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Plant Physiology

The following web page represents a copy of my notes that formed the basis of lectures given during the first portion of the Biology of Plants (BOT 1103) lecture course.  Please refer to your own notes, handouts, and to the textbook (Stern, K., R., J. E. Bidlack, and S. H. Jansky. 2008.  Introductory Plant Biology, McGraw-Hill. 616 pp. - reading assignments are in the syllabus) for additional information.  This web page does not include information found in various handouts and related materials (e.g., films, charts, overhead projections, etc.) that you will receive during the course of the semester.  You will be evaluated over this information as well.  If you note any errors in the following document, I'd appreciate it if you would bring this to my attention.  Email address: mhuss@astate.edu.

  • ENERGY AND METABOLISM

    Metabolism - controlled capacity to acquire and use energy. 

    Energy - the capacity to do work. 

    Potential energy - stored energy. 
    Kinetic energy - energy of movement. 
    Heat - energy of random motion - not available for doing work. 
    Both potential and kinetic energy are available to do work, in other words, they are both forms of free energy. Free energy is the energy in a system available for doing work.  Heat is a loss of free energy in the sense that it can not do work. 

    EXAMPLES: 
  • Water running out of a dam runs a turbine to make electricity or run a mill.
  • In the covalent bonds of a glucose molecule. 
  • In an electron excited to a higher "orbit" or energy state by being struck with a photon of
    sunlight. 

    First Law of Thermodynamics - Law of Conservation of Energy - total energy in the universe is constant. Energy can be transformed from one form to another but not destroyed. 

    Second Law of Thermodynamics - In the universe as a whole the total amount of free energy is declining (e.g., batteries run out of juice, a clock winds down). Free energy is being converted to heat. Increasing randomness or disorder = ENTROPY. 

    Each time there is an energy transformation, some free energy is lost as heat. 

    CHEMICAL REACTIONS AND ENERGY 

    Endergonic processes or reactions (not spontaneous). For example: 

    CO2 + water + energy ==> glucose + oxygen.

    Exergonic processes or reactions (spontaneous) For example: 

    glucose + oxygen ==> CO2 + water + energy

    Activation energy needed to get many kinds of chemical reactions going (e.g., sparks in spark plugs of an engine; heat and flames for firewood or charcoal briquettes). 

    Catalyst - lowers the activation energy without being consumed. 

    Enzymes - biological catalysts (protein). Protein has an active site - physical location on the
    surface of the enzyme where substrates bind to the protein. 

                            E + S ======> ES =====> E + P

    Lock and Key hypothesis - "older view of enzymatic action" 

    Induced-fit hypothesis - enzyme undergoes conformational changes in the course of bonding with the substrate, which improves the fit and makes ES more reactive. 

    Cofactors - large nonprotein organic molecules that function as coenzymes (e.g., vitamins).
    Examples of nonprotein cofactors involved in electron and hydrogen ion transport are: 

    NAD+ + H+ + 2 e- ------> NADH nicotinamide adenine dinucleotide 
    NADP+ + H+ + 2 e--------> NADPH nicotinamide adenine dinucleotide phosphate 
    FAD+ + 2H+ + 2 e-- ------> FADH2 flavin adenine dinucleotide 

    Another group of molecules involved in electron transport are cytochromes. Cytochromes -
    contain metals (Fe, Cu) and generally function as carrier proteins bound to membranes. 

    CHEMICAL REACTIONS ARE CONTROLLED IN LIVING SYSTEMS 

    enzymes - mediate chemical reactions 
    energy transfers are done in steps 

    Example: There are two ways to get from the top of a very tall building to the bottom floor. Keep in mind that a person on the top floor of a building has a lot of potential energy relative to the ground level. 
  • Jump out the window! 
  • Take the stairs and expend the potential energy a little at a time until you get to the
    bottom floor. 

    Both methods get you to the bottom floor but one method is destructive, while the other is not. These two situations are analogous to uncontrolled vs. controlled energy transfers. Potential energy transferred gradually so more work is done than heat. 

    In organisms, electron transport systems "intercept" excited electrons and make use of the energy release. Series of oxidation (removal of electrons)/reduction (addition of electrons) reactions, usually across a membrane. 

    ATP (adenosine triphosphate) delivers or picks up energy for almost all metabolic pathways. 

                        ATP ===> ADP + Pi + 7.3 kilocalories of energy

PHOTOSYNTHESIS

Autotrophs - "self feeder". Examples include photosynthetic organisms (cyanobacteria, green algae and plants). 

Heterotrophs - "other feeder". Examples include bacteria, some protists (e.g., amoebae, paramecia, slime molds), fungi, non-photosynthetic parasitic plants (dodder, Indian pipe) and animals. 

Metabolism - ability to acquire and use energy. 

Each time an energy transfer takes place, some energy is lost in the form of heat. 
 
Why are plants green? The presence of the pigment chlorophyll (a and b) in the thylakoid makes the chloroplast appear green in color. Sunlight, within the visible spectrum of the electromagnetic spectrum, is composed of different wavelengths or colors of light. When light hits a leaf, some of the light is absorbed, some passes through the leaf (like light through a filter), and the rest is reflected away. The reason that leaves are green is due to the fact that colors like red and blue are absorbed, while green and yellow are reflected back to you. When light lands on the retina of your eye, your brain perceives this as the color green. This phenomenon also explains why we see any object as being one color versus another.  Objects that are red reflect the red wavelengths of light and absorb all other wavelengths; objects that are black reflect little light back, most light is absorbed; while those objects that appear to be white reflect most of the light away, little light is absorbed. 

Sunlight contains energy, which under the right conditions can be converted to other forms of energy: e.g., chemical bond energy, electricity, kinetic energy, heat, etc. 

Light possesses both wave-like and particle-like properties. Packets of energy called photons. The shorter the wavelength of light, the more energy it contains per photon. The longer the wavelength of light, the less energy it contains per photon. Wavelengths of light are measured in very small units called nanometers. 

Photosynthesis - a set of enzyme-mediated reactions in which light energy from the sun is
converted into the chemical bond energy of found in glucose and ATP. 

The overall chemical reaction for photosynthesis is: 

                6CO2  + 12H2 O + sunlight energy =====> C6 H12 O6  (glucose) + 6H2 O + 6O2 

In eukaryotes, the organelle associated with photosynthesis is the chloroplast. Chlorophylls as and b, the light absorbing pigments (along with accessory pigments like the carotenoids) are embedded in the thylakoid membrane that form the grana and membrane system (thylakoid interior - pH of 5.0) within the chloroplast. The grana and thylakoid membrane system are surrounded by the stroma (pH of 8.0). 

Photosynthesis proceeds in two major steps: (1) During the light dependent reactions [photchemical reactions], light energy is captured by pigments in chloroplasts and water is split to yield hydrogen ions, electrons, and oxygen. When light strikes the electrons of the magnesium atom, it gets excited. Electrons are jumped to a higher energy level and the energy transferred by fluorescece (via photons of longer wavelengths and less energy) from one pigment molecule to the next in an antennae complex until it is "captured" at the reaction center. This energetic electron is funneled to electron transport system imbedded in the thylakoid membrane of the chloroplast. The electrons originate from the breakdown of water (photolysis), which liberates hydrogen ions and oxygen. The electrons move from one electron acceptor to another in the thylakoid membrane, starting at Photosystem II (P680) and moving through Photosystem I (P700). The energetic electrons and hydrogen ions are used to generate ATP (by noncyclic photophosphorylation and chemiosmosis - hydrogen passing through the channel protein, ATPase or ATP synthase) and NADPH (from NADP+). The energy originally in light, is now present in the chemical bonds associated with ATP and NADPH. The aforementioned method (noncyclic photophosphorylation) occurs in eukaryotic plants - algae, mosses, ferns, conifers, and flowering plants. 

(2) The light dependent reactions [biochemical reactions] are associated with the thylakoid membrane system. The energy in ATP and NADPH is used during the light independent reactions, to "fix" the carbon in CO2  by producing a more complex carbohydrate. The series of enzyme-mediated chemical reactions responsible for the capture and incorporation of "gaseous carbon" into "solid" carbon compounds is called the CALVIN CYCLE (also referred to as C3 photosynthesis). The enzyme that initially starts off the process by reacting with CO2 is called ribulose bisphosphate carboxylase. This is the most abundant naturally-occurring protein produced on the earth.  Know what PGA, GAL3P, and RuBP stand for! 

In leafy plants, glucose is used to stored as sucrose or starch. Glucose is transported through phloem (food-conducting vascular tissue). Another type of vascular tissue found in the leaves, stems and roots is the xylem, which conducts water. Plants have a body design that enhances and coordinates the process of photosynthesis. Above ground, these plants are highly branched to increase the surface area for the absorption of sunlight and CO2. Below ground, plants have a highly branched root system to anchor the plant and increase the surface area for the absorption of water. Glucose and the stored chemical energy it contains are used to build more complex organic compounds (e.g., polysaccharide, lipids, proteins, nucleotides). Energy, when needed (e.g., germination of seeds and growth, at night when no photosynthesis is possible, in underground roots), contained in glucose is liberated by mitochondria through aerobic respiration . Other organisms take advantage of the food produced by producers, such as plants to meet their own energy needs. 

Photorespiration, C4, and CAM Photosynthesis

Leaves are organs of photosynthesis - responsible for harvesting light and carbon dioxide.
Stomata open to allow for gas exchange (carbon dioxide in; water and oxygen out). Food is
transported out of the leaf through the vascular system. 

When weather is cool and water is readily available, the stomata open, causing concentrations of
carbon dioxide to go up while oxygen levels go down. 

        High CO2 /low O2  ===> CO2  + RuBP (5 carbons) is converted by ribulose bisphosphate
        carboxylase to 2 molecules of PGA (3 carbons). 

When days are hot and dry, stomata close causing concentrations of CO2 go down, while oxygen goes up. 

        Low CO2 /high O2  ===> O2  + RuBP yields 1 PGA (phosphoglycerate) and phosphoglycolic acid. 

In the mitochondria, phosphoglycolic acid is hydrolyzed to glycine (2 carbon amino acid). Two glycine molecules combine forming CO2 (which may or may not be captured by RuBP carboxylase) and serine (3 carbon amino acid). Serine is converted to PGA at the expense of ATP) which then enters the Calvin cycle. THIS PROCESS IS REFERRED TO AS PHOTORESPIRATION. Oxygen inhibits photosynthesis, because RuBP carboxylase can act as an oxygenase - Warburg effect. Energy in the form of ATP and carbon are squandered. 

C4 photosynthesis evolved to compensate for the negative effects of photorespiration. 

Krantz Anatomy - veins encased by thick-walled photosynthetic bundle-sheath cells that are in turn surrounded by tin-walled mesophyll cells (common in grasses, like cereal grains and
sugarcane). 

In the mesophyll cell, PEP carboxylase causes CO2 to combine with phosphoenolpyruvate (PEP) to make oxaloacetic acid (OAA). This 4 carbon compound moves into the bundle sheath cell, where carbon dioxide is dumped and moves into the Calvin cycle. C3 photosynthesis proceeds as normal but in an environment saturated with carbon dioxide with minimal oxygen. Food is dumped into the phloem, which is near the site of production, another advantage to these types of plants. 

Advantages of C4 photosynthesis over C3 alone. 

PEP carboxylase does not combine with oxygen, so this molecule does not interfere with the harvesting of CO2.  Rubisco is insulated from high concentrations of oxygen, because it is only produced in the bundle sheath cells, and not in the mesophyll.   C4 plants are more efficient in their use of nitrogen, because they don't need as much Rubisco as C3 plants.  C4 photosynthesis is an adaptation against photorespiration that is common in hot dry environments and it appears to have evolved independently in at least 17 angiosperm families (including dicots and monocots). Common examples include, corn, sorghum, sugarcane, millet, and pigweed. C4 photosynthesis is only more efficient than C3 plants in conditions that are hot, bright, and dry. 

Crassulacean Acid Metabolism (CAM) 

Crassulacean Acid Metabolism (CAM) - unusual photosynthesis first discovered in a member of the Crassulaceae (a group of succulent plants). Malic acid stored in vacuoles, because pH drops to as low as 4 would otherwise damage the cell. 

CO2 is harvested at night and stored in vacuoles as malic acid (4 carbon acid). During the day CO2 is liberated into the chloroplast where it enters the Calvin cycle. CAM photosynthesis is an extreme adaptation to prevent water loss while maximizing photosynthesis. Common in desert and tropical plants, including Kalanchöe, pineapple, Spanish moss, two genera of ferns, cacti, orchids, and Agave. 

PLANT GROWTH REGULATORS (PLANT HORMONES)

  • Internal and external signals that regulate plant growth are mediated, at least in part, by plant growth-regulating substances, or hormones (from the Greek word hormaein, meaning "to excite").

  • Plant hormones differ from animal hormones in that: 

  • No evidence that the fundamental actions of plant and animal hormones are the same.

  • Unlike animal hormones, plant hormones are not made in tissues specialized for hormone production. (e.g., sex hormones made in the gonads, human growth hormone - pituitary gland) 

  • Unlike animal hormones, plant hormones do not have definite target areas. 

  • Animal hormones usually have specific effects, while those of plants seldom, if ever, have specific effects (e.g., auxins stimulate adventitious root development in a cut shoot, or shoot
    elongation or apical dominance, or differentiation of vascular tissue or etc.). 

  • HORMONES ARE NECESSARY FOR, BUT DO NOT CONTROL, MANY ASPECTS OF PLANT GROWTH AND DEVELOPMENT. - BETTER NAME IS GROWTH
    REGULATOR. 

  • THE EFFECT ON PLANT PHYSIOLOGY IS DEPENDENT ON THE AMOUNT OF
    HORMONE PRESENT AND TISSUE SENSITIVITY TO HORMONE 

  • THE EFFECT ON PLANT PHYSIOLOGY IS DEPENDENT ON THE AMOUNT OF
    HORMONE PRESENT AND TISSUE SENSITIVITY TO HORMONE 

FIVE MAJOR CLASSES OF PLANT HORMONES

  • AUXIN (cell elongation)
  • GIBBERELLIN (cell elongation + cell division - translated into growth) 
  • CYTOKININ (cell division + inhibits senescence) 
  • ABSCISIC ACID (abscission of leaves and fruits + dormancy induction of buds and
    seeds)
  • ETHYLENE GAS (promotes senescence, epinasty, and fruit ripening) 
     

1. AUXIN => INDOLE-3-ACETIC ACID (IAA) 

  • First plant hormone discovered.    
  • During the 1870's, Charles and Francis Darwin studied coleoptiles (primary shoot) of canary grass and oats, both of which grow toward unidirectional light (phototropism). In 1881, the Darwins published their findings in a book entitled The Power of Movement of Plants. Conclusion: The growth of coleoptiles toward light is somehow controlled by the tip of the coleoptile. 
  • Peter Boysen-Jensen (1913) - Cut tips of coleoptiles and placed an intervening piece of agar between the cut tip and the cut coleoptile. The "influence" passed through the agar and reestablished the phototropic response.  He also concluded that "influence" was a water soluble substance, because "butter-barriers" (water soluble substance could not pass through) and metal or mica barriers (not an electrical response) did not allow the response to take place. 
  • Arpad Paál (1919) - Asymmetrical placement of cut tips on coleoptiles resulted in a bending of the coleoptile away from the side onto which the tips were placed (response mimic the response seen in phototropism). 
  • Frits Went (1926) - Demonstrated the diffusion of some chemical from the tips of coleoptiles into agar. Agar allowed to take up "influence" from cut tips, then tips discarded, and blocks placed on cut coleoptiles, response initiated. 
  • Phototropic response due to chemical produced by the tip, and not the physical presence of the tip. Went named this chemical auxin (from the Greek word auxein, meaning "to grow").  Auxin influences phototropism: light striking one side of a coleoptile causes auxin the migrate to the shaded side of the coleoptile, where it stimulates growth and causes growth toward light. 
  • The first auxin to be identified chemically was isolated from human urine. Functions of
    auxins in animals is not known. 
  • IAA - naturally-occurring auxin 2,4-D (2,4-dichlorophenoxyacetic acid) and NAA
    (naphthaleneacetic acid) - synthetic auxins. 
  • Synthetic auxins have been used as herbicides for killing dicot plants. Grasses are
    monocots and symptoms are not as severe. 
  • Agent Orange - 1:1 ratio of 2,4-D and 2,4,5-T used to defoliate trees in Vietnam War.
    Dioxin usually contaminates 2,4,5-T, which is linked to miscarriages, birth defects,
    leukemia, and other types of cancer. 
  • Auxin manufactured by parenchyma cells of the cortex, pith, and vascular tissues.  Moves through the tissue polarly from the roots up into the stem (acropetal - towards the shoot tip), and the tip of the shoot toward the base of the stem (basipetal). 

    EFFECTS OF AUXIN 
  • Proper functioning of auxins depends on its interaction with calcium ion. What consequences would calcium deficiencies have on a plant? Calcium stored in the vacuoles. Auxins produced by the plant would be less effective, if calcium ions are lacking. Calcium activates calmodulin, a protein, which regulates many processes in plants, animals, and microbes.
  • Acid-growth hypothesis - Cellular elongation in grass seedlings and herbs occurs because cell walls are loosened by the activation of enzymes and drop in pH mediated by presence of auxin. 
  • Apical dominance - Axillary buds at the base of their petioles are inhibited near the shoot apex, but less so farther from the tip. Apical dominance is disrupted in some plants by removing the shoot tip, causing the plant to become bushy instead of cone-shaped (like a Christmas tree). 
  • Abscission - shedding of leaves during senescence (aging and death) 
  • Auxin interacts with other hormones (cytokinin, gibberellins) to prevent senescence. As auxin levels drop, senescence begins, and production of ethylene and abscisic acid causes the formation of an abscission zone (suberized cells, cellulase and pectinase). Deciduous plants lose their leaves every fall, while evergreens retain their leaves for 2 or more years. Leaves (needles) on evergreens are not dropped all at once. 
  • Differentiation of vascular tissue - Vascular cambium is activated in the spring by auxin produced by young developing leaves. 
    • High auxin/low gibberellin promotes differentiation of xylem. 
    • Low auxin/low gibberellin promotes differentiation of phloem. 
    • Nonhormonal factors + auxin  - Auxin + small amounts of sucrose (2%) favors differentiation of xylem.   Auxin + moderate amounts of sucrose (3%) favors differentiation of xylem and phloem.  Auxin + large amounts of sucrose (4%) favors differentiation of phloem. 
  • Fruit development - Seeds produce auxins which stimulates fruit development and ripening. Parthenocarpic (seedless) fruits can be produced in tomato and cucumbers. The achenes of strawberry produce auxins, without these the receptacle of this aggregate fruit will not expand. That's why there are "no seedless" strawberries. 
  • Adventitious roots - Cuttings can be stimulated to produce adventitious roots by dipping them in auxin (powder or mixed as a lanolin paste). 

    2. GIBBERELLINS 
  • In 1930's, Ewiti Kurosawa and colleagues were studying plants suffering from bakanae, or "foolish seedling" disease in rice. Disease caused by fungus called, Gibberella fujikuroi, which was stimulating cell elongation and division. Compound secreted by fungus could cause bakanae disease in uninfected plants. Kurosawa named this compound gibberellin. 
  • Gibberella fujikuroi also causes stalk rot in corn, sorghum and other plants. Secondary metabolites produced by the fungus include mycotoxins, like fumonisin, which when ingested by horses can cause equine leukoencephalomalacia - necrotic brain or crazy horse or hole in the head disease. Considered to be a carcinogen. 
  • Many different types of gibberellins - many are probably inactive precursors of active forms of gibberellins (or gibberellic acid). 

    EFFECTS OF GIBBERELLINS 
  • Stimulation of shoot elongation by GA vs. auxin 
  • GA controls internode elongation in mature regions of trees, shrubs, and a few grasses,
    whereas IAA regulates elongation in grass seedlings and herbs. 
  • GA induces cellular division and cellular elongation; auxin induces cellular elongation alone.
  • GA-stimulated elongation does not involve the cell wall acidification characteristic of IAA-induced elongation 

            TAKE HOME MESSAGE - Different hormones can elicit similar effects via different mechanisms. 

  • Flowering - Phenomenon of bolting (rapid elongation of stem during flowering process of rosette [short internodes, leaves pack together like a cabbage]. Bolting is common in biennials like radish, cabbage, and carrot. 
  • Seed Germination - Especially in cereal grasses, like barley. Not necessarily as critical in dicot seeds. 
  • Gibberellins can break dormancy or substitute for a cold treatment in some plants (e.g., tobacco, lettuce, oats). 
  • Fruit Formation - "Thompson Seedless" grapes grown in California are treated with GA to increase size and decrease packing. 

    3. CYTOKININS 
  • Gottlieb Haberlandt in 1913 reported an unknown compound that stimulated cellular division. 
  • In the 1940s, Johannes van Overbeek, noted that plant embryos grew faster when they were supplied with coconut milk (liquid endosperm), which is rich in nucleic acids. 
  • In the 1950s, Folke Skoog and Carlos Miller studying the influence of auxin on the growth of tobacco in tissue culture. When auxin was added to artificial medium, the cells enlarged but did not divide. Miller took herring-sperm DNA. Miller knew of Overbeek's work, and decided to add this to the culture medium, the tobacco cells started dividing. He repeated this experiment with fresh herring-sperm DNA, but the results were not repeated. Only old DNA seemed to work. Miller later discovered that adding the purine base of DNA (adenine) would cause the cells to divide. 
  • Adenine or adenine-like compounds induce cell division in plant tissue culture. Miller, Skoog and their coworkers isolated the growth facto responsible for cellular division from a DNA preparation calling it kinetin which belongs to a class of compounds called cytokinins. 
  • In 1964, the first naturally occurring cytokinin was isolated from corn called zeatin. Zeatin and zeatin riboside are found in coconut milk. All cytokinins (artificial or natural) are chemically similar to adenine. 
  • Cytokinins move nonpolarly in xylem, phloem, and parenchyma cells. Cytokinins are found in angiosperms, gymnosperms, mosses, and ferns. In angiosperms, cytokinins are produced in the roots, seeds, fruits, and young leaves. 
  • For historical overview of cytokinin, see the following article:  1955 Kinetin Arrives.pdf.  Article courtesy of Richard Amasino.  Dr. Amasino is a Distinguished Professor of Biochemistry at the University of Wisconsin-Madison.

    EFFECTS OF CYTOKININS 
  • Cellular Division - Cytokinins hasten the transition of cells from the G2 phase to mitosis. This effect requires the presence of auxins. 
  • Effects on cotyledons - Cytokinins promote cellular division and expansion in cotyledons. Increases cell wall plasticity but does not involve wall acidification (as the case with auxins). 
  • Organogenesis 
    • Cytokinins and auxin affect organogenesis. 
      • High cytokinin/auxin ratios favor the formation of shoots. 
      • Low cytokinin/auxin ratios favor the formation of roots. 
  • Senescence 
    • Cytokinins delay the breakdown of chlorophyll in detached leaves by preventing the genes that produce chlorophyll from being turned off.
  • The presence of calcium ions Ca2+ increases the sensitivity of tissues to cytokinins.  


4. ETHYLENE 

  • In the 1800s, it was recognized that street lights that burned gas, could cause neighboring plants to develop short, thick stems and cause the leaves to fall off. In 1901, Dimitry Neljubow identified that a byproduct of gas combustion was ethylene gas and that this gas could affect plant growth.
  • In R. Gane showed that this same gas was naturally produced by plants and that it caused faster ripening of many fruits.
  • Synthesis of ethylene is inhibited by carbon dioxide and requires oxygen. 
  • Fruit ripening 
    • Only climacteric fruits (tomato, apple, banana, orange) respond to ethylene, which is
      characterized by increased rates of respiration, softening of fruit tissue, changes in sweetness and color. 
  • Flowering 
    • Ethylene inhibits flowering in most species, but promotes it in a few plants such as pineapple, bromeliads, and mango. Spraying plants with ethephon, a compound that breaks down into ethylene, using fires or presence of ripening fruits can stimulate this process. 
  • Abscission The increased production of ethylene at the abscission zone triggers the breakdown of the middle lamella, and thereby initiates abscission in leaves and fruits. 
  • Sex Expression 
    • Ethylene and gibberellins can determine the sex of a flower. Cucumber buds treated with ethylene become carpellate (female) flowers, whereas those treated with gibberellins become staminate (male) flowers. 

  • Stem Elongation 

    Shaking a plant increases ethylene production which in turn causes thigmomorphogenesis. The plants form short thick stems. Effect is mediated by ethylene.  
  • Roots and shoots
    • Ethylene buildup in the roots, stimulates production of pectinases and cellulase which helps create many intercellular spaces characteristic of hydrophytes. Even though ethylene production is reduce in waterlogged (anaerobic) soil, what little that is produced is trapped and elicits the above response. In shoots, ethylene causes parenchyma cells on the upper side of the petiole to expand and point the leaf down (epinasty).

5. ABSCISIC ACID (ABA) 

  • In 1940s, scientists started searching for hormones that would inhibit growth and development, what Hemberg called dormins. In the early 1960s, Philip Wareing confirmed that dormin to a bud would induce dormancy. F.T. Addicott discovered that this substance stimulated abscission of cotton fruit. he named this substance abscisin. (Subsequent research showed that ethylene and not abscisin controls abscission).  Abscisin is made from carotenoids and moves nonpolarly through plant tissue. 
  • Closure of Stomata 
    • During drought, leaves make large amounts of ABA which causes stomata to close.  Potassium ions leaving guard cells cause water move by osmosis as well.  Cells lose turgor pressure causing the stomata to close. 
  • Bud and Seed Dormancy 

    ABA induces bud and seed dormancy in many plant species and may work in conjunction with other plant hormones (IAA and cytokinins). 

    ABA counteracts the stimulatory effects of other hormones. These effects may occur because ABA is a calcium antagonist. 

    HOW PLANTS RESPOND TO ENVIRONMENTAL STIMULI

    I. Tropisms - plant growth toward or away from a stimulus such as light or gravity. 

    PHOTOTROPISM 

    Blue light less than 500 nm in wavelength induces phototropism. IAA accumulates on the shaded side of the shoot and causes elongation of cells. Consequently the shoot bends toward the light. 

    GRAVITROPISM 

    Charles and Frances Darwin discovered that plants require the root cap in order to respond to gravity. Sedimentation of starch-laden amyloplasts in root cap cells without amyloplasts are still able to respond to gravity. Role of amyloplasts uncertain in perception of gravity. Role of amyloplast uncertain in perception of gravity. Ca    ++ and auxin are involved in bending of roots toward gravity. Ca++ and IAA inhibits cell elongation in roots. So up the side of the root where IAA does not accumulate, elongates and causes the root to bend towards gravity. Roots are positively gravitropic and grow downward, whereas stems are negatively gravitropic and grow away from the pull of gravity. 

    HYDROTROPISM - mediated by the root cap, movement of roots toward water in soil. 

    THIGMOTROPISM - growth response to touch, tendrils (garden peas and beans) and stems (bindweed and morning glory) wind around on object they are using for support. IAA and ethylene are involved. 

    II. Nastic Movements - response to environmental stimuli that are independent of the direction of the stimulus. Pre-determined response. 

    SEISMONASTY - a nastic movement resulting from contact or mechanical shaking. 

  • Sensitive plant - Mimosa pudica, touching the leaf causes the leaflet to fold and the petiole to droop.  Touching the leaf initiates an electrical signal that moves along the petiole.  Cells become more permeable to K+ and other ions. Parenchyma cells (motor cells) are located in a joint-like structure (pulvinus) located at the base of each leaflet and the petiole.  Movement of ions out of motor cells decreases the water potential in the surrounding extracellular space via osmosis.  Loss of water causes the cells to shrink creating movement. 
  • VENUS'S FLYTRAP. 

    Three hairs located on the surface of each of the trap leaves. An insect triggering the hairs causes an electrical signal to travel into the modified leaf tissue. Hydrogen ions are transported to cell walls of the outer epidermal cells along the outside of the hinge. These acidified cells enlarge. The epidermal cell walls along the inner side of the hinge are not acidified and do not enlarge. The resulting tension causes the trap to close. 
  • Alternate hypothesis suggests that mesophyll cells of trap are compressed, causing the trap to be under tension. The tension is released when the insect touches the hair triggers. 

    NYCTINASTY - sleep movements: prayer plant - lower leaves during the day and raises leaves at night; Oxalis (shamrock) and legumes.

    SEASONAL RESPONSES OF PLANTS TO THE ENVIRONMENT
    Photoperiodism - (ability of plants to measure seasonal changes in daylength). 

    Flowering in many plants is induced by length of day and night.

    Day-neutral plants flower with no regard to photoperiod. 

    Short-day plants flower only if light periods are shorter than some critical length. Ragweed plants flower when exposed to 14 hours or less of light per day. Asters, poinsettias, potatoes, and goldenrods (FALL PLANTS). [LONG-NIGHT PLANTS] 

    Long-day plants flower in the spring or early summer; they flower only if light periods are longer than a critical length, which is usually 9-16 hours. Lettuce, radish, gets, clover, corn, and iris  [SHORT-NIGHT PLANTS] 

    Interrupt the dark period of a long night, for as little as 1 minute and a long night plant will not flower, but a short night plant will flower.   Phytochrome appears to be the light receiving pigment in the leaves that controls photoperiod. Phytochrome exists in two interconvertible forms, Pr  (inactive form) and Pfr (active form).  Sunlight contains more red light than far-red light. 

    Pr ===> red light ===> Pfr (promotes flowering in short night plants, but inhibits flowering in long night plants) 

    Pfr ===> far-red light (more slowly converted in dark) ===> Pr 

    Phytochrome responses + internal clock regulate photoperiod and flowering. 

    Florigen - hypothetical hormone that is produced by leaves and induces a flowering response.
    Evidence exists that it exists, but no one has ever isolated it. 

    Phytochrome also is important in seed germination in some plants. Lettuce and weed seeds.  Presence of Pr inhibits germination, while its conversion to Pfr in red light induces germination. 

    Red light ===> germination 
    Far-red light ===> no germination 
    Red ===> far-red ===> red ===> germination 
    Red ===> far-red ===> red ===> far-red ===> no germination 

    Those seeds not buried deep in the ground get exposed to red light, and this signals germination. 

    Etiolated seedlings convert to normal stem growth when grown in the light and not the dark.  Response is mediated by phytochrome. Pr converts to Pfr and normal growth is restored. 

    Final note: Photoperiod also controls dormancy and leaf senescence in many plants. 

    CIRCADIAN RHYTHMS 

    Regular daily rhythms are called circadian rhythms. Opening of flowers, folding of leaves, stomatal opening, protein synthesis, secretion of nectar, hormone synthesis cellular division etc... 

    Biological clock of a plant is probably set by phytochrome, and rhythmic changes in transport and metabolism in cells due to daily activities. 

Genetic Engineering and Bioengineering

  • Transformation  - introduction of a new gene into a bacterium, fungus, plant or animal by some mechanism. (i.e., bombardment, electroporation, virus- or bacteria-mediated gene transfer).
Transgenic organisms are produce by:
  1. Addition of new genes (to express a new gene product).
  2. Suppression of a gene or a gene product.
Potential benefits of transgenic organisms (GMOs - Genetically-modified organisms)

Genetic engineering can produce organisms that are:

  • able to synthesize oils, starches, hormones (e.g., bacteria that produce human insulin for use by diabetics; plants can synthesize special proteins) and bioplastics
  • edible vaccines from vegetables and milk
  • able to synthesize enzymes for food processing and other uses
  • more nutritious foods (e.g., plants with a higher protein content, and wider profile of essential amino acids - methionine-rich beans or lysine-rich corn; golden rice to help enrich the diets of those not able to eat foods rich in beta-carotene or Vitamin A to prevent blindness caused by a nutritional deficiency)
  • plants able to fix their own nitrogen for growth
  • freeze resistant plants
  • pest resistant plants
  • herbicide resistant plants
  • disease resistance in animals and plants
  • gene therapy to help cure certain diseases linked to the under or over abundance of a protein product by a genetic disorder.

Potential problems

  • Allergies to transformed plant and animal products.
  • Accidental movement of novel genes into wild relatives from domesticated plants and animals.
  • Consumer resistance to using genetically-modified products, especially food and drugs.
  • Ethical and moral considerations. (e.g., exploitation of genetic resources for personal gain).

EXAMPLES OF GENETIC ENGINEERING  

Bioluminescent Tobacco (Are Christmas Trees Next? - http://www.biocrawler.com/encyclopedia/Bioluminescence)

From http://www.biocrawler.com/encyclopedia/Image:Glowing_tobacco_plant.jpg:

"An image of a tobacco plant (SEE BELOW) which has been genetically engineered to express a gene taken from fireflys (specifically: Photinus pyralis) which produces luciferase. The image is an "autoluminograph" produced by placing the plant directly on a piece of Kodak Ektachrome 200 film. When the plant is watered with a luciferin containing nutrient medium, tissue specific luminescence is observed. This image was first published in a November 1986 issue of the journal Science in a paper titled "Transient and stable expression of the firefly luciferase gene in plant cells and transgenic plants". [1] (http://www.pgec.usda.gov/Ow/A10-1986OwetalScience.pdf) by David W. Ow, Keith V. Wood, Marlene DeLuca, Jeffrey R. de Wet, Donald R. Helinski and Stephen H. Howell. The research was funded by grants from the US Dept. of Agriculture and the National Science Foundation."

For more information on Genetically-Modified Plants visit:

This web page was assembled by Dr. Martin J. Huss - Last modified on November 6, 2007