In this article, we will discuss about the application of somaclonal variation in some crop plants and horticulturally plants.
In tissue culture, somaclonal variation has been presented a lot of significant contributions to plant science. Somaclonal variation among regenerated plants from callus and protoplast culture has been suggested as a useful source of potentially valuable germplasm for plant breeding and plant improvement.
The major benefit of somaclonal variation is to create variation in adapted genotypes. Recognition of new genotypes at the whole plant level and their efficient exploitation would however be very useful in breeding programme. Agronomically desirable many traits in several crop plants have been raised from tissue culture. Many of these traits will find a place in new improved varieties.
Some examples of somaclonal variation in crop plants as well as in some horticulturally important plants has been discussed below:
The occurrence of somaclonal variation in rice (Oryza sativa) had been independently reported several times. Numerous variations have been observed in somaclones derived from an assumed homozygous parent; a dihaploid derivative (a self-doubled haploid or homozygous diploid) from anther culture.
About 1,121 somaclones, selected progeny were examined immediately in three successive selfed generations and these segregated for characters such as plant height, maturity, heading date and grain yield. Within two generations of selfing, many of the variants had become true breeding. Based on progeny analysis of five characters, 72% of the regenerates differed from the parent in at least one character; 28% differed for two or more traits.
Schaeffer et al. (1984) found significant variation among anther-derived dihaploid of rice. Significant improvements relative to the parent were observed for seed weight, seed proteins percentage, tiller number, panicle length and time of flowering. At the International Rice Research Institute, recent research has involved analysis of somaclonal variation in several rice cultivars.
Mutants were observed for many characters such as panicle, grain and leaf morphology and tiller arrangement. Several kinds of chlorophyll-defective mutants were also observed. The progeny have also been screened for tolerance to salinity and aluminium toxicity.
It has recently become possible to regenerate variant plants from tissue cultures of wheat. Variations were manifested for both morphological characters and for traits under simple genetic control such as gliadin proteins in seed, grain colour etc. and polygenic control such as plant height, heading date and yield. Both heterozygous and homozygous mutants were screened in the primary regenerates of wheat.
The classical genetic, chromosomal and recently molecular research in maize has enabled a penetrating analysis of somaclonal variation. In one analysis of 77 somaclones regenerated from maize tissue cultures, 17 defective endosperm or seedling mutants were identified. In another analysis of 51 somaclones, eight segregated for recessive kernel mutations and one segregated for a mutation which caused premature wilting.
Maize studies have also provided conclusive evidence that the mitochondrial genome can undergo genetic changes during cell culture. Cytoplasmic male sterile line of maize is very sensitive to T-toxin produced by the causal organism of southern corn leaf blight, Drechslera maydis. Normal cytoplasm plants are male sterile and resistant to the T-toxin.
The toxin has been used as a selective agent in tissue culture of T-cytoplasm maize lines. Plants regenerated from the selected cell lines were resistant both to the toxin and to infection by D. maydis. Toxin resistant, male fertile plants were regenerated from Cms – T maize culture grown without exposure to toxin. The conversion during tissue culture to toxin resistance, male fertility was maternally inherited and shown to be associated with the mitochondria.
For the improvement of potato crop Shepard et al (1980) suggested that it will be more profitable to improve a popular variety selectively rather than to create a new one. The potato somaclones were also screened for both late and early blight resistance. The parent ‘Russet Burbank’ is highly susceptible to both these diseases. From among more than 800 plants, a range of variation to late blight (Phytophtliora infestans) was found.
About 2% of the somaclones were also able to transmit the disease resistant character through subsequent tuber generations. In addition, several other disease resistant variants were recovered. Such variants are resistant to early blight (Alternaria solani) and to multiple races of Phyiophthora infestans.
A large number of somaclonal variations have been raised from leaf derived callus tissue of tomato. Variants were screened for a number of characters such as male sterility, joint- less pedicel, fruit colour, indeterminate growth etc.
Many somaclonal variations have been observed among plants regenerated from cultured immature embryo, apical meristem of oat. Variants were selected for plant height, heading date, leaf striping, awns, etc.
The occurrence of soma- clonal variation in Brassica spp. has been independently reported several times. Variants were found which affected flowering time, growth habit, waxiness glucosinolates, Phoma hngam tolerance.
In Nicotiana sp., plant regeneration was possible from anther culture, protoplast culture and leaf callus culture. From regenerated plant population, somaclonal variants were selected for a number of characters such as plant height, leaf size, yield grade index, alkaloids, reducing sugars, specific leaf chlorophyll loci etc.
A triploid hybrid was obtained by crossing a diploid Lolium perenne with a tetraploid L. multiflorum. Callus tissue derived from this hybrid produced more than 2,000 plants in five years. These somaclones exhibited a wide variation in leaf shape, size, floral development, growth vigour and longevity.
Some variants possessed characteristics of both the parents which were agronomically valuable and their progenies also showed the same variations, whereas these characteristics were not observed in triploid hybrids reared up in a conventional way.
A high degree of variability in tissue culture regenerated plants from 5 cultivars of Pelargonium zonale was observed. Changes were found in plant and organ size, leaf and flower morphology, essential oil constituents, fasciation, pubescence and anthocyanin pigmentation. One of the variants had been released, as a new cultivar known as ‘Velvet Rose’.
Tokumasu and Kato (1976) using somatic callus cultures of a dihaploid Geranium plant had obtained homogenous plants amongst which two were variants for essential oil constituents. The potentiality of somaclonal variation has increased dramatically with clear evidence of expression of selected traits in selected plants and seed progeny.
Among the economically important traits which have been selected in vitro and recognized as having potential impact are now being employed to enhance the efficiency of plant breeding. Somaclonal variations provide also a powerful option for plant improvement and this may be the best approach of plant improvement over conventional methods.