The majority of higher plants begin life as seeds. When seeds germinate, the embryonic tissues begin to grow and undergo differentiation until, ultimately, the various parts of the mature plant are formed.
Every aspect of this growth and development is regulated by a group of chemical messengers called hormones. These plant hormones, or phytohormones, function as plant regulators. A plant regulator is an organic compound, other than a nutrient, which in small amounts promotes, inhibits, or otherwise modifies a basic plant process.
Hormones are produced in one area of the plant and transported to another area, where their effects are exerted. There are five major classes of identifiable plant hormones, and others will surely be identified in the future: auxins, cytokinins, gibberellins, ethylene, and abscisic acid.
Although there are numerous plant responses to the auxins, one of the primary functions of this group of hormones is to cause increases in cell length by loosening cell walls and increasing the synthesis of cell wall material and protein.
In order for plants to grow, cells produced at the stem or root meristems must undergo this process of elongation. No cell elongation can take place in the absence of auxin.
The cell elongation promoted by auxins results in regular growth, and it is also responsible for various tropisms. For example, phototropism causes plants to grow toward a source of light. Gravitropism is a response to gravity and causes the roots to grow downward.
Besides cell elongation, auxins will initiate root growth at the base of the stem. Auxins inhibit growth of the lateral buds; as long as auxins are being transported down the stem from the apical bud, the lateral buds will not develop.
This phenomenon, known as apical dominance, accounts for the fact that plants will not bush out until the apical buds are removed. While there are a number of natural and synthetic compounds that exhibit auxin activity, the major, naturally occurring auxin is a compound called indole acetic acid (IAA).
Cytokinins are referred to as the cell division hormones, and while cell division will take place only in the presence of one of these hormones, the cytokinins stimulate a number of other plant responses as well.
These hormones have been shown to retard senescence in detached leaves and to create metabolic sinks, areas within a tissue where increased metabolism takes place. As a result, there is an increase in transport of metabolites to the area. For example, amino acids will be transported to a site where the presence of cytokinins has increased protein synthesis.
In many plants, these hormones stimulate the production of larger, greener leaves by causing leaf cells to expand and by promoting both chloroplast development and chlorophyll synthesis. A number of naturally occurring substances that exhibit cytokinin activity have been identified. Of these, ribosyl zeatin is one of the most abundant in plants.
Many dwarf plants exhibit a decrease in height because they contain low levels of the stem elongation hormones called gibberellins. These hormones, of which more than thirty have been identified, increase the amount of water taken up by the cells of stems.
As the individual cells swell from the increased water content, the stem grows longer. In addition to stem elongation, the gibberellins elicit a number of other plant responses.
The seeds of numerous plant species exhibit dormancy, which can be broken by gibberellins. The length of the daylight period (photoperiod) is crucial to the flowering response inmany plants.
Fall- and winter-flowering plants require short days, while the plants that flower in the spring and summer must be exposed to long days. Some plants must also be subjected to a prolonged period of cold before flowering can occur.
The gibberellins can substitute for the long day or the cold requirement in many plants. Additionally, the gibberellins can produce thicker stem growth in certain woody plants and increase the number of fruits that develop in some species.
One of the most important functions of ethylene is fruit ripening. Some fruits produce almost no ethylene until a few days before ripening and then release large amounts. Such fruits are said to be climacteric.
Nonclimacteric fruits continuously produce more moderate amounts of the hormone throughout the ripening period. In all types of fruit, ethylene must be present before ripening can occur.
Ethylene also causes ripened fruit to abscise (separate from the parent plant) and is even involved in dehiscence (removal of the husk) of some types of fruit, such as pecans and walnuts.
In addition to its role in fruiting, ethylene can initiate root development, cause leaves to droop, inhibit plant motion, and increase metabolic activity in some plants. Because ethylene inhibits auxin transport, it will also release apical dominance.
The name “abscisic acid” was chosen because one of the major activities of this compound is to promote leaf abscission. During the autumn of the year, the concentration of this hormone increases in many plants. This high concentration causes the leaves to senesce, turn yellow, and abscise. Abscisic acid also inhibits growth and promotes both bud and seed dormancy.
In the vast majority of hormone responses, enzyme activation or gene induction (the turning on of genes that produce new enzymes or other proteins) can be detected. This suggests that the hormones are initially acting as “first messengers.”
These first messengers react with the cellular membranes, and as a result of these reactions, the membranes activate “second messengers” within the cell. The second messengers then activate a group of substances referred to as inducers.
One group of inducers is the protein kinases, a class of enzymes that add phosphate to other proteins. Protein kinases activate various enzymes or induce DNA (deoxyribonucleic acid) regulatory proteins to control genes responsible for cell elongation, cell division, flowering, fruit ripening, or one of the many other responses to the hormones.
Each class of hormones activates a different set of protein kinases to produce a different set of responses. Most of the information concerning protein kinases is based on studies of animal systems, but it is highly likely that a similar type of mechanism is also present in plants.
Horticultural and agricultural applications of hormone technology are widespread. Adilute solution of auxin is used to promote the root development necessary to propagate stem cuttings vegetatively. The cytokinins have been used to enhance sex ratios among the plants in the cucumber family, producing more female plants and thus more fruit.
For years, ethylene has been used to enhance flowering and, in turn, increase yields in pineapples. Ethylene has also been used to promote root formation in stem cuttings and has been applied in commercial operations to husk walnuts.
Because gibberellins increase growth in the cellular layer that produces thicker stems in pines, seedlings can be sprayed with the hormone and be made ready for transport in two years rather than the normal three or four.
The action of gibberellins on the production of amylase (an enzyme that breaks down starch to glucose) has proved to be useful in the brewing industry. The malting process can be accelerated because barley seeds treated with a gibberellin exhibit an increased rate of starch digestion.
One of the most economically important uses of the gibberellins has been in the grape industry. Yields have been increased by as much as one-third in grape vineyards sprayed with gibberellins. Gibberellins have also been used to increase length and water content in celery and sugar content in Hawaiian sugarcane.
Normally, hormone action is associated with the promotion of, enhancement of, or increase in some plant activity. This is the case, however, only when the concentration of the hormone is extremely low.
At high concentrations, hormones actually have detrimental effects and can cause plant death. The knowledge of this has led to the development of a number of hormone-type herbicides.
The most effective, by far, are the auxin-type herbicides such as 2,4-dichlorophenoxy acetic acid and picloram. These herbicides have provided excellent control of broadleaf weeds in grasses and food grains, because grasses are unaffected by them, while broadleaf weeds die as a result of overgrowth.
There are probably a number of plant hormones that have not yet been identified. As these are discovered, and as those currently known are better understood, it will become possible to control even more aspects of plant growth and development.