Chapter 6 Plant Growth, Development and Regulation Section 1 Regulation of growth and development by plant hormones First, Plant Hormones The major known types of plant hormones are auxins, gibberellins, cytokinins, abscisic acid, and ethylene. (1)Auxins 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, auxin normally stimulates the enlargement of cells by increasing the plasticity of cell walls. 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, 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 differentiationof 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 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. (2)Gibberellins 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. 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. 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. 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 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. (3) 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. Certain bacteria stimulate the growth of galls 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. (4)Abscisic Acid ABA was originally believed to promote the formation of abscission layers in leaves and fruits. However, the evidence suggesting that 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. (5) Ethylene The production of ethylene by plant tissues varies considerably under different conditions. A surge of ethylene lasting for several hours becomes evident after various tissues, including those of fruits, are bruised or cut, and applications of auxin can cause an increase in ethylene production of two to ten times. As pea seeds germinate, the seedlings produce a surge of ethylene when they meet interference with their growth through the soil. This apparently causes the stem tip to form a tighter crook, which may aid the seedling in pushing to the surface. Ethylene apparently can trigger its own production. If minute amounts are introduced to the tissues that produce the gas, a tremendous response by the tissues often results. These tissues may then produce so much ethylene that the part concerned can be adversely affected. Flowers, for example, may fade in the presence of excessive amounts, and leaves may abscise. Second, Other Hormones or Related Compounds A number of compounds called oligosaccharins, which are released from cell walls by enzymes, influence cell differentiation, reproduction, and growth in plants and therefore must be considered hormones. However, oligosaccharins produce their effects at concentrations of up to 1,000 times less than those of auxins, and the effects are not only highly specific, but the responses to them are essentially identical in all species. Brassinosteroids, which have a gibberellin-like effect on plant stem elongation, are known from legumes and a few other plants. Yams, which incidentally are a source of DHEA a hormone whose production by humans tends to decrease with age are also the source of batasins. Batasins promote dormancy in bulbils, which are produced from axillary buds in lilies and a few other plants. Section 2 Plant Vegetative Growth and Regulation First,plant movements (1)Growth Movements Growth movements result from varying growth rates in different parts of an organ. They are mainly related to young parts of a plant and, as a rule, are quite slow, usually taking at least 2 hours to become apparent, although the plant may have begun microscopic changes within minutes of receiving a stimulus. (2)Movements Resulting Primarily from Internal Stimuli Nutations Charles Darwin once attached a tiny sliver of glass to the tip of a plant growing in a pot. Then he suspended a piece of paper blackened with carbon over the tip, and as the plant grew, he raised the paper just enough to allow the tip to touch the paper without hurting the plant. He found that the growing point traced a spiral pattern in the blackened paper. We know now that such nutations are common to many plants.
Nodding Movements Members of the Legume Family, such as garden beans, whose ethylene production upon germinating causes the formation of a thickened crook in the hypocotyl, exhibit a slow, oscillating movement as the seedling pushes up through the soil. This nodding movement apparently facilitates the progress of the growing plant tip through the soil. Twining Movements although twining movements are mostly stimulated internally, external forces, such as gravity and contact, may also play a role. These movements occur when cells in the stems of climbing plants, such as morning glory, elongate to differing extents, causing visible spiraling in growth. Tendril twining, which is initiated by contact, results from an elongation of cells on one side of the stem and shrinkage of cells on the opposite side, followed by differences in growth rates. Some tendrils are stimulated to coil by auxin, while others are stimulated by ethylene. Contraction Movements In lilies, for example, seeds germinating at the surface ultimately produce bulbs that end up 10 to 15 centimeters below ground level because of the activities of contractile roots. There is some evidence that temperature fluctuations at the surface determine how long the contracting will continue. When the bulb gets deep enough that the differences between daytime and nighttime temperatures are slight, the contractions cease. The aerial roots of some banyan trees straighten out by contraction after the roots have made contact with the ground. The shrinking of roots has been shown to take place at the rate of 2.2 millimeters a day in sorrel. Nastic Movements When flattened plant organs, such as leaves or flower petals, first expand from buds, they characteristically alternate in bending down and then up as the cells in the upper and lower parts of the leaf alternate in enlarging faster than those in the opposite parts. Such non-directional movements are called nastic. Nastic movements may involve differential growth or turgor changes in special cells. Epinasty is the permanent downward bending of an organ, often the petiole of a leaf, in response to either an unequal flow of auxin through the petiole or to ethylene. Nastic movements that involve changes in turgor pressure include sleep movements and contact movements, (3)Movements Resulting from External Stimuli Permanent movements resulting from external stimuli coming from one direction are commonly referred to as tropisms. Tropic movements can be divided into three phases: 1. In the initial perception phase, the organ receives a greater stimulus on one side. 2. Then, transduction occurs, during which one or more hormones become unevenly distributed across the organ. 3. Finally, asymmetric growth occurs as a result of the uneven distribution of the hormone causing greater cell elongation on one side. Second, Apical Dominance For centuries, gardeners and nursery workers have often deliberately removed terminal buds of plants to promote bushier growth, knowing that the buds are involved in apical dominance. Apical dominance is the suppression of the growth of
the lateral or axillary buds, each of which can form a branch with its own terminal bud. Apical dominance is believed to be brought about by an auxin like inhibitor in a terminal bud. It is strong in trees with conical shapes and little branching toward the top and weak in trees that branch more often. Section 3 Light and temperature on the growth of plants Temperature and Growth Each species of plant has an optimum temperature for growth although the optimum may vary throughout the life of the plant and a minimum temperature below which growth will not occur. Each species also has a maximum temperature above which injury may result. The growth of many field crops is roughly proportional to prevailing temperatures--making it possible to predict harvest times although the number of days until maturity varies considerably with the locale. In 1855, the Swiss botanist A. P. de Candolle established a basis for the harvest dates of crops. His method, based on summing the temperature means for each day, makes it possible to follow a crop s progress with some precision. This method has been refined by multiplying the temperature means by the number of hours of daylight. Section 4 Plant reproductive growth and regulation First, photoperiodism That all the plants flowered, and further investigations showed that the length of day was directly related to the onset of flowering in many plants. They published the results of their investigations in 1920 and later called the phenomenon they had discovered photoperiodism. The critical length of day for the initiation of flowering is often about 12 to 14 hours, although it can vary considerably. Plants that will not flower unless the day length is shorter than the critical length are called short-day plants. They include asters, chrysanthemums, dahlias, goldenrods, poinsettias, ragweeds, sorghums, salvias, strawberries, and violets. Plants that will not flower unless periods of light are longer than the critical length are called long-day plants. These include garden beets, larkspur, lettuce, potatoes, spinach, and wheat. Such plants usually flower in the summer but will also flower when left under continuous artificial illumination. Accordingly, leafy vegetables, such as lettuce and spinach, need to be harvested in the spring and grown again in the fall in temperate latitudes if bolting is to be avoided. Potato breeders in the United States grow their plants in northern states where the long summer days initiate flowering, but since the potatoes themselves are produced independently of flowering, the plants may be grown for crop purposes at any latitude where other conditions are favorable. Indian grass and several other grasses have two critical photoperiods; they will not flower if the days are too short, and they also will not flower if the days are too long. Such species are referred to as intermediate-day plants. Other plants, particularly those of tropical origin, will flower under any length of day, providing, of course, they have received the minimum amount of light necessary for normal growth. Such plants
are called day-neutral plants and include garden beans, calendulas, carnations, cyclamens, cotton, nasturtiums, roses, snapdragons, sunflowers, and tomatoes, as well as many common weeds, such as dandelions. With some plants, small differences in day length may be critically important. Some varieties of soybeans, for example, will not flower when days are 14 hours long but will flower if the day length is increased to 14 1/2 hours. This difference could amount to less than 320 kilometers of latitude, with certain varieties grown in the southern states not producing fruit in the northern states, and vice versa. Second, photochromes and cryptochromes Phytochromes are extraordinary pale blue proteinaceous pigments that apparently occur in all higher plants and are associated with the absorption of light. Only minute amounts are produced, mostly in meristematic tissues. Phytochromes occur in two stable forms, either of which can be converted to the other: Pred, or Pr, is a form that absorbs red light; Pfar-red, or Pfr, is an active related form that absorbs the far-red light found at the edge of the visible light spectrum. When either form absorbs light, it is converted to the other form, so that Pr becomes Pfr when it absorbs red light, and Pfr becomes Pr when it absorbs far-red light. Pr is stable indefinitely in the dark. The normal effect of light in nature is to cause more Pr to become Pfr than vice versa. Pfr converts back to Pr in the dark over a period of several hours or else becomes inactivated, but its conversion in the presence of appropriate light is instantaneous. A second group of blue, light-sensitive pigments known as cryptochromes also play a role in circadian rhythms and evidently interact with phytochromes in plants in controlling reactions to light. Much of the research since cryptochromes were discovered has centered around fruit flies, cyanobacteria, and fungi. Cryptochromes have also been identified in humans. Third, a flowering hormone It was theorized that plants produce one or more flowering hormones, which may then be transported to the apical meristems where flower buds are initiated. Despite many years of research and all the circumstantial evidence, however, a flowering hormone has never been isolated from a plant, nor has it otherwise been proved to exist. This then led to speculation that photoperiods may bring about a shunting of nutrients that initiate flowering or that flowering is triggered by changes in relative proportions of other hormones. The theory that a flowering hormone exists has now been discarded, and a simple explanation involving hormones for the phenomenon of photoperiodism has proved to be nonexistent. Instead, it is now believed that no hormones are involved in flowering and that flowering is triggered by the direct effects of phytochrome on gene expression. This leads to the accumulation of and/or disappearance of specific RNA s, which, in turn, is thought to initiate flowering. Section 5 Plant mature, aging and regulation Senescence The breakdown of cell components and membranes that eventually leads to the death of the cell is called senescence. As mentioned in Chapter 7, the leaves of
deciduous trees and shrubs senesce and drop through a process of abscission every year. Even evergreen species often retain their leaves for only 2 or 3 years, and the above-ground parts of many herbaceous perennials senesce and die at the close of each growing season. Amounts of cytokinin are added. By varying the amounts of cytokinin, it is possible to stimulate the pith cells to differentiate into roots or into buds from which stems will develop. Gibberellins and ABA are important in the regulation of germination of seeds. Gibberellins promote cell elongation, thereby enabling the developing root to break through the seed coat. After water has been imbibed by cereal seeds, gibberellins released by the embryo stimulate the secretion of enzymes that digest endosperm. ABA has been shown to inhibit the synthesis and release of these enzymes. The relative amounts of gibberellins and ABA in seeds other than cereal grains also influence whether the seeds germinate or remain dormant.