Plant Hormones Notes

Plant Hormones

  • Organic substances, other than nutrients, that modify plant physiological processes in minute amounts.
  • A group of unrelated chemical substances affecting plant growth, development, and other physiological processes.
  • Produced by a plant and active elsewhere than at the site of production.

Auxin

  • Derived from the Greek word "auxein," meaning "to grow."
  • Synthesized in apical meristems, young leaves, and developing seeds.
  • Polar (Basipetal) Auxin Transport:
    • Directional and active flow of auxin molecules through plant tissues.
    • As a weak acid, its protonation state is dictated by the pH of the environment.
    • Polarity is set up in the cell as efflux carriers are positioned asymmetrically on the plasma membrane, usually located only at the base of plant cells, directing flow towards the roots.
  • Nonpolar Auxin Transport:
    • Occurs through the phloem.
    • Auxin moves in the assimilate that translocates through the phloem from its source (usually the shoot) to a sink (e.g., the root).

Auxin Actions

  1. Tropisms: Direct growth responses to environmental stimuli.

    • Phototropism: Growth in response to light.
    • Gravitropism: Growth in response to gravity.
    • Higher auxin concentration on one side of a stem typically causes that side to elongate; the effect is opposite in roots, where higher auxin concentration inhibits elongation.

  2. Growth and Development:

    • Embryo Development: From the first mitotic division of the zygote, auxin gradients guide the patterning of the embryo into parts that will become the plant's organs (shoot apex, primary leaves, cotyledon(s), stem, and root).
    • Vascular Tissue Differentiation: Auxin controls cell differentiation of vascular tissue.
    • Leaf Development and Arrangement: Accumulation of auxin initiates the formation of new leaves in the apical meristem; already-developing leaves deplete surrounding cells of auxin to prevent new leaves from forming too close, establishing the characteristic pattern of leaves; auxin controls the precise patterning of epidermal cells in developing leaves.
    • Root Initiation and Development: Localized accumulation of auxin in epidermal cells of the root initiates the formation of lateral or secondary roots; stimulates the formation of adventitious roots in many species. Auxin concentration gradient in the region of differentiation promotes elongation and differentiation of root cells. In the tip, it promotes the production of new cells at the meristem. Cytokinin acts as an auxin antagonist.
    • Shade Avoidance: Stimulates cell elongation in parts of plants that have access to light as part of the shade-avoidance response.
    • Interactions with Other Growth-Regulating Hormones: Required for the function of other growth-regulating hormones like cytokinin; cytokinin promotes cell division but only in the presence of auxin.

  3. Apical Dominance: The inhibition of axillary bud (lateral bud) formation is triggered by the downward transport of auxins produced in the apical meristem.

  4. Flowering and Fruit Development: Promotes flowering, fruit setting, and ripening. As seeds mature, they release auxin to the surrounding flower parts, which develop into the fruit that covers the seeds.

  5. Prevention of Abscission:

    • Regulated by interactions between auxin and ethylene.
    • During the growing season, young leaves and fruits produce high levels of auxin, which blocks the activity of ethylene, keeping them attached to the stem.
    • As seasons change, auxin levels decline, permitting ethylene to initiate senescence (aging).

Auxin: Commercial Applications

  • Propagation
  • Fruit Setting and Dropping
  • Herbicides

Cytokinin

  • Derivatives of the purine, adenine.
  • Promote cytokinesis (cell division).
  • Zeatin is an example of naturally occurring cytokinin; kinetin is a synthetic cytokinin.
  • Synthesized in root tips and other young structures where cell division is occurring (e.g., embryos and fruits) and by wounded tissue.
  • Transported through the xylem.

Cytokinin Actions

  1. Seed Germination: The endosperm of monocot seeds (e.g., corn/maize) contains large stores of the precursor to the cytokinin zeatin. When the corn kernel germinates, zeatin moves from the endosperm to the root tip where it stimulates vigorous mitosis.

  2. Plant Development:

    • Involved in leaf formation and delays senescence in leaf tissues.
    • Promotes chloroplast development.
    • Counters the effects of auxin when regulating shoot and root development.
      • Inhibits apical dominance by stimulating axillary bud development.
      • Inhibits the formation of lateral roots, while auxin initiates lateral roots.
    • When cytokinin is applied to a callus (mass of undifferentiated cells), shoots form; if auxin is applied, roots form.

  3. Gravitropism:

    • Similar to that of auxin.
    • When a root is turned on its side, cytokinins accumulate on the lower side, inhibiting elongation there, causing it to bend downwards.

Gibberellin (GA)

  • A growth-promoting substance isolated from cultures of a fungus that parasitizes rice plants.
  • Synthesized in the root and stem apical meristems, young leaves, and seed embryos.
  • Transported through the vascular tissue.

Gibberellin Actions

  1. Shoot Elongation: Results from both cell division and cell elongation. When applied in low concentrations to a bush or "dwarf" bean, the stem begins to grow rapidly, overcoming genetic limitations in many dwarf varieties.

  2. Seed Germination: Breaks dormancy (a state of inhibited growth and development) in the seeds of plants that require exposure to cold or light to germinate.

  3. Bolting: induces bolting by affecting both temperature and day length

Gibberellin: Commercial Applications

  • Fruit Production
  • Bolting and Flowering
  • Early Seed Production

Abscisic Acid (ABA)

  • Response to stressful environmental conditions (dehydration, cold temperatures, shortened day lengths, exposure to salt water or salinated soil).
  • Synthesized in mature leaves and roots.
  • Transported through the vascular tissue.

Abscisic Acid Actions

  1. Maintaining Dormancy:

    • Seed Maturation and Inhibition of Germination: Essential for seed maturation and enforces a period of seed dormancy by blocking germination and promoting the synthesis of storage proteins. Prevents premature germination during unseasonably mild conditions. Mangrove species with viviparous germination have reduced ABA levels during embryo formation.
    • Bud Dormancy: Promotes the development of winter buds; mediates the conversion of the apical meristem into a dormant bud. Developing leaves become stiff bud scales that protect the meristem from mechanical damage and drying out during the winter.

  2. Response to Water Stress:

    • Cellular Protection from Dehydration: Turns on the expression of genes encoding proteins that protect cells (in seeds and vegetative tissues) from damage when they become dehydrated.

Ethylene

  • A smaller and simpler molecule that is a volatile gas.
  • Synthesized in aging tissues (wilting or ripening tissues) and nodes of stems.
  • Transported through the air as a gas.

Ethylene Actions

  1. Fruit Ripening: Stimulates the conversion of starch and acids to sugars. Some store unripe fruit in a sealed paper bag to accelerate ripening, due to the ethylene released.

  2. Abscission: When auxin levels decline, ethylene triggers senescence and programmed cell death at the site of leaf attachment. The abscission layer (abscission zone) forms at the base of the petiole or fruit stalk, comprising a separation layer and a protective layer. Nutrients are absorbed into the stem before abscission. The separation layer breaks down, and the leaf falls; the protective layer, reinforced with suberin, serves as a seal.

Ethylene: Commercial Applications

  • Fruit Ripening
  • Inhibiting leaf dropping by removing ethylene from greenhouses using fans and ventilation.

Other Signaling Molecules

  1. Brassinosteroids:

    • Synthesized primarily in young tissues; do not travel far.
    • Positively influence apical dominance, seed germination, gravitropism, lateral root formation, vascular tissue differentiation, and resistance to freezing.
    • Inhibit root growth and fruit dropping.

  2. Systemin:

    • A short polypeptide distributed systemically upon production.
    • Activates plant responses to wounds from herbivores and causes the plant to produce jasmonic acid.

  3. Jasmonates:

    • Work with systemin to mediate responses to drought, damage by ground-level ozone, and ultraviolet light.

  4. Salicylic Acid:

    • Initiates a systemic acquired response (SAR) to infection by parasites or pathogens.
    • The hypersensitive response (HR) is a specific, localized response to infection.

  5. Oligosaccharins:

    • Short chains of simple sugars that play a role in plant defense against bacterial and fungal infections.
    • Act locally at the site of injury and can also be transported to other tissues.
    • Enzymes (chitinase and glucanase) attack the fungal wall, releasing oligosaccharins that elicit the production of defense compounds (phytoalexins).
    • Fungal pectinase releases biologically active oligosaccharins from the plant cell wall.

  6. Strigolactones:

    • Promote seed germination in some species and inhibit lateral apical development in the absence of auxins.
    • Play a role in the establishment of mycorrhizae (mutualistic association of plant roots and fungi).

  7. Florigen:

    • A systemic signal that initiates flowering, involved in the formation of storage organs and contributes to plant architecture.
    • Synthesized in leaves and transported to the shoot apical meristem where it promotes flowering in response to daylength cues.
    • Represented as a protein product encoded by the FLOWERING LOCUS T (FT) gene.
    • Regulation of flowering time is an important target for plant breeding because the control of flowering to a favorable time provides successful grain production. Flowering at unfavorable seasons causes loss of yield.