9.3 Growth in Plants

meristems

Tissues in plant consisting of undifferentiated cells capable of indeterminate growth

Two types of meristems

* apical meristems
* lateral meristems

Apical meristems

Occur at shoot and root tips and are responsible for primary growth. (e.g. production of new leaves)

Lateral meristems

Occur at the cambium and are responsible for secondary growth (widening) (e.g. production of bark)

Growth in stem

* occurs in sections called nodes
* remaining meristem tissue will form an inactive axillary bud
* these buds have the potential to form new branching shoots, complete with leaves and flowers.

Differentiation of dividing meristem

Gives rise to a variety of stem tissues and structures - including leaves and flowers

Where are plant hormones that control growth released from?

Shoot apex

Name for hormone that controls growth in root and shoot

Auxin

Function auxins

Promotes growth in shoot apex (top of a stem) via cell elongation and division

What does the production of auxins prevent?

Growth in lateral axillary buds, a condition known as apical dominance

Function 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
* Terminal bud = top of plant
* Axillary bud = side bud
* Different species of plants will show different levels of apical dominance

Auxin efflux pumps (PIN3)

* Can set up concentration gradients within tissues - changing the distribution of auxin within the plant
* 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 in shoot vs root

Shoot: stimulates cell elongation

Root: Inhibits cell elongation

How does auxin influence cell growth?

By changing the pattern of gene expression

How does auxin regulate flexibility of the cell wall in order to promote plant growth?

* Proton pump is activated which secretes H+ ions into cell wall
* Decreases pH and causes cellulose fibres within the cell wall to loosen
* 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

Phototropism

Growth movement in response to a unidirectional light source

Geotropism

Growth movement in response to gravitational forces

Hydrotropism

Response to water gradient

Thigmotropism

Response to tactile stimulus

Auxins and phototropism

* Light receptors trigger the redistribution of auxin to the dark side of the plant
* Therefore the dark side of the shoot elongates and grows towards the light
* The dark side of the root becomes shorter and the roots grow away from the light.

Geotropism and auxins

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

Micropropagation

* A technique used to produce large numbers of identical plants from a selected stock plant
* When plant tissues are cultured in the laboratory in order to reproduce asexually

Steps to micropropagation

* Specific plant tissue is selected from a stock plant and sterilized
* The tissue sample (explant) is grown on sterile nutrient agar gel
* The explant is treated with growth hormones to stimulate shoot and root development
* The growing shoots can be continuously divided and separated to form new samples
* Once the root and shoot are developed, the cloned plant can be transferred to soil.

Use of micropropagation

* Rapid bulking
* Virus-free strains
* Propagation of rare species

Use of micropropagation - Rapid bulking

* desirable stock plants can be cloned via micropropagation to conserve selected characteristic
* More reliable than selective breeding
* Produces large quantities

Use of micropropagation - 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

Use of micropropagation - 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