CLONING HIGHER PLANTS FROM ASEPTICALLY CULTURED TISSUES AND CELLS
Authors: KRIKORIAN A. D. 
Published: January 2008

1. For a number of years considerable attention has been given both in the scientific and lay press to dramatic advances in the rapid clonal multiplication of plants by means of so‐called ‘modern’ tissue culture techniques. Because the words clone, cloning and tissue culture mean different things to different people, there is considerable confusion even among biologists as to what has actually been achieved.

2. An historical approach has been used to put the accomplishments into perspective.

3. Organ culture, tissue culture, cell or suspension culture and protoplast culture all offer varying degrees of potential for clonal multiplication but the most substantial progress has been made with explanted stem tips or lateral buds which can be stimulated to produce numerous precocious axillary branches. These, in turn, can be separated or subdivided and induced to root and thus yield populations of genetically and phenotypically uniform plantlets.

4. Similarly, undifferentiated calluses can sometimes be induced to form shoots and/or roots adventitiously. This method of stimulating organ development on callus is based on the now classic work of Skoog and Miller done in the I950s with theWisconsin 38strain of a ‘Havana’ cultivar of tobacco. Because of the growing criticism that when callus cultures are maintained for more than a few subcultures genetic or chromosomal aberrations are induced and these nullify the ultimate goal ‐ namely to get large numbers of genetically identical plants ‐ callus procedures are generally being relinquished in favour of precocious axillary branching methods.

5. The number of plantlets obtainable through the procedures with precocious axillary shoots or callus culture are fewer, in theory, than the number of plantlets obtainable from a somatic embryo system starting with cells grown in suspension culture. But the fact is that cell culture techniques required to produce somatic embryos are also demanding and very much in their infancy as far as their applicability to clonal multiplication is concerned.

6. Even so, slowly but steadily, advances are being made in learning how to stimulate formation of somatic or adventive embryos from totipotent cells grown in suspension culture. In cases where it has proven feasible, not only may somatic embryos be generated in great quantities, but their discrete bipolar nature often obviates the need for time‐consuming, laborious procedures to assure their success in a conventional soil or greenhouse setting.

7. While protoplast cultures are of very limited direct use for clonal propagation of higher plants, the procedures hold some hope as a vehicle whereby select useful genes can be introduced into plants. Parasexual hybridization, that is the production of hybrids by fusing totipotent protoplasts of different origin, offers no imminent prospects for the development of useful and novel plant types but may be applicable in the long term in very specific situations.

8. It is concluded that many problems exist in the producing and growing of totipotent or morphogenetically competent cell suspensions, but the potential benefits that could accrue when investigators learn to overcome these problems far outweigh any disadvantages that they may have. In short, cloning via cultured free cells should be seen in the context of potentialities rather than limitations.