8.1.7 The Effects of IAA

• Just like animals, the survival of plants is dependent on their ability to respond to changes in their environment; this maximises their survival chances e.g.
  • Growing towards light maximises the rate of photosynthesis and therefore glucose production
  • Producing harmful or foul-tasting chemicals in response to being eaten by a herbivore reduces the likelihood of being eaten
  • Roots growing towards water maximises the plant's ability to gain water
• Plants can respond to several types of stimuli e.g.
  • Light
  • Gravity
  • Physical objects
  • Herbivory
  • Water
  • Physical touch
• Plants can respond to stimuli in various ways, including by altering their growth
  
  • E.g. a seedling will bend and grow towards the light because there is more growth on the shaded side than on the illuminated side
• This type of directional growth response is referred to as a tropism
  • Phototropism is a growth response to light
  • Geotropism is a growth response to gravity
    • The response to gravity is also known as gravitropism
• Tropisms can be positive or negative
  • Positive tropisms involve growth towards a stimulus
    • E.g. positive phototropism is a growth response towards light
  • Negative tropisms involve growth away from a stimulus
    • E.g. negative geotropism is a growth response away from gravity i.e. upwards

Plant growth factors

• The growth responses of plants rely on chemical substances that are released in response to a stimulus
• These chemical growth factors act in a similar way to the hormones that are found in animals
  • Plant growth factors are sometimes referred to as plant hormones as they are chemical messengers
• Growth factors are produced in the growing parts of a plant before moving from the growing regions to other tissues where they regulate cell growth in response to a directional stimulus
  • E.g. auxin is a growth factor that stimulates cell elongation in plant shoots and inhibits growth in cells in plant roots
• Other examples of plant hormones along with some of their regulatory roles include
  • Giberellins 
    • Stem elongation
    • Flowering
    • Seed germination
  • Cytokinins 
    • Cell growth and division
  • Abscisic acid (ABA) 
    • Leaf loss
    • Seed dormancy
  • Ethene 
    • Fruit ripening
    • Flowering

Indoleacetic acid

• Indoleacetic acid, or IAA, is a type of auxin
  • Auxins are a group of plant growth factors that influence many aspects of plant growth
• It is thought that IAA brings about plant responses such as phototropism by altering the transcription of genes inside plant cells
  • Altering the expression of genes that code for proteins involved with cell growth can affect the growth of a plant
• IAA is produced by cells in the growing parts of a plant before it is redistributed to other plant tissues
  • IAA can be transported from cell to cell by diffusion and active transport 
  • Transport of IAA over longer distances occurs in the phloem
• The redistribution of IAA is affected by environmental stimuli such as light and gravity, leading to an uneven distribution of IAA in different parts of the plant
  • This brings about uneven plant growth

IAA in plant shoots

• Light affects the growth of plant shoots in a response known as phototropism
• The concentration of IAA determines the rate of cell elongation within the stem
  • A higher concentration of IAA causes an increase in the rate of cell elongation 
  • If the concentration of IAA is not uniform across the stem then uneven cell growth can occur
• When light shines on a stem from one side, IAA is transported from the illuminated side of a shoot to the shaded side
• An IAA gradient is established, with more on the shaded side and less on the illuminated side
• The higher concentration of auxin on the shaded side of the shoot causes a faster rate of cell elongation, and the shoot bends towards the source of light

IAA stimulates cell elongation in shoots

IAA in roots

• Roots respond to gravity in a response known as geotropism
• In roots, IAA concentration also affects cell elongation, but higher concentrations result in a lower rate of cell elongation
  • Note that this is the opposite effect to that of IAA on shoot cells
• IAA is transported towards the lower side of plant roots
• The resulting high concentration of auxin at the lower side of the root inhibits cell elongation
• As a result, the lower side grows at a slower rate than the upper side of the root, causing the root to bend downwards

IAA inhibits cell elongation in shoots. Note that you do not need to know about the role played by amyloplasts in detecting the direction of gravity