, PTV, SAF). The sources of data were Chessin et al., 1967; Desc. 153; Francki et al., 1971; Hariharasubramanian et al., 1973; Kado et al., 1968; Kurachi et al., 1972; Linnasalmi & Toiviainen, 1974; S. J. Morris, pers. comm.; Mosch et al., 1973; Nozu et al., 1971; Rombauts & Fraenkel-Conrat, 1968; Tsugita, 1962; Tung & Knight, 1972; van Regenmortel, 1967; and Wang & Knight, 1967.
The relative positions of the ‘individuals’ in the third dimension of the ordination are indicated by their sizes. The first dimension contains 32% of the ‘comparative information’ of the ordination, the second contains 21% and the third 20%; altogether 73%.
Diagram illustrating an ordination of the serological differentiation indices (SDI’s) in Table 2 of van Regenmortel (1975); the SDI between HRV and CGMMV was taken as 6.5 (M. H. V. van Regenmortel, pers. comm.). The first dimension contains 43% of the ‘information’ in the ordination, the second 28% and the third 18%; altogether 89%.