ABSTRACT
Plant growth regulators (PGRs) are a group of organic substances that are capable of exerting large effects in plants in terms of gene expression, growth, and development, although they occur in relatively low concentrations, usually in nano-(10-9) to micro-(10-6) molar amounts. Endogenous PGRs, those that the plant biosynthesizes for itself and are naturally occurring, are called “hormones.” The word “hormone” is derived from the Greek word hormone, which means “that which sets in motion.” These plant hormones or phytohormones control processes such as cell division and enlargement, root and bud initiation, dormancy, owering, and ripening. Horticulturists make use of the unique features of synthetic growth regulators and apply them exogenously (originating outside of the plant) to generate desired responses in the plant. When a plant is developing from a germinating seed, endogenous growth regulators or hormones are responsible for directing the development of shoots and roots via cell division and enlargement. In traditional plant propagation with cuttings, PGRs have
enhanced adventitious rooting for even many hard-toroot species. PGRs have been used to stimulate germination, enhance owering, and enlarge fruit. In tissue culture, we make use of the ability to direct growth using growth regulators to stimulate the development of nonzygotic embryos, the growth and development of callus, the proliferation of axillary shoots, and the development of adventitious roots. This has enabled the application of tissue culture techniques to fundamental studies of how cells function, the growth and development of the various tissues of the plant, commercial clonal plant propagation, germplasm storage, the development of pathogen-free plants, the use of bioreactors to produce secondary plant products, genetic transformation, and even the production of articial seed by means of non-zygotic (somatic) embryogenesis (Figure 4.1).
