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Here is a term paper on ‘Somaclonal Variation’ for class 11 and 12. Find paragraphs, long and short term papers on ‘Somaclonal Variation’ especially written for school and college students.
Term Paper on Somaclonal Variation
Term Paper Contents:
- Term Paper on the Meaning of Somaclonal Variation
- Term Paper on the Factors Determining the Chance of Mutation during in Vitro Culture
- Term Paper on the Reasons for Somaclonal Variation
- Term Paper on the Isolation of Somaclonal Variants
- Term Paper on the Applications of Somaclonal Variation
- Term Paper on the Advantages and Drawbacks of Somaclonal Variation
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Term Paper # 1. Meaning of Somaclonal Variation:
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When plants are grown in culture, much genomic variability is produced. This variability can be found in callus and suspension cultures, tissue cultures, protoplast cultures and other in vitro culture systems. Such variation observed among plants regenerated from tissue cultures is called somaclonal variation.
Frequent genetic modifications are known to occur during the process of cell or tissue culture. These modifications tend to be inherited as mutations among the progeny of regenerated plants. The source of explant is a critical factor or variable for somaclonal variation.
Plant cell, tissues and somatic tissues developed from various explants are sources for generating somaclonal variation. Somaclonal variation has been reported in a number of plant species such as sugarcane, potato, rice, wheat, Brassica, tobacco, tomato and others for various agronomic traits like disease resistance, plant height, maturity and for many physiological and biochemical traits (Table 8.1).
Term Paper # 2. Factors Determining the Chance of Mutation during in Vitro Culture:
(1) The Method of Vegetative Propagation Used:
Genetic stability is preserved if the in vitro propagation is carried out using single node or the axillary bud method. If by the use of growth regulators adventitious shoots are formed, then the probability of mutations taking place increases.
Further, methods of production of these adventitious shoots also influence the mutation frequency. For instance, if a shoot arises from a single cell then chances of mutation are greater as compared to those arising from more cells. Vasil (1986) indicated that probability of somaclonal variation is high with members of Gramineae when adventitious shoots arise in callus culture.
(2) The Genotype Used:
Whereas some plants seldom exhibit the tendency to mutate after adventitious shoot formation (e.g., hyacinth, lily, etc.), others like Begnonia show high mutation rate. Some cultivars of plants like potato and oat mutate easily. “Russet Burbank” cultivars of potato that have originated from leaf protoplasts were always mutated. Similar genetic instability was observed in Nicotiana tabacum, when plants were regenerated from protoplasts derived from cotyledon tissue.
(3) Growth Regulators:
Some growth regulators such as 2, 4-D, NAA and synthetic cytokinins lead to a high number of mutation. High amount of BA induced somaclonal variation in tobacco. These have their effect on cell division, degree of deorganised growth and selective proliferation of cell types.
(4) Tissue Types:
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The risk of mutation is high when the starting material happens to be differentiated tissue such as pith, as cell differentiation in vivo tends to produce polyploid cells. Polyploid shows chromosomal abnormalities more frequently than diploids. The chance of mutation is less with undifferentiated tissue such as procambium, pericycle and cambium.
(5) Number of Sub-Culture:
Repeated sub-culturing in vitro enhances the chance of mutation as observed for protocorm culture of orchids, adventitious and axillary shoot formation, and for callus-, suspension- and single cell-cultures.
Term Paper # 3. Reasons for Somaclonal Variation:
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Variations in somaclones may take place on account of various reasons:
(1) Pre-existing genetic variation in the explant tissue.
(2) Variants may be produced due to subtle changes because of gene mutations in cultures. Somaclonal variants for single recessive gene mutations are reported for maize, rice and wheat. Somatic crossing over followed by segregation may be responsible for homozygosity which in turn causes phenotypic expression of the recessive nature. Several gene mutations are reported in wheat somaclones.
(3) Numerical and structural changes in chromosomes during in vitro growth. Polyploidy, euploidy, inversion, deletion as well as translocation are reported to cause variation in regenerated plants.
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(4) Somatic crossing over involving symmetric and asymmetric recombination. Tissue culture may enhance the frequency of somatic crossing over.
(5) Changes in DNA, isoenzyme and protein profiles have been correlated with somaclonal variations in plants including potato, rice, barley and maize.
(6) Transposable elements have been reported to induce changes in strains of alfalfa and wheat.
(7) Intracellular mutagenic agents produced during in vitro growth.
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(8) Molecular basis of somaclonal variation. Changes in chromosome number or structure, gene mutation, plasma gene mutation, alteration in gene expression, gene amplification, meiotic crossing over – all are capable to lead to somaclonal variation. Further, somaclonal variation can be characterized based on morphological, isoenzymes and DNA markers like RAPDs, RFLPs, etc. Alteration of a single base pair results in changes of a single amino acid in polypetide sequence, thus producing somaclonal variants.
(9) Variation in DNA methylation could be responsible for tissue culture-induced mutagenesis. The DNA methylation and base sequence changes are frequent in maize callus and in regenerated plants.
(10) Changes in organelle DNA. The classical example of such variation in tissue culture is reported in cytoplasmically controlled male sterility. A single mutation to male fertility and toxin insensitivity was due to frame shift mutation in mitochondrial DNA.
(11) Epigenetic variation. Cultured cells upon exposure to stress factor result in altered expression of traits, and these changes are temporary and are not reflected in off-springs. Such epigenetic changes in tissue culture could be due to DNA amplification, DNA methylation or transposable elements. Phillips et al. (1994) suggested that somaclonal variation occurs by a stress-response mechanism.
Term Paper # 4. Isolation of Somaclonal Variants:
It is grouped in two broad categories:
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1. Screening, and
2. Cell Selection.
1. Screening:
This involves the observation of large number of cells or regenerated plants for detecting variants; this is the only visible technique for isolation of mutants for yield and yield traits. This procedure is applied for isolating cell clones which produce high amount of certain bio-chemicals.
Table 8.2 lists some of the examples where somaclonal variation is caused in various morphological traits:
2. Cell Selection:
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In this case, a selection pressure is the basis of cell selection that allows survival or growth of desired variants only, e.g., selection of cells resistant to toxins, herbicides, high salt concentration etc. When the selection medium allows only the mutant cells to survive, it is termed as positive selection, whereas, in case of negative selection wild type cells divide and are killed by a counter selection agent like arsenate.
The basis for selection of auxotroph mutants is the negative selection. Generally, two types of selection are applied– Single step selection, and Multi-step selection. In the former one, the inhibitor is added to culture medium three times in excess compared to maximum inhibitory concentration (MIC) and cultures are maintained for several sub-culture regimes with the inhibitor. In a multi-step selection, a sub-lethal concentration (less than MIC) is added to culture medium.
In the subsequent sub-cultures, a gradual increase in inhibitor level is maintained. This method has been employed in tobacco for herbicide tolerance, and in maize for amino acid analogue resistance.
Term Paper # 5. Applications of Somaclonal Variation:
(1) Increased Genetic Variability for Agronomic Traits:
Several useful somaclonal variants showing resistance to diseases, insects and tolerance to herbicides have been isolated (Table 8.3).
(2) In Vitro Selection:
In vitro selection has been used to select agronomically desirable somaclones, including tolerance to pathotoxins, herbicides and diseases.
Some examples include:
(i) Tobacco variants resistant to Pseudomonas syringae and Alternaria alternata have been obtained following selecting protoplast-derived callus on medium having pathotoxins.
(ii) Somaclonal variants with increased resistance to Fusarium oxysporum in celery have been obtained.
(iii) Herbicide resistant variants have been obtained in cell culture.
(3) ‘Elite’ Germplasm and Commercial Cultivars:
Variants with improved traits have been obtained/developed giving rise to new useful germplasm, as well as cultivars (Table 8.4).
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(4) Improved Ornamental Plants:
Somaclonal variation is known to be employed commercially in ornamental plants. A wide range of somaclonal variation for flower size, plant morphology, plant height in Begoniax elatior plants regenerated from callus have been reported. Flower variation is reported in tissue culture – derived plants of carnation, Chrysanthemum and Gerbera.
Term Paper # 6. Advantages and Drawbacks of Somaclonal Variation:
Advantages:
(1) Cost – effectiveness.
(2) Source of genetic variability; helpful in crops with a narrow genetic base.
(3) Successfully removes defects in otherwise well-adapted cultivars.
(4) Improves various vegetatively and seed propagated species.
(5) Produces new variants.
Drawbacks:
(1) Instability of variants.
(2) Poor plant regeneration.
(3) Some somaclones possess undesirable features like sterility, etc.
(4) Variation is usually not new.
(5) Unpredictability associated with nature of somaclonal variation.
