Plant Hormones Summary

Plant Hormones

Overview of Plant Hormones

  • Plant hormones are similar to animal hormones but with some differences.
  • Signaling systems involve:
    • Perceiving a signal.
    • Transducing or transmitting and amplifying the signal.
    • Developmental or behavioral response.
  • Plants respond to internal (e.g., age) and external cues (e.g., light, cold).
  • Perception often occurs via pigments (e.g., chlorophyll).
  • Transmission is frequently hormone-mediated.
  • Growth regulation is achieved through tropisms.

Characteristics of Plant Hormones

  • Small, simple molecules with diverse structures
  • Not produced in specialized glands; generally produced throughout the plant
  • Have protein receptors on target cells
  • Small amounts can have large effects.

Classic Five Plant Hormones

  • Auxins
  • Ethylene
  • Cytokinins
  • Gibberellins
  • Abscisic Acid

Auxins

  • Involved in phototropism (growth towards light) and gravitropism (growth in response to gravity).
  • Auxin molecules are unevenly distributed across the plant tip.
    • Accumulate on the shady side, promoting cell elongation and bending towards the light.
  • Key roles:
    • Embryo development.
    • Root and lateral root development.
    • Fruit growth.
    • Gravitropism and phototropism.
  • Apical Dominance: Auxin produced at the shoot apex inhibits lateral bud growth.
  • Synthetic auxins are used as herbicides (e.g., 2,4-D).

Ethylene

  • Simple, volatile hormone involved in fruit ripening, leaf abscission, and flower senescence.
  • Can affect nearby plants due to its volatile nature.
  • Fruits are either:
    • Climacteric (ripening accompanied by an ethylene burst).
    • Non-climacteric (ethylene still important but no burst).
  • Ethylene precursors can be used to initiate fruit ripening in crops like cotton.

Gibberellins

  • Stimulate cell division and elongation, especially in internodes.
  • Involved in the green revolution through breeding for reduced plant height in wheat and rice.
    • Dwarf wheat has reduced sensitivity to gibberellic acid.
  • Functions:
    • Stem elongation.
    • Fruit development (parthenocarpy - seedless fruit).
    • Seed germination.

Cytokinins

  • Promote cell division and growth.
  • Promote growth of auxiliary buds (antagonistic to auxin).
  • Delay leaf and flower senescence.
  • Auxin and cytokinin act as antagonists, balancing plant growth.

Abscisic Acid (ABA)

  • Stress hormone involved in response to abiotic stresses like drought and salt.
  • Causes closure of stomata under water stress.
  • Inhibits germination (antagonistic to gibberellins).

Plant Behavior: Red Light Sensing and Flowering Time

  • Plants flower in response to day length (photoperiod).
    • Short-day plants.
    • Long-day plants.
  • Phytochrome is a pigment in leaves that perceives light.
    • Red light activates phytochrome.
    • Far-red light and darkness deactivate phytochrome.
  • Florigen (FT protein) is a protein that moves from the leaf to the shoot apex to trigger flowering.
  • It directly regulates gene expression.