somaclonal variations


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somaclonal variations by Aisha Ashraf Department of botany , University of punjab Lahore , Pakistan


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Somaclonal variation:

Somaclonal variation

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Aisha Ashraf BO9-O6 Bs.6 th semester Department of botany University of the punjab

Contents :

Contents Introduction Definition Mechanism of somaclonal variation Causes of somaclonal variation Genetic causes Physiological causes Biochemical causes Factors that influence the somaclonal variation Assessment of somaclonal variation isolation and detection of somaclonal variants

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Applications of somaclonal variation Advantages Disadvantages Work on somaclonal variation Conclusion References

Introduction :

Introduction 1976 “Calliclones” by Skinvin and Janice Protoclones 1980 Shepard et al . The term Somaclone first given by Larkin & scowcroft in 1981. 1982 , variations may be transmitted to subsequent generation

How to define somaclonal variations:

How to define somaclonal variations The variation among the tissues and plants derived from in vitro somatic cell cultures and variant plants recovered from any form of cell cultures. Variations arising in cell cultures , regenerated plants and their progenies. Variations arises either from pre-existing variations or by variation induced during cell culture

Mechanism of somaclonal variations:

Mechanism of somaclonal variations Genetic variations Change in ploidy e.g euploidy and aneuploidy Mainly caused by failure of spindle formation , lagging of chromosome at anaphase and fusion of spindles in multinucleate cells.

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Change in chromosome structure Deletions Inversion Duplication Translocation Mainly Caused by chromosomal breakage During cell cycle stages and sometimes by Activation of transposable elements Point mutations DNA methylation

Deletion and duplication :

Deletion and duplication


Inversion and translocation http.//

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Epigenetic variations Temporary ,Non –heritable Various environmental stresses Temporary modifications in phenotype DNA amplification ,DNA methylation also contribute to epigenetic variation

DNA Methylation:

DNA Methylation

CAUSES OF somaclonal variations:

CAUSES OF somaclonal variations The pre-existing genetic variations in the explants tissue. Spontaneous point mutations , somatic crossing over , chromosomal arrangements , transposable elements, DNA methylation, exposure to plant regulators etc.

Genetic causes of variations:

Genetic causes of variations Genotype of parent Karyotypic changes Point mutation Transposable element activation Amplification of DNA Mitotic crossing over

Transposable element activation :

Transposable element activation Imprecise excision and insertion leads to rearrangement of adjacent chromosomal sequences Changes in gene expression and structure Activation results due to Chromosomal breakage Late replication of heterochromatin Changes in methylation pattern

Amplification of DNA:

Amplification of DNA Genes can amplify themselves during differentiation due to environmental stresses Due to increased production of mRNAand protein from that gene Hence copy number of particular sequence vary during and after cell culture Amplification of genes for herbicide resistance detected in alfalfa

Point mutations:

Point mutations Single gene mutations have now been detected Chaleff and Mauvais in 1984 obtained chlorosulfan-resistance tobacco mutant due to acetolacetate synthase A single dominant gene resistant to Fusrium wilt in observed in tomato (1987 by spivey)

Mitotic crossing over:

Mitotic crossing over Leads to homozygous recessive single gene mutations Dulier in 1982 studied Nicotiana tobacum with chlorophyll deficiency markers in heterozygous condition Regen erants at a1 and y loci combination of deletion and mitotic recombination Causes are Environmental factors and certain agents

Altered gene expression in multigene families:

Altered gene expression in multigene families Cultural conditions may regulate expression of multigene families Genetic somaclonal variation has obtained for gliadin and beta amylase in wheat

Cytoplasmic genetic changes:

Cytoplasmic genetic changes Changes in mt-DNA and chloroplast DNA In maize CMS and sensitivity to toxin of Helminthosporium maydis are controlled by mt-DNA Chloroplast is conserved and stable

Physiological causes:

Physiological causes Usually epigenetic Various stresses may result into transient altered expression Prolonged exposure to auxins 2,4-D and 2,4,5-T Cultural conditions

Biochemical Causes:

Biochemical Causes Lack of photosynthetic ability due to alteration in carbon metabolism. Biosynthesis of starch Carotenoid pathway Nitrogen metabolism Antibiotic resistance Alteration in normal methylation pattern

Factors influence the somaclonal variations:

Factors influence the somaclonal variations Explant source Genetic constitution of donor plant Culture conditions Degree of departure from organized growth

Explant source:

Explant source S.C.V may be induced or due to pre-existing variations in explant

Genetic constitution of donor plant :

Genetic constitution of donor plant S.C.V are largely genotype dependant Polyploids show higher chromosomal variability as compared to diploids and haploids Genomes carrying transposable elements exhibit more variations

Culture conditions :

Culture conditions Concentration of growth regulators have effects on Cell division Degree of disorganized growth Proliferation of specific cell types

Degree of departure from organized growth :

Degree of departure from organized growth Long term maintained callus cultures and cell suspension cultures may become genetically unstable. Hence the plants regenerated from them exhibit greater variations

Applications of somaclonal variation:

Applications of somaclonal variation somaclonal variation in crop improvement somaclonal variation in improvement of ornamental plants New variant retaining all the favourable characters along with an additional useful trait Development of elite germplasm and commercial cultivars Enhancing alien gene introgression into cultivated species In vitro selection of herbicide , disease , insect , salts , resistant somaclones

Somaclonal variation in crop improvement :

Somaclonal variation in crop improvement Recovery of novel or rare variants e.g Atrazine resistance in maize Improved lysine and methionine content in cereals Seedling vigour in lettuce Jointless pedicels in tomato etc.

Some examples of somaclonal variants possessing improved traits in crops :

Some examples of somaclonal variants possessing improved traits in crops Potato Resistance to Fusarium oxysporium Resistance to Phytophtora infestans Sugarcane Resistance to eyespot , fiji desease and downy mildew Sweet potato Darker and stable skin colour Wheat Resistance to Helminthosporium maydis , tolerance to heat stress

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Rice Resistance to salt Improved lysine content Tomato Resistance to fusarium race 2 Brassica Herbicide Resistance Salt tolerance Celery Resistance to fusarium wilt

Somaclonal variation in improvement of Ornamental plants:

Somaclonal variation in improvement of Ornamental plants Jain et al. 1993 reported S.C.V plants for flower size , flower colour , plant morphology , plant height and number of flowers per plants from leaf disc callus

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Calluna vulgaris is an economically important plant in Germany polyploid level is to be developed for creating new attractive phenotypes with larger flowers

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Epicactus hybrids have been developed by multiple crossings of different species.

Development of elite germplasm and commercial cultivars :

Development of elite germplasm and commercial cultivars Somaclones with improved traits has resulted in new useful germplasm and cultivars Cultivar of sweet potato Bell sweet a variety of bell pepper having improved traits e.g no seeds Resistance to Pircularia

Enhancing alien gene introgression into cultivated species:

Enhancing alien gene introgression into cultivated species Important in enhancing inter-specific crosses where the genomes show no homology So the chromosomal exchange results In new combinations and transfer of alien chromosome segment

In vitro selection:

In vitro selection Used to select the agronomically desirable traits In vitro selection for herbicide and disease resistance Selective and mutagenic agents having ability to enhance the occurrence of resistant mutants, are added in the medium e.g. Tobacco resistant to sulfonylurea Potato resistant to early and late blight

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In vitro selection for metals and salt resistance Develop biochemical mutants with abiotic stress resistance e.g. aluminium tolerance in carrot, salt tolerance in tobacco and maize. In vitro selection for insect and environmental stress

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Assessment of Somalconal Variation:

Assessment of Somalconal Variation Assessed at phenotypic level S,C.V determined as %age of aberations for one or more characters Molecular techniques used to evaluate S.C.V e.g RFLP RAPD-PCR AFLP

How to isolate the somaclonal variants:

How to isolate the somaclonal variants Different approaches to detect and isolate the variants Morphological distinct cells e.g non-green or the cell having pigments (anthocyanins) Computer based automated cell sorting devices used to screen 1000-2000 cell/second Positive selection Negative selection

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Positive selection is subdivided into following categories Direct selection Rescue selection Stepwise selection Double selection

Advantages :

Advantages Somaclonal variations are stable and occur at high frequencies. Somaclonal variations may show novel mutations. Can be performed in all types of cells, ie; vegetatively or sexually or asexually propagated plants. Somaclonal variations may reduce two years the time required for the release of new variety compared to mutation breeding.

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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 :

Disadvantages Many Somaclonal variants show undesirable features such as reduced fertility, growth rate etc. The variation is not always heritable The variation is generally cultivar dependent Selected clones show unpredictable and uncontrollable variants.

Work done on somaclonal variations in Punjab University:

Work done on somaclonal variations in Punjab University Dr Javaid Iqbal : (University of Punjab ) Biochemical and molecular markers of somaclonal variants in sugarcane. Biochemical and molecular Investigations on S.C.V of induced mutants of potato

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Dr Humaira Afrasiab (University of Punjab) Biochemical and molecular markers in somaclonal variants and induced mutants in potato.

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Rana Abrar Hussain :(PhD,PU) Biochemical and molecular investigation of somaclonal variants in sugarcane

Conclusion :

Conclusion Somaclonal variations being used as a novel source of heritable genetic variations suitable for upgrading existing cultivars of crop plants. Breeders are finding it attractive because of its simplicity. These variations will allow to introduce new characteristics into all major plant varieties thereby developing new improved varieties in a shorter period of time.

References :


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Books : Jain,S.M ,D,S Brar and Ahloowali.B.S. (1998)Somaclonal variation and induced mutation in crop improvement.kluwer academic publishers,Boston 32:1-39,169-189 G.S Chahal ,S.S Gosal (2006)Principles and Procedures of Plant Breeding,Narosa publishing House 435-441

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Bhojwani S.S, Razdan M.K, (2004)Plant tissue culture:Theory and Practice,a revised edition ,Page 246-267. Yadav Dr.P.K and Tyagi Dr.Rajiv,Biotechnology of plant tissue culture, Discovery publishing house new dehli 11oo21,page 183-189

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