Somaclonal Variations

 

Cell or tissue cultures undergo frequent genetic changes – polyploidy, aneuploidy, chromosomal breakage, deletion, etc., and that these are also expressed at biochemical and molecular levels. Plant cell and tissue cultures, therefore, provide increased genetic variability relatively rapidly and without applying sophisticated technology.

The genetic variability present among the cultured cells, plants derived from such cells or progeny of such plants is called somaclonal variation. Plants derived from such cells are referred to somaclones.

 First report of morphological variation in plants regenerated from cell culture was made by Heinz and Mee in 1971 in sugarcane. Larkin and Scowcraft proposed the term Soma -clonal variation to describe all those variations which occurs in plants regenerated from any form of cell culture (or) it refers to the heritable changes which accumulated in the callus from somatic explants and expression in the progeny of invitro regenerates obtained from callus.

The plants derived from cell and tissue cultures are termed as somaclones and the plants displaying variation as somaclonal variants. The somaclonal variation may be transient (epigenetic) or genetic only the latter is transmitted to the next generation and is thus important for crop improvement. The epigenetic changes are expressed at cell culture stage but usually disappears when plants are regenerated to reproduced sexually, Somaclonal variation is a wide spread phenomenon reported in Monocots (wheat, rice, and maize) Dicots (Tomato, Tobacco, and Brassica) and asexually propagated crop plants except sugarcane.

 Types of somaclonal variation

 Based on the tissue from which variation originate Somaclonal variation can be divided into the following types 1) Gametoclonal variation : variation observed among the plants regenerated from gametic cultures a) Androclonal variation observed among the plants regenerated from anther (or) pollen culture b) Gynoclonal variation:- from ovule (or) ovary culture

2) Protoclonal variation:- variation observed among the plants regenerated from protoplast cultures

3) Calliclonal variation:- variation observed among the plants regenerated from callus cultures

 Origin of Somaclonal variation

Somaclonal variation may occurs either for single trait (or) more than one trait at a time. Variation may arrives due to any one (or) a combination of following mechanisms

1) Somaclonal variation may occur due to chromosomal abnormality in cultured cells and plants regenerated from them

2) The chromosomal abnormality may be due to changes in chromosomal number (ploidy level) and chromosomal structure Gene mutations Variation may also be due to additive effects of mutated genes change in base sequence (or) activation of some gene amplification results alternation in gene expression. changes in plasmagenes which include plasmagene mutations and rearrangement in cytoplasm gene can also be altered by in vitro culture condition Activation of transposable elements DNA methylation Mitotic crossing over

 The variations observed in the plants regenerated from cultured cells are derived broadly from two sources 1) Pre – existing variability. Some of the variation could be due to the inherent cellular heterogeneity of explant

2) In vitro induced variability Culture conditions may bring about new genetic changes

Advantages of somaclonal variation

 1) Frequency of variation under in vitro condition are often considerably higher than the incidence of spontaneous mutation or Chemically induced mutation.

2) Some times unique mutations have been generated through tissue culture which could not be obtained through crossing because of their non – availability in germplasm Eg:- Jointless pedicel mutant in tomato

3) Use of Somaclonal variation may reduce the time required for release of new variety by two years as compared to mutation breeding

 4) It can be used to isolate new genotype that retain all the favourable characters of the existing cultivers while adding one additional trait that means it may not involve a drastic change in genetic background

5) It occurs for trait both nuclear (or) cytoplasmic origin

6) Saves time by reducing lengthy procedures of hybridization and selection

 7) In wide crosses it provides a mechanism of gene integration

 8) Mature embryos of the wide cross can be callused and the desirable gene integrated plants can be selected

9) Highly efficient as it can screen very large, number of cells rapidly with small effort time, cost labour, and space requirement.

10) Characters can be selected at the cell level which can not be allowed at plant level

11) This is the only approach for isolation of biochemical mutants in plants

12) It is a simple and cheap form of plant biotechnology as compared with somatic hybridization and genetic transformation

13) The in vitro regenerates Eg:- of tomato and potato showed increased rates of recombination. This could be useful in generating novel genetic variation by breaking undesirable close linkages and by shuffling the genes linked in the repulsion phase

Applications

      1. Somaclonal variation has been described for a variety of both qualitative and quantitative traits

1.     Isolation of regenerants resistant to diseases. Maize lines having Texas male sterile cytoplasm are susceptible to southern leaf blight caused by Helminthosporium maydis which produces a toxin that binds to mitochondria.

2.     Maize cells resistant to this toxin have been selected and plants regenerate from them were resistant to leaf blight caused by Helmithosporium maydis.

3.     3. A tomato line resistant to bacterial wilt caused by psuedomonas solanacearum were isolated by screening of plants regenerated from un selected calli

4.     4. A fiji disease resistant sugarcane line were isolated from the variety pindar is released as a new variety called “ono”

5.     5. Variation may arise for useful morphological characters. An improved scented Geranium variety named velvet Rose has been developed from Rober’s lemon rose

6.      6. Isolation of variants resistant to abiotic stresses. Plant tissue culture techniques have been successfully used to obtain salt tolerant cell lines (or) variants in several plant species plant cells resistant to 4 to 5 times the normal toxic salt concentration (Nac1) have been isolated. In many cases, the plants regenerated from them were also tolerant to saline condition. Eg:- Tobacco plants regenerated from high salts (0.88%) tolerant cell lines were also salt tolerant.

7.     7. Low temperature is another important environmental factor effecting survival and performance of crop plants. Cell lines resistant to chilling have been isolated in several cases. Eg:- Chillies, Nicotiana sylvestris.

8.     8. Development of varieties with improved seed quality. A variety Ratan of Lathyrus which has low neurotoxin content has been develop through somoclonal variation

9.     9. Isolation of mutants for efficient nutrient utilization. Tomato cell lines which are able to grow normally in phosphate deficient condition due to high secretion of enzyme. Acid phosphatage have been isolated through in vitro selection.

10.   Achievements

11.  A list of somaclonal variants released as variety for commercial cultivation

12.  Crop Somaclonal variety Parent variation Salient features

13.  Sugarcane Ono Pindal Resistant fiji disease

14.   Sweet potato Scarlet Selected from shoot tip culture derived clones contains darker and more stable skin colour

15.   Geranium Velvet rose Robbers lemon Rose Polyploid somaclone, sturdiness vigour and attractiveness

16.  Ciltronella java Bio-13 Not known 37 percent more oil content 39 percent more citronella content released by CIMAP, Lucknow Brassica juncea Pusai Jaikisan Varuna Bolder seeds. 17.4 percent yield advantage released by IARI New