MERISTEM CULTURE

MERISTEM CULTURE-VIRUS ELIMINATION

Meristem:

- A localized group of actively dividing cells, from which permanent tissue system, i.e., root, shoot, leaf, and flower, are derived.

Meristem culture:

- In vitro culture of a generally shiny, dome-like structure measuring less than 0.1 mm in length when excised, most often excised from the shoot apex.

 

Meristem tip culture

Only the meristematic dome and 1 pair of subtending leaves should be excised.

If larger pieces are taken, it is likely that the virus will be transmitted. The size of a meristem plus the subtending leaves ranges from 0.1-0.5 mm. The apical dome itself measures from 0.1-0.25 mm depending on the species. There is a balance in size. The meristem tip must be small enough to eradicate viruses and other pathogens, yet large enough to develop into a shoot. Although roots may form on the shoot directly in the same medium, often the shoot has to be transferred to another medium in order for roots to develop.

Meristem tip culture is used successfully to remove viruses, bacteria, and fungi from plants. In a majority of cases heat therapy is combined with meristem tip culture in order to produce the greatest number of plants that are “virus free”.

The term meristem, shoot tip, meristem tip are often interchanged. Here we will use the term shoot tip to refer to an apical tip ranging from 1-3 cm. The meristem is strictly the meristematic dome without any primordial leaves. The term meristem tip will be used to denote the meristem together with 1-2 primordial leaves and measuring between 0.1 and 0.5 cm in height.

In case of meristematic propagation, elimination of virus particles in explant cells is reached within a short time. In many cases meristematic cells do not contain virus particles because of non-existing vascular connection with other plant parts. For plant treatment proper meristematic apex without adjacent leaf primordia (size - 0.2-1.0 mm) is used as starting explant.

Thermotherapy /Heat treatment: Heat treatment is used in those plants in which viruses cannot be eradicated just by meristem tip culture alone.

In plant tissue cultures viruses can also be eliminated with incidence of higher temperatures (heat treatment.).In such case explants are exposed to the incidence of higher temperatures, which are not lethal for plant cells, but they are lethal for viruses Mostly used temperature range is 50-52^oC with exposition about 10-30 minutes. In case this method is applied on whole plants, lower temperatures have to be used (32-40^oC) with exposition about 4 - 30 days(depends on plant species and virus type).

Chemotherapy : Virus-free plants can be also obtained when antivirus matters are added into nutrient solution (Ribavirin or 2-thiouracil).

Mostly combination of thermo-therapy and meristem culture is used for virus-free plants production (e.g., cassava, bananas, citruses, strawberry, Irish potato, apples, chrysanthemas, garlic). Heat therapy combined with meristem tip culture is able to eradicate viruses, bacteria, and fungi but does not remove viroids. Unlike viruses, viroids are RNA without a protein coat – thus they are known as ‘naked’ RNA and are very difficult to eradicate. Usually the infected plant must be destroyed.

Shoot tip grafting (STG) /Micrografting

In a number of species including those of citrus, attempts at meristem culture remained unsuccessful. As an alternative, shoot tips of 0.14-0.18 mm in length isolated aseptically from a diseased plant were grafted on to young etiolated root stock seedlings grown in vitro. STG had been used to produce virus free plants in other crops including peach, and apple.

Why virus eradication works

Several hypotheses exist to explain why heat therapy and meristem tip culture when used together are effective in eradicating viruses.

  Virus distribution is uneven in a plant and is much less in a meristem.

  Viruses cannot travel quickly enough through plasmodesmata to keep up with actively growing tip.

Plants in which virus eradication is commonly used

Garlic, pineapple, dahlia, cymbidium, orchid, carnation ,strawberry,hyacinth sweet potato, iris, lily, apple, cassava,banana, narcissus , perlagonium (geranium),gooseberry, raspberry, sugarcane,potato, grape and ginger.

One of meristem propagation advantages is that one of facilitating of plant material international exchange.

Virus Indexing

The biological assays are reasonably accurate, but too slow and difficult but to use for screening a large number of plants. Alternatively, techniques involving electron microscope, use of indicator plants, serology or a combination of both were developed for virus detection in plant tissues. Among serological techniques, enzyme linked immunosorbent assay (ELISA) and nucleic acid hybridization techniques are popular.