Plant Hormones
Overview
Plant hormones function as general growth stimulators or inhibitors. Unlike animal
hormones, plant hormones are not produced in definite organs, do not have specific target
tissues, and are not regulators of homeostasis. Five major classes of plant hormones have
been extensively studied and are reasonably well understood. However, others exist. Plant
hormones interact with one another in complex (and often poorly understood) ways to
produce the mature, growing plant.
Historical Review
The Darwins, studying phototropism in coleoptiles, discovered the action of a growth
stimulator. They found that curvature in the stem is due to the effect of light on the tip of the
coleoptile; they proposed that some signal is transmitted downward from the tip to the
elongating region of the coleoptile. Later, it was demonstrated that the signal is a mobile
substance of some kind. Subsequently, studies showed that phototropism is due to the
production (in the apical region) of a chemical he named auxin. Auxin proved later to be
indoleacetic acid (IAA).
Cytokinins were discovered near the turn of the century by trial and error. Extracts of
coconut milk (part of the endosperm of the coconut seed) were found to cause growth and
development of plant cells in tissue culture.
About a century ago, an aberration was noted in rice seedlings grown in Asia :
occasionally, the seedlings grew unbelievably tall and toppled over. This became known as
"foolish seedling disease", and was discovered in Japan (1926) to be due to a fungus,
Gibberella, which was present on some seeds. Subsequently, it was learned that the
syndrome is due to the infected individuals acquiring an overdose of the chemical,
gibberellic acid, produced by the fungus (and also produced in much smaller quantities in
plants).
Although abscisic acid (ABA) was named because it was originally thought that ABA
causes abscission, which occurs in deciduous plants in the autumn, such a role was never
actually demonstrated.
Citrus farmers used to "cure" citrus by using kerosene stoves - they thought the enhanced
ripening was due to heat. However, it was later learned that non-kerosene heaters did not
provide the same effect. Subsequently, it became apparent that the effects of curing could
be replicated using ethylene, a byproduct of kerosene combustion, even when no heat
was used.
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Classes of Hormones
Five classes of plant hormones have been identified: auxin, cytokinin and gibberellin (all
chemically related growth stimulators), abscisic acid and ethylene (growth inhibitors,
hence "aging stimulators"). Generally they work by affecting cell division, elongation and
differentiation. Effects of the chemicals vary depending on target area, developmental
stage, hormone concentration, and interactions with other hormones. At the cellular level,
the hormones result in gene expression, effects on enzymes, or modifications of cell
membrane properties.
Auxin is involved in several aspects of plant development; a primary function is to
stimulate the elongation of cells in young developing shoots. Threshold concentrations are
10-3 - 10-8 M. Higher levels initiate production of ethylene, another hormone which
generally counteracts the effects of auxin. Rate of transit within stems is more rapid than
possible by diffusion yet too slow to be due to mass transport (i.e. within phloem).
Cytokinins are produced in actively growing tissues, particularly roots, embryos and fruits.
In conjunction with auxin, cytokinin stimulates cell division and determines the course of
differentiation. For example, cytokinin alone has no effect on cells in tissue culture (they get
large but do not divide); when both are added, division occurs- when C = A, division but no
differentiation occurs; when C > A shoots develop; when C < A, roots develop. The
hormone is antagonistic to auxin in the phenomenon of apical dominance.
Over 70 gibberellins have been isolated, including the most studied, gibberellic acid (GA)
which is produced in the apical portions of roots, stems, and in young leaves. It is active in
promoting internode elongation. Its effects are enhanced when auxins are also present. Many
dwarf varieties of plants are apparently genetic mutants which lack the gene for gibberellin
synthesis, since application of synthetic GA can lead to normal growth in such varieties.
Abscisic acid (ABA) is synthesized primarily in fruits, root caps and mature leaves. We
consider it to be an inhibitory hormone in that it tends to have effects which are
antagonistic to those of auxins, cytokinins and gibberellins. It is involved in causing leaf
primordia of stem tips to develop into scales of terminal buds prior to dormancy in the
autumn. It is also involved in the function of stomata, especially when plants are stressed
at the time of wilting.
Ethylene, a gas, is a plant hormone which has a number of effects related to senescence.
Two of the most prominent are that it is produced during fruit ripening (and also hastens
the process) and it is also involved in leaf abscission. Many of the processes originally
attributed to auxin have been found due to ethylene, which is produced as a response to
auxin at high concentrations.
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Some Mechanisms of Action
Auxin migrates downward in the stem (and laterally) away from light. It results in elongation
in the cells it contacts (hence, bending of the stem). The mechanism of action is that it
increases the plasticity of the cell wall. The change in plasticity is due to a cascade of
events which occur when auxin comes into contact with the cell :
1) the affected cell transports H+ to the cell wall region,
resulting in a localized decrease in pH
2) crosslinks in cellulose molecules are broken due to the activation
of an enzyme with a low pH optimum
3) turgor results in an increase in cell diameter. In addition transcription of mRNA
results, leading to changes in cell shape due to production in proteins related to
cell growth. This phenomenon, described as the acid growth hypothesis, may
result in cell shape changes in as little as 20 minutes after addition of auxin.
During seed germination, the embryo produces a signal (gibberellin), which causes the
aleurone layer of the seed to synthesize and secrete digestive enzymes into the
endosperm. Then, the enzymes digest the storage compounds, giving the embryo access
to the nutrients it needs for growth and development.
Commercial Uses of Hormones and Analogues
We frequently use synthetic auxins in horticulture simply by applying them to plants.
Reasons for application are to prevent abscission and senescence, promote root formation
in cuttings, destroy weeds (the synthetic auxin 2-4,D causes broad leaved dicots to literally
"grow to death" by elongation and 2-4-5,T, agent orange, kills woody seedlings), induce
fruit production even when no fertilization by pollen has taken place.
Gibberellins are used commercially to increase fruit size and set; increase cluster size in
grapes; delay the ripening of citrus; speed up flowering of strawberries; stimulate partial
digestion of starches in the germination of barley during beermaking.
Ethylene, the gas liberated from ripening fruit can cause the same or other types of fruit in
the vicinity to ripen prematurely. Hence, we can pick fruit green (and hard for easy
transport) and ripen it rapidly at our convenience using ethylene. Since carbon dioxide has
the opposite effect, CO2 is used as a "ripening inhibitor" during shipping. A recent twist on
this story is that molecular biologists can add an antisense RNA to fruit which inhibits
transcription of one of the genes required for ethylene synthesis, thus providing the same
inhibitory effect which can also later be reversed by use of ethylene.
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Tropisms and Photoperiodism
Tropisms are responses to the environment which affect plant form.

Phototropism - growth response to a unidirectional source of light,
mediated primarily by auxin

Gravitropism - growth response of the root to gravity, generally causing shoots to
grow up (negative gravitropism) and roots to grow down (positive gravitropism).

Thigmotropism - response to touch. Tendrils in plants normally grow straight, but
when physical contact by tendril cells with an object occurs, differential growth of
cells on the two sides of the tendril results. Hence, coiling occurs due to a poorly
understood effect of hormone concentrations.

Turgor movements are reversible changes due to changes in turgor pressure
within cells located at the joints of leaves. Stimulation causes these cells to become
flaccid because they rapidly lose potassium, resulting in loss of water.

Photoperiodism is a response (seed germination, ending of bud dormancy, onset
of senescence, flowering, et al.) to a specific photoperiod. "Short day" plants start to
form flowers when the days become shorter than a critical day length; "long day"
plants begin to form flowers when the days become longer than a certain critical
length; in day neutral plants, flowering times are unresponsive to photoperiod. The
phenomenon is due to a phytochrome pigment, interconvertible into 2 forms, P r and
Pfr, which absorb red light (illuminating the leaf in the day time) and far red light
(illuminating the leaf in the night time), respectively. Pfr is biologically active (when
biological reactions require phytochrome, Pr won't help). In photoperiodism, Pfr
blocks the flowering response. During the day, Pr is converted to Pfr, which is
converted back to Pr during the night. When nights are long enough, enough Pfr is
converted back to Pr to allow for flowering. Although the mechanism of action is not
fully understood, apparently the conversion of phytochrome allows the plant to
detect photoperiod changes that may result in flowering.
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