conditions. With this method, acclimation of the rooted shoots can be carried out
simultaneously. In vitro rooting consists of rooting the plants in axenic
conditions. Despite the cost factor, in vitro rooting is still a very common practice in
many plant species because of its several advantages. Tissue culture conditions facilitate
administration of auxins and other compounds, avoid microbial degradation of applied
compounds, allow addition of inorganic nutrients and carbohydrates, and enable exper-
iments with small, simple explants. Several factors are known to affect rooting. The most
important factor is the action of endogenous and exogenous auxins. In many cases a
pulse treatment with auxins for a short period has also been sufficient for root induction.
Phenolic compounds are known to have a stimulatory effect on rooting. Among the phe-
nolic compounds, phloroglucinol, known as a root promoter, has a positive effect on rooting
(Hegedus and Phan 1987). Catechol, a strong reducing agent, has been reported to regulate
IAA oxidation and thus affect rooting in plant tissue culture (Hackett 1970).
5.9 Acclimation
Once plants are generated by tissue culture, they have to be transferred to the green-
house or field. This requires that the plants be hardened-off before transfer to the
field. During this acclimation process, plants are first transferred to a growth
chamber or greenhouse and covered by domes to minimize the loss of water. Tissue
culture conditions are at approximately 100% humidity, whereas relative humidity
outside the vessels is typically much lower. In addition, the plants must be
“weaned” off the rich media so they can grow as normal plants in soil. Once the
plants are acclimatied under greenhouse conditions, they are ready for transfer to the
field. Acclimation is a very important step in tissue culture since it is possible to
lose plants if they are not properly hardened-off.
5.10 Conclusions
Plant tissue culture is an essential tool in plant biotechnology that has enabled mass clonal
propagation, production of secondary metabolites, preservation of germplasm, and pro-
duction of virus-free plants. Moreover, it serves as an indispensable tool for regenerating
transgenic plants. All this has been possible by manipulating plant tissues and various
kinds of media developed by plant tissue culturists and by the use of plant hormones. It
has been one of the very exciting discoveries for plant biologists and will continue to be
most useful in the coming years.
Acknowledgments
The author would like to thank Dr. Leo D’Souza from the Laboratory of Applied Biology,
St Aloysius College, Mangalore, India for suggestions on the manuscript and for kindly
providing most of the illustrations for this chapter. Many thanks to Dr. Wayne Parrott
and Benjamin Martin from the University of Georgia for providing the somatic embryo-
genesis photos.
128 TISSUE CULTURE: THE MANIPULATION OF PLANT DEVELOPMENT