AP2, PI, AP3, AG, and SEP proteins. How do these proteins function to specify floral
organs? AP3 and AG encode MADS box genes, a family of transcription factors expressed
in yeast and plants that most likely function by turning on other, specific genes required to
build a sepal, petal, stamen, or carpel. The ABC model predicts that expression of these
genes should be confined to the specific whorls where they function. This prediction has
been verified by observing the in situ mRNA expression patterns of the genes. For
example, AP2 is expressed early in whorls 1 and 2.
It is important to note that several homeotic genes controlling floral development
have been isolated from other plants, includingAntirrhinum(snapdragon), supporting
the importance of the ABC model in other species. For example, the Antirrhinum
deficiens(DefA) gene probably functions similarly to AP3 fromArabidopsis(Krizek and
Fletcher 2005).
4.6 Hormone Physiology and Signal Transduction
4.6.1 Seven Plant Hormones and Their Actions
Signal transductionis the cascade of events that allow a signal, usually from outside the
cell, to be interpreted by the cell. Signal transduction cascades usually result in a final bio-
logical response, and often the response can be measured. Besides light and abiotic stress,
the plant hormones are the major developmental and physiological signaling molecules in
the plant. The seven major plant growth hormones are small molecules rather than proteins
or peptides, and in some cases they are similar to certain animal cell hormones (Fig. 4.9).
For example, brassinolide (Br) is a sterol, much like estrogen and testosterone, which func-
tion as sex hormones in animals. Br is critical for normal plant growth and development in
plants, playing a role in stem elongation, leaf development, pollen tube growth, vascular
differentiation, seed germination, photomorphogenesis, and stress responses.
Auxin, or indole 3-acetic acid, was the first plant hormone discovered and contains an
indole ring much like the melatonin hormone of animals. Auxin is known to stimulate
cell elongation and cell division, differentiation of vascular tissues, root initiation and
lateral root development. Auxin can also mediate the bending responses to light and
gravity, and within the apical bud it suppresses the growth of lateral or axillary meristems.
It can delay senescence, and interfere with leaf and fruit abscission. It can induce fruit
setting and delay ripening in some fruits. It can also stimulate the production of another
plant hormone,ethylene.
Cytokininis generally considered the second most important plant growth-regulating
hormone, following auxin. Cytokinin is similar to adenine and was first discovered in 1941
as the active component in coconut milk that promoted growth of plant cells in tissue
culture. Cytokinin can promote cell division and shoot growth and can delay senescence.
Abscisic acid (ABA) was first identified in a search for an abscission-promoting
hormone. This is not the function of ABA, and as noted earlier, it functions in promoting
dormancy and in sensing drought and other stresses. ABA is derived from mevalonic acid
and carotenoids and is thus similar in structure to the developmental factor from animals
calledretinoic acid. Transport of ABA can occur in the vascular tissues. ABA stimulates
closure of the stomatal pore, and can inhibit shoot growth. In seeds, it promotes dormancy
and stimulates the production of seed storage proteins. It is mostly antagonistic togibber-
ellic acid(GA) and can inhibit the response of grains to GA. ABA is also involved in
4.6. HORMONE PHYSIOLOGY AND SIGNAL TRANSDUCTION 101