FOR NEARLY 30 YEARS after the discovery of auxin in 1927, and more
than 20 years after its structural elucidation as indole-3-acetic acid, Western
plant scientists tried to ascribe the regulation of all developmental
phenomena in plants to auxin. However, as we will see in this and subsequent
chapters, plant growth and development are regulated by several
different types of hormones acting individually and in concert.
In the 1950s the second group of hormones, the gibberellins (GAs),
was characterized. The gibberellins are a large group of related compounds
(more than 125 are known) that, unlike the auxins, are defined
by their chemical structure rather than by their biological activity. Gibberellins
are most often associated with the promotion of stem growth,
and the application of gibberellin to intact plants can induce large
increases in plant height. As we will see, however, gibberellins play
important roles in a variety of physiological phenomena.
The biosynthesis of gibberellins is under strict genetic, developmental,
and environmental control, and numerous gibberellin-deficient
mutants have been isolated. Mendel’s tall/dwarf alleles in peas are a
famous example. Such mutants have been useful in elucidating the complex
pathways of gibberellin biosynthesis.
We begin this chapter by describing the discovery, chemical structure,
and role of gibberellins in regulating various physiological processes,
including seed germination, mobilization of endosperm storage reserves,
shoot growth, flowering, floral development, and fruit set. We then
examine biosynthesis of the gibberellins, as well as identification of the
active form of the hormone.
In recent years, the application of molecular genetic approaches has
led to considerable progress in our understanding of the mechanism of
gibberellin action at the molecular level. These advances will be discussed
at the end of the chapter.