1. Introduction

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 (Laibach, 1925, 1929), 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 in 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 plus trees and raising enough planting material for field trials. In tissue culture, propagation occurs under pathogen and pest-free conditions.

An important contribution made through tissue culture is the revelation of the unique property of plant cells, called “cellular totipotency”. The totipotency of plant cells was predicted in 1902 by Haberlandt and the first true plant tissue culture on agar was established. Since then plant tissue culture techniques have greatly evolved. The technique has developed around the concept that a cell has the capacity and ability to develop into a whole organism irrespective of their nature of differentiation and ploidy level. Therefore, it forms the backbone of the modern approach to crop improvement by genetic engineering. The principles involved in plant tissue culture are very simple and primarily an attempt, whereby an explant can be to some extent freed from inter-organ, inter-tissue and inter-cellular interactions and subjected to direct experimental control.

Regeneration of plants from cultured cells has 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. Pollen raised 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.

The entire plant tissue culture techniques can be largely divided into two categories based on to establish a particular objective in the plant species: