PLANT HORMONES

Mature and growing plants' growth and development is governed primarily by HORMONES, which come in five basic "flavors:"

  • auxins - responsible for cell division and growth in cell size
  • cytokinins - responsible for increase in cell division
  • ethylene - responsible for senescence (aging) processes
  • abscisic acid - responsible for dormancy of various types
  • gibberellins - responsible for cell division and growth in cell size

    AUXIN - the first plant hormone discovered


    Darwin & Darwin did the first experiments to study the effects of the mysterious growth factor. (See diagrams from class!)

    Went did additional experiments and named the mysterious chemical AUXIN from the Greek auxein ("to increase").

    We now know that light causes transport of auxins away from an area which is exposed to light.

    The naturally-occurring auxin is indole acetic acid (IAA).

  • Auxin is responsible for cell elongation - inhibits lateral buds, which is why apical meristems exhibit APICAL DOMINANCE.

  • Auxin transport is...

    A. Polar (unidirectional)

  • probably via cell membrane channels still transport auxin
  • via parenchyma cells of vascular tissue
  • Auxin is responsible for the differentiation of cells into vascular tissue

  • Auxin stimulates the vascular cambium to divide and differentiate
  • Auxin stimulates adventitious root growth (but application to growing roots inhibits growth)
  • Seeds produce auxin, leading to fruit growth, ripening
  • Auxin inhibits leaf abscission

    CYTOKININS

    Naturally occurring "kinetins" eventually were discovered in corn. These are the cytokinins (so named because of their involvement in cytokinesis). Since then, many have been identified.

  • Cytokinins promote mitosis in growing or damaged tissues
  • In the presence of IAA (auxin) and cytokinin, rapid mitosis without differentiation occurs (IAA + cytokinin exists in meristematic tissue)
  • If the concentration of IAA is greater than the concentration of cytokinin, then a root forms
  • If the concentration of IAA is less than the concentration of cytokinin, then a shoot forms

  • Cytokinins delay senescence

  • Cytokinins are produced in roots and travel throughout the plant via xylem

    GIBBERELLINS

    1926 - E. Kurosawa - studied a parasitic fungus (Gibberella fujikuroi) which was found to produce a substance which caused plants to "bolt" (to develop elongated stems)

    There are about 70 naturally-occurring giberellin compounds.

  • Some promote stem elongation, but differently from auxin
  • They stimulate flowering
  • When synthetically applied, they increase fruit size

  • They activate digestive enzymes in embryos
    (embryo enzymes digest endosperm starch, making it available as sugars)
  • PARTHENOCARPIC fruit production (auxin does this, too).

    ETHYLENE

    The effects of ethylene on plants were known long before Darwin discovered auxin!

    1800's - gas leaking from gas mains caused nearby trees to lose their leaves

    Ethylene:

  • is produced at stem nodes, senescing tissues

  • is involved with aging and maturation:

    a. fruit ripening (used commercially to ripen fruit)

    b. leaf senescence

    c. other plant part senescence

  • leaf abscission

  • sex determination in flowers [at least in members of Family Cucurbitaceae (cucumbers, melons, et al.)]

    ABSCISIC ACID (ABA)

    NOTE: The name is a misnomer! This hormon is not responsible for leaf abscission!

  • inhibits plant growth: opponent to cytokinin

  • young seeds have lots of ABA. Results:

    a. production of storage proteins

    b. inhibition of germination

    ...however, seeds may not produce ABA

  • causes stomatal closure by pulling K+ from guard cells. H20 follows and the cells shrink (this occurs when there is water stress)

  • promotes bud scale formation in preparation for dormancy

  • produced in leaf, root cap, stem. Transported via xylem, phloem

  • gibberellin reverses the effects of ABA.

    HOW DO PLANTS KNOW WHEN TO FLOWER?

    Some species bloom in the summer (when days are long), whereas others bloom in the winter (when days are short). Others bloom at times in between. How does each individual "know" how to flower exactly when it's conspecifics are flowering, thus increasing its own chance of pollination?

    What factors might induce plants to flower?

  • temperature?
  • moisture?
  • oxygen?
  • light period?

    Which of these is the most reliable gauge of the season?

    Although temperature, water, and other environmental conditions may vary from year to year, day length is constant!

    The secret to flowering: PHOTOPERIODISM

    Phytochrome is a proteinaceous, blue-green pigment, which can exist as one of two forms (same chemical formula, different physical shape):

  • PR
  • PFR It changes from one form to the other (i.e., it isomerizes) in response to light.

    PR absorbs short red wavelengths (660 nm). It is the more stable form of the two phytochrome isomers. In the absence of light, the pigment will slowly revert to this form if it has been changed to...

    PFR, which absorbs long red wavelengths (730 nm). It is this form of the pigment which elicits a physiological response. This form of the pigment will spontaneously revert to PR, which is the more stable isomer.

    Let's have a look at the system:

    IN ALL PLANTS...

  • In the winter when days are short, [PR] > [PFR]

  • In the summer when days are long, [PR] < [PFR]

    AND...

  • In short-day (i.e., winter flowering) plant species, PFR inhibits flowering.
  • In long-day (i.e., summer-flowering) plant species, PFR induces flowering.

    THEREFORE...

  • In WINTER, PFR concentration is low, the flowering inhibition it exerts in winter-flowering plants is "lifted." They bloom!

  • In SUMMER, PFR concentration is high, the flowering induction it exerts in summer-flowering plants starts working. They bloom!

    There are many "day-neutral" plant species in which light is NOT the factor involved in triggering flowering (the mechanism is unknown)