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Internal Control: Plant Hormones

Unlike animals, plants can manufacture all the chemical substances they need for survival, given only light, water, carbon dioxide and trace elements from the soil. They manufacture all the amino acids, proteins, carbohydrates, nucleic acids and other compounds , and among these are hormones (from the Greek horman, meaning "to stimulate").

By definition (originally from animals), a hormone

In plants, some hormones operate in the same tissues in which they are manufactured, and others are transported for use to different locations.

Plants don't have glands to produce hormones: various tissues throughout the plant body produce hormones.

Recall the generalized model for a hormone-triggered signal transduction pathway:

  • a hormone binds to a specific protein receptor, either embedded in the plasma membrane or in the cytoplasm (depending on the receptor and the system).
  • binding of the hormone to the receptor causes the protein's conformation to change
  • this stimulates the production of "relay molecules" in the cytoplasm
  • relay molecules trigger various responses to the original signal

    A given plant hormone may:

    The sensitivity of a plant tissue to a given hormone may be altered either by


    Meet the Hormones

    Mature and growing plants' growth and development is governed primarily by five major classes of HORMONES


    In addition to the five hormone classes above, other chemical substances exhibiting hormone-like activity have more recently been discovered.

  • brassinosteroids

  • salicylates and jasmonates

  • polyamines
  • systemin
      A small polypeptide (18 amino acids long!) that acts as a transcription factor (an inducer) to stimulate production of at least 15 different genes that code for protective chemical compunds. Systemin is transported from the site of herbivore damage to other tissues where it induces gene transcription of defense factors, including jasmonate.

  • Nitric Oxide (NO)


    The Original Big Five

    As mentioned before, the original five classes of hormones (the only ones known for many decades) were... Let's meet them in more detail.

    Auxin

    Darwin & Darwin (1881) did the first experiments to study the effects of the mysterious growth factor.

    Additional experiments by Frits Went (1926) demonstrated that the mystery factor was produced in shoot meristems. Went named the compound auxin (from the Greek auxein meaning "to increase")

    We now know that light causes transport of auxins away from an area exposed to light, and in that area, auxins cause cell elongation.


    The most common naturally occurring auxin is Indole Acetic Acid (IAA), but other forms occur, and many IAA-like compounds are used commercially because they are more stable than natural IAA, which is readily broken down by plant enzymes, fungi and bacteria.

    The compound shown in the middle is the infamous Agent Orange, widely used as a defoliant during the Vietnam War (along with similar compounds "Agent Purple," "Agent Green" and "Agent Pink", collectively known as the "Rainbow Herbicides" and so named for the identifying stripe of color painted on the barrels containing them), was contaminated with and breaks down into highly toxic and carcinogenic dioxin.

    The compound on the far right is the active ingredient in commercial preparations such as "RootTone." It is used to stimulate the growth of adventitious roots and to reduce fruit drop in commercial crops. This synthetic auxin, too, can break down into toxic, carcinogenic compounds.

    Site of Production - primarily leaf primordia, meristems, and maturing seeds, though all plant tissues can produce small amounts of this hormone.

    Transport - IAA is the only plant hormone known to be transported in a polar (i.e., unidirectional) fashion. Transport takes place via the vascular parenchyma cells, and may be

    IAA also may be transported in a non-polar fashion via the phloem conducting cells.

    Effects

  • differentiation of meristem into vascular tissue

  • promotes leaf formation and arrangement

  • induces flowering

  • inhibits leaf abscision

  • inhibition of lateral bud growth via stimulation of a "nutrient sink" in the growing tip (this results in apical dominance).

  • stimulation of pericycle to produce lateral roots (although a high concentration applied to already growing roots will inhibit their growth)

  • fruit development and ripening (from auxins produced by maturing seeds). (Parthenocarpic (seedless) fruit can be induced to ripen with application of IAA, and some mutant commercial forms produce IAA in their tissues, ripening without the aid of seeds.

    Auxin's Mode of Action Revealed
    Until very recently, the exact mechanism of action of auxin was not fully understood. But recent research is helping to solve this long-standing mystery.

    Known:

    1. Short-lived (5-10 minute lifespan), rare, plant proteins known as Aux/IAA proteins are believed to be DNA-binding transcription factors involved in the regulation of secondary (as opposed to primary or early) auxin-response genes.

    2. Aux/IAA proteins are believed to be repressors.

    3. Auxin binds to a specific plasma membrane receptor, T1R1, which is a type of f box protein. Thus bound, it promotes (ubiquitin-dependent) degradation of Aux/IAA repressor proteins.

    4. With repressors removed, auxin-response genes are transcribed and translated, and response to auxin is initiated.

    5. F-box receptors in addition to T1R1 have recently been identified, all of them with an affinity for auxin. (Mutant plants lacking any of these receptors are insensitive to auxin.)


    Parthenocarpy and Stenospermocarpy
    Ever wonder how your banana, pineapple, watermelons, oranges or grapes can develop without seeds? Wonder no more.

    Parthenocarpy - in some species, fruits mature without ovule fertilization. In some species, pollination is required to stimulate fruit production, in others, pollination produces larger, tastier parthenocarpic fruit, and in still others, pollination is not required. (Most seedless citrus must be pollinated; bananas and pineapples need not be.)

    Parthenocarpic fruits do occur in nature, and some species produce both parthenocarpic and seeded fruit in the same crop. Why waste the energy? It could be an important defense against fruigivorous insects.

    Stenospermocarpy - This process requires both pollination and fertilization, because it results from the abortion of the developing embryo in the seed, at which point the seed stops developing. Seedless grapes are a result of stenospermocarpy, and you can see teh remnant of the aborted seed in most seedless grapes.

    Seedless grapes are usually smaller than seeded grapes because the developing seeds produce another hormone, gibberellin, that promotes the fruits increase in size and sugar content. By spraying seedless grapes with gibberellins artificially, the farmer can produce large, marketable grapes.


    Cytokinins

    The cytokinins were first discovered in the liquid endosperm of coconuts (coconut water, not coconut milk) by Johannes van Overbeck in 1941. Applied to growing plant embryos, the mystery substances in the coconut endosperm stimulated growth of both tissues and cells.

    After about another decade of fruitless (har!) research, Folke Skoog was able to purify, but not isolate the substance. Carlos Miller later analyzed a DNA breakdown product that had similar activity to the mystery coconut substance, and this led to the discovery that the substance he found (kinetin) was related to the plant hormones (though probably not found naturally in plants) that were later named cytokinins for their role in promoting cell division. Miller isolated a natural cytokinin from corn and named it zeatin, after the corn genus. It is still the most active and powerful of the known plant cytokinins.

    Where found? - Growing tissues and wounded areas.

    Transport - Acropetal, from root to shoot, via the vascular tissues. Effects

  • delays senescence (loss of chlorophyll) in leaf tissues

  • Interacts with IAA to facilitate plant cell growth and differentiation.


    Ethylene

    The effects of ethylene on plants were known long before Darwin discovered auxin. In the late 1800s, when streets were lit at night with gas lights, it was noted that leaves on trees growing near leaking gas mains lost their leaves. Dimitry Neljubov (1901) demonstrated that the causative agent was a simple hydrocarbon, ethylene:

    Later experiments revealed that ethylene governs a number of plant functions, including Source/Transport - Ethylene's natural precursor is the amino acid methionine, and the biosynthetic pathway is relatively simple:

    Note the interesting things that will stimulate ACC to break down into the active form, ethylene.

    Effects

  • antagonist to IAA, it inhibits cell enlargement in most species

  • triggers stem elongation in many aquatic species (keeps them from drowning!)

  • triggers an increase in air space development in the submerged tissues of mesophytes caught in a flood! (a stop-gap emergency supply of oxygen!). The air spaces are generated by the destruction of parenchyma tissues in the stem cortex.

  • promotes leaf, flower, and fruit abscission by causing degeneration of cells in the abscission zone of these structures. (Auxin will reverse or inhibit this function.)

  • promotes ripening of fruit

  • Family Cucurbitaceae: ethylene is involved in the sex determination in monoecious members of the cucumber family. High [gibberellin] leads to maleness, and high [ethylene] leads to femaleness.


    Abscisic Acid (ABA)

    The "abscisic" part is a misnomer. This hormone is not responsible for leaf, flower or fruit abscission. ABA is, rather, the Hormone of Dormancy.

    Transport - ABA is produced in leaves, root cap and stems. It is transported via xylem, phloem to the target tissues.

    Effects

  • inhibits plant growth: it is the opponent to cytokinin

  • young, developing seeds contain high concentrations of ABA which

  • under water-stress conditions, ABA causes stomatal closure

  • promotes bud scale formation in preparation for dormancy

  • gibberellin reverses the effects of ABA.


    Gibberellins

    In 1926, Japanese pathologist E. Kurosawa was studying a parasitic fungus (Gibberella fujikuroi) that caused "Foolish Seedling Disease." Affected plants would "bolt"--grow pale, long, weak stems very quickly, then topple and die. Kurosawa discovered that it was a substance produced by the fungus that made the plants bolt.

    The substance was isolated in 1934 by chemists T. Yabuta and Y. Sumiki, who named it gibberellin (gibberellic acid, or GA), after the fungus. It was later discovered that almost all plants produce gibberellins, and that they are present in various concentrations in various plant tissues. At least 125 naturally occurring gibberellins have so far been identified.

    Effects

  • Some GAs promote stem elongation, but their mechanism is different from that of auxin; they enhance cell elongation

  • artificial gibberellin application to seeds can replace the "after-ripening" necessary for some species before germination can begin

  • in some species, gibberellins are involved in the mobilization of stored protein and starch in the endosperm for use by the embryo
  • can stimulate parthenocarpic fruit (even in some species in which auxin cannot, such as mandarin oranges, almonds, and peaches).

  • involved in promoting flowering in some species


    As mentioned at the start of this lecture, most hormones operate at the molecular level by acting as transcription factors or by affecting transcription factors, and hence, DNA transcription and translation.

    As you'll learn in Genetics (if you haven't already), it's All About RNA. Stay tuned.