In this article we will discuss about the translocation and transmission of virus.
Translocation of Virus:
Viruses move from one cell to the neighbouring plant cell through plasmodesmata. During movement, they multiply in most cells. Initially they infect parenchyma cells where they multiply and then pass in the neighbouring cells.
In leaf parenchyma cells, the virus moves approximately 1 mm (about 8-10 cells/day). They can move more distance through the phloem (about 15 cm in first 6 minutes). After entering the phloem they can move in both upward and downward directions, thereby they can reach easily the apical meristem as well as the storage organs like rhizome, tuber etc.
Transmission of Virus:
The movement of virus from one host to other host is called transmission. The transmission is dependent upon the external agents.
The transmission of plant viruses takes place by the following means:
(i) By vegetative propagating organs like tuber (potato spindle tuber virus), bulbs (Cattle disease virus), etc.
(ii) Through collateral hosts like lady’s finger and Croton bonplandianum (Yellow vein mosaic virus).
(iii) Through soil (Tobacco, Potato, and Wheat mosaic viruses).
(iv) Through seeds (Bean mosaic virus, Barley strip mosaic virus, Curly top virus of sugar beet, Ring spot virus of soyabean).
(v) Through pollen grains (Stone fruit ring spot virus, Bean mosaic virus).
(vi) Through fungi (Tobacco necrosis virus transmitted by Olpidium brassicae, the root pathogen; Synchytrium brassicae transmits the potato virus).
(vii) Through rubbing and friction between closely growing healthy and infected plants (Potato virus x, Tobacco mosaic virus, etc.).
(viii) Through insects (Yellow mosaic of mung by white fly, Bemisia; Maize mosaic virus by an aphid, Myzus; Maize dwarf mosaic virus by an aphid, Brachycaudus, etc.).
Some common viral diseases:
A. Plants:
(i) Tobacco mosaic,
(ii) Sugarcane mosaic,
(iii) Yellow vein mosaic of bhindi,
(iv) Leaf curl of papaya,
(v) Leaf mosaic of curcurbits,
(vi) Bunchy top of banana,
(vii) Chirke of cardamom,
(viii) Potato leaf roll,
(ix) Tulip break,
(x) Tomato black-ring,
(xi) Yellow mosaic of mung.
B. Humans:
(i) Small pox (Variola),
(ii) Poliomyelitis (Poliovirus),
(iii) Rabies (Rabies virus),
(iv) Encephalitis (Alpha virus — mosquito-borne),
(v) Common cold (Rhino virus and Human corona virus),
(vi) Dengue fever and Yellow fever (Flavi virus — mosquito bone),
(vii) Hepatitis (Hepatitis A, B, C, D, E, F, G),
(viii) Measles (Measles virus).
C. Animals:
(i) Hog cholera,
(ii) Virus pneumonia of pig,
(iii) Rabies,
(iv) African horse sickness,
(v) Myxomatosis of Rabbits,
(vi) Granulosis of Butterflies and Moths.
Viruses or virus-like particles in Cryptogams and Gymnosperms:
A. Algal viruses (Cyanophages):
The ds-DNA virus from Cyanophyceae (Piectonema boryanum) was first discovered by Safferman and Morris (1963). Later, the same virus was also reported in Lyngbya and Phormidium. Thus the first algal virus was named as strain LPP-1 (based on the initial name of the genera, Lyngbya, Piectonema, Phormidium). Similarly, N, was isolated from Nostoc musco- rum and SM, from Synechococcus and Microcystis.
B. Fungal viruses (Mycophages):
The fungal viruses were first reported in the fruit body of Agaricus brunnescens (syn. A. bisporus) by Holling (1962).
Since then, virus or virus-like particles have been reported from many species of fungi belonging to Mastigomycotina (Plasmodiophora brassicae), Zygomycotina (Rhizopus, Mucor), Ascomycotina (Saccharomyces ludwigii, S.cerevisiae), Basidiomycotina (Agaricus brunnescens, Puccinia graminis, Ustilago maydis etc.) and Deuteromycotina (Helminthosporium maydis, Cephalosporium crysogenum etc.).
C. Viruses of Pteridophytes:
Since the first isolation of virus in fern (1953), the virus-like symptoms are also observed in many ferns like Phylitis scoiopendrium, Polypodium vulgare, etc.
D. Viruses of Gymnosperms:
Some virus-like diseases have also been reported in pine trees.