Most of the crop plants, especially those propagated by vegetative means, are systematically infected with one or more pathogens. A large number of viruses are not transmitted through seeds. In such cases, it should be possible to obtain virus free plants from infected individuals by using their seeds as the propagules. However, most of the horticultural plants are propagated through vegetative means. Hence, there is a need to develop a method of virus free plant in those plants.
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 proposed for the escape of the meristem from virus invasion are
- Viruses readily move in plant body through the vascular system which is absent in meristem. The alternate method of cell-to-cell movement of the virus through plasmodesmata is rather too slow to keep pace with the actively growing tip.
- High metabolic activity in the actively dividing meristem cells does not allow virus replication.
- The virus inactivating systems in the plant body has higher activity in the meristem than in any other region.
- High endogenous auxin level in the shoot apices may inhibit virus multiplication.
The knowledge of the gradient virus distribution in the shoot tips enabled Holemes (1948) to obtain virus free plant from infected individuals of Dahlia through shoot-tip culture. Although mainly used for virus elimination, meristem tip culture has also enabled plants to be freed from other pathogens including mycoplasmas, bacteria and fungi (and certain viroids). Before the meristem tip culture technique was developed the in vivo eradication of viruses was achieved by heat treatment of whole 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”.
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.
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. For plant treatment proper meristematic apex without adjacent leaf primordia (size – 0.2-1.0 mm) is used as starting explant
Shoot tip grafting (STG) /Micrografting
- In a number of species including those of citrus peach and apple attempts at meristem culture remained unsuccessful in virus elimination. 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.
- The biological assays are reasonably accurate, but too slow and difficult. 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.
|Plant species||Virus eliminated|
|Allium sativum||Garlic Mosaic Virus|
|Brassica oleracea||Cauliflower Mosaic Virus|
|Dahlia spp.||Complex viruses|
|Glycine max||Soybean mosaic virus|
|Musa spp.||Cucumber mosaic virus|
|Nicotiana tobaccum||Tobacco mosaic virus