1.1.  Background

For thousands of years man has been dependent on plants for food, shelter, medicine and many other purposes. Throughout history he has cultivated and selected the useful plants more suited to his needs. The latest advances in plant biotechnology provide potential to make improvements much more quickly than by conventional plant breeding. Studies on all aspects of plant development and multiplication in whole plants are often complicated by interactions between the various processes that underlie growth and development. It is, therefore, desirable to simplify matters so that controlling influences can be easily identified and studied. This can be done by isolating and culturing parts of the plants in vitro. Plant tissue culture has become popular among horticulturists, plant breeders and industrialists because of its varied practical applications. It is also being applied to study basic aspects of plant growth and development. The discovery of the first cytokinin (kinetin) is based on plant tissue culture research.

The earliest application of plant tissue culture was to rescue hybrid embryos and the technique became a routine aid with plant breeders to raise rare hybrids, which normally failed due to post-zygotic sexual incompatibility. Currently, the most popular commercial application of plant tissue culture is clonal propagation of disease-free plants. In vitro clonal propagation, popularly called micropropagation, offers many advantages over the conventional methods of vegetative propagation: (1) many species (e.g. palms, papaya) which are not amenable to in vivo vegetative propagation are being multiplied in tissue cultures, (2) the rate of multiplication in vitro is extremely rapid and can continue round the year, independent of the season. Thus, over a million plants can be produced in a year starting from a small piece of tissue. The enhanced rate of multiplication can considerably reduce the period between the selection of a plus tree and raising enough planting material for field trials. In tissue cultures, propagation occurs under disease and pest-free conditions.

An important contribution made through tissue culture is the revelation of the unique capacity of plant cells, called “cellular totipotency”. It means that all living plant cells are capable of regenerating whole plants irrespective of their nature of differentiation and ploidy level. Tissue culture also provides the best means to elicit the cellular totipotency of plant cells and, therefore, it forms the backbone of the modern approach to crop improvement by genetic engineering. Regeneration of plants from cultured cells has also found many other applications. Plant regeneration from cultured cells is proving to be a rich source of genetic variability, called “somaclonal variation”. Several somaclones have been processed into new cultivars. Regeneration of plants from microspore/pollen provides the most reliable and rapid method to produce haploids, which are extremely valuable in plant breeding and genetics. With haploids, homozygosity can be achieved in a single step, cutting down the breeding period to almost half. This is particularly important for highly heterozygous, long-generation tree species, such as neem. Studies on in vitro production of haploids of tropical woody perennials have met with very little success. Pollen plants also provide a unique opportunity to screen gametic variation at sporophytic level. This approach has enabled selection of several gametoclones, which could be developed into new cultivars. Even the triploid cells of endosperm are totipotent, which provides a direct and easy approach to regenerate triploid plants difficult to raise in vivo.