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Somaclonal Variation




  • Genetic variations in plants that have been produced by plant tissue culture and can be detected as genetic or phenotypic traits.

    Basic Features of Somaclonal Variations

    • Variations for Karyotype, isozyme characteristics and morphology in soma clones may also observed.
    • Calliclone (clones of callus), mericlone (clones of meristem) and protoclone (clones of Protoplast)were produced.
    •  Generally heritable mutation and persist in plant population even after plantation into the field

    Mechanism of Soma clonal Variation

    Steps involved in induction and selection of Soma clonal Variations 

     

    Soma clonal Variation

    In addition to the variants/mutants (cell lines and plants) obtained as a result of the application of a selective agentin the presence or absence of a mutagen, many variants have been obtained through the tissue-culture cycle itself.These soma clonal variants, which are dependent on the natural variation in a population of cells, may be genetic orepigenetic, and are usually observed in the regenerated plantlets (Larkin & Scowcroft 1981). Somaclonal variationitself does not appear to be a simple phenomenon, and may reflect pre-existing cellular genetic differences or tissueculture-induced variability. The variation may be generated through several types of nuclear chromosomal re-arrangements and losses, geneamplification or de-amplificationj: n’on-reciprocal mitotic recombination events, transposabllelement activation, apparent point mutations, or re-activation of silent genes in multigene families, as well as alterationsin maternally inherited characteristics (Larkin et al. 1985; Scowcroft et al. 1987; Karp 1994).

    Many of the changes observed in plants regenerated invitrohave potential agricultural and horticultural significance.

    These include alterations in plant pigmentation, seed yield, plant vigour and size, leaf and flower morphology,essential oils, fruit solids, and disease tolerance or resistance. Such variations have been observed in many crops, includingwheat, triticale, rice, oats, maize, sugar cane, alfalfa, tobacco, tomato, potato, oilseed rape and celery (Thorpe 1990; Karp 1994). The same types of variation obtained from somatic cells and protoplasts can also be obtainedfrom gametic tissue (Evanset al. 1984; Morrison & Evans 1988). One of the major potential benefits of somaclonal variationis the creation of additional genetic variability in coadapted, agronomically useful cultivars, without the need to resort to hybridization (Scowcroft et al. 1987). This method could be valuableifselectionispossiblein vitro, orif rapid plant-screening methods are available. It is believed that somaclonalvariants can be enhancedfor some charactersduring culture in vitro, including resistance to disease pathotoxins and herbicides and tolerance to environmentalor chemical stress. However, at present few cultivars of any agronomically important crop have been produced throughthe exploitation of somaclonal variation (Karp 1994).

    Causes of Soma clonal Variations

    *     Physiological Cause

    *     Genetic Cause

    *     Biochemical Cause

    Physiological Cause

    • Exposure of culture to plant growth regulators.
    • Culture conditions

    Genetic Cause

    1. Change in chromosome number
        • Euploidy: Changes chromosome Sets
        • Aneuploidy: Changes in parts of chromosome Sets

           Polyploidy: Organisms with more than two chromosome sets

           Monoploidy: Organism with one chromasomes set

    1. Change in chromosome structure
        • Deletion
        • Inversion
        • Duplication
        • Translocation

    3.      Gene Mutation

    • Transition
    • Trans version
    • Insertion
    • Deletion

    4.      Plasma gene Mutation

    5.      Transposable element activation

    6.      DNA sequence

    • Change in DNA
      • Detection of altered fragment size by using Restriction enzyme
    •  Change in Protein
      • Loss or gain in protein band
      •  Alteration in level of specific protein
    •  Methylation of DNA
      •  Methylation inactivates transcription process

    Biochemical Cause

    • Lack of photosynthetic ability due to  alteration in carbon metabolism
    • Biosynthesis of starch via carotenoid pathway
    • Nitrogen metabolism
    • Antibiotic resistance

     Detection and Isolation of Soma clonal Variants

    1. Analysis of morphological characters
        • Qualitative characters: Plant height, maturity date, flowering date and leaf size
        • Quantitative characters: yield of flower, seeds and wax contents in different plant parts

    2.      Variant detection by cytological Studies

        • Staining of meristematic tissues like root tip, leaf tip with feulgen and acetocarmine provide the number and morphology of chromosomes.

    3.      Variant detection by DNA contents

        • Cytophotometer detection of feulgen stained nuclei can be used to measure the DNA contents

    4.      Variant detection by gel electrophoresis

    • Change in concentration of enzymes, proteins and hemical products like pigments, alkaloids and amino acids can be detected by their electrophoretic pattern

    5.      Detection of disease resistance variant

    • Pathogen or toxin responsible for disease resistance can be used as selection agent during culture.

    6.      Detection of  herbicide resistance variant

    • Plantlets generated by the addition of herbicide to the cell culture system can be used as herbicide resistance plant.

    7.      Detection of  environmental stress tolerant variant

    • Selection of high salt tolerant cell lines in tobacco
    • Selection of water-logging and drought resistance  cell lines in tomato
    • Selection of temperature stress tolerant in cell lines in pear.
    • Selection of mineral toxicities tolerant in sorghum plant (mainly for aluminium toxicity)

     

    Advantages of Soma clonal Variations

    • Help in crop improvement
    • Creation of additional genetic varitions
    • Increased and improved production of secondary metabolites
    • Selection of plants resistant to various toxins, herbicides, high salt concentration and mineral toxicity
    • Suitable for breeding of tree species

     Disadvantages of Soma clonal Variations

    • A serious disadvantage occurs in operations which require clonal uniformity, as in the horticulture and forestry industries where tissue culture is employed for rapid propagation of elite genotypes
    • Sometime leads to undesirable results
    • Selected variants are random and genetically unstable
    • Require extensive and extended field trials
    • Not suitable for complex agronomic traits like yield, quality etc.
    • May develop variants with pleiotropic effects which are not true.

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