The principle applications of plant tissue culture can be discussed under the following subheadings:
The importance and application of plant cell and tissue culture in plant science are vast and varied. During last few years of research in plant cell, tissue and organ culture have seen the emergence of technology from technique. Plant tissue culture has become an important tool in biotechnology.
The establishment of micro propagation for rapid propagation, the use of shoot-tip culture to produce nuclear stock free from pathogens especially viruses and the application of a variety of procedures including anther and pollen culture to speed up the process of producing better varieties, protoplast culture for hybrid plant production and genetic manipulation have all contributed to the acceptance of plant tissue culture as a valuable tool for plant improvement. Further, a considerable commercial interest has been developed in the exploitation of this new technology.
Application # 1. Micropropagation:
The regeneration of whole plant through tissue culture is popularly called micropropagation. This is a technique where a callus mass has been initiated from a single explant taken from any living part of a donor plant and within a very short time and space, a large number of plantlets can be produced from such callus tissue. Again, it is possible to make a large number of callus pieces from the origin stock culture during subculturing.
Then it is possible to produce hundreds of plantlets that develop on each of these callus pieces. Therefore, the most obvious advantage of micropropagation is the numerical one. Large numbers of plant species or varieties can be propagated all the year around. The plant breeder or grower is no longer restricted by season in the production of large numbers of plants.
Application # 2. Clonal Propagation:
In vitro clonal propagation is a type of micropropagation. The cultured plants raised from tissue culture are derived asexually and also multiply within culture vessel by asexual means. Asexual reproduction gives rise to plants which are genetically identical to the parent plant. Multiplication of genetically identical copies of a cultivar by asexual reproduction is called clonal propagation and a plant population derived from a single donor plant in tissue culture constitutes a clone. So, the variability that can arise from sexual reproduction and seed formation in a seed plant is omitted.
More specifically a single plant with desirable characters can be selected from a breeding programme and propagated so that further trials and selections can be carried out as quickly as possible. Plants with long seed dormancy can be raised faster by in vitro clonal propagation than in vivo seed propagation. In vitro clonal propagation is the only commercially viable method of micropropagation in orchid plants. Clonal multiplication of cultivar is very important in horticulture and silviculture.
Application # 3. Production of Genetically Variable Plants:
There is a major tendency of the callus tissue towards the numerical variation of chromosomes in the cells that occur after a number of serial subcultures.
These variations may be due to:
(1) The cells of various ploidy and genetic constitution of the initial explant may be induced to divide.
(2) Cultural conditions may contribute to irregularities.
The chromosomal instability in the cultured cells play an important role in polyploidization of cells and genetically variable plants can be raised from such polyploidized cells by subsequent micropropagation. Thus, tissue culture is, providing to be rich and novel source of variability with a great potential in crop improvement without resorting to mutation or hybridization.
Variants selected through tissue culture are calliclones (from callus culture), protoclones (from protoplast culture) but Larkin and Scowcroft proposed a general term Somaclones for plant variants achieved from tissue cultures, irrespective of their origin. Such variant plants may show some useful characters such as resistance to a particular disease, herbicide resistance, stress tolerance etc. Such changes are valuable for crops which are normally propagated by vegetative methods. Plant breeders can exploit such variants for their breeding programme.
Application # 4. Plant Pathology and Plant Tissue Culture:
There have been many valuable contributions of plant tissue culture to the problems of plant pathology.
One outstanding success is the virus eradication by apical meristem culture and second success of tissue culture in plant pathology is the result of its application to their problems of plant tumors especially crown gall.
Virus Eradication:
In virus infected plants, the distribution of viruses in plant body is uneven. It is well known that the apical meristems are generally either free or carry a very low concentration of viruses. The apical meristems culture is the only way to obtain a clone of virus free plant which can be multiplied vegetatively under control conditions that would protect them from the chance of reinfection.
Virus eradication by apical meristem culture has enormous horticultural and agricultural value. In the horticultural world, the production of plants for the cut flower industry must be maintained virus free and are used for the propagation and breeding.
In the agricultural world, the production or yield of a crop can fall due to viral infection. Tissue culture techniques could be of value in restoring the original characters of the variety by removing the infection and so bringing it back into the commercial market. These virus tested stocks could provide ideal material for the national and international distribution of plants either for further propagation or use as breeding material.
Application # 5. Study of Crown Gall by Plant Tissue Culture:
Smith and Townsend (1907) discovered that crown gall or plant tumor formation can be induced by a bacterium, Agrobacterium tumefaciens. Braun (1941) showed that in sunflower Agrobacterium could induce tumors not only at the inoculated point but at considerable distance, where secondary tumors free of bacteria are formed.
Cells of these medium without adding auxin and cytokinin, where as normal tissue requires auxins and in some cases cytokinins. Crown gall tissues deprived of bacteria give rise to tumors by grafting. But the basic mechanism of tumor transformation has not been clarified. It is hoped that plant tissue culture technique can throw some light on the basic mechanism of tumor transformation.
Application # 6. Plant Breeding, Plant Improvement and Plant Tissue Culture:
The conventional breeding methods are most widely used for the crop improvement. But these methods have to be supplemented with plant tissue culture techniques to increase their efficiency or to be able to achieve the objective which is not possible through the conventional methods. Embryo culture is useful for shortening the breeding cycle, overcoming seed dormancy and incompatibility. It is also useful in the propagation of orchids.
In meristem culture, shoot apical meristem along with some surrounding tissue grown in vitro is used for clonal propagation recovery of virus free plants, potentially useful in germplasm exchange and long-term storage of germplasm through freeze preservation. Anther and Pollen culture has a potential application in plant breeding and plant improvement programme for the production of haploid as well as homozygous diploid plant.
All year round rapid clonal propagation using plant tissue culture techniques has highlighted the possibilities for new plant improvement techniques. Another most important approach is the mutation of tissue culture cells to produce a mutant line from which plants can be raised. Production of mutant line is highly desirable for plant breeding.
Application # 7. Production of Useful Biochemicals:
Man depends on plants for many compounds other than food such as medicines, pigments, vitamins, hormones, flavoring agents, latex and tannins. It is possible to culture cells from a plant that naturally produces a certain biochemical and cause the culture to produce that chemical in vitro conditions. We do not yet understand the regulation mechanisms that control the production of most biochemical substances and so that we can manipulate them.
Number of cell cultures have been found to produce specialized biochemicals which are found in the intact parent including alkaloids such as nicotine, atropine, ephedrine, caffeine, codeine, their precursors and derivatives. Production of cardiac glycosides, other steroids, benzoquinones, latex, phenolics, anthocyanins, organic acids, anti-tumor agents, anti-microbials and various flavours, odours have also been reported.
Application # 8. Application of Plant Tissue Culture in Forestry:
Forest is an important renewable natural resource for man because it provides several forest products like fuel, timber, paper, fodder etc. But with the spread of industrial civilization and the rapid growth of population, unfortunately forests tend to disappear. This should be stopped immediately and the conservation of forests should be done by adopting some scientific and technical methods. The most common traditional measure is the rapid propagation and scientific plantation of forest trees.
But there are some problems to propagate forest trees by the conventional methods. To overcome such problems, plant tissue culture technique has been employed and exploited for rapid multiplication of forest trees within very short time and to produce new varieties. By this technique, millions of potential forest trees could be maintained in a few test tubes. For ex- micropropagation of forest trees like Sequoia, sempervirens, Tectona grandis, Eucalyptus citriodora etc.
Micropropagation means the shoot multiplication cycle is very short i.e., within 2-6 weeks each cycle results growth in the number of shoots. By apical meristem culture in Manihot esculenta commonly known as cassava, we can eliminate virus and raise the number of virus free plants. By embryo culture, a hybrid between Pinus lambertiana × P.armandi and P.lambertiana × P.koraiensis can be grown successfully. It is a disease resistant pinus plant. Endosperm culture in Populus tremuloids (plywood yielding plant) resulted in producing a better pulp wood quality plant compared to diploid plant obtained from seeds.
Application # 9. Importance of Plant Tissue Culture in Biotechnology:
Biotechnologists are trying to modify the genetics of the cultured cells by one of the three ways such as:
a. Mutagenesis and selection of cell lines in cell suspension culture.
b. Transplantation of foreign genetic material in protoplasts by means of genetic engineering.
c. Somatic hybridization by the fusion of protoplasts brings together in a single plant, genes of different species that are too unrelated to allow mixing, thereby allowing creation of hybrid or cybrid plants for the improvement of crop species.
Another goal of biotechnology using plant tissue culture technique is to produce self-fertilizing plant that would provide their own usable nitrogen. Generally plants rely on a few types of bacteria and cyanobacteria to fix atmospheric nitrogen into a biologically usable form. Genetically engineered plants that contain the bacterial genes for nitrogen fixation could be grown well even in nitrogen deficient soils.
Genetic engineers are also developing plants that produce their own pesticides. The value of plant protein to human diet is being improved by creating corn or beans that manufacture a complete protein, one with all amino acids essential to the human diet.
In another effort the luciferase gene from the firefly (Photinus pyralis) was used for light production in transected plant cells and transgenic plants. A complementary DNA (cDNA) clone of the firefly luciferase gene under the control of a plant virus promoter (cauliflower mosaic virus 35 S-RNA promoter) was introduced into plant protoplast cells (Daucus carota) and into plants (Nicotiana tabacum) by the use of (Agrobacterium tumefacience) tumor induced plasmid.
In the above experiment stable expression of the firefly luciferase gene in plant cell and transgenic plants has been achieved. The transgenic plant incubated in luciferin also emits light like firefly. The successful introduction of animal DNA into plant genome has opened a new avenue in the field of biotechnology.
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