SOMACLONAL VARIATION AND CROP IMPROVEMENT

Larkin and Scowcroft (1981) proposed the term somaclone to describe the plants originating from any type of tissue culture. Genetic variation (Genotypic and Phenotypic Variability) found to occur between somaclones in plant tissue cultures was then called somaclonal variation.

 The usefulness of variation was first demonstrated through the recovery of disease resistant plants in potato (resistance against late blight and early blight) and sugarcane (resistance against eye-spot disease, Fiji disease and downy mildew)

Genetic variation - mutations or other changes in the DNA of the tissue those are heritable .This is only transmitted to the next generation and is thus important for crop improvement. Therefore it is necessary to study the transmission of variation to sexual progeny to facilitate the estimation of its utility for improvement of a sexually propagated crop. In several crops R0,R1 and R2 progenies were analyzed for genetic analyses and 3:1 segregation leading to the isolation of true breeding variants was observed.

Epigenetic variation-  non-heritable phenotypic variation. Epigenetic changes can be temporary and are ultimately reversible. However, they may also persist through the life of the regenerated plant.

Physiological variation-Temporary in response to stimulus and disappear when it is removed.

Causes for variation

Changes of mother plant origin

Chimeral rearrangement of tissue layers.  These layers can be rearranged during rapid cellular proliferation. Therefore, regenerated plants may contain a different chimeral composition or may no longer be chimera at all. Cell variation also occurs if callus is initiated from explants containing differentiated and matured tissue s that have specialized function.

Explant derived variation

The most stable cultures are obtained from meristematic tissue of a mature plant or tissues of a very young organ of meristematic nature.Polyploid cells can give more variability than diploids

Genetic changes arising in culture

Ploidy Changes Three phenomena that occur during mitosis lead to most changes in ploidy:

endomitosis (sister chromatids separate within the nuclear membrane, but there is no spindle formation nor cytoplasmic division)

endoreduplication (chromosomes at interphase undergo extra duplications)

spindle fusion (giving binucleate or multinucleate cells).

Gross structural rearrangements appear to be a major cause of somaclonall variation. These involve large segments of chromosomes and so may affect several genes at a time.

Structural changes in the DNA sequence

Chromosomal rearrangements, point mutations, or transposition of transposable elements can occur during culture. These changes can occur spontaneously or can be induced with chemicals or radiation

Effect of the genotype

Effects of the culture process itself (lengthy culture periods, growth and other aspects of the culture medium may also affect the ploidy of the cultured cells. Medium that places cells under nutrient limitation will favor the development of "abnormal" cells. Chromosomal alterations, like ploidy changes, increase with increased lengths of culture.

Selection of somaclonal variants on subjecting the cells to selection pressure

Selection of cells in the presence of

Resistance to herbicide - Herbicide

Resistance to salt - Sodium chloride / Aluminium

Resistance to drought - PEG / Mannitol

Resistance to frost - Hydroxy proline resistant lines

Resistance to pathogens - Pathotoxin / Culture filtrate

Crop improvement through somaclonal variation for desirable characters

Crop	Characters modified
Sugarcane	Diseases (eye spot, fiji virus,downy miledew, leaf scald)
Potato	Tuber shape, maturity date, plant morphology, photperiod, leaf color, vigour, height, skin color, Resistance to early and late blight
Rice	Plant height, heading date, seed fertility, grain number and weight
Wheat	Plant and ear morphology, awns,, grain weight and yield, gliadin proteins,amylase
Maize	T toxin resistance, male fertility, mt DNA
Medicago sativa	Multifoliate leaves, elongated petioles, growth, branch, no.of plants, dry matter yield.
Tomato	Leaf morphology, branching habit, fruit colour, pedicel, male fertility, growth
Avena sativa	Plant height, heading date,awns
Hordeum spp	Plant height and tillering
Lolium hybrids	Leaf size, flower,vigour, survival