Auxin and Phytohormones

What is a phytohormone?

A phytohormone is a chemical messenger that coordinates growth, development, and responses between different parts of a plant.

Who first defined plant hormones?

Frits Went and Kenneth Thimann in 1937.

What do phytohormones regulate in plants?

• Cell division

• Cell elongation

• Differentiation

• Pattern formation

• Organogenesis

• Reproduction

• Stress responses (abiotic and biotic)

Name the main classes of phytohormones.

• Auxin

• Cytokinin

• Gibberellins

• Strigolactones

• Brassinosteroids

• Ethylene

• Abscisic acid (ABA)

• Salicylic acid

• Jasmonic acid

How do environmental stimuli affect plants?

Environmental stimuli (light, gravity, stress) alter gene expression and physiology, often via phytohormones.

What is the main natural auxin in plants?

Indole-3-acetic acid (IAA).

What major processes does auxin regulate?

• Growth

• Phototropism

• Gravitropism

• Branching

• Embryonic patterning

• Stem cell maintenance

• Organ initiation

How does auxin affect shoot branching?

Auxin inhibits branching in the shoot (apical dominance).

How does auxin affect root branching?

Auxin promotes lateral root formation and branching.

What is apical dominance?

The suppression of side shoot growth by auxin produced at the shoot apex.

How do auxin and cytokinin interact?

They act antagonistically (opposing effects) in many developmental processes.

Where do auxin and cytokinin interact strongly?

At the root apex, where they regulate growth and differentiation.

Why is auxin–cytokinin interaction important in tissue culture?

The ratio of auxin to cytokinin determines whether roots, shoots, or callus form.

What does totipotent mean?

A cell can give rise to a whole plant.

What is callus?

Undifferentiated plant cells grown in tissue culture.

What hormone ratio promotes shoot formation in tissue culture?

High cytokinin : low auxin.

What hormone ratio promotes root formation?

High auxin : low cytokinin.

Which auxin is usually used in tissue culture instead of IAA?

NAA (a synthetic auxin).

What do gibberellins mainly regulate?

Stem elongation and growth.

Why are gibberellins important in agriculture?

Genes controlling gibberellin synthesis were key to the Green Revolution.

What do strigolactones do?

Inhibit shoot branching.

What are brassinosteroids involved in?

• Cell elongation

• Seed germination

• Pollen tube growth

• Vascular differentiation

• Stress tolerance

What is phototropism?

Directional growth in response to light.

What type of phototropism do shoots show?

Positive phototropism (growth towards light).

Which receptors are involved in phototropism?

Phototropins (blue-light receptors).

Who carried out early experiments on phototropism?

Charles Darwin.

How is auxin transported in plants?

Through polar transport using PIN proteins.

Where are PIN proteins located in cells?

On one side of the plasma membrane.

Why is PIN protein location important?

It determines the direction of auxin movement.

How does light affect PIN proteins?

Light causes PIN proteins to relocate, redirecting auxin flow.

Why is auxin considered a key plant hormone?

It integrates growth, development, and environmental responses through controlled transport and concentration gradients.