Plant hormones 8C

Why do plants need to respond to their environment?

To maximise survival chances, e.g., growing towards light for photosynthesis, producing chemicals to deter herbivores, and flowering at the right time for reproduction.

What are some stimuli that plants respond to?

Light, gravity, physical objects, herbivory, water, and physical touch.

How do plants respond to stimuli without a nervous system?

They rely on chemical substances like phytochromes, auxins, and gibberellins.

What are the two forms of phytochrome?

PR (inactive, absorbs 660 nm red light), PFR (active, absorbs 730 nm far-red light).

How does light affect phytochrome conversion?

PR absorbs red light → converts to PFR; PFR absorbs far-red light → converts back to PR; In darkness, PFR slowly reverts to PR.

How does phytochrome influence seed germination?

Red light (660 nm) converts PR to PFR, triggering germination. Far-red light (730 nm) converts PFR back to PR, preventing germination.

How does phytochrome regulate flowering?

Long-day plants flower when short nights maintain high PFR levels. PFR acts as a transcription factor, activating genes that promote flowering.

How does phytochrome regulate transcription?

PFR moves into the nucleus and binds to PIF3 (phytochrome-interacting factor 3). This complex initiates transcription of light-responsive genes.

What is Indoleacetic Acid (IAA)?

A type of auxin that influences plant growth, including phototropism and geotropism.

How does IAA move in plants?

Transported via diffusion, active transport, and phloem.

How does IAA cause phototropism in shoots?

Light causes IAA to accumulate on the shaded side. Higher IAA concentration increases cell elongation, bending the shoot towards light.

How does IAA cause geotropism in roots?

Gravity causes IAA to accumulate on the lower side of roots. High IAA concentration inhibits cell elongation, causing the root to bend downwards.

How does IAA regulate transcription?

It alters gene expression for proteins involved in cell growth.

What are gibberellins?

Plant growth regulators involved in seed germination, stem elongation, and flowering.

How do gibberellins initiate seed germination?

The embryo absorbs water, triggering gibberellin production. Gibberellins diffuse to the aleurone layer, stimulating amylase gene transcription. Amylase hydrolyses starch into maltose → glucose → energy for embryo growth.

What role does gibberellin play in transcription?

It increases transcription of genes coding for amylase, allowing starch breakdown in seeds.