9.3 Growth in plants

9.3.1 Meristems and Indeterminate Growth

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

  • They are analogous 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)

  • Apical meristems give rise to new leaves and flowers, while lateral meristems are responsible for the production of bark

  • Lateral meristems occur at the cambium and are responsible for secondary growth (i.e. plant widening / thickening)

9.3.2 Apical Dominance

• 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

• 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

  • The growth of the stem and the formation of new nodes is controlled by plant hormones released from the shoot apex

9.3.3 Auxin 

• Auxin group of hormones produced by the tip of a shoot or root (i.e. apical meristems) that regulate plant growth

  • Auxin promotes cell growth in shoots and roots, and the direction of growth can be influenced by stimuli such as light and gravity.

• 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 

• Auxin has different actions in the roots of plants versus the shoots of plants:

  • shoots, auxin stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger)

  • 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

  • roots, auxin inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller)

9.3.4 Tropisms

• 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 

    • light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant

  • Geotropism (or gravitropism) is a growth movement in response to gravitational forces 

    • auxin will accumulate on the lower side of the plant in response to the force of gravity

  • Other tropisms include hydrotropism (responding to a water gradient) and thigmotropism (responding to a tactile stimulus)

• In shoots, high auxin concentrations promote cell elongation, 

  • 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, 

  • 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

9.3.5 Micropropagation

• Micropropagation is a technique to produce a large number of genetically identical plants using tissue from the shoot apex and growing them on nutrient agar gels with growth hormones.

• The shoot apex tissue is placed on a nutrient medium, and then multiple shoots are developed which can be transferred to soil.

• Micropropagation can be used for rapid bulking up of new varieties, the production of virus-free strains of existing varieties, and the propagation of orchids and other rare species.

• Cryopreservation is a technique that uses liquid nitrogen to store plantlets, which is similar to the function of a seed bank.

• 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