Plant hormones are signal molecules, present
in trace quantities. Changes in hormone concentration
and tissue sensitivity mediate a whole range of
developmental processes in plants, many of which
involve interactions with environmental factors.
This chapter is concerned with the biosynthetic,
catabolic, and conjugation pathways that together
control the homeostasis of plant hormone pools.
Attention will be focused on the first five plant
hormones discovered— gibberellins, abscisic acid,
cytokinins, indole-3-acetic acid, and ethylene—
as well as on compounds shown more recently to
have a regulatory role in plant development, namely,
brassinosteroids, polyamines, jasmonic acid, and
salicylic acid (Fig. 17.1). Each of these compounds
has its own particular properties, so the pathways
regulating their production and degradation are
quite diverse and have been elucidated by synergistic
use of many disciplines, including chemistry,
biochemistry, plant physiology, genetics and,
more recently, molecular genetics. The most powerful
tool has been the use of mutant plants that are
unable to catalyze an enzymatic step leading to
the formation or degradation of a bioactive hormone.
Such mutants have been invaluable in hormone analysis
studies and in phenotypic rescue experiments using
intermediates in the biosynthetic pathway. They
also provide the essential material for cloning
the genes that encode the biosynthetic enzymes.
In addition, inhibitors of plant hormone biosynthesis
have been used to dissect biosynthetic reactions
for which relevant mutants have not yet been identified.
More recently, transgenic technology has provided
insights into pathways by altering the expression
of genes involved in the biosynthesis or catabolism
of hormones. Furthermore, plants expressing transgenic
constructs in situations in which the promoters
of the biosynthetic genes are linked to reporter
genes have been used to visualize the specific
tissues where bioactive hormones are produced.
In summary, each hormone biosynthetic pathway
has been elucidated by using similar techniques
but with biases toward certain methodologies according
to the nature of the biosynthetic pathway and
the availability of mutants.