Plant Hormones

The Complete Grape Growing System

The Complete Grape Growing System

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A relatively small number of plant hormones regulate plant development, and each type of hormone can have multiple effects. It is believed that hormones act by chemically binding to specific receptors. The observed effect is thought to be initiated by this hormone-receptor association, which triggers a series of biochemical events, including turning genes on and off. The biochemical events are called signal transduction and may include changes in the complement of enzymes produced in a tissue or changes in transport across membranes.

The major known types of plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.

Auxins

In 1881, Charles Darwin and his son Francis were fascinated by the fact that coleoptiles (more or less tubular, closed sheaths protecting the emerging shoots of germinating seeds of the Grass Family—Poaceae) bend toward a light source. They noticed that the bending did not occur if they placed little metal foil caps over the coleoptile tips but the bending resumed when the caps were removed. They concluded that the coleoptile tips must be sensitive to light. Charles Darwin passed away in 1882, but others following up on his work demonstrated that a water-soluble "influence" was produced in the coleoptile tips and that electrical triggering of the bending, which had been suspected, did not exist.

In 1926, Frits Went, a young plant physiologist in Holland, cut off the tips of oat coleoptiles and placed the tips, cut surface down, on flat portions of a gelatinlike growth medium called agar (a substance obtained from marine algae). After a few hours, he removed the tips and cut the agar into little square blocks, which he then placed on decapitated coleoptiles. He found that if he placed the block squarely over the cut top of a coleoptile, it grew straight up, but if he placed the block off center, the tip bent away from the side on which the block had been placed (Fig. 11.1). He also found that the more coleoptile tips he placed on the agar to begin with, the more pronounced was the response. His

Oat Coleoptiles

Figure 11.1 Went's experiment with oat coleoptiles. A. A germinated oat "seed" with an intact coleoptile. B. The tip of a coleoptile was cut off, placed on a small agar block, and left for an hour or two. C. When this agar block was placed squarely on a decapitated coleop-tile, growth was vertical. D. When the agar block was placed off center so that only half of the decapitated coleoptile was in contact with it, the tip bent away from it. This experiment demonstrated that something affecting growth diffused from the coleoptile tip into the agar and from the agar to the decapitated coleoptile.

Figure 11.1 Went's experiment with oat coleoptiles. A. A germinated oat "seed" with an intact coleoptile. B. The tip of a coleoptile was cut off, placed on a small agar block, and left for an hour or two. C. When this agar block was placed squarely on a decapitated coleop-tile, growth was vertical. D. When the agar block was placed off center so that only half of the decapitated coleoptile was in contact with it, the tip bent away from it. This experiment demonstrated that something affecting growth diffused from the coleoptile tip into the agar and from the agar to the decapitated coleoptile.

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experiments demonstrated conclusively that something produced in the coleoptile tips moved out into the agar and that this substance was responsible for the bending observed. Went named the substance auxin (from the Greek word aux-ein, meaning "to increase"), a term now in widespread use.

Auxins were the first plant hormones to be discovered, with several others coming to light later. At least three major groups apparently promote (but do not control) the growth of plants. Depending on the concentration, some may also have an inhibitory effect.

Auxin molecules have a structure similar to those of the amino acid tryptophan, which is found in both plant and animal cells. For many years, auxins were thought to be synthesized from tryptophan by living cells. In 1991, however, it was demonstrated that mutant plants incapable of producing tryptophan can develop auxin without it, and the precise nature of the synthesis remains to be determined.

Production of auxins occurs mainly in apical meristems, buds, young leaves, and other active young parts of plants. It is sometimes difficult to predict how cells will respond to auxins because responses vary according to the concentration, location, and other factors. For example, if an auxin of a specific concentration promotes shoot growth in a certain plant, the same auxin of identical concentration will usually inhibit its root growth. At appropriate concentrations, auxins normally stimulate the enlargement of cells by increasing the plasticity (irreversible stretching) of cell walls.

Hormone concentrations were measured at first by bio-assays, which relate the response of a sensitive plant part to the amount of hormone applied to the part. For example, a fixed number of coleoptile tips can be placed on a specific sample of agar for a certain time period, and a measured cut block of the agar can then be placed off center on a decapitated coleoptile. The concentration can be determined from the angle of bending of the coleoptile that occurs within a certain time frame when it is compared with the bending brought about by a similar block containing a known amount of auxin (Fig. 11.2).

Hormone concentrations today are more frequently determined by vaporizing a sample and moving it through a tube of liquid or powdered material by a technique known as gas chromatography. As the sample moves through the column, the hormone and other components of the sample separate, and the amount of hormone present then can be determined relatively precisely.

Auxins also may have many other effects, including triggering the production of other hormones or growth regulators (especially ethylene), causing the dictyosomes to increase rates of secretion, playing a role in controlling some phases of respiration, and influencing many developmental aspects of growth. Auxins promote cell enlargement and stem growth, cell division in the cambium, initiation of roots, and differentiation of cell types. Auxins delay developmental processes such as fruit and leaf abscission, fruit ripening, and inhibit lateral branching. Sensitivity to auxins is less in many monocots

Aba And Leaf AbscissionIaa Agarblock Method

Figure 11.2 How a bioassay of auxin is made. A. A specific number of coleoptile tips are cut and placed on a measured portion of agar (B) for a set period of time. C. The coleoptile tips are then removed, and the agar is cut into blocks of specific size. An agar block is placed off center on a decapitated coleoptile held by a clamp; the leaf within the coleoptile is pulled up slightly to support the agar block. After another set period, the angle of curvature is measured. D. The angle of curvature is compared to that produced when a similar agar block containing a known amount of auxin is placed off center on another coleoptile for the same period of time.

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Gardenia Cutting
Figure 11.3 Effect of auxin applied to the base of a Gardenia cutting 4 weeks after application. Left. A treated cutting. Right. An untreated cutting.

than it is in dicots, and shoots are less sensitive than roots. Higher concentrations, however, will kill almost any living plant tissues. The effects of auxins combined with those of other regulators produce many of the growth phenomena.

Movement of auxins from the cells where they originate requires the expenditure of energy stored in ATP molecules. The migration is relatively slow (about 1 centimeter per hour), although the active transport "pumping" mechanism involved carries the hormone up to 10 times faster than it would be by simple diffusion alone. The movement is polar, which refers to the movement of auxins away from their source, usually from a stem tip toward the base, with movement in roots proceeding toward the tip. This polar movement occurs even when a stem is inverted. The auxins apparently are ordinarily not carried through the sieve tubes of the phloem but proceed from cell to cell, particularly through parenchyma cells surrounding vascular bundles.

In the past, it was believed that several natural auxins occurred in nature, but until relatively recently, most scientists thought that indoleacetic acid (IAA) was the only active auxin. Furthermore, it was believed that other organic acids that act like auxins were actually converted to IAA in the plants. Current research, however, indicates that plants do actually produce three other growth regulators that are not converted to IAA but bring about many of the same responses as IAA. One, called phenylacetic acid (PAA), is often more abundant but less active than IAA. Another, 4-chloroindoleacetic acid (4-chloroIAA), is found in germinating seeds of legumes. The third, indolebutyric acid (IBA) occurs in the leaves of corn and various dicots. It previously was known only in synthetic form. At least three auxin precursors also have activities similar to those of auxins.

IAA and a number of organic acids that are not plant hormones regulate growth and have also been synthesized. They are used widely in agriculture and horticulture. At the proper concentrations, IAA and other growth regulators stimulate the formation of roots on almost any plant organ. Nurseries apply these substances in paste or powdered form to the bases of cut segments of stems to stimulate a more vigorous production of roots than would occur naturally (Fig. 11.3). Synthetic growth regulators presently in use include NAA (naphthalene acetic acid), 2,4-D (2,4-dichlorophenoxyacetic acid), and MCPA (2-methyl-4-chlorophenoxyacetic acid).

Orchardists spray fruit trees with auxins to promote uniform flowering and fruit set, and they later spray the fruit to prevent the formation of abscission layers and subsequent premature fruit drop. A substantial saving in labor expenses results from being able to go through an orchard or a pineapple plantation and pick all of the fruit at one time. If auxins are applied to flowers before pollination occurs, seedless fruit can be formed and developed. Some orchardists use auxins for controlling the number of fruits that will mature in order not to have too many small ones, and some even control the shapes of plants for sales appeal.

Weeds were controlled by hand labor and by the use of caustic or otherwise poisonous substances until it was discov ered that synthetic auxins, including 2,4-D, 2,4,5-T, 2,4,5-TP, NAA, and MCPA, when sprayed at low concentrations, kill some weeds. Broad-leaved plants such as dandelions, plantains, and others (for reasons that are not clearly understood) are more susceptible to low concentrations of these plant growth regulators than narrow-leaved plants such as grasses. They have been used on lawns to kill weeds without noticeably affecting the grass. The precise mechanisms of action of these herbicides (plant killers) are still largely unknown.

Thus far, there is little evidence that 2,4-D has directly adverse effects on humans and other animal life, but such is not the case with 2,4,5-T, which was banned in 1979 for most uses in the United States by the Environmental Protection Agency. The controversy over the use of 2,4,5-T began in the 1960s during the Vietnam War when Agent Orange, a 1-to-1 mixture of 2,4-D and 2,4,5-T, was used to defoliate jungles. Subsequent tests and experiments with 2,4,5-T have produced leukemia, miscarriages, birth defects, and liver and lung diseases in laboratory animals. The diseases and defects are apparently caused by TCDD (2,3,7,8-tetrachlorodibenzoparadioxin), a dioxin contaminant that evidently is unavoidably produced in minute

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amounts during the manufacture of 2,4,5-T. TCDD harms or even kills laboratory animals in doses as low as a few parts per billion.

Gibb erellins

In 1926, Eiichi Kurosawa of Japan reported the discovery of a new substance that was causing what was referred to as bakanae, or "foolish seedling," a serious disease of rice. The stems of rice seedlings infected with a certain fungus grew twice as long as those of uninfected plants, but the stems were weakened so that they eventually collapsed and died. Kurosawa found that extracts of the fungus applied to unin-fected plants brought about the same growth stimulations as the fungal disease. Nine years later, other Japanese scientists were able to crystallize the substance, which was named gibberellin, after the scientific name of the fungus that produced it (Gibberella fujikuroi). It was not until the 1950s, however, that the true chemical structure of gibberellin was elucidated by a British group headed by Brian Cross and an American group led by Frank Stodola.

There are now more than 110 known gibberellins, customarily abbreviated to GA. Each individual GA is identified by a subscript (e.g., GA6). They have been isolated from immature seeds (especially those of dicots), root and shoot tips, young leaves, and fungi. Most GA produced by plants is inactive, apparently functioning as precursors to active forms. No single species has thus far been found to have more than 15 kinds of GA, which probably also occur in algae, mosses, and ferns. None are known in bacteria. GA moves through xylem and phloem and, unlike that of auxin, the movement is not polar.

Acetyl coenzyme A, which is vital to the process of respiration, functions as a precursor in the synthesis of GA. Interest in the hormonal properties of GA is high, for their ability to increase growth in plants is far more impressive than that of auxin alone, although traces of auxin apparently need to be present for GA to produce its maximum effects.

Most dicots and a few monocots grow faster with an application of GA, but coniferous trees such as pines and firs generally show little, if any, response. If a GA, at the appropriate concentration, is applied to a cabbage, the plant may become 2 meters (6 feet) tall, and bush bean plants can become pole beans with a single application. There is not usually, however, a corresponding stimulation of root growth. Many genetically dwarf plants grow as tall as their normal counterparts after an application of the appropriate GA.

Gibberellins not only dramatically increase stem growth, but they are also involved in nearly all of the same regulatory processes in plant development as auxins (Fig. 11.4). In some kinds of plants, flowering can be brought about by applications of GA, and the dormancy of buds and seeds can be broken. Some GA appears to lower the threshold of growth; that is, plants may start growing at lower temperatures than usual after an application of GA. For example, an application to a lawn could cause it to turn green 2 or 3 weeks earlier in the spring.

Gibberellin Hormone Effect Cabbage
Figure 11.4 Effect of gibberellins on flowering. Left. Cabbage plants grown outside in cool temperatures. Right. Cabbage plants grown in a warm greenhouse. The plants grew tall but did not flower until treated with gibberellins. (© Sylvan H. Wittwer/Visuals Unlimited)

In some varieties of lettuce and cereals, the seeds normally germinate only after being subjected to a period of cold temperatures. An application of GA can eliminate the cold requirement for germination. It is common for plants after germination of seed to produce juvenile foliage at first and then different adult foliage as the plant matures. In such plants, GA treatment of buds that would normally develop into adult branches causes the foliage to develop in juvenile form.

Several growth retardants available commercially inhibit or block the synthesis of GA. Applications of growth retardants result in stunted plants because stem elongation is inhibited. When applied to certain commercially grown crops such as chrysanthemums, flowers with thicker, stronger stalks are produced.

GA has been used experimentally to increase yields of sugar cane and hops and has revolutionized the production of seedless grapes through increasing the size of the fruit and lengthening fruit internodes, which results in slightly

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wider spaces between grapes in the bunches. Better air circulation between grapes reduces their susceptibility to fungal diseases, and the need for hand thinning is eliminated. They have been used in navel-orange orchards to delay the aging of the fruit's skin and have been found to increase the length and crispness of celery petioles (the parts that are most in demand as a raw food). GA is now used to increase seed production in conifers and to enhance starch digestion by increasing the rate of malting in breweries.

Field experiments have shown that if GA is applied to plants at the same time selected herbicides are applied, the GA may reverse the effects of the herbicides. The relatively high cost of GA has limited the extent of its use in horticulture and agriculture.

Cytokinins

By the close of the 19 th century, botanists suspected that there must be something that regulates cell division in plants. In 1913, Gottlieb Haberlandt of Germany discovered an unidentified chemical in the phloem of various plants. This chemical stimulated cell division and initiated the production of cork cambium. In 1941, Johannes van Overbeek discovered that coconut milk (a liquid endosperm; endosperm is discussed in Chapter 23) contained something that increased the growth rate of tissues and embryos, and within a few years, several substances that accelerated cell division were isolated from coconut milk. In 1955, a substance named kinetin was found, in the presence of auxin, to stimulate the proliferation of parenchyma cells in tobacco pith. Kinetin was isolated from heat-treated herring sperm and is not a naturally occurring substance. Its discovery was important, however, because it demonstrated that a simple chemical could promote cell division. In 1964, the identity of a substance very similar to kinetin was also determined in kernels of corn. These various stimulants to cell division came to be known as cytokinins.

The several cytokinins now known differ somewhat in their molecular structure and possibly also in origin, but they are similar in composition to adenine. You'll recall that adenine is a building block of one of the four nucleotides found in DNA, although none of the cytokinins appears to be derived from DNA. Some cytokinins do, however, occur in certain forms of RNA. Cytokinins are synthesized in root tips and germinating seeds.

If auxin is present during the cell cycle, cytokinins promote cell division by speeding up the progression from the G2 phase to the mitosis phase, but no such effect takes place in the absence of auxin. Cytokinins also play a role in the enlarging of cells, the differentiation of tissues, the development of chloro-plasts, the stimulation of cotyledon growth, the delay of aging in leaves, and in many of the growth phenomena also brought about by auxins and gibberellins. Cytokinins move throughout plants via the xylem, phloem, and parenchyma cells.

Certain bacteria stimulate the growth of galls (tumors) on plants, either by producing cytokinins and auxins that promote the unorganized growth of tumor cells or by transferring bacterial and auxin genes to the DNA of the host plant. The incorporation of the transferred genes results in host tissue that produces more auxin and cytokinin than normal, which, in turn, results in tumor growth.

Despite their role in cell division and enlargement, however, there is a total absence of evidence that cytokinins initiate or promote animal cancers or have any other effect on animal cells.

Experiments have shown that cytokinins prolong the life of vegetables in storage. Related synthetic compounds have been used extensively to regulate the height of ornamental shrubs and to keep harvested lettuce and mushrooms fresh. They also have been used to shorten the straw length in wheat, so as to minimize the chances of the plants blowing over in the wind, and to lengthen the life of cut flowers. Many have not yet been approved for general agricultural use.

Abscisic Acid

Although the promotion of growth by auxins and gib-berellins had been amply demonstrated by the 1940s, plant physiologists began to suspect that something else produced by plants could have opposite (inhibitory) effects. In 1949, Torsten Hemberg, a Swedish botanist, showed that substances produced in dormant buds (i.e., buds whose development is temporarily arrested) blocked the effects of auxins. He called these growth inhibitors dormins.

In 1963, three groups of investigators working independently in the United States, Great Britain, and New Zealand discovered a growth-inhibiting hormone, which was in 1967 officially called abscisic acid (ABA). Later, it was shown that ABA and dormins were one and the same.

ABA is synthesized in plastids, apparently from carotenoid pigments. It is found in many plant materials but is particularly common in fleshy fruits, where it evidently prevents seeds from germinating while they are still on the plant. When ABA is applied to seeds outside of the fruit, germination usually is delayed. Because the stimulatory effects of other hormones are inhibited by ABA, cell growth is usually also inhibited. Like the movement of gibberellins and cytokinins, that of ABA throughout plants is nonpolar.

ABA was originally believed to promote the formation of abscission layers in leaves and fruits. However, the evidence suggesting that ethylene (discussed in the following section, "Ethylene") is far more important than ABA in abscission now is overwhelming, and, despite its name, ABA has little, if any, influence on the process.

ABA apparently helps leaves respond to excessive water loss. When the leaves wilt, ABA is produced in amounts several times greater than usual. This interferes with the transport or retention of potassium ions in the guard cells, causing the stomata to close. When the uptake of water again becomes sufficient for the leaf's needs, the ABA breaks down, and the stomata reopen. ABA produced in times of drought is transported from the shoot to the root and causes an increase in both root growth and water uptake. Despite the growth-inhibiting effects of ABA, there is no evidence that it is in any way toxic to plants.

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Responses

  • maria
    How come the shoot grows when the tip is cut off and placed on an agar bloack?
    8 years ago
  • Heike
    How auxin kill dicot but not monocyte?
    8 years ago
  • ANTHONY
    How agriculture use plant hormones for cutting process?
    7 years ago
  • corrado
    How agriculturist use plant hormones for cutting processes?
    7 years ago
  • fredegar
    Have more pronounced role in the stem elongation than auxins?
    7 years ago
  • sebastian
    How gibberellin promote hapnostic movement?
    4 years ago
  • betty
    What will happen if metal foil cap is removed from coleoptiles?
    3 years ago
  • leighton stevenson
    Which plant hormone causes oats growth movement?
    3 years ago
  • GILBERT
    What is the observation of the tip of the shoot was intact?
    3 years ago
  • linda
    What is coleioptile?
    2 years ago

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