Somaclonal variation in plants,trees

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Somaclonal variation : 

Somaclonal variation N.KANAGARAJ BSF-06-007

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Somaclonal variation It is the term used to describe the variation seen in plants that have been produced by plant tissue culture. Chromosomal rearrangements are an important source of this variation.

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Regeneration from callus, leaf explants or plant protoplasts leads to the generation of considerable variation, described as somaclonal variation.

Different Steps Involved in Utilization of Somatic Variation Flow Chart : 

Different Steps Involved in Utilization of Somatic Variation Flow Chart

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The variations can be A] genotypic B] epigenetic in origin

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A] Genetic alterations are: 1.changes in chromosome 2.chromosome structure 3.DNA sequence

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1.changes in chromosome numbers; Euploidy- changes in whole chromosome sets Aneuploidy- changes in parts of chromosome sets Ploidy: number of basic chromosome sets (a diploid has 2 sets; a hexaploid has 6 sets) Euploid: organism containing multiples of the basic chromosome set Monoploid: organisms with one chromosome set (in essentially diploid taxa) Polyploid: organism containing more than two chromosome sets

2.Structural Changes in Chromosomes : 

2.Structural Changes in Chromosomes Deletion Inversion Duplication Translocation

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3.DNA sequence (base mutations). Change at DNA level Detected using restriction enzyme analysis (altered fragment size) Changes at protein levels Loss or gain of protein band Altered levels of specific proteins DNA methylation Methylation of a gene inactivates its transcription

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B] Epigenetic change Due to gene expression not gene alteration Often temporary Some examples Morphological change - leaf shape Earlier flowering Improved adventitious rooting Increased vigor Plageotropic growth

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Specific epigenetic processes Paramutation Imprinting Gene silencing Cellular differentiation X chromosome inactivation Position effect Reprogramming Transvection Regulation of histone modifications Maintain different characteristics

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Advantages : Benefit of somaclonal variation is in plant improvement It is leads to the creation of additional genetic variability Increased production of secondary metabolites Characteristics for which somaclonal mutants can be enriched during in vitro culture include resistance to disease pathotoxins, herbicides and tolerance to environmental or chemical stress.

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Somaclonal variation in trees Somatic variation may be a better mans for tree improvement Direct gene insertion or agrobacteruim –mediated transformation have many technical difficulties that must be overcome with this methods. It have the potential to significantly change tree breeding programs by reducing the time required for selecting and improving desired traits .

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Applying soma clonal technology to forest trees offers an even greater advantage because of the long generation times of trees and the opportunity to introduce desired traits not possible through traditional breeding. Somaclonal technology is simpler to exploit than other types of genetic engineering.

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To Increase Somaclonal Variation: Callus and suspension cultures for several cycles Regeneration of large number of plants from long-term cultures Testing of selected somaclones for genetic stability Multiplication of genetically stable somaclones for developing new cultivars

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Disadvantages: No control - may change in undesirable direction Variations are random in nature May not be genetically stable Require extensive field testing May not occur for complex agronomic traits

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Control of Somaclonal Variation: Regular reinitiation of clones from new explants might reduce variability over time. Avoid 2,4-D in the culture medium, as this hormone is known to introduce variation. The duration of the culture cycle may influence the variability.

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References: november23,2007 - 20k

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