OEB52 Quiz #10: Symbiosis + environmental perception

What is symbiosis?

Close physical association between two or more different organisms

Three types of symbiosis?

• Mutualism → both benefit

• Commensalism → one benefits, other unaffected

• Parasitism → one benefits, one harmed

Why is symbiosis important evolutionarily?

Responsible for major innovations in life (endosymbiosis)

What do plants gain?

• Nutrients (N, P)

• Protection from pathogens

• Pollination & dispersal

Most limiting nutrients?

Nitrogen (N) and Phosphorus (P)

Who can fix nitrogen?

Prokaryotes ONLY

Reaction of nitrogen fixation?

N₂ → NH₄⁺ (usable form)

What bacteria are involved? symbiosis legumes

Rhizobium

Where does fixation occur? symbiosis legumes

Root nodules

Why is nitrogen fixation tricky?

Requires low oxygen (anaerobic conditions)

How is nitrogen fixation maintained?

Plant hemoglobins regulate O₂

What are mycorrhizae?

Symbiosis between fungi + plant roots

Plant gets:

• ↑ nutrient uptake (especially phosphorus)

• Protection

• Larger effective root system

Fungus gets:

Sugars (carbon)

Types of Mycorrhizae

Endomycorrhizae (VAM)

Ectomycorrhizae

Endomycorrhizae (VAM)

• Fungus enters root cells

• Most common (~65%)

• Exchanges happen via arbuscules

Ectomycorrhizae

• Fungus surrounds root

• Does NOT enter cells

• Common in woody plants

Do fungi cross the endodermis?

No

What are fungal endophytes?

Fungi living inside plant tissues

Function fungal endophytes?

• Protect against herbivores

• Increase stress tolerance

Pollination Symbiosis

• Fig + wasp

• Yucca + moth

Why is agriculture symbiosis?

Humans + crops both benefit

• Humans → food

• Plants → protection, spread

Why do plants perceive environment?

To regulate:

• Germination

• Flowering

• Growth

• Dormancy

four categories of environemntal perception

• Tropisms

• Photomorphogenesis

• Circadian rhythms

• Photoperiodism

What are tropisms?

Growth toward/away from stimulus

Types of tropisms

• Phototropism → light

• Gravitropism → gravity

• Thigmotropism → touch

What detects light?

Phototropins (blue light receptors)

What happens in phototropism mechanism?

Auxin redistributes → uneven growth → bending

Auxin Rule

• Shoots: ↑ auxin → ↑ growth

• Roots: ↑ auxin → ↓ growth

What senses gravity?

Statoliths (amyloplasts)

Mechanism of gravitropism

Settle → signal → auxin redistribution → bending

Direction of Growth

• Roots → positive gravitropism (down)

• Shoots → negative gravitropism (up)

What triggers Thigmotropism?

Touch → Ca²⁺ signaling → auxin redistribution

what is photomorphogenesis

Development controlled by light

Example: Etiolated (dark-grown) vs light-grown plants

Types of Photoreceptors

• Phototropins → blue light

• Phytochromes → red/far-red

• Cryptochromes → blue/UV

Two forms of phytochrome system?

• Pr (red-absorbing)

• Pfr (far-red absorbing, active)

what triggers shade avoidance?

Low red:far-red ratio (shade)

Effects:

• Taller growth

• Less branching

• Earlier flowering

What are circadian rhythms?

24-hour internal biological cycles

What is photoperiodism?

Response to day length

Key molecule for flowering control?

Florigen (FT protein)

What gene regulates flowering?

CONSTANS (CO) → activates FT