CLONING PLANTS BY TISSUE CULTURE: EARLY YEARS, CURRENT STATUS AND FUTURE PROSPECTS

The basic ideas of plant tissue culture were articulated by Haberlandt more than 80 years ago. As he had envisioned, plant cell cultures became generally achievable following the identification of IAA as auxin and the discovery of kinetin, the first cytokinin. And as Haberlandt predicted, somatic cell embryogenesis, or manifestation of totipotential, has been confirmed in cell cultures. Current methods of regenerating plants for rapid propagation are based on manipulating auxin/cytokinin balances to achieve adventitious organogenesis, as disclosed in 1957 by Skoog and Miller; removing apical dominance with high levels of cytokinin, as first accomplished in 1958 by Wickson and Thimann; and, in a very few instances, inducing somatic cell embryogenesis. The first plant cloning application of tissue culture dates back to 1952, when Morel and Martin attained elimination of dahlia virus by shoot apex culture. Morel extended the virus exclusion to cymbidium and, in doing so, perceived its applicability to rapid clonal propagation of orchids. Extension of the tissue culture method to other crops followed the impressive successes by commercial orchidologists. The application to other crops began with ferns and other ornamentals, then spread slowly to vegetables, fruits, field crops, and forest trees. The number of species that are now propagatable by tissue culture is near 1000, although not all have been tested for commercial feasibility.

The method is not without serious flaws. Current procedures of plant multiplication are too laborious and slow. Faster methods, e.g., somatic cell embryogenesis, are not always achievable or applicable and may result in high incidences of mutants and other variants. Successful transfer of propagules to soil is not assured. And pathogen exclusion by current practices is largely coincidental.

Nevertheless, tissue culture can become the principal alternative in propagating all clonal cultivars. This status will be reached when labor and costs can be reduced and plant multiplication increased substantially. Some steps in the current practice will be mechanizable. Clonal seeds, produced by encapsulating somatic cell embryos, might become the method of propagating some species.