Hormonal control of growth and development

Growth and organogenesis in vitro is highly dependent on the interaction between naturally occurring endogenous growth substances and an analogous growth regulator added to the medium. It is often necessary to alter the growth regulator composition and/or concentration for in vitro culture according to the species or variety of plant being grown, the explant origin, the type of culture, and other medium constituents. A balance between auxin and cytokinin growth regulator is most often required for the formation of adventitious shoot and root meristems. The interactions found are often complex, and more than one combination of two substances is likely to produce optimum results. Figure 1 gives an example of the relative amounts of an auxin and a cytokinin that are often required to bring about some kind of morphogenesis.