Effects of Plant Hormones (Edexcel International A Level Biology)

• 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
  • Producing flowers at the right time of year increases the chances of reproducing successfully
• Plants can respond to several types of stimuli e.g.
  • Light
  • Gravity
  • Physical objects
  • Herbivory
  • Water
  • Physical touch
• Unlike many animals, plants do not possess a nervous system; the responses of plants rely on chemical substances that are released or altered in response to a stimulus

Phytochrome

• Phytochromes are plant pigments that react to different types of light, leading to different plant responses
• Phytochrome pigments exist in two forms
  • P_R is the inactive form of phytochrome, it absorbs light from the red part of the spectrum (wavelength 660 nm)
  • P_FR is the active form of phytochrome, it absorbs light from the far red part of the spectrum (wavelength 730 nm)
• Absorption of different wavelengths of light causes a reversible conversion between the P_R and P_FR forms of phytochrome
• • When P_R absorbs red light (660 nm) it is converted into P_FR
  • When P_FR absorbs far red light (730 nm) it is converted back into P_R 
  • In the absence of red light, the unstable P_FR gradually converts back into P_R

Phytochrome and germination

• Early observations of light exposure and seed germination showed that seeds exposed to red light would germinate, while seeds exposed to far-red light would not.
• It is now thought that phytochrome is responsible for these observations
  • Exposure to even a short burst of red light converts P_R into P_FR, triggering germination in seeds
  • Far-red light causes P_FR to be converted back into P_R, reversing the effects of any red light exposure and preventing germination

Phytochrome and Flowering

• Flowering in plants is controlled by the stimulus of night length
  • Nights are shorter during the spring and summer and longer in the autumn and winter
  • Some plants flower when nights are short and some flower when nights are long
• When the nights reach a certain length, genes that control flowering may be switched on or off, leading to the activation or inhibition of flowering
  • Genes that are switched on are expressed, leading to production of the polypeptides for which they code, while genes that are switched off are not expressed, so the polypeptides for which they code are not produced
• The length of night can be detected by a plant because it determines the quantities of different forms of a pigment called phytochrome in the leaf

• During the day levels of P_FR rise
  • Sunlight contains more wavelengths at 660 nm than 730 so the conversion from P_R to P_FR occurs more rapidly in the daytime than the conversion from P_FR to P_R
• During the night levels of P_R rise
  • Red light wavelengths are not available in the darkness and P_FR converts slowly back to P_R

P_R is converted to P_FR in a reversible reaction which controls flowering

• E.g. long day plants
  • Long day plants flower when the nights are short e.g. in summer
    • When nights are short, the day length is longer, hence the term 'long day plants'
  • In long day plants high levels of the active form of phytochrome activate flowering
  • Flowering occurs due to the following process
    • Days are long so P_R is converted to P_FR at a greater rate than P_FR is converted to P_R
    • The active form of phytochrome, P_FR, is present at high levels
    • High levels of P_FR activate flowering 
      • P_FR activates expression of genes that stimulate flowering
        • It is thought that P_FR acts as a transcription factor
      • The active gene is transcribed and translated 
      • The resulting protein causes flowers to be produced rather than stems and leaves

Phytochrome and transcription

• Evidence suggests that in addition to their role in germination and flowering, phytochromes are also involved in plant responses to light and gravity
• Phytochromes are thought to influence gene expression in plants by acting as transcription factors
  • P_R is converted into P_FR in the presence of red light
  • P_FR can move into the nucleus via the nuclear pores
  • P_FR binds to a protein in the nucleus known as phytochrome-interacting factor 3 (PIF3)
  • Once bound to P_FR, PIF3 can initiate transcription
• It is thought that P_FR and PIF3 together are able to activate various different genes and so control many different aspects of plant growth and development

Growth factors

• 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

• 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
  • Gibberellins 
    • 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, e.g.
    • Apical dominance; the suppression of the growth of side shoots by auxins in the growing shoot tip
    • Promoting the growth of roots at low concentrations and inhibiting the growth of roots at high concentrations
    • Phototropism in shoots
• 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 by increasing the stretching ability of cell walls
  • 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 high concentrations of IAA 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

Gibberellins

• Gibberellins are a type of plant growth regulator involved in controlling seed germination, stem elongation, flowering, and fruit development
• When a barley seed is shed from the parent plant, it is in a state of dormancy, containing very little water and being metabolically inactive
  • This allows the seed to survive harsh conditions until the conditions are right for successful germination, e.g. the seed can survive a cold winter until temperatures rise again in spring
• The barley seed contains
  • An embryo
    • This will grow into the new plant when the seed germinates
  • An endosperm
    • This is a starch-containing energy store surrounding the embryo
  • An aleurone layer
    • This is a protein-rich layer on the outer edge of the endosperm

• When the conditions are right the barley seed starts to absorb water to begin the process of germination
• This stimulates the embryo to produce gibberellins
• Gibberellin molecules diffuse to the aleurone layer and stimulate the cells there to synthesise amylase
  • In barley seeds it has been shown that gibberellin does this by causing an increase in the transcription of genes coding for amylase
• The amylase hydrolyses starch molecules in the endosperm, producing soluble maltose molecules
• The maltose is converted to glucose and transported to the embryo
• This glucose can be respired by the embryo, providing the embryo with the energy needed for growth

Gibberellins in barley seeds cause the synthesis of amylase enzymes which break down starch stores in the endosperm