Protoplast Fusion and Somatic Hybridization: Importance and Limitation

In this article, we will discuss about the importance and limitation of the protoplast fusion and somatic hybridization.

Protoplast fusion and somatic hybridization have opened up a new avenue in plant science. It is now a well-known fact that the somatic hy­bridization in plants can be used in the improve­ment of plants.

One of these is the production of hybrids which is not possible through normal sexual fusion or fertilization process.

In other words, it includes the formation of somatic hy­brids between two species which are sexually in­compatible. Thus protoplast fusion provides a method of combining the different genomes of different genera and species, with the potential of overcoming sexual incompatibility barrier be­tween plants.

The cytoplasmic mix obtained from proto­plast fusions is novel with the opportunity for the production of cybrids coupled with the op­portunity for the formation of mitochondrial re­combinants. Mitochondria can segregate or recombine their DNAs to form a new type of mitochondria.

Chloroplasts segregate but do not ap­pear to undergo recombination. Thus, with the production of hybrid or cybrid, the mixing of cy­toplasm of both parental protoplast can improve the extra-nuclear genetic elements. In sexual hy­bridization, only maternal cytoplasm, i.e. the cytoplasm of egg cell takes part in the formation of hybrid (Fig 13.6).

Cybrids are generally produced due to elimination of total genome of one parent after the fusion of two protoplasts. If one parental nucleus completely disappears, the cytoplasm of the two parental protoplasts is still hybridized and the fusion product is known as cybrid or cyto­plasmic hybrid or heteroplast.

But the use of a certain compound like cytochalasin has been found to completely extrude the nucleus from the protoplast thus producing enucleate protoplasts. The fusion of the enucleate protoplast with nu­cleate protoplast may lead to the production of male sterile somatic cybrid where male sterility is present in the cytoplasm.

In the experiment, cybrid may also arise by the following ways:

(a) Fusion between a normal nucleate proto­plast and a protoplast containing a non­viable nucleus.

(b) Elimination of one of the nuclei after heterokaryon formation.

(c) Selective elimination of chromosomes at the later stage.

The formation of cybrid has some appli­cation in plant improvement programme. The importance of cybridity has been confirmed by breeding experiments. The transfer of cytoplas­mic male sterile cytoplasm by protoplast fusion to somatic hybrid should be of interest to the plant breeders. This may be of critical impor­tance in male sterility based hybrid seed produc­tion.

Chromosome elimination in fusion product can be used for gene mapping as in fusion prod­ucts of animal cells. Studies of fusion product can give informa­tion about compatibility or incompatibility of the nuclei or cytoplasm.

It has in recent years been repeatedly em­phasized that plant tissue culture per se appears to be an unexpectedly rich source of genetic vari­ation; this has stimulated effort to find out whe­ther such genetic variation can be enhanced by protoplast cloning. This opportunity will undoubtedly lead to the production of new genetic variation.

By protoplast fusion, it is possible to transfer some useful genes such as disease resistance, nitrogen fixation, rapid growth rate, protein quality, frost hardiness, drought resistance etc. from one species to another and thereby widen the genetic base for plant breeding.

Hybrid vigour is well known in sexual hy­bridization and it has been suggested that so­matic hybridization may produce even greater vigour in hybrids. A critical evaluation of this suggestion is required since it could result in en­hanced yields in many crops.

In case of vegetatively reproducing plants, the genetic variation can be induced through protoplast fusion of two species, varieties or two different genera. In case of sugarcane, which is vegetative, the production of somatic hybrids be­tween different varieties followed by regeneration of whole plants can produce improved varieties which may be highly beneficial to the sugarcane industry. Similar advantage may be obtained in potato and other horticultural plants for their improvement.

Limitation of Somatic Hybridization:

It was once suggested that somatic hybrids would be of great value in crop improvement. But the experimental reports are not very en­couraging. At present, techniques for selection and manipulation of somatic hybrid cells and re­generation of hybrid plants from them is limited to a few special cases where they can be manip­ulated very easily in culture. So far the produc­tion of somatic hybrid of agronomically impor­tant plants is not possible.

The main objective of protoplast fusion and somatic hybridization was to overcome the pre- fertilization barrier to sexual incompatibility or any genomic incompatibility. Therefore, it would be undoubtedly expected to achieve very wide crosses through protoplast fusion and it will solve many problems relating to crop improvement. But practically inter-generic crosses between widely related plants, which are not compatible sexually, are not possible.

Few interspecific somatic hybridization where plants are sexually compatible or incom­patible due to natural reproductive isolation, are only achieved. In certain wide crosses, elimination of chro­mosomes from the hybrid cell is another limi­tation of somatic hybridization. So, desirable hybrids are no longer available.

In protoplast fusion experiment, the per­centage of fusion between two different parental protoplast is very low. Although some attempts has been made to increase the percentage of fused cells, still it is also a limitation of somatic hybridization.

Lastly, for hybrid identification, selection and isolation at the culture level, there is no standardized method which is applicable for all material.