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Thermotherapy
Culturing plants for a period of time at high temperatures (35- 40°C) has been shown to eliminate viruses from several vegetatively propagated plant species. Thermotherapy heat treatment is usually effective only against isometric and thread-like viruses and not against rod-shaped viruses. This procedure has been used in potato via a simple technology reported to eliminate most viruses that affect the crop. Thermotherapy, however, is more efficient when used in combination with tissue culture. Apparently, the reduction of virus concentration in plants subject to heat treatment is due to the negative effect of high temperature on the multiplication and distribution of viruses in the plant. For this reason, agents such as PSTVd that require high temperature for replication and accumulation in plant tissues cannot be eliminated by this treatment, rather, their concentration is increased. On the contrary, elimination of PSTVd by treatment with low temperature (between 5 and 10°C) followed by the excision and culture of meristems has been reported. Growing host plants at higher temperatures significantly reduces replication of many plant viruses by disrupting viral ssRNA and dsRNA synthesis. Higher temperatures (35-37°C) cause disruption in the production and/or activity of virus- encoded movement proteins (MPs) and coat proteins (CPs). MPs are involved in cell- to-cell movement of viruses through plamodesmata and plant vascular system, while CPs plays role in the reconstitution of virus particles from replicated viral nucleic acids. Therefore, thermotherapy of infected plants followed by meristem culture improves virus freedom even from relatively large-size meristems. Reduction in virus titer is higher, if the infected plants are exposed to elevated temperature for longer periods. Current virus elimination programmes involve either growing of whole plants or in vitro cultures at temperatures close to the threshold of normal plant growth. The exact temperature and length of treatment vary with the virus and the heat tolerance of the host plant. Meristem culture combined with thermotherapy is widely used for virus elimination in potato. The source plants infected with viruses are incubated in a growth chamber under light intensity of 30-50 μmol m-2s-1 at 35-37°C for 2-6 weeks. After respective periods of thermotherapy, the meristems are excised and cultured on nutrient medium for regeneration. Cold therapy followed by apical meristem culture has also been shown to successfully eliminate several viruses from infected plants. Viroids, some of which are quite resistant to elevated temperatures, have been effectively eliminated by cold therapy. Low temperature therapy (4-7°C) followed by meristem excision and regeneration has been used to eliminate potato spindle tuber viroid (PSTVd) from infected potato plants.
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