Gibberellins are a group of thirty or more closely related plant hormones that promote cell enlargement, often causing dramatic increases in plant height. One of the common forms is gibberellic acid (Figure 4-7). They will not cause curvature in Avena coeloptiles but will cause dwarf corn plants to elongate and grow to normal size. Since auxins will not affect dwarf corn plants, the two growth hormones can be clearly distinguished by scientists.
The number of plant processes known to be affected by gibberllins is not as great as those known to be influenced by auxins, but their influence is nonetheless impressive. Rosetted plants can be made to grow tall following the application of gibberellic acid. Dormant seeds that normally require a cold treatment (stratification) for germination can be made to germinate without the treatment when gibberellic acid is applied instead. Also, various other plant processes normally initiated in response to temperature and/or photoperiod stimulations can be prompted instead by the application of gibberellins.
Rosetted plants are those having good leaf development but retarded internodal growth. Under natural growing conditions, such plants will respond to long-day photoperiods or a period of cold treatment with a dramatic elongation of the internodes (bolting) and the production of flowers atop a stem five or six times the original height of the plant. When kept under short-day conditions or in warm temperatures, the plants remain rosetted. Since the treatment of plants with gibberellin under conditions normally promotive of rosetting can induce bolting and flowering, it is suggested that the stage of growth of the plant is controlled by the amount of natural gibberellin present within the plants. Both cell division and cell enlargement occur in the plants treated with the gibberellin.
Both gibberellins and auxins, when applied to the pistils of certain flowers, can cause them to set fruit without being pollinated. The process is called parthenocarpy, and the resulting seedless fruit are called parthenocarpic fruit. Examples most readers would recognize are the naval orange, the banana, and the seedless grape. In ordinary pollination, growth hormones are introduced to the pistil from pollen produced on the stamen or synthesized in the ovary following a stimulus from pollen.
Through observation and study of these and other plant processes, scientists are able to theorize about the role of gibberellins in plants. Gibberellins have been known to stimulate cell division and cell enlargement in young embryos of germinating grains. Evidence suggests that the gibberellin activates genes that are normally repressed. As the genes become active, new ribonucleic acid (RNA, the carrier of genetic information) and new enzymes result. The new enzymes digest and release materials stored in the endosperm. These materials then become available to the developing embryo, thereby promoting cellular activity that might otherwise be inhibited. It is doubtful whether such an explanation covers all the effects of gibberellins, however.
Cytokinins are naturally occurring hormones of which the best known is kinetin (Figure 4-8). The principal role of the cytokinins is the division
figure 4-7. Chemical structure of GA3 (gibberellic figure 4-8. Chemical structure of kinetin acid) (Delmar/Cengage Learning) (Delmar/Cengage Learning)
of cells, but an expanding body of evidence suggests their involvement in many other plant growth processes. That involvement seems to be in association with gibberellins, auxins, and other growth regulators.
In their interaction with auxins, cytokinins have been found to promote either cell enlargement or cell division. The initiation and growth of roots and shoots as well as the overcoming of apical dominance have all been linked to the presence of cytokinins.
Cytokinins can be distinguished from auxins and gibberellins with a diagnostic test that involves the culture of tobacco parenchymal cells in a nutrient solution. When kinetic and auxin are added to the solution, cell division is accelerated in the tobacco tissue. No effect on dwarf corn plants or Avena coleoptile curvature results when cytokinins are applied, so the three tests are important biological detectors for the growth regulating hormones.
Other Growth Regulators
In addition to the three naturally occurring hormones already described, other natural and synthetic plant hormones are known to exist. Among them are:
• Abscisic acid—a naturally occurring growth inhibitor that counters the effects of gibberellins. It can promote the premature abscission of leaves and fruits.
• Ethylene—influences geotropism, leaf abscission, and the ripening of fruit. It is a natural hormone.
• Antigibberellins—a group of synthetic compounds that have a dwarfing effect on plant growth.
It is likely that further research will find that many plant growth responses stem from the synergistic action of two or more growth regulators. Since they can coexist within the same tissue and often promote similar responses in the plant, their combined action seems predictable.