Discuss the applications of Plant Tissue Culture in Agriculture, Horticulture & Industry

Applications: Plant tissue culture is an essential component of plant biotechnology. The possibility to regenerate whole plants from protoplasts, single cells, tissue & organs, in-vitro, has opened out entirely new approaches to plant improvement.

Application of the plant tissue culture in Agriculture
a) Production of Rare Hybrids:- Hybridization is a well-established plant breeding procedure to obtain superior plants by combining useful characters distributed in different plants.

Hybrid embryo normally aborts on account of the failure of endosperm development or due to embryo endosperm incompatibility. In such instances, it can be excised from the young seed & cultured in-vitro.

Embryo culture has been successfully applied to several sexually incompatible crosses. In several interspecific crosses of Brassica abortion of hybrid embryo occurs at such an early stages that it is not possible to excise & culture the embryo. A hybrid can be obtained by culturing the ovules (ovule culture) or ovaries (ovary culture) enclosing the hybrid embryo. Embryo culture has been used to raise hybrids between the sexually incompatible parents, Gossypium hirsutum & G. arboretum.

b) Somatic hybridization & cybridization:- Somatic hybridization involves the fusion of somatic cells & regeneration of plants from the fusion products. Plant cells are bounded by a rigid cellulose wall & are cemented together by a pectin-rich matrix to form tissues. An essential step in the fusion of plant cells is to bring together the plasma membrane by degrading the cellulosic wall. Thus, the first step in somatic hybridization is the isolation of plant protoplasts.

A highly significant application of protoplast fusion is the production of asymmetric hybrids by partial genome transfer from an irradiated donor protoplast to an acceptor protoplast & the selective transfer of cytoplasmic genes. Selective transfer of cytoplasmic traits is achieved by the fusion of normal protoplasts of the recipient parent with the donor's protoplasts in which the nucleus has been rendered inactive by irradiation or with it's enucleated such protoplasts or miniplasts. Such hybrids are called cybrids.

c) Haploid production:- Haploids are extremely important in genetics & plant breeding. In haploids, it is possible to detect recessive mutants which do not express themselves in the diploid state due to the presence of the dominant allele. In cross pollinated plants & F1 hybrids with a high degree of heterozygosity, the fixation of a particular trait through the conventional method of backcrossing takes 7-8 years.

Application of plant tissue culture in Horticulture
a) Clonal propagation:- The conventional method of clonal propagation are slow & often not applicable. For example, the only in-vivo method for clonal multiplication of cultivated orchids, which are complex hybrids is 'back-bulb' propagation. It involves separating the oldest pseudo bulbil to force the development buds. This process allows, at best doubling the plant number every year.

In-vitro clonal propagation, popularly called Micropropagation. Micropropagation generally involves three steps: (i) Shoot multiplication (ii) rooting & (iii) transplantation

(i) Shooting multiplication: The most popular method of shoot multiplication is forced proliferation of axillary shoots. For this cultures are initiated from apical or nodal cuttings carrying one or more vegetative buds. In the presence of cytokinin alone or in combination with a low concentration of an auxin, such as IAA or NAA, the pre-existing buds grow & produce 4-6 shoots within 3-4 weeks.

(ii) Rooting: Shoots produced through axillary branching or adventitious differentiation are rooted in-vitro on a medium containing a suitable auxin, such as IAA, NAA or IBA. Alternatively, where possible the shoots are treated with auxin & directly planted in the potting mixture for in-vivo rooting.

(iii) Transplantation: The shoots or plantlets multiplied on a medium containing organic nutrients, show poor photosynthetic capability. In practice, the plants are maintained under high humidity for 10-15 days after they removed from culture vessels. During the next few weeks, the humidity around the plants is gradually lowered, before they are transferred to natural conditions.

b) Production of Disease-free plants:- Under normal conditions plants are infected by a wide range of pathogens such as bacteria, fungi, viruses, viroids & insects like nematodes & insects. Many perennial plants & those propagated by vegetative means are systematically infected with one or more pathogens, which reduce yield, vigor & quality of the plant. If explants for micro propagation are derived from an infected plant, the pathogens can multiply & spread to a large number of plants. It is therefore essential to use disease free stock plants for micropropagation. Eradication of viruses & other pathogens is also desirable from the point of view of international exchange of plant materials.

Application of plant tissue culture in Industry
Cultured plant cells retain their metabolic potential & synthesize secondary products of commerce. Cell cultures can also be used as factories for bioconversion of intermediate compounds into more valuable products. Shikonin, an expensive compound, obtained from the roots of Lithospermum erythrorhizon, has been used by the Japanese tradition as a vegetable dye & in cosmetics & toiletries. To reduce dependence on import of this plant material, the Japanese scientists have developed a tissue culture method for the commercial production of Shikonin. In tissue cultures the yield of high-value compounds can be enhanced by feeding the cells with precursors of their biosynthetic pathway (Biotransformation), manipulation of the culture conditions & selecting high yielding cell lines. One important fundamental contribution of plant tissue culture is the discovery of cytokinins.