Structure of Some Plant Hormones
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, Boysen-Jensen demonstrated that the signal is a mobile substance of some kind. Subsequently, studies by Frits Went 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 by Kurosawa (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). A 3D molecular model of gibberellic acid is shown elsewhere.
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.
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). The mechanism of transport is via polar transport, which involves use of energy, pH microgradients across the plasma membrane and between adjacent cells, carrier proteins, and ionization of auxin.
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 prominant 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.
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 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.
Signal Transduction
A significant aspect of hormone action in both plants and animals relates to the transduction of the extracellular signal (a chemical which diffuses to the cell surface) to an intracellular signal which has some effect(s) on the specific activities of the cell. Hence, the signal transduction causes events whereby the stimulus is linked to the response. Analyses of pathways for both plants and animals reveal striking similarities. They generally consist of three steps: reception, transduction, and induction. Reception consists of the binding of the hormone to the receptor site of the target cell (defined as any cell exhibiting a receptor for the chemical in question- cells lacking the receptors have no way to sense the presence of the signal). During transduction there is a conversion of the signal to one or more "second" signals within the cell which may result in induction of some process.
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.
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). Apparently, gravity is "sensed" within cells containing amyloplasts which contain starch and play the role of statoliths, settling to the bottom region of cells. For some reason, the accumulation of amyloplasts results in a localized increase in Ca++ which somehow sets the bending process into motion, possibly by affecting IAA transport.
- 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. Thigmomorphogenesis is similar in that localized or generalized mechanical stimuli result in cessation of elongation. For example, a stem in a windy location will be shorter and stockier than a stem in a sheltered location.
- Turgor movements are reversible changes due to changes in turgor pressure within cells of motor organs called pulvini, located at the joints of leaves. Stimulation causes the pulvini to become flaccid because they rapidly lose potassium, resulting in osmotic efflux. Interestingly, an action potential is transmitted from the point of origin to other parts of the leaf, resulting in turgor movements elsewhere. This transmission is somewhat similar to the transmission of signals in the nerve cells of animals. Turgor movements allow both better orientation of plant parts for enhanced photosynthesis and other phenomena (venus flytrap and mimosa).