The plants infected with bacteria and
fungi can be treated by bactericidal and fungicidal compounds, there is no
commercially available treatment to cure virus-infected plants. A large numbers
of viruses are not transmitted through seeds. Therefore, it would be possible
to obtain virus free plants from infected individuals by using seeds as
propagules. However, genetic variation often occurs from the sexually
reproduced plants when propagated by seeds.
Generally, clonal multiplication of
cultivars can be achieved by vegetative propagation. However, where the entire
population of the clone is infected the only way to obtain pathogen-free stock
is to eradicate the pathogen from vegetative parts of the plants and regenerate
full plants from such tissues. Once pathogen free plants are obtained they can
be multiplied indefinitely under conditions which would protect them from
chance reinfection.
It is well
known that the distribution of viruses in plants is uneven. In infected plants
the apical meristems are generally either free or carry a very low
concentration of the viruses. The reasons for low concentration of virus in
apical meristerms are:
- (a) Viruses readily move in a plant body
through the vascular system which is absent in the meristem.
(b) Meristems are always in a state of continuous division because of
which virus cannot replicate in such a short period.
(c) High metabolic activity in the actively dividing meristem
cells does not allow virus replication.
(d) A high endogenous auxin level in shoot apices may inhibit
virus multiplication.
Methods of Virus Elimination in Plants:
Often, apical meristems are not always free of virus and it can't be
considered as a universal occurrence. There are certain viruses like, Tobacco
Mosaic Virus (TMV), Potato Virus X (PVX) and Cucumber Mosaic Virus (CMV), which
invade the meristematic region of the growing tips and interrupts the growth of
the meristematic tissue.
Different methods are used for elimination of viruses:
Thermotherapy:
The
meristematic dome of the shoot tips is not always free of viruses. Some of the
viruses that invade the meristematic region are carnation mottle virus, TMV,
potato virus X, cucumber mosaic virus and odontoglossum ringspot virus (ORSV).
In such cases, meristem-tip culture alone is not effective in virus
elimination. A combination of meristem-tip culture and thermotherapy has been
helpful in raising virus-free plants in many such cases.
Shoot tips or nodal explants from infected plants are
cultured in vitro after proper sterilization and subjected to heat therapy at
30–400 C in a thermal chamber for varying periods. The new shoots
that appear are then used for isolating the meristem-tip and cultured on
appropriate media for growth and development.
Alternatively,
meristem-tips are excised from mother plants and subjected to thermotherapy.
Apple cultivar ‘Idared’ could be made free of ACLSV and ASPV, following the in
vitro thermotherapy technique. This technique is regularly followed for virus elimination
in strawberry plants.
Mother plants are exposed
to heat treatment before excising the meristem-tips.
Thermotherapy of the mother plants prior to meristem-tip culture has an added
advantage. It allows initiating cultures with comparatively larger meristem-tip
explants, which greatly increases the proportion of explants surviving and
developing into virus free plants.
Potato Virus S (PVS) and Potato Virus X (PVX), which
are not readily eliminated by meristem-tip culture or thermotherapy alone, could
be eradicated by taking meristem-tips from heat-treated mother plants.
Chemotherapy
Chemical control of viral diseases under field
conditions has not been successful so far. However, application of a range of
chemicals, such as antibiotics, growth regulators, amino acids, purine and
pyrimidine analogues, has met with some success in inactivation of viruses and
inhibition of virus multiplication in tissue cultures.
So far,
ribavirin (1-b-D-ribofuranosyl-1,2,4- triazole-3-carboxamide; trade name
‘Virazole’), a base analogue, has proved to be the most effective compound
against plant viruses. This broad-range antiviral compound has been shown to be
effective in eradicating a large number of plant viruses, such as PVS, PVX and
PVY from potato, Chlorotic Leaf Spot Virus (CLSV;) and ASGV from Apple, PNRSV from plum, ORSV and
Cymbidium Mosaic Virus (CyMV) from Cymbidium and Sugarcane Mosaic Virus (SCMV)
from sugarcane.
In sugarcane,
50 mg l-1 ribavirin combined with meristem culture raised the frequency of
virusfree plants from 95 to 62 % with meristem alone (Balamuralikrishnan et al.
2002). Apple Stem Grooving Virus (ASGV), which could not be removed by
thermotherapy, was successfully eliminated by culturing shoots of Malus
domestica on a medium containing 10 lM each of two viricidal compounds,
ribavirin (a base analogue) and quercetin (a flavonoid) for 9–12 weeks
Cryotherapy
Cryotherapy is
a novel application of plant cryopreservation techniques to eradicate
pathogens, such as viruses, phytoplasmas and bacteria, at a high frequency by a
brief exposure of shoot tips to liquid nitrogen using cryopreservation
protocols.
Since the first
demonstration of Plum Mosaic Virus (PMV) elimination from an interspecific
Prunus rootstock by cryotherapy of shoot tips). This technique has been
successfully applied to eradicate Cucumber Mosaic Virus (CMV) and Banana Streak
Virus (BSV) from Musa, Grapevine Virus A (GVA) from grapes, PLRV and PVY from
potato and Sweet Potato Chlorotic Stunt Virus (SPCSV) and Sweet Potato Feathery
Mottle Virus (SPFMV) from sweet potato.
In cryotherapy,
shoot tips (up to 2 mm long) are precultured, directly or after encapsulation
in Ca-alginate, in increasing concentration of sucrose (0.25, 0.5 and 0.75 M
for 1 day each) to impart desiccation tolerance, vitrified for dehydration and
plunged into liquid nitrogen, at -196 C, for about 1 h. The shoot tips are
retrieved from the super low temperature by rapid thawing in hot water (40 C)
for about 3 min and cultured on a suitable medium for shoot growth and rooting.
The plants thus obtained are indexed for viruses.
The explanation offered for the elimination of viruses
in the cultures of larger shoot tips following cryotherapy is that exposure to
-196 C kills the older, highly vacuolated cells, which carry the infection, and
the smaller, highly compact cells with high nucleus to cytoplasm ratio in the
apical meristem, which are generally free of viruses, are able to grow and
regenerate healthy plants.
Electrotherapy
Electrotherapy has been used for elimination of PVX
from potato, CMV from banana and Tomato Yellow Leaf Curl virus (TYLCV) from
tomato.
The technique of electrotherapy involves exposure of
shoots to electrical impulses (5–15 mA) for 5–10 min. The shoots are then
surface sterilized and inoculated in a suitable medium for regeneration. The
regenerated plants are indexed for viruses.
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