Plant Hormones


Plant hormones (or phytohormones) are chemical substances that are used to control growth and development in plants

They are produced in relatively small amounts and can be transported in the xylem or phloem if the target cell is different

Plant hormones may have different effects in different areas of the plant, making it difficult to definitively assign a function for a given phytohormone


Types of Plant Hormones

Auxins (e.g. IAA)

  • Auxins are a class of plant hormones principally involved in plant growth and elongation
  • They may increase the rate of cell elongation in response to directional stimuli, and are thus important in tropic responses (e.g. phototropism)
  • They promote apical dominance - where the apex / tip of a plant grows while the lateral buds don't develop
  • They may increase the rate of cell division


Cytokinins

  • Cytokinins are important in growth as they promote cell division (cytokinesis)
  • They play a role in ensuring the roots and shoots grow at equal rates (important for continued survival) and also stimulate growth of fruit
  • Whereas auxins are responsible for primary (vertical) growth, cytokinins are responsible for secondary (lateral) growth - they control branching


Gibberellins

  • Gibberellin (or gibberellic acid) is necessary for seed germination
  • In the presence of water, it stimulates the production of amylase (converts starch into maltose), allowing for the formation of ATP (via glucose)
  • The energy produced in the embryo - as a result of the action of gibberellin - is used to facilitate germination
  • The glucose produced may also be used to synthesis cellulose - for cell wall formation
  • Gibberellin also causes stem elongation by promoting both cell elongation and cell division


Ethylene

  • A gas which acts as a hormone and stimulates maturation and ageing (senescence) 
  • It is responsible for the ripening of certain fruit (auxins and gibberellins promote fruit growth but inhibit ripening)
  • It is also responsible for the ageing and loss of leaves (abscission) and the death of flowers


Abscisic Acid (ABA)

  • Abscisic acid principally inhibits plant growth and development
  • It promotes the death of leaves (abscission) and is responsible for seed dormancy
  • It generally initiates stress responses in plants (like winter dormancy in deciduous trees)
  • ABA prevents excess water loss in dehydrated plants by causing the closure of stomata


General Overview of Role of Plant Hormones in Growth and Development


External Factors

  • The growth and development of a plant is often dependent on external environmental factors
  • For instance, germination is dependent on the presence of water and some plants may only flower after exposure to cold temperatures (vernalisation)
  • Tropisms are growth responses prompted by directional stimuli (e.g. phototropism = response to light ; geotropism = response to gravity)


Phototropism

  • Phototropism is the growing or turning of an organism in response to a unidirectional light source
  • Auxins (e.g. IAA) are plant hormones that are produced by the tip of a shoot and mediate phototropism
  • Auxin makes cells enlarge or grow and, in the shoot, are eradicated by light
  • The accumulation of auxin on the shaded side of a plant causes this side only to lengthen, resulting in the shoot bending towards the light
  • Auxin causes cell elongation by activating proton pumps that expel H+ ions from the cytoplasm to the cell wall
  • The resultant decrease in pH within the cell wall causes cellulose fibres to loosen (by breaking the bonds that hold them together)
  • This makes the cell wall flexible and capable of stretching when water influx promotes cell turgor
  • Auxin can also alter gene expression to promote cell growth (via the upregulation of expansins)


The Role of Auxin in Phototropism


Photoperiodism

  • Flowering is controlled by phytochrome, which is affected by light (photoperiodicity)
  • Phytochrome exists in two forms:
    •  A red (Pr) form absorbs red light (~660 nm) and is converted into a far red form (Pfr)
    • A far red (Pfr) form absorbs far red light (~730 nm) and is converted into a red form (Pr)
  • The Pfr form is the active form of phytochrome, while the Pr form is the inactive form of phytochrome
  • Sunlight contains more red light, so the Pfr form is predominant during the day, with the gradual reversion to the Pr form occurring at night
  • In long day plants, the active Pfr form is a promoter of flowering and so flowering is induced when the night period is less than a critical length and Pfr levels are high
  • In short day plants, the active Pfr form is an inhibitor of flowering and so flowering is induced when the night period is greater than a critical length and Pfr levels are low


The Control of Flowering in Plants