9.3 Plant Growth

• Meristems are tissues in a plant consisting of undifferentiated cells capable of indeterminate growth

  • They are analagous to totipotent stem cells in animals, except that they have specific regions of growth and development
  • Meristematic tissue can allow plants to regrow structures or even form entirely new plants (vegetative propagation)
• Meristematic tissue can be divided into apical meristems and lateral meristems:

  • Apical meristems occur at shoot and root tips and are responsible for primary growth (i.e. plant lengthening)
  • Lateral meristems occur at the cambium and are responsible for secondary growth (i.e. plant widening / thickening)
  • Apical meristems give rise to new leaves and flowers, while lateral meristems are responsible for the production of bark
• The apical meristems give rise to primary growth (lengthening) and occurs at the tips of the roots and shoots

  • Growth at these regions is due to a combination of cell enlargement and repeated cell division (mitosis and cytokinesis)
  • Differentiation of the dividing meristem gives rise to a variety of stem tissues and structures – including leaves and flowers
• In the stem, growth occurs in sections called nodes – with the remaining meristem tissue forming an inactive axillary bud

  • These axillary (lateral) buds have the potential to form new branching shoots, complete with leaves and flowers
• • Auxin efflux pumps can set up concentration gradients within tissues – changing the distribution of auxin within the plant
  • These pumps can control the direction of plant growth by determining which regions of plant tissue have high auxin levels
  • Auxin efflux pumps can change position within the membrane (due to fluidity) and be activated by various factors 
• Auxins are a group of hormones produced by the tip of a shoot or root (i.e. apical meristems) that regulate plant growth
• • In the shoots, auxin stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger)
  • In the roots, auxin inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller)
• Auxin is a plant hormone and influences cell growth rates by changing the pattern of gene expression with a plant’s cells

  • Auxin’s mechanism of action is different in shoots and roots as different gene pathways are activated in each tissue
• In shoots, auxin increases the flexibility of the cell wall to promote plant growth via cell elongation

  • Auxin activates a proton pump in the plasma membrane which causes the secretion of H+ ions into the cell wall
  • The resultant decrease in pH causes cellulose fibres within the cell wall to loosen (by breaking the bonds between them)
  • Additionally, auxin upregulates expression of expansins, which similarly increases the elasticity of the cell wall
  • With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size
• Tropisms describe the growth or turning movement of an plant in response to a directional external stimulus 
• • Phototropism is a growth movement in response to a unidirectional light source
• • Geotropism (or gravitropism) is a growth movement in response to gravitational forces
• • Other tropisms include hydrotropism (responding to a water gradient) and thigmotropism (responding to a tactile stimulus)
• Both phototropism and geotropism are controlled by the distribution of auxin within the plant cells:

  • In geotropism, auxin will accumulate on the lower side of the plant in response to the force of gravity
  • In phototropism, light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant
• In shoots, high auxin concentrations promote cell elongation, meaning that:

  • The dark side of the shoot elongates and shoots grow towards the light (positive phototropism)
  • The lower side of the shoot elongates and roots grow away from the ground
• In roots, high auxin concentrations inhibit cell elongation, meaning that:

  • The dark side of the root becomes shorter and the roots grow away from the light (negative phototropism)
  • The lower side of the root becomes shorter and the roots turn downwards into the earth
• Micropropagation is a technique used to produce large numbers of identical plants (clones) from a selected stock plant

  • Plants can reproduce asexually from meristems because they are undifferentiated cells capable of indeterminate growth
  • When a plant cutting is used to reproduce asexually in the native environment it is called vegetative propagation
  • When plant tissues are cultured in the laboratory (in vitro) in order to reproduce asexually it is called micropropagation
• The process of micropropagation involves a number of key steps:

  • Specific plant tissue (typically the undifferentiated shoot apex) is selected from a stock plant and sterilised
  • The tissue sample (called the explant) is grown on a sterile nutrient agar gel
  • The explant is treated with growth hormones (e.g. auxins) to stimulate shoot and root development
  • The growing shoots can be continuously divided and separated to form new samples (multiplication phase)
  • Once the root and shoot are developed, the cloned plant can be transferred to soil
• Rapid Bulking

  • Desirable stock plants can be cloned via micropropagation to conserve the fidelity of the selected characteristic
  • This process is more reliable that selective breeding because new plants are genetically identical to the stock plant
  • This technique is also used to rapidly produce large quantities of plants created via genetic modification
• Virus-Free Strains

  • Plant viruses have the potential to decimate crops, crippling economies and leading to famine
  • Viruses typically spread through infected plants via the vascular tissue – which meristems do not contain
  • Propagating plants from the non-infected meristems allows for the rapid reproduction of virus-free plant strains
• Propagation of Rare Species

  • Micropropagation is commonly used to increase numbers of rare or endangered plant species
  • It is also used to increase numbers of species that are difficult to breed sexually (e.g. orchids)
  • It may also be used to increase numbers of plant species that are commercially in demand