Topic 6: Organism response: Tropism and Plant Growth Factor

A tropism

the growth of a part of an organism like in a plant in response to a directional stimulus

Like taxes tropisms are named for their specific stimuli they are in response to:

• phototropism directs growth towards or away from light

• gravitropism (geotropism) directs growth towards or away from gravitational pull

Different parts of the plant shows differnt tropisms:

• the shoot exhibits positive phototropism and negative gravitropism, to optimise light absorption for photosynthesis

• the root shows negative phototropism and positive gravitropism, to increase the probability that roots will grow into the soil, where they can absorb more water and minerals

IAA

• indoleacetic acid is a type of auxin

• plant growth factor produced in small quantities

• controls plant cell elongation

• produced by the cells in shoot tip and root tip

• hormone like substances - plant growth factor

• different stimuli cause uneven distribution of IAA, which in turn causes the plant’s response

Water: roots grow towards water

positive hydrotropism

Phototropism in shoots

1. Cells in the tip of the shoot produce IAA, which is initially transported evenly down the shoot

2. Light causes IAA to diffuse to the shaded side of the shoot

3. A greater concentration of IAA on the shaded side cause cell to elongate more

4. The shaded side of the shoot elongates faster causing the shoot to bend towards the light

Phototropism in roots (negative phototropism)

1. Higher concentration of IAA inhibits cell elongation in the roots

2. In roots, the elongation of cells is greater on the light side than on the shaded side and so roots bend away from light

3. Roots demonstrate negative phototropism

Gravitropism in roots

1. Cells in the tip of the root produces IAA, which is initially transported along all sides of the root

2. As a result of gravity, a greater concentration of IAA builds up on the lower side of the root

3. IAA inhibits cell elongation on the lower side

4. The relatively greater elongation on the upper side causes the roots to bend towards the gravity

IAA increases the plasticity (ability to stretch) of the cell walls of young plant cells

Mature cells have a rigid cell wall which does not respond to IAA

Acid growth hypothesis:

• auxin activates the proton pump in the cell membrane

• hydrogen ions are actively transported from the cytoplasm into spaces in the cell wall

• this lowers the pH, making it more acidic causing cell wall to become more plastic allowing the cell to elongate

• expansins are special proteins that loosen connections in the rigid cellulose network of the cell wall (activated by lower pH)

Darwin and Darwin experiment conclusions:

1. The shoot is positively phototropic

2. Tip of the shoot is sensitive to the light - the tip must either detect the stimulus or produce the messenger (or both) as its removal prevents a response

3. The tip is the light sensitive region of the shoot, but response (bending) occurs lower down the shoot

Boysen-Jenson experiment conclusions:

1. When the tip of the coleoptile was separated by a cube of gelatin, which allowed chemicals to pass, the seedlings responded normally, bending towards light

2. If the tip was separated from the lower coleoptile by an impermeable barrier, such as mica, no phototropic response occurred

3. The ‘messenger’ was therefore a mobile chemical substance

Mica - conducts electricity but not chemicals

Gelatin - conducts chemicals but not electricity

Paal experiment conclusion:

Displacement of the tip means that the chemical initially only moves down the side of the shoot that is contact with the tip. This side grows more rapidly, causing bending away from that side

Went experiment conclusions:

1. Agar block contained a chemical produced in the coleoptile

2. When agar block was placed on one side of the cut tip, the shoot grew bending away from the side the agar was on

3. This chemical stimulated growth as it passes down the coleoptile

Went chose the name auxin for this chemical messenger

Briggs experiment theories:

1. Light inhibits IAA production in the tip so it is only produced on shaded side

2. Light destroys the IAA as it passes down the light side

3. IAA is transported from the light side to the shaded side of the shoot

Plant growth factors, sometimes called plant hormones or plant growth regulators,

are chemicals that influence plant development mainly by modulating cell elongation and division

Differences between plant and animal hormones:

• plant hormones usually act locally at the site of production, while animal hormones usually act on distant organs

• plant hormones are produced in a variety of tissue throughout the plant, while animal hormones are produced in specialised cells within glands

Types of tropisms:

• phototropism

• gravitropism

• hydrotropism

Phototropism

this is the directional growth of parts of the plant towards or away from light

Gravitropism

this is directional growth of parts of the plant towards or away from the gravitational pull

Hydrotropism

this is the directional growth of parts of the plant towards or away from moisture

The tropic responses are critical for plant survival, enabling them to optimise photosynthesis, anchor themselves, and access water and nutrients efficiently

For instance, positive hydrotropism in the roots is beneficial for plants as it allows the roots to absorb more water, which is an essential medium for transport and a key reactant in photosynthesis

IAA stimulates cell elongation in plant shoots

but inhibits cell elongation in plant roots

Positive phototropism in plant shoots:

1. IAA is produced in cells in the tip of the plant shoot

2. IAA is transported down the plant shoot

3. Light stimulates IAA to move from the light side of the shoot to the shaded side

4. IAA becomes concentrated and stimulates more cell elongation on the shaded side of the shoot

5. The shoot bends towards the light

Negative phototropism and positive gravitropism in roots:

1. IAA is produced in the cells in the tip of the plant root

2. IAA is transported along the plant root

3. Any light available stimulates IAA to move from the light side of the root to the shaded side

4. Gravity also pulls IAA from the upper side of the root to the lower side of the root

5. IAA becomes concentrated and inhibits cell elongation in the lower, shaded side of the root

6. The root bends away from any available light and downwards towards the pull of gravity