HL Bio - Unit #5.1: Plant Hormones

Troposim

The process of controlled growth within plants based off of external stimuli

Types of Tropism Growth

1. Negative Tropism

2. Positive Tropism

Negative Tropism

Growth away from the stimulus

Positive Tropism

Growth towards the stimulus

Types of Tropism based on the stimuli

1. Phototropism

2. Gravitropism

3. Touch stimuli

Positive Phototropism

Shoots tend to grow towards the highest light intensity in its environment to get more light, due to auxin being released within the shaded areas of the shoot making it grow more from there and then elongating that side until the shoot begins to “curve” towards the light

Gravitropism

Growth in response to gravitational forces

Most are positive gravitropic (geotropic) as the grow downward in the direction of gravity

Turgor/hydrostatic pressure:

The force within the cell that pushes the plasma membrane against the cell wall (cells wall must stay thin when the cell is still growing inorder to give it the space to grow)

• Turgidity helps provide shoots/roots with the resistance against gravity and wind and provides the strength for roots to push through the soil

Touch stimuli

Where the plant touching something will causes a reaction

• Eg, tendrils on climbing plants that will coil around any potential support

Phytohormones

plant hormones that Act as chemical messengers for smaller ranges

• normally only in the area that they’re produced

Auxin (IAA)

Main growth hormone within a plant

produced by shoot apical meristems which promotes growth of the apex/top of the shoot and keeps going up

shoot apical meristems

The proliferating embryonic cells of the top/aerial part of the stem called the coleoptile

How Auxin changes gene expression

When auxin binds to its receptor (TIRI), it triggers the degradation of Aux/IAA proteins. This allows ARFs to repress genes related to growth.

Apical Dominance

Inhibiting lateral/axillary buds or growth

• Ensures that all energy to the plant is being used to grow towards the light source

Auxin in shoots

Stimulates cell elongation (expanding)

So the shoot can lean towards the light

Auxin in roots

Inhibits cell elongation (contracting)

So the root can continuously grow downward rather than go upwards

Auxin Efflux Carriers

Helps maintain the concentration gradients of phytohormones 

• Plants can control the distribution of auxin efflux carriers which will help it transport auxin across a tissue and create the concentration gradient

Process of Auxin moving around the plant

1. Auxin enters cells by passive diffusion if its carboxyl group (COOH) remains undissociated

2. Once auxin has entered, due to the alkaline nature of plant cytoplasm the carboxyl group dissociates (loses a proton → COO^-) not allowing the auxin to pass through the membrane 

3. Plant cells produce proteins called Auxin Efflux Carriers which pumps the auxin across the plasma membrane into the cell wall (Active transport), due to the acidic nature of the cell wall the auxin reverts back to it’s uncharged state (COOH) and can be diffused into the adjacent cell

Promotion of cell growth by Auxin:

• Cell walls are made using microfibrils when the wall needs to thicken extra microfibrils are made and passed to the place of growth

• Microfibrils are crosslinked by other carbohydrates and its strength is dependent on its pH

  • Lower pH (acidic) = weaker links → Allowing walls to extend 

• Auxin promotes the synthesis of proton pumps within the plasma membrane that transport H⁺ ions to the apoplast to acidify the inside allowing for the walls to elongate

microfibrils

bundles of cellulose molecules

Ethylene

Promotes the ripening process within fruits and their development

• Creates a positive feedback loop: Ethylene is produced → fruit ripens → ripened fruits produces more ethylene → further ripens the fruits → so on

• Ethylene is produced as a vapor so it can diffuse to other fruits and begin their ripening processes → helps synchronize the ripening of fruits on plants

WHy do fruits ripen

Fruits are ripened on plants for animals to eat so the seeds can be propagated as the animal moves the fruit with the seeds away 

and to fully develop the seeds

Jasmonic Acid

Triggers the secretion of enzymes in response to external stress

Gibberellin

Affects the rate of cell division and stem growth 

• Changes the water potential of a plant

Gibberellic Acid

Produced in embryonic cells to promote growth by converting protein reserves within the cotyledons into hydrolytic enzymes which mobilize the stores food reserves

Brassinosteroids

These hormones are involved in regulating the division, elongation, and differentiation of numerous cell types throughout the entire plant life cycle

Abscisic Acid

Plant hormone that regulates numerous aspects of plant growth, development, and stress responses

Cytokinins

Another growth stimulating hormone

Produced in the roots and are then transported to the shoot (through the xylem) where it stimulates growth

Synergism of Auxin and Cytokinins

Both work together to further increase the amount the plant grows

• Cell division in the apices (tips) of the stems and roots

• Cell enlargement in the apices of the stems and roots

Antagonism between Auxin and cytokinins

One of them will inhibit the production/work of the other hormone

• Development of branches of roots/new roots

  • Inhibits cytokinin and uses auxin

• Development of lateral buds of the stem

  • Uses cytokinin and inhibits auxin