Elementary plant Biochemistry and Biotechnology (2+1)

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.

Origins and mechanisms of somaclonal variability

• Such variation was thought to be due to epi genetic factor such as exposure to plant growth regulators (PGRs) and prolonged culture time

Somaclonal variation can be of two sorts:

• Genetic (i.e. heritable) variability – caused by mutations or other changes in DNA.
• Epigenetic (i.e. non-heritable) variability – caused by temporary phenotypic changes.

Genetic variability
Various molecular mechanisms are responsible for genetic variability associated with somaclonal variation:

• Changes in ploidy
• Structural changes in nuclear DNA
• Epigenetic variability

The importance of somaclonal variation

1. A novel source of genetic Variation. However successful utilization of somaclonal variation heavily depends upon its systematic evaluation and judicious utilization in breeding programmes. E.g. scented geranium viz., velvet rose; pure thronless blackberries – Lincoln logon (Rubus); Hasuyume – protoplast derived rice cultivar; potato – Russet barbank
2. It is of interest as a basic genetic process, since it contradicts the concept of clonal uniformity.
3. Third soma clonal variation is unwanted when the objective is micropropagation of elite genotypes or genetic transformation that partly involved tissue culture. Under such circumstances, prevention or at least minimization of variation is of utmost importance. To achieve this, the frequency, nature and magnitude of somaclonal variation in relation to manipulation of media components, explant source, culture conditions etc. should essentially be understood.
4. Somaclonal variation are employed in disease resistance improvement. For example, in sugar cane, it was used to develop eye spot disease resistance caused by Helmenthosporea sacchari, downey mildew, and fizi virus; in potato, protoplast derived russet bar bank resistant to phytopthora infestans (Late blight of potato).
5. Somaclonal variation can also be used to impart abiotic stress resistance such as drought, salinity, aluminium, herbicide tolerance and insect resistance.
 
Why variation occurs?

• Factors that contribute to soma clonal variation are of categories i.e. physiological genetic and biochemical.

Physiological causes of variation

• Variations induced by physiological factors were identified quite earlier. Such variations are those induced habituation to PGR in culture and culture conditions and are epigenetic. They may not be inherited in Mendelian fashion. Prolonged exposure of explant tissue to powerful auxin such as phenoxyacetic acid (e.g., 2, 4-D or 2, 4, 5-T) often results in variation among the regenerants.
Genetic causes of variation

• Tissue culture plants show certain variations which are results of alteration at the chromosomal level. Although the explant tissue may be phenotypically similar, plant often have tissue comprised of diverse cell type or cells. That is there are cytological variation among the cell types within the explant tissue, such pre existing conditions often result in plant regenerates from the tissue that are dissimilar.

Biochemical cause of variation

• Biochemical variations are predominant type of variations in tissue culture. Biochemical variations include alteration in carbon metabolism leading to lack of photosynthetic ability (albinos in cereals such as rice), starch biosynthesis carotenoid pathway.

Selection of somaclones

• Although cytological and phenotypic analyses can be used to evaluate somaclonal variation, recently molecular techniques (such as RFLP, RAPD, SSR, AFLP) have been used with increasing frequency.


Benefits

• The major likely benefit of somaclonal variation is in plant improvement. Somaclonal variation leads to the creation of additional genetic variability. Characteristics for which somaclonal mutants can be enriched during in vitro culture include resistance to disease or patho- toxins, herbicides and tolerance to environmental or chemical stress, as well as for increased production of secondary metabolites.

Disadvantages

• A serious disadvantage of somaclonal variation 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.

Ways of reducing somaclonal variation

• It is well known that increasing numbers of subculture increases the likelihood of somaclonal variation, so the number of subcultures in micropropagation protocols should be kept to a minimum. Regular re-initiation of clones from new explants might reduce variability over time. Another way of reducing somaclonal variation is to avoid 2,4-D in the culture medium, as this hormone is known to introduce variation.


• Vitrification [hyperhydracity] may be a problem in some species. In case of forest trees, mature elite trees can be identified and rapidly cloned by this technique. High production cost has limited the application of this technique to more valuable ornamental crops and some fruit trees.