The Physiology of Flowering Plants - KHAM PHA MOI

184 PLANT GROWTH HORMONESof effects only when the interactions of these compounds with otherplant growth hormones are considered.7.2.4 CytokininsThe cytokinins were discovered after many years of effort in attemptingto find compounds which control cell division in plants (cytokinesis –cell movement – an important feature of cell division: Section 8.2.2).Plants contain many different cytokinins which are based around6-substituted adenine derivatives (Fig. 7.2) and these may be conjugatedto a variety of other compounds. Cytokinins are important in cellculture where, at low concentrations, they stimulate cell division.Synthetic forms, such as kinetin, are widely used both commerciallyand in laboratory studies for plant tissue culture.The list of actions of cytokinins is very long and in many waysoverlaps that of GAs and auxins, including roles in cell division andelongation, the promotion of parthenocarpy and flowering, breakingof dormancy, the control of apical dominance and the delay ofsenescence. Again, this overlap is most probably a result of interactionswith other plant growth hormones rather than a duplicationof role. Perhaps one of the best-studied examples of such an interactionis the relationship between auxin and cytokinin concentrationin determining the development of plant tissue in culture. Skoog andMiller (1957) demonstrated that it is the ratio of auxin to cytokinin,rather than the absolute concentrations of these plant growth hormones,which determines the tissues which will be produced.7.2.5 Plant tissue cultureModern plant breeding makes extensive use of tissue culture techniques.If plant tissue is placed in a nutritive medium, and suppliedwith precise amounts of auxin and cytokinin, it will develop as amass of undifferentiated cells known as callus. If these cells are thentransferred to a medium in which the concentration of cytokinin hasbeen increased relative to that of auxin, the callus is stimulated todifferentiate into shoots. After a period of growth, the shoots can betransferred into a medium with a high auxin content, stimulatingroot development. In this way an entire plant can be regeneratedfrom a single cell, or many genetically-identical clones can be generatedfrom a single plant.The ability to manipulate plant growth and development underpinsmany procedures used in plant breeding. In the 1950s, cellsisolated from the pith of tobacco (Nicotiana tabacum) stems were culturedand then induced to form new plants – this process is referredto as somatic embryogenesis, as embryonic material is generatedfrom vegetative (somatic) cells. It is even possible to generate haploidplants (i.e. plants with only a single copy of each chromosome) byculturing pollen grains, which can then be induced to double theirchromosome number by exposure to drugs such as colchicine. Thistechnique allows a true-breeding, homozygous line of plants to beproduced in a few months, rather than the years required by