9.3 PLANT GROWTH

Meristems: Regions in plants where small undifferentiated cells continue to divide and grow

2 types of Meristems:

• Apical meristems

Occurs at the tips of roots and stems

Growth allows roots and stems to elongate

Shoot produces new leaves and flowers

• Lateral meristems

Growth in thickness of plants [woody plants - trees & shrubs]

Vascular cambium lies between xylem and phloem

Growth of a plant is either cell enlargement or an increase in the number of cells (mitotic cell division)

Determinate: Animal embryos fixed parts develop (2 legs and 2 arms)

Indeterminate growth: Plants can grow continiously 

Apical meristems continue to grow throughout lifecycle, the stems and leaves produce many branches, leaves or flowers

Shoot: Stem and leaves

On the shoot there is shoot apical meristems present at the tip

Plant shoots grow by: Mitosis and Cell division

• Some cells stay in the meristem and continue going through the cell cycle producing more cells
• New cells cause the other cells to become displaced to the edge of the meristem
• Cells at the edge stop dividing. They undergo rapid growth and differentiation to either become a stem or a leaf

• Leaves are initiated as small bumps at the side of the apical dome
• The leaf primorda (bumps) continue cell division and growth until mature leaves

Tropisms: Growth or movement to directional external stimuli

Can be positive (towards stimulus) or negative (away from stimulus)

Common stimuli are: Chemicals, light, touch and gravity

Phototropism: Plant growth in response to light

Shoots exibit positive phototropism (grow towards light)

Roots exhibit negative phototropism

If there are many plants in a single area the seeds must grow towards the light for photosynthesis

Auxins (hormones) trigger positive phototropism

Auxin: Growth promoter. Used to control the growth at the shoot tip

Auxins increase flexibility of plant cell walls in young shoots

This results in cell elongation on the side of the shoot necessary to cause growth towards light

Auxin cannot diffuse through the plasma membrane - moved through membrane by Active Transport using a trans-membrane protein: Auxin Efflux pump

• Phototrophins (pigments) detect differences in the intensity of blue light
  
• This triggers the movement of Auxin by Active Transport via efflux pumps in the plasma membrane
• Efflux pumps pump Auxin from cytoplasm to cell wall
• Auxin efflux pumps move into the plasma membrane of shoot cells [where elongation is going to take place] on the shady side
• This sets up a concentration gradient of auxin (more in shade)
• More elongation (bend towards light) from the shady side which allows the shoot to grow towards the light

Concentration gradients of Auxin is necessary to control the direction of plant growth. Therefore auxin must be unevenly transported

• Auxin is produced in the tip of the shoot in the apical meristem

• Auxin causes the cells to elongate, promoting formation of leaves, causing apical dominance

• Auxin accumulates on the shady side of the shoot

• Protein pumps (Auxin efflux pumps) in the cell membrane are positioned on the shady side of each cell in the shoot. They pump Auxin out of the cell on the shady side which moves the Auxin to the shady side of the shoot

• Auxin binds to auxin receptors in plant cells - transcription of specific genes are promoted

• Expression of specific genes causes secretion of hydrogen ions into the cell walls

• This loosens the connections between cellulose fibres allowing for cell expansion

• Auxin is a plant hormone
• Produced by the shoot tip
•  Auxin being produced causes transport of hydrogen ions from cytoplasm to the cell wall
• Hydrogen pumping breaks the bonds between the cell wall fibres
• This makes the cell walls flexible
• Auxin makes the cell walls grow

• Gene expression is also altered by Auxin to promote cell growth
• Positive phototropism is growth towards light
• Shoot tip senses the direction of the brightest light
• Auxin is moved to the shady dark side
• This causes the cells on the dark side to elongate

Micropropogation: Propogation with very small pieces of tissue taken from the shoot apex of the plant

1. A small piece of tissue is removed from plant that is being cloned. This tissue comes from shoot tip. Tissue is sterilized. All apparatus and growth media must be sterilized to prevent infections - called the aseptic technique

2. Tissue is placed on sterile nutrient agar gel contains high auxin concentration. This stimulates cell growth and devision

3. A callus grows

4. Callus is transferred to nutrient agar gel that contains less auxin but has a lot of cytokinin which stimulates plantlets with roots and shoots to develop

5. The plantlets are seperated and transferred to soil

Produces large numbers of identical plants from stock plants

• New variaties can be bulked up much more quickly

• Virus free strains of existing variaties can be produced

• Large numbers of rare plants can be produced (example orchids)