defending the plant, such as thepathogenesis-related 1(Pr1) gene. Microbial and viral
pathogens can also trigger JA synthesis, thus the study of JA-mediated events in the
plant cell are of interest to plant pathologists who wish to engineer transgenic plants that
are disease-resistant.
Gibberellic acid(GA) and ethylene are two plant hormones with no similar molecular
counterparts in other eukaryotic organisms. GA was first discovered from fungi that can
stimulate plant cell elongation and cause significant and “leggy” growth of rice plants.
GA is a series of 136 diterpene compounds that contain 19 or 20 carbons in four or five
ring systems. These are named for the order in which they were discovered (GA1, GA2,
etc.). The other functions of GA, as mentioned previously, are in general antagonistic to
the actions of ABA. For example, ABA promotes seed dormancy, while GA is required
in most cases to break seed dormancy. The actions of GA on barley germination have
been well studied where it has been shown that GA promotes expression of the
a-amylase genes required to break down starch in barley aleurone, an important process
in the grain-malting business. GA also plays a prominent role in stimulating flower devel-
opment under long days.
Ethylene, a hydrocarbon gas, is a very simple molecule that is best known for its stimu-
lation of fruit ripening and promotion of the seedling triple response. Indeed, people of
ancient cultures understood the actions of ethylene and could burn incense in a closed
room to stimulate fruit ripening. The triple response of seedlings is a specific developmental
program wherein an apical hook forms in the shoot, and the root becomes thicker. These
adaptations may increase survival under certain conditions. In addition, ethylene can stimu-
late the release of dormancy, adventitious root formation, flower opening, and flower and
leaf senescence.
4.6.2 Plant Hormone Signal Transduction
The first eukaryoticsignal transductionpathways to be characterized were the peptide
growth hormone pathways of animal cells. This most likely resulted from the discovery
that animal oncogenes sometimes encoded altered growth factors, growth factor receptors,
or other signal transduction components that regulate cell growth. A paradigm signal trans-
duction pathway is shown in Figure 4.10 to facilitate understanding of how signal transduc-
tion works. Signals from outside a cell can be perceived, sometimes by receptors that span
the plasma membrane. After stimulation of such receptors, information can be relayed by a
series of small molecules or proteins to the cell nucleus, where activation of specific tran-
scription factors can stimulate new gene expression programs. The resulting gene
expression results in the production of new proteins that can function in the final biological
responses to the signal.
Because plant hormones are small molecules rather than proteins, and because the plant
cell wall encloses the plasma membrane, it was suggested that plant hormone signal trans-
duction pathways would be significantly different from those of animals. While this is true
in general, it is important to keep in mind that most, if not all, of the individual components
of plant signal transduction pathways have similar counterparts in other eukaryotes. Plant
receptors linked to plant hormone action were not discovered until the 1990s. The acceler-
ated pace of experimentation that followed resulted in three major paradigms of plant
hormone signal transduction that will no doubt be joined by other types of pathways in
the future (Chow and McCourt 2006; Gibson 2004).
4.6. HORMONE PHYSIOLOGY AND SIGNAL TRANSDUCTION 103