Plant Growth Regulators
Plants respond to the environment by: Sending signals between different parts of the plant. Tracking the time of day and the time of year. Senseing and responding to gravity and direction of light, etc. Responding to environmental cues by adjusting their growth pattern and development. These control systems evolved through interactions with the environment. I. Hormones and Their Actions A. Hormone - An organic compound produced by one part of an organism that is translocated to other parts where in low concentrations it triggers a response in target cells and tissues. B. Hormones control plant growth and development by affecting division, elongation, and differentiation of cells. 1. Effects depend on site of action, stage of plant growth and hormone concentration. 2. Reaction to hormones depends on hormonal balance (relative concentration of one hormone compared with others). 3. The hormonal signal is amplified, perhaps by affecting gene expression, enzyme activity, or membrane properties. C. The concept of differential target cell sensitivity to hormones 1. Anthony Trewayas in the early 1980s proposed that target cell sensitivity was more important in determining the effects of a hormone than its concentration within the target cell. 2. Since that time it is now excepted that both target cell sensitivity and hormone concentration are important in the action of a hormone. 3. There are three parts of a hormonal response system: a. The hormone must be present in sufficient quantities in the target cells. b. The hormone must be recognized by and bound tightly to the target cells that respond to the hormone. This action occurs through hormone-binding receptor proteins in the membranes of the plant cell. It appears that the plasma membrane is most important, but the tonoplast and endoplasmic reticulum also have been noted to have hormone-binding receptor proteins. c. The receptor protein must cause other metabolic changes that lead to amplification of the hormonal signal. These receptor proteins function by changing their conformation after they are activated by the hormone. The altered conformation then allows the receptor proteins to activate other proteins or nucleic acids (DNA or RNA) D. It has been found that hormones effect gene activity. 1. This gene activation can result in a large degree of amplification of the hormonal signal. 2. There are various controls points at which a hormone could affect the expression of a gene: a. Transcription: DNA to RNA b. Translation: RNA to Protein c. Post-translational modification of proteins. E. Sites of hormonal activity usually involve membranes: 1. Hormones act first in the plasma membrane where receptor proteins are located. 2. In animals there are two such receptor protein process that have been identified. Each one involves a second- messenger. a. Cyclic-AMP (cAMP): involves the enzyme Adenylate cyclase. Adenylate cyclase uses ATP to form cAMP. cAMP is then involved in activation of protein kinases. Protein kinases function by activating or inactivating other enzymes by a process of phosphorylation. Thus far, the cyclic-AMP process has not been found to be important in plants. b. Inositol Triphosphate (IP3): involves the binding of the hormone to a receptor protein in the plasma membrane of the target cell. The bound hormone- receptor complex then activates phospholipase c (PLC). Phospholipase c hydrolyzes phosphoinositol (a nonabundant membrane phospholipid) into inositol- 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Both of these compound that cause a cascade effect. (1) IP3 is water-soluble and moves to the vacuole and causes the release of Ca2+ into the cytosol. [NOTE: in animals IP3 stimulates Ca2+ release from the endoplasmic reticulum.] These increased Ca2+ levels in the cytosol causes activation of enzymes, including several different kinds of protein kinases. When Ca2+ concentration rise in the cytosol, four Ca2+ combine to form a chelate with calmodulin. This Ca-calmodulin complex can activate other enzymes. such as protein kinases, NAD+ kinase. One enzyme that is activated is ATPase which functions to transfer excess Ca2+ out of the cell. (2) DAG is not water-soluble, so it functions within the plasma membrane where it activates Protein Kinase c (PKC). This kinase uses ATP to phosphorylate other enzymes that regulate metabolism. F. So far, five classes of plant hormones have been identified. 1. Auxin (such as IAA) 2. Gibberellins (such as GA,) 3. Cytokinins (such as kinetin) 4. Abscisic acid 5. Ethylene II. Auxin A. Auxin = A hormone that promotes elongation of young developing stems or coleoptiles. 1. The term "Auxin" comes from the Greek auxin with means "to increase". This terms was coined by Frits Went in 1926. Frits Went had been working as a graduate student on the phototropic responses of coleoptiles in response to auxin application. 2. Phototropism = Growth toward or away from light (e.g. growth of a shoot toward light). a. Results form differential growth of cells in opposite sides of a shoot or, in the case of a grass seedling, coleoptile. b. Cells on the darker side elongate faster. c. Experiments on phototropism led to the discovery of auxin as a plant hormone. 3. Charles and Francis Darwin removed the tip of the coleoptile from a grass seedling (or covered it with an opaque cap) and it f ailed to grow toward light. a. Concluded that the coleoptile tip was responsible for sensing light, and, since the curvature occurs some distance below the tip, the tip sends a signal to the elongating region. 4. Peter Boysen-Jensen separated the tip from the remainder of the coleoptile by a block of gelatin, preventing cellular contact but allowing chemical diffusion. a. Seedlings behaved normally. b. If an impenetrable barrier was substituted, no phototropic response occurs. c. Demonstrated that the signal was a mobile substance. 5. F.W. Went removed the coleoptile tip, placed it on an agar block, and then put the agar (without the tip) on decapitated coleoptiles kept in the dark. a. If block was placed off -center, the plant curved away from the side with the block. b. Concluded the agar block contained a chemical that diffused into it from the coleoptile tip, and that this chemical stimulated growth. c. Went called this chemical an Auxin. 6 Auxin was purified and characterized by Kenneth Thimann. B. Natural Auxin is a compound named indoleacetic acid (IAA). 1. There are three other compounds that are structurally similar to IAA that also act as auxins: a. 4-chloroindoleacetic acid (4-chloroIAA): found in young seeds of legumes. b. Phenylacetic acid (PAA): widespread and more abundant than IAA. c. Indolebutyric acid (IBA): found in corn leaves and many dicots. 2. Auxin Precursors: indoleacetaldehyde, indoleacetonitrile, and indole ethanol. 3. Synthetic Auxins: a-naphthalene acetic acid (NAA), 2,4- dichlorophenoxyacetic acid (2,4-D), 2-methyl-4- chlorophenoxyacetic acid (MCPA). a. NAA: is used as an auxin source in tissue culture b. 2,4-D is used as a herbicide on broadleaf (dicot) weeds in narrowleaf (monocot) crops. It is especially useful to eliminate broadleaf weeds in lawns. Another related compound is 2,4,5-T. During the Viet Nam Conflict ("WAR") 2,4-D and 2,4,5-T were the active ingredients for Agent Orange. Since that time the U.S. Environmental Protection Agency has forced removal of 2,4,5-T from the market because it contains dioxin (a mutagen and carcinogen) as a by- product of manufacturing. c. Other herbicides that are auxin derivatives are: MCPA, Picloram, and Dicamba d. The effect of these herbicides on broadleaf plants occur by an alteration of DNA transcription and RNA translation that disrupt enzymes needed for coordination of plant growth. As a result plants treated with these compounds respond by producing twisted and deformed leaves, petioles, and stems. C. The synthesis, concentration control, and degradation of IAA. 1. IAA is structurally similar to tryptophan (an amino acid) and it is indeed the precursor of IAA. The enzymes necessary for the conversion of tryptophan to IAA are found in young tissues such as meristems, growing leaves and fruits. It is not surprising that auxin concentrations are highest in these tissues. 2. Concentration control of auxin in plant tissues is achieved by: a. The rate of synthesis. b. Temporary inactivation by conjugate formation, or what has often been called bound auxin. There are numerous IAA conjugates: peptide (indoleacetyl aspartic acid, esters (IAA-inositol and IAA- glucose). IAA can be released from these conjugates by hydrolase enzymes. c. Conjugates are storage forms of IAA, and are important in the transport of IAA. 3. Degradation of IAA: a. Oxidation by O2 and loss of the carboxyl group as CO2 to form 3-methyleneoxindole. This reaction is catalyzed by IAA oxidase. There are several IAA oxidase isozymes which are all nearly identical to the peroxidase that is involved in lignin synthesis. Synthetic auxins and auxin conjugates are not degraded by IAA oxidase. b. Another pathway of degradation involves the oxidation of carbon 2 in the heteroxylic ring to form oxindole- 3-acetic acid. D. The transport of auxin in the plant. 1. IAA is translocated through parenchyma cells in contact with vascular bundles. IAA will move through sieve tubes if it is applied to the surface of a leaf that is exporting sugars. But, the normal transport of IAA in plant stems is along the petioles of the leaf to the cells along the vascular bundles. 2. Auxin movement is slow, about 1 cm per hour. This, however, is 10X faster than diffusion. 3. Polar Auxin transport requires metabolic energy. a. IAA is actively transported down a stem by Auxin carriers located on the basal ends of cells (carriers are absent on the apical ends). b. Even in root the transport is in an acropetal (apex- seeking) direction. c. Antiauxins such as 2,3,4-triiodobenzoic acid (TIBA) and a-naphthylthalamic acid (NPA) inhibit this polar transport. 4. Mechanism for polar transport of auxin: a. Cells use plasma membrane ATPase to pump H+ from the cytosol into cells walls. The lower pH of the cell walls keeps the carboxyl group of auxin less dissociated than in the cytosol. The noncharged auxin moves from the cell wall into the cytosol by a cotransport mechanism with H+. b. Once inside the cytosol the higher pH causes the carboxyl group on auxin to dissociate and become negative. As the concentration of charged auxin builds up, the outward flow becomes favored thermodynamically. c. Polar transport requires auxin to move out only at the basal end of the cell. It is assumed that a carrier transports auxin basipitally within the cell and then transports it out into the cell wall, where the low pH starts the cycle again. d. Some flavonoids (quercetin, apigenin, kaempferol) have been found to inhibitors of the basal transporter of auxin. e. Auxin transport is important for: (1) vascular cambium activity in woody plants during the spring and early summer, (2) differentiation of xylem and phloem in leaf petioles, (3) stem cell growth, (4) lateral bud inhibition. E. Roots and their responses to auxins: 1. Auxin concentrations in roots are similar to other parts of the plant. 2. Auxin inhibits root growth at low concentrations. For many years it was thought that this was due to increased ethylene production which is stimulated by increases in auxin. Ethylene retards root and stem elongation. But, recent research has shown that auxin inhibits growth of roots by an unknown mechanism that is independent of ethylene. 3. Roots can synthesize enough auxin for their growth. There is probably a balance between growth of root and shoot systems that is strongly influenced by auxin from the stems. Auxin from the stems influences root initiation and adventitious root development on stems. F. Lateral-bud development and auxin effects: 1. Apical Dominance: the inhibitory influence the apical bud exerts on the lateral (axillary) buds. This apical dominance slows the development of the lateral buds. It also causes lateral branches to grow horizontally. This horizontal growth reduces shading of the lower branches and thus increases photosynthetic productivity of the plant. 2. The IAA concentration in lateral buds increases after the plant is decapitated. This is contrary to what would be predicted - one would suspect that auxin concentration would decrease when the apical bud is removed. 3. Lateral buds have been found to be deficient in cytokinins. There may be an interaction between these two growth hormones. 4. Abscisic acid, ethylene, and gibberellins have also been found to be important in apical dominance. G. Auxin Mechanism of Action: 1. Growth requires water absorption, cells must maintain a water potential more negative than that of the surrounding solution. Cell walls of auxin-treated cells yield more easily, so the pressure potential required to force cell expansion is not as great as untreated cells. Auxins cause wall loosening, the more rapidly extensible or plastic nature of cell walls. 2. The most studied mechanism of action has been: the acid- growth hypothesis that states that cell elongation is due to stimulation of a proton pump which acidifies the cell wall. Auxin treated cells pump H+ into the cell walls, thus lowering the pH so that the wall loosens and can grow faster. a. Acidification activates enzymes that break the crosslinks between the walls cellulose myofibrils. b. This loosens the wall, allowing turgor pressure to elongate the cell. c. There is a difference between growth promotion of cells caused by cytokinins and cell wall acidification. Other growth hormones cause wall loosening and cell expansion in some species by some unknown mechanism 3. Which cells respond to auxin? a. Epidermal cells: these elongate in response to auxin treatment. b. Subepidermal cell layers (hypodermis, cortex, pith) contain cells that are under pressure. These cells are ready to elongate because of their internal pressure. Elongation is restricted because their cell wall polysaccharides are continuous with those of the epidermal cells that are unable to stretch. The subepidermal cell layer elongates just enough to keep the epidermal cell walls under tension. Auxins cause epidermal cell wall polysaccharides to loosen and elongate. The elongation of the epidermal cells is followed by the elongation of the connecting subepidermal cells such that the stem elongates in unison. 4. How do auxins control the cell responses: a. Auxins have been found to cause rapid changes in genetic activity. b. Auxins change the kinds of proteins formed in the cell. The main control seems to be at the level of transcription. c. Auxin works by binding to an auxin receptor protein found mainly in the endoplasmic reticulum and to a lesser extend in the plasma membrane. This receptor protein is a dimer of two polypeptides of about 20 kDa each. III. Cytokinins A. A Very Brief History of Cytokinin Discovery: 1913: Gottlieb Haberlandt first found a growth hormone that stimulated cell division (cytokinesis) in plants. These substances are now called cytokinins. In the 1940's Johannes vanOverbeek found cytokinins in the endosperm of immature coconuts. In the 1950's Folke Skoog and F.C. Steward used this information about cytokinins in endosperm in their plant tissue culture work. D.S. Letham (1974) isolated two very active forms of cytokinins, Zeatin and Zeatin-riboside, from the coconut endosperm. Carlos Miller (1954) found a substance that was named Kinetin, in aged or autoclaved herring-sperm DNA. This work helped lead to the defining of the structure of cytokinins. B. Chemistry of Cytokinins: 1. Cytokinins are not found in DNA or breakdown products of DNA, even though they are involved in cytokinesis. Some cytokinins do occur in transfer-RNA and ribosomal-RNA. 2. Cytokinins have been found in seed plants, mosses, brown and red algae, diatoms, fungi, bacteria, and primates. More than 30 free and bound forms of cytokinins have been identified. In pathogenic fungi and bacteria, cytokinins influence the disease process. In non- pathogenic fungi and bacteria cytokinins influence the mutualistic relationship with plants (mycorrhizae and root nodules). 3. Most common and physiologically active natural forms are: zeatin, dihydrozeatin, isopentenyl adenine (IPA). 4. Synthetic cytokinins: Kinetin and benzyladenine (BA) 5. Cytokinin Structure: a. Cytokinins can exists as free-bases or and nucleosides or nucleotides b. Substituted adenine compounds C. Cytokinin metabolism: 1. Isopentenyl-AMP is the precursor of Cytokinins. It is formed by the enzyme isopentenyl AMP synthase from isopentenylpyrophosphate and AMP. 2. Cellular levels of cytokinins are influenced by their degradation and conversion to inactive derivatives (other than nucleosides and nucleotides). Cytokinins are degraded by cytokinin oxidase. The most common conjugates are glucose (cytokinin glucosides) or alanine. 3. The cytokinin glucosides may be storage forms or transport forms of cytokinins. The alanine conjugates are not stored, but are formed irreversibly as removal products of cytokinin. Conjugates represent physiologically inactive cytokinins. D. Sites of Cytokinin Synthesis and Transport: 1. Cytokinin levels are highest in young organs such as seeds, fruits, young leaves, and root tips. 2. Roots tips are active in synthesis of cytokinins. The cytokinins are transported from the root tips, through the xylem, to all plant parts. 3. Shoot are also able to synthesize some of the cytokinins they need. 4. Cytokinins are mainly transported through the xylem, however, they have been identified in sieve tubes of the phloem. For example, when the petiole of a mature leaf is cut, cytokinins move to the base of the petiole through the phloem. E. Promotion of Cell Division and Organ Formation by Cytokinins: 1. A high cytokinin-to-auxin ratio in tissue culture will produce meristematic cells in callus culture. These cells will give rise to buds, stems, and leaves. A low cytokinin-to-auxin ratio will favor root formation. 2. Organogenesis: formation of shoots or adventitious roots by callus culture. Embryogenesis: formation of embryos by callus culture. 3. Agrobacterium tumefaciens (bacterium): contains the Ti plasmids that codes for isopentenyl AMP synthase. Infection of a plant with this bacterium will result in tumorous outgrowths (galls) on stems called crown gall disease. F. Delay of senescence and increase of nutrient-sink activity by cytokinins: 1. If a mature leaf is detached from the stem of a plant it will lose chlorophyll, RNA, proteins, and lipids from the chloroplast membrane system. This will cause the leaf to turn yellow, a process associated with senescence. If the leaf is treated with cytokinins, this senescence process will be delayed. 2. Cytokinins maintain the integrity of the tonoplast (vacuole) membrane. This is important since the vacuole stores proteases that could hydrolyze proteins in the cytoplasm and organelles, especially organelle membranes. 3. Cytokinins are also able to protect membranes from degradation by preventing oxidation of unsaturated fatty acids. Cytokinins inhibit formation of or speed the breakdown of free radicals (superoxide and hydroxy radicals) that would normally attach the lipids in membranes. 4. Cytokinins stimulate transport of solutes from older plant parts to younger plant parts. Migration of solutes occurs through the phloem. Young plant parts are able to mobilize solutes towards themselves because they have high cytokinin concentrations. Thus the young tissues of a plant are "sinks" for solutes. 5. Some pathogenic fungi produce cytokinins in order to mobilize nutrients from the plant to the fungi. 6. The ability of cytokinins to retard senescence has been used in agriculture. Cytokinins are used to retard the senescence of cut flowers and fresh vegetables. Cytokinins increase the storage life of Brussels spouts and celery (not done in the U.S.). G. Promotion of lateral-bud development in dicots by cytokinins: 1. Application of cytokinin to a lateral bud that is under the control of a shoot apex will cause the lateral bud to grow. It is not clear if this is what occurs in nature. H. Enhancement of cell expansion in dicot cotyledons and leaves by cytokinins: 1. When dicot seeds germinate, they stay yellow (etiolated) until they emerge above ground. When cotyledons are exposed to light their growth increases as a response to phytochrome and cytokinins. Growth is by cell expansion caused by water uptake because the dry weight of the cotyledons does not change. I. The normal growth of stems and roots requires cytokinins even though application of exogenous cytokinins sometimes decreased elongation. J. Promotion of chloroplast development and chlorophyll synthesis by cytokinins: 1. Cytokinins enhance the development of etioplasts (proplastids that eventually will form chloroplasts upon maturation) into chloroplasts by promoting grana formation and increasing the rate of chlorophyll formation. 2. Cytokinins enhance the formation of one or more proteins in the chloroplast membrane system to which chlorophyll binds and is stabilized. K. Mechanism of action for cytokinins: 1. It has been found that cytokinins may have a primary effect that is followed by many secondary effects which depend on the physiology of the target cell. 2. The hormonal signal is amplified by the cell. 3. Cytokinins promote RNA and enzyme formation. 4. Cytokinins promote cell division by decreasing the transition time from G2 to mitosis by increasing the rate of protein synthesis. It is well known that there is a control point at the end of G2 which is controlled by the stockpiling of enzymes and proteins that will be need during mitosis. Benzyladenine (BA) may control DNA synthesis since it causes a decrease in the length of the S phase in the cell cycle. 5. Cytokinins also act on translation of mRNA. Cells treated with cytokinins form large polysomes that aid in mRNA translation. 6. Cytokinins also appear to effect mRNA levels by influences in transcriptional control of some genes (turning on some and turning off others). Cytokinins may also affect the stability of mRNA. 7. Cytokinins have been reported to affect the amount of mRNA: a. Upregulation of chlorophyll a/b binding protein (part of the LHCII thylakoid membrane) b. Upregulation of small subunit or RuBisCo. c. Downregulation of phytochrome (by cytokinins and red light) 8. Cytokinins cause increased plasticity of cells walls. The cell walls are loosened so that turgor pressure can expand them faster. This is not an acidification process in the cell walls as seen with auxin. IV. Gibberellins A. More than 84 different gibberellins, many naturally occurring, have been identified. 1. First discovered in Japan in the 1930s where it caused a disease in rice plants called "foolish seedling". Eventually it was found that a fungus (Gibberella fujikuroi) caused the disease through a compound named gibberellin. Although Gibberellin was found about the same time as Auxin, it did not receive much publicity in the west until after WWII. 2. All gibberellins are acidic and are derivative from the ent-gibberellane structure. (19 or 20 carbon atoms in four or five rings with one carboxyl group). The most active form is GA3, which has been named gibberellic acid. 3. Gibberellins are found in angiosperms, gymnosperms, ferns, mosses, algae, and fungi. They have also been reported in some bacteria. B. Metabolism of Ginbberellins: 1. Gibberellins are isoprenoid compounds (diterpenes) that are formed through the mevalonic acid pathway. They are formed in the endoplasmic reticulum. 2. The 19 carbon gibberellins are more active than the 20 carbon gibberellins. 3. There are several commercial growth retardants that inhibit stem elongation (stunting) by blocking gibberellin synthesis: Phosphon D, Amo-1618, Cycocel, ancymido, and paclobutrazol. 4. Conjugated gibberellins are inactive. Glucoside conjugates are stored or translocated before they are released at the proper time and place. 5. Many time active gibberellins can be changed into less active forms. 6. Sites of gibberellin synthesis in the plant: a. Most plant cells have the ability to synthesize gibberellins b. Immature seeds: usually contain large amounts of gibberellins that arise from biosynthesis in the seed itself, not from transport. c. Young leave are major sties for gibberellin synthesis. d. Roots synthesize gibberellins. Root gibberellins have little effect on root growth but it inhibits adventitious-root (roots arising from above ground stems) formation. There is evidence that roots supply the stems with a large amount of gibberellin that is translocated through the xylem. 7. Gibberellins are transported through the plant in the xylem and phloem in a non-polar fashion. C. Growth promotion by gibberellin in intact plants: 1. Gibberellins have been found to greatly increase the stem growth of dwarf plants, or biennials that are in the rosette stage. 2. Many of the gibberellin-synthesis mutants that have been found are dwarf because one or more steps in the synthesis of gibberellin is missing or non-functional. 3. Most plant species require GA1 for stem elongation. Gibberellin-sensitivity mutants have sufficient amounts of gibberellin in their tissues but cannot respond to it. It is possible that a receptor protein is missing or non-functional in these mutants. D. Promotion of germination in dormant seeds and growth of dormant buds by gibberellins. 1. Buds in temperate zone plants become dormant in late summer or early fall. To break the dormancy of these buds the plant must undergo a cold treatment in the winter, or proper exposure to photoperiod. 2. Seeds often show dormancy when first shed from the plant. These seeds must also undergo some treatment (cold temperature, scarification, acid treatment, exposure to sufficient moisture or proper photoperiod, etc.) before they can germinate. 3. Breaking of bud and seed dormancy can be overcome by gibberellin treatment. In some seeds, the elongation of the radicle by gibberellin treatment allows it to push through the seed coat. E. Gibberellin affects on flowering: 1. Flowering in a plant depends on several factors: plant age, environmental cues (freezing), and photoperiod. Some plants will flower only under short day, some flower only during long days, and some flower during any length of day (day neutral). 2. Gibberellins are able to cause flowering in long-day plants and plants that need a cold period. F. Mobilization of food and mineral elements in storage cells of seeds is stimulated by gibberellins. 1. After seed germination, root and shoot systems use mineral nutrients, fats, starch, and proteins present in seed storage cells. These reserves are used before the seedling is able to absorb minerals from the soil and before photosynthesis has begun. 2. Mineral nutrients are readily translocated via the phloem into the roots and shoots. Fats, polysaccharides, and proteins are not readily translocated. These nutrients are metabolized into smaller, more mobile molecules such as sucrose, amino acids, and amides. Gibberellin is responsible for these conversions in many seeds, especially monocots. 3. The embryo of monocots is surrounded by the endosperm. The endosperm is a nutritive storage tissue with little metabolic ability. the endosperm is surround by a metabolically active layer called the aleurone. the aleurone layer provides hydrolytic enzyme that digest starch, proteins, phytin, RNA, and cell-wall materials in the endosperm. 4. Gibberellin stimulates the secretion of hydrolytic enzyme into the endosperm from the aleurone layer. Mineral elements in reserve in the endosperm are also made available by gibberellin activity. 5. Gibberellins are synthesized in the scutellum (cotyledon) and parts of the embryo. The scutellum is more important than the aleurone layer in providing enzymes for digestion of the endosperm tissues. 6. In breaking both seed dormancy and bud dormancy, gibberellins act antagonistically with abscisic acid, which inhibits plant growth. G. Other effects of gibberellin: 1. Gibberellins cause parthenocarpy in fruits (seedless fruit development) a. Fruit development is controlled by both gibberellins and auxins. b. In some plants both must be present for fruit set. c. Commercial application in spraying Thompson seedless grapes. 2. Gibberellins in young leave help renew activity of the vascular cambium in woody plants during the spring. 3. Gibberellins delay aging (senescence) in leaves and citrus fruits 4. Gibberellins have been found to influence leaf shape, especially in plants that show a difference between their juvenile and adult forms (heterophylly). 5. Gibberellins have been implicated in flower petal development. H. The many a varied effects of gibberellins on plant growth and development suggest that gibberellins have more than one primary site of action. 1. Stimulation of cell division in shoot apexes: gibberellins have been found to promote cell division by stimulating cells in the G1 phase to enter into the S phase. Gibberellins have also been found to shorten the time a cell stays in the S phase. 2. Gibberellins promote cell growth by affecting enzymes that increase hydrolysis of starch, fructans, and sucrose. 3. Gibberellins can increase cell-wall plasticity 4. Gibberellins probably function by activating genes responsible for enzyme that run these physiological processes. I. Uses of gibberellins in agriculture: 1. Prevent rind disorders in navel-oranges. 2. Promote seed production in Pinaceae seed orchards. 3. Improve the size of Thompson seedless-grapes. 4. Increase the rate of malting in breweries. 5. Increase the length and crispness of celery stalks. 6. Increase sugarcane growth and sugar yields in Hawaii. V. Abscisic Acid (ABA) A. Abscisic acid signals physiological stress such as: water, salinity, cold, and frost. Abscisic acid helps in embryogenesis, formation of seed-storage proteins, and prevention of seed germination and bud growth. It is universal in vascular plants, mosses, green algae, and fungi. B. Abscisic acid helps prepare plants for winter by suspending both primary and secondary growth. 1. Directs leaf primordia to develop scales that protect dormant buds. 2. Inhibits cell division in vascular cambium. C. Effects of ABA on bud dormancy and leaf abscission: 1. ABA levels increase in leaves and buds when bud dormancy occurs. ABA synthesis in leaves is controlled by day length, and translocated to buds. 2. ABA probably has no direct role in leaf abscission but acts to cause premature senescence of cells in the leaf which in turn causes production of ethylene. D. ABA also acts as a stress hormone, closing stomata in times of water-stress. 1. ABA content rises when leaves are under water stress. Roots under water stress form ABA that is transported through xylem to the leaves where it causes stomatal closure. ABA from water stressed roots comes mainly from shallow roots, thus acting as an advanced signal to the plant that water is in short supply. 2. ABA works by inhibiting the ATP-dependent proton pump in the plasma membrane of the guard cells. ABA stops the K+ influx into the guard cells, so K+ and water leak out of the guard cells, reducing turgor, and closing the stomates. 3. The water stress signals the plasma membranes to activate certain nuclear genes that increase ABA synthesis. The plasma membrane responds to Ca+ and phosphoinositols (IP3), indicating that this is a calmodulin response. E. Abscisic acid involvement in salt and cold stress: 1. Salt stress: causes formation of different proteins like osmotin. These proteins help protect the plant against salt stress. 2. Cold Stress: activation of genes that control ABA in cold and heat stress are similar to the responses noted in water stress. F. Effect of abscisic acid on embryo development in seeds: 1. Embryo development is divided into three stages: a. mitosis and cell differentiation b. cell expansion and accumulation of food reserves (protein, fat, starch) c. maturation: dehydration and dormancy 2. ABA is linked to normal maturation pathway in seed development. ABA inhibits precocious germination of developing seeds. 3. ABA stimulates storage of seed proteins that are needed for later dehydration and dormancy. 4. The ratio of ABA:gibberellins determines whether seeds remain dormant or germinate. G. Abscisic acid metabolism and transport: 1. ABA is a 15-carbon sesquiterpenoid from the mevalonic acid pathway. It is synthesized in plastids like the chloroplasts. It has common metabolic intermediates with gibberellins, sterols, and carotenoids. ABA is synthesized as a degradation product of carotenoids in plastids (chloroplasts - leaves, chromoplasts-fruits, leucoplast-roots, proplastids-seeds). 2. ABA is inactivated by attachment to glucose. ABA- glucose esters are restricted to vacuole. Also, ABA is inactivated by oxidation with oxygen to form phaseic acid and dihydrophaseic acid. 3. ABA is transported in the xylem and phloem. Transport has also been found in parenchyma outside the vascular system. There is no polarity in ABA transport. 4. ABA has three major effects on plant tissues: a. effects plasma membrane of roots: root membranes are positively charged probably due to the inhibition of a plasma membrane ATPase. There is probably an interaction with auxin-induced acid growth. b. inhibition of protein synthesis: results in effects on growth and development such as control of seed and bud dormancy. c. specific activation and deactivation of genes. VI. Ethylene A. A gaseous hormone that diffuses through air spaces between plant cells. High auxin concentrations induce release of ethylene, which acts as a growth inhibitor. 1. During fruit ripening, ethylene triggers senescence, and then the aging cells release more ethylene. The signal to ripen spreads from fruit to fruit since ethylene is a gas. 2. Fruit ripening phenomenon has been know for a long time. Ancient Chinese knew that burning incense would ripen fruit. The Jamaican Agricultural Department knew in 1910 that oranges could not be stored with bananas on ships because the oranges would cause the bananas to overripen. Bonfires where used to help ripen pineapples in Hawaii and Puerto Rico, and mangos in the Philippines. 3. Ethylene released by soil fungi promotes seed germination, controls seedling growth, and retards disease caused by soil-borne organisms. B. Synthesis of ethylene in plants: 1. All parts of the seed plant can produce ethylene. Stems produce large amounts of ethylene when they are laid horizontally. Roots do not produce much ethylene. Leaf ethylene production increases gradually over the growing period, until they senesce and abscise. Flowers synthesize ethylene just before they fade and wither. Fruits produce ethylene just before the respiratory climacteric (a large increase in cellular respiration just before the fruit ripens). In non- respiratory climacteric fruits (citrus fruits, cherries, grapes), ethylene has little effect on fruit ripening. However, ethylene is used in the citrus industry to de-green oranges and lemons. 2. Stress will also cause a plant to produce ethylene. This can come from mechanical stress such as rubbing (wind) or, pressure (trees leaning on each other), pathogens (fungi, bacteria, virus, insects), environmental stress (waterlogging or drought). Figs can be ripened by slashing the fruit. 3. Ethylene is synthesized form methionine. Its direct precursor is 1-amino-cyclopropane-1-carboxylic acid (ACC). 4. Inhibitors of ethylene production: aminoethoxyvinylglycine (AVG) and aminooxyacetic acid (AOA). Both block the synthesis pathway from methionine to ethylene. 5. Control of ethylene synthesis: a. Auxin promotes ethylene production b. Auxin promotes ACC formation, which is the direct precursor of ethylene. 6. Ethylene stimulates its own formation in senescing organs (autocatalytic). Thus there is some truth to the saying that "One rotten apple can spoil the whole bunch." Diffusion of ethylene through intercellular spaces in a fruit coordinates ripening. 7. Light inhibits ethylene production in leaves while carbon dioxide promotes ethylene synthesis. C. Leaf abscission is an adaptation that prevents deciduous trees from desiccating during winter when roots cannot absorb water from the frozen ground. 1. Before abscission, the leafs essential elements are shunted to storage tissues in the stem. 2. Environmental stimuli are shortening days and cooler temperatures. 3. When a leaf falls, the breakpoint is an abscission zone near the petiole base. a. Weak area since the small parenchyma cells have very thin walls and there are no fiber cells around the vascular tissue. b. Mechanics of abscission controlled by a change in the balance of ethylene and Auxin. c. Auxin decrease initiates changes in the abscission layer. Cells then produce ethylene. d. Ethylene induces synthesis of enzymes that digest the polysaccharides in the cell walls, further weakening the abscission zone. e. Wind and weight cause the leaf to f all. 4. Even before the leaf falls, a layer of cork forms a protective scar on the twig's side of the abscission layer. D. Ethylene effects on plants in waterlogged soils and submerged plants. 1. Waterlogged roots produce less ethylene since waterlogged soils are hypoxic. Ethylene synthesis is inhibited because oxygen is required to convert ACC to ethylene. 2. The ethylene that is synthesized is trapped in the root because the ethylene cannot escape readily though water. 3. Ethylene causes cortical cells to synthesize cellulase which hydrolyzes cellulose. The cortical cells lose their protoplast, forming air-filled aerenchyma tissue. 4. As ACC accumulates it is transported in the xylem to the shoots where it is metabolized into ethylene. 5. Leaf epinasty occurs because the parenchyma cells on the upper surface of the petioles elongate in the presence of ethylene relative to the lower surface. Ethylene also retards stem elongation, increases radial expansion, causes leaf senescence, and promotes adventitious root formation. E. Effect of ethylene on stem and root elongation: 1. Ethylene inhibits elongation of stems and roots and subsequently causes them to become thicker by increasing radial expansion. This is caused by the cellulose microfibrils being laid down in a more longitudinal orientation. 2. Important response of dicot seedlings emerging from soil. The hook in the epicotyl or hypocotyl is formed as a response to ethylene. The hook allows the seedling to push through the soil without damaging the meristematic tissue at the tip. The ethylene causes the seedling to increase in thickness so that it is stronger. 3. This increased thickening is also found in the mesocotyls of monocots. 4. Ethylene causes loss of gravitropic response in seedlings, which allows them to bend around obstructions in the soil as they are growing. 5. Aquatic plants respond to ethylene production by causing elongation of their stems and roots. This allows aquatic plants to have their leaves kept above the water line. F. Ethylene effects on flowering: 1. Normally, ethylene inhibits flowering in plants. However, in mangos and bromeliads (example: pineapple), ethylene induces flowering. 2. Commercially: Ethrel (ethephon) is used to promote flowering in Hawaiian pineapple fields which facilitates a single mechanical harvesting. Ethrel also used in late season on tomatoes to cause uniform ripening and enable mechanical harvesting. G. Other ethylene effects: 1. Ethylene causes flower senescence, probably because after pollination there is an increase in the rate of ethylene production in flowers. 2. Ethylene promotes adventitious-root formation, also an effect of auxin. 3. Ethylene causes changes in the expression of flower sex. Ethylene promotes formation of female flowers. 4. Ethylene helps break seed dormancy. Found as a function of some soil fungi. H. Relation of ethylene to auxin effects: 1. Auxin increases ethylene production. Raises the question of whether some plant responses to auxin are merely responses to ethylene. 2. Some ethylene responses that relate to auxin production: leaf epinasty, inhibition of stem and leaf elongation, flower induction (bromeliads and mangos), inhibition of hook opening in dicot seedlings, female flower production, senesces of flowers. 3. Some ethylene responses that relate to other hormones (auxin, cytokinins, abscisic acid): abscission of leaves, flowers, and fruits. I. Antagonists of ethylene action: 1. High concentrations of carbon dioxide inhibits ethylene responses in the plant. High carbon dioxide concentrations are often used in fruit packing houses to prevent overripening of fruits and vegetables. 2. Silver ion (Ag+) also inhibits the action of ethylene. Silver ions in the form of silver nitrate (AgNO3) inhibit abscission of leaves, flowers, and fruits in plants. 3. Synthetic antagonists of ethylene are: volatile olefin, transcycloctene, 2,5-norbornadiene. J. Ethylene mode of action: 1. Ethylene causes an increase in synthesis of enzymes in different target tissues. For example ethylene stimulates cellulase and other cell wall degrading enzymes in the leaf abscission. Different enzymes are produced that aid in fruit ripening and flower senescence in the presence of ethylene. During wounding, presence of ethylene is related to synthesis of phenylalanine ammonia lyase, an enzyme active in phenolic compound synthesis needed for healing. In cells infected with fungi, ethylene stimulates the production of b-1,3-glucanase and chitinase which degrade fungal cell walls. 2. Ethylene appears to stimulate mRNA transcription. 3. Ethylene binds one or more membrane receptor proteins that contain copper at their active sites. VII. Other plant growth substances (not necessarily hormones):
triacontanol, brassins, salicylic acid, turgorin, and polyamines A. Triacontanol: 30-carbon, saturated primary alcohol. Found to increase growth of several agricultural plants. B. Brassins and Brassinosteroids: growth promoters. Act by increasing sensitivity to auxins. C. Salicylic Acid: Active ingredient in aspirin. Promotes cyanide-resistant respiration that causes heat production and volatilization of compounds in Arum lilies which attract insect pollinators. Important in causing resistance to certain plant pathogens (tobacco necrosis virus, and fungi) D. Turgorin: act on the turgor pressure in the pulvinus cells of plants that have nastic movements. E. Polyamines: polyvalent cations containing two or more amino groups. Examples are: putrescine, cadaverine, spermidine, spermine. Involved in promoting cell division, stabilizing membranes and protoplasts, promoting development of fruit, reducing water stress, and delaying leaf senescence. The positively charged amino groups allow them to combine to the phosphates in DNA and RNA, thus controlling transcription and translation. VIII. Other inhibitory plant growth regulators: A. Lunularic acid: found in the gemmae of liverworts as well as many species of lower plants, but not algae. Prevents germination of the gemmae and growth of the thallus as a response to day length. Long days: lunularic acid concentrations are high and growth is slow. Short days: lunularic acid concentrations are low and growth is rapid. B. Batasins: found in yams where it causes dormancy in bulbils (vegetative reproductive structures). C. Jasmonic acid: found in oil of jasmine. Inhibits the growth of plant parts and promotes leaf senescence. IX. Some Unanswered Questions About Plant Hormones A. So much is unknown about internal chemical signals of plants that some argue it is premature to call these growth regulators hormones. B. For example, an increase in Auxin concentration in the zone of elongation has not been detected. C. According to one hypothesis, responses to these growth regulators is due more to changes in the sensitivity of local cells to regulators already present than to the arrival of these regulators from other parts of the plant.