9.3 Plant Growth Flashcards
What are meristems?
Meristems are tissues in a plant consisting of undifferentiated cells capable of indeterminate growth
What type of cells are meristems similar to?
They are analagous to totipotent stem cells in animals
What is one difference between meristems and totipotent stem cells?
except that they (meristems) have specific regions of growth and development
What can meristematic tissue allow plants to do?
Meristematic tissue can allow plants to regrow structures or even form entirely new plants (vegetative propagation)
Into what two categories can meristematic tissue be put into?
Meristematic tissue can be divided into
apical meristems and lateral meristems
Where are apical meristems located?
Apical meristems occur at shoot and root tips
What are apical meristems responsible for?
are responsible for primary growth (i.e. plant lengthening)
Where are lateral meristems located?
Lateral meristems occur at the cambium
What are lateral meristems responsible for?
are responsible for secondary growth (i.e. plant widening / thickening)
What do apical meristems create?
Apical meristems give rise to new leaves and flowers,
What do lateral meristems produce?
bark (on trees)
How does primary growth at the apical meristems occur (what processes)?
Growth at these regions is due to a combination of cell enlargement and repeated cell division (mitosis and cytokinesis)
How do apical meristems result in many other tissues?
Differentiation of the dividing meristem gives rise to a variety of stem tissues and structures – including leaves and flowers
Where does growth occur in the stem?
In the stem, growth occurs in sections called nodes
What does the meristem that is not used in the stem form?
with the remaining meristem tissue forming an inactive axillary bud
What may axillary buds form?
These axillary (lateral) buds have the potential to form new branching shoots, complete with leaves and flowers
What controls the growth of the stem?
The growth of the stem and the formation of new nodes is controlled by plant hormones released from the shoot apex
What is the main group of plant hormones involved in plant growth called?
One of the main groups of plant hormones involved in the shoot and root growth are AUXINS (e.g. indole-3-acetic acid / IAA)
What do auxins do?
When auxins are produced by the shoot apical meristem, it promotes growth in the shoot apex via cell elongation and divisio
What does auxin “inhibit”?
The production of auxins additionally prevents growth in lateral (axillary) buds, a condition known as apical dominance
Why is apical dominance necessary?
Apical dominance ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants
When does the inhibition of the axillary bud diminish?
As the distance between the terminal bud and axillary bud increases, the inhibition of the axillary bud by auxin diminishes
Do all plants show the same level of apical dominance?
NO
Different species of plants will show different levels of apical dominance
What are auxins?
Auxins are a group of hormones produced by the tip of a shoot or root (i.e. apical meristems) that regulate plant growth
How can auxin be distributed around the plant?
Auxin efflux pumps can set up concentration gradients within tissues – changing the distribution of auxin within the plant
What does the action of auxin efflux pumps result in?
These pumps can control the direction of plant growth by determining which regions of plant tissue have high auxin levels
Do auxin efflux pumps stay in the same place?
NO
Auxin efflux pumps can change position within the membrane (due to fluidity) and be activated by various factors
Does auxin have the same mechanism of action in the shoots and roots?
NO
Auxin has different mechanism of action in the roots of plants versus the shoots of plants:
How does auxin function in the shoots?
In the shoots, auxin stimulates cell elongation and thus high concentrations of auxin promote growth (cells become larger)
How does auxin function in the roots?
In the roots, auxin inhibits cell elongation and thus high concentrations of auxin limit growth (cells become relatively smaller)
How does auxin influence cell growth rates?
Auxin is a plant hormone and influences cell growth rates by changing the pattern of gene expression with a plant’s cells
Why is auxin’s mechanism of action different in the roots and shoots?
Auxin’s mechanism of action is different in shoots and roots as different gene pathways are activated in each tissue
In the shoots, what does auxin do to the cell wall?
In shoots, auxin increases the flexibility of the cell wall to promote plant growth via cell elongation
- What does auxin activate to start the process of cell elongation?
Auxin activates a proton pump in the plasma membrane which causes the secretion of H+ ions into the cell wall
- What does the secretion of H+ ions cause?
auxin
The resultant decrease in pH causes cellulose fibres within the cell wall to loosen (by breaking the bonds between them)
- How does auxin increase the elasticity of the cell wall?
Additionally, auxin upregulates expression of expansins, which similarly increases the elasticity of the cell wall
- How do all these steps change the cell?
auxins
With the cell wall now more flexible, an influx of water (to be stored in the vacuole) causes the cell to increase in size
What are tropisms?
Tropisms describe the growth or turning movement of an plant in response to a directional external stimulus
What is phototropism?
Phototropism is a growth movement in response to a unidirectional light source
What is a geotropism/gravitropism?
Geotropism (or gravitropism) is a growth movement in response to gravitational forces
What is a hydrotropism?
responding to a water gradient
What is a thigmotropism?
responding to a tactile stimulus
What are phototropisms and geotropisms controlled by?
controlled by the distribution of auxin within the plant cells:
How does auxin play a role in geotropisms?
In geotropism, auxin will accumulate on the lower side of the plant in response to the force of gravity
How does auxin play a role in phototropism?
In phototropism, light receptors (phototropins) trigger the redistribution of auxin to the dark side of the plant
What does a high auxin concentration in shoots do?
In shoots, high auxin concentrations promote cell elongation
How is a positive phototropism achieved in shoots?
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
What does a high auxin concentration in roots do?
In roots, high auxin concentrations inhibit cell elongation
How is a negative phototropism achieved in roots
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
What is micropropagation?
Micropropagation is a technique used to produce large numbers of identical plants (clones) from a selected stock plant
In what way do plants reproduce from meristems? Why?
Plants can reproduce asexually from meristems because they are undifferentiated cells capable of indeterminate growth
What is vegetative propagation?
When a plant cutting is used to reproduce asexually in the native environment it is called vegetative propagation
How is micropropagation different from vegetative propagation?
When plant tissues are cultured in the laboratory (in vitro) in order to reproduce asexually it is called micropropagation (not native environment)
- What is the first step of micropropagation?
micropropagation
Specific plant tissue (typically the undifferentiated shoot apex) is selected from a stock plant and sterilised
- What is done with the tissue sample?
micropropagation
The tissue sample (called the explant) is grown on a sterile nutrient agar gel
- What is added to the explant?
micropropagation
The explant is treated with growth hormones (e.g. auxins) to stimulate shoot and root development
- What happens in the multiplication phase?
micropropagation
The growing shoots can be continuously divided and separated to form new samples (multiplication phase)
- When can the plant be transferred to soil?
micropropagation
Once the root and shoot are developed, the cloned plant can be transferred to soil
What is the purpose of micropropagation?
Micropropagation is used to rapidly produce large numbers of cloned plants under controlled conditions:
What is rapid bulking?
Desirable stock plants can be cloned via micropropagation to conserve the fidelity of the selected characteristic
Why is rapid bulking better than selective breeding?
This process is more reliable that selective breeding because new plants are genetically identical to the stock plant
Apart from cloning stock plants, what can rapid bulking be used for?
This technique is also used to rapidly produce large quantities of plants created via genetic modification
Why are plant viruses harmful? (basic)
Plant viruses have the potential to decimate crops, crippling economies and leading to famine
In what way do plant viruses spread?
Viruses typically spread through infected plants via the vascular tissue – which meristems do not contain
How can micropropagation be useful in producing plants with virus-free strains?
Propagating plants from the non-infected meristems allows for the rapid reproduction of virus-free plant strains
What type of species of plants can micropropagation create more of? 3
Micropropagation is commonly used to increase the numbers of rare or endangered plant species
It is also used to increase the numbers of species that are difficult to breed sexually (e.g. orchids)
It may also be used to increase the numbers of plant species that are commercially in demand
