9.3 Growth in Plants

Question 1

What are meristems?

Answer 1

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)

Question 2

Apical meristems

Answer 2

occur at shoot and root tips and are responsible for primary growth (i.e. plant lengthening)
give rise to new leaves and flowers

Question 3

Lateral meristems

Answer 3

occur at the cambium and are responsible for secondary growth (i.e. plant widening / thickening)
are responsible for the production of bark

Question 4

Apical growth

Answer 4

Growth at apical meristems 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

Question 5

Role of auxin in plant growth in shoot apex

Answer 5

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

One of the main groups of plant hormones involved in shoot and root growth are auxins (e.g. indole-3-acetic acid / IAA)

When auxins are produced by the shoot apical meristem, it promotes growth in the shoot apex via cell elongation and division

The production of auxins additionally prevents growth in lateral (axillary) buds, a condition known as apical dominance
Apical dominance ensures that a plant will use its energy to grow up towards the light in order to outcompete other plants
As the distance between the terminal bud and axillary bud increases, the inhibition of the axillary bud by auxin diminishes
Different species of plants will show different levels of apical dominance

Question 6

What are auxins?

Answer 6

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

Question 7

Auxin efflux pumps

Answer 7

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

Question 8

Effect of auxin in shoots

Answer 8

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

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

Question 9

Effect of auxin in roots

Answer 9

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

Question 10

Tropisms

Answer 10

describe the growth or turning movement of an plant in response to a directional external stimulus
is growth is towards the stimulus it is positive and if it is away it is negative

Question 11

Phototropism

Answer 11

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
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
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

Question 12

Geotropism or gravitropism

Answer 12

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

Question 13

Hydrotropism

Answer 13

responding to a water gradient

Question 14

Thigmotropism

Answer 14

responding to a tactile stimulus

Question 15

Micropropagation

Answer 15

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 plant tissues are cultured in the laboratory (in vitro) in order to reproduce asexually it is called micropropagation

Question 16

Process of micropropagation

Answer 16

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

Question 17

Purposes of micropropagation

Answer 17

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