Plant Hormones

• Tropisms can be positive or negative;

  • positive tropisms will grow towards a stimulus
  • negative tropisms will grow away from a stimulus
• Plants respond to stimuli using chemical growth factors, also called plant hormones, which act as chemical messengers similar to animal hormones
• Examples of plant hormones and their roles;

  • Giberellins - stem elongation, flowering, seed germination
  • Cytokinins - cell growth and division
  • Abscisic acid (ABA) - leaf loss, seed dormancy
  • Ethene - fruit ripening, flowering
• Growth factors are made in growing regions, then they move to other tissues to regulate growth in response to stimuli (for example; auxin promotes cell elongation in shoots but inhibits growth in roots)
• Indoleacetic acid (IAA) is a type of auxin which brings about plant responses by altering the transcription of genes inside plant cells
• Auxins are a group of plant growth factors that influence many aspects of plant growth
• IAA is produced in growing regions and is transported via diffusion, active transport, and through the phloem. Its redistribution is influenced by environmental stimuli such as light and gravity, causing uneven growth.
• IAA in shoots:

  Light causes IAA to move to the shaded side of the shoot (a phototropism response). When light is even, the IAA molecules are evenly distributed.

  When light is uneven, the IAA moves towards the shaded side which increases the rate of cell elongation, causing the shaded side to grow at a faster rate, pushing the tip towards the light.
• IAA in roots (geotropism):

  in roots, high IAA concentration result in a lower rate of cell elongation (opposite of shoot cells). IAA is transported towards the lower side of plant roots, resulting in the lower side growing at a slower rate than the upper side of the root - causing the root to bend downwards (towards centre of gravity)
• Auxin stimulates cell signalling cascade:

  1. Auxin binds to receptor in the cell surface membrane of target cell
  2. second inactive messenger is activated by binding of auxin to receptor
  3. transcription factors are activated by second messenger
  4. mRNA for relevant gene is transcribed - leading to synthesis of relevant protein
  5. genes produce proteins for cell division, expansion and differentiation
• Flowering in plants is controlled by the stimulus of night length (some flower when nights are short and some when nights are long)
• A phytochrome molecule consists of a protein bonded to a non-protein light-absorbing pigment molecule. The pigment molecule exists in two different isomers (Pr and Pfr)
• The Pr inactive form of phytochrome absorbs red light
• The Pfr active form of phytochrome absorbs far-red light
• Absorption of red light converts Pr into Pfr
• Absorption of far-red light converts Pfr into Pr
• Pfr accumulates in the light as more red light is absorbed.
• In the dark, any Pfr present is slowly converted to Pr
• During the day time:

  • unfiltered sunlight is rich in red light but deficient in far-red light
  • this means rapid conversion of Pr to Pfr but little conversion of Pfr back to Pr
  • so Pfr accumulates in the daytime
• During the night time:

  • Pfr slowly decays back into Pr
  • The ratio of Pfr to Pr allows the plant to determine the length of the night
• Phytochromes regulate responses such as:

  • seed germination
  • stem elongation
  • chlorophyll formation
  • flowering
• length of day = photoperiod
• Pfr promotes flowering in long day plants
• Pfr inhibits flowering in short day plants
• short day plants flower in the winter as nights are longer, which means that there is time for more Pfr to be converted back into Pr so little Pfr remains - inhibiting flowering
• long day plants flower in the summer as the nights are shorter, which means that there is little time for all Pfr to be converted back to Pr, so Pfr remains to promote flowering
• High levels of Pfr activates flowering; it binds to receptors and activates expression of genes that stimulate flowering, so the active gene is transcribed and translated into a protein that causes flowers to be produced.