Plant Signals & Responses [Chapter 39]

Lecture 3

What is a tropism?

A growth movement toward or away from a stimulus

What is phototropism?

A plant's growth response to light direction and intensity

How do plants respond to different wavelengths of light?

They grow toward white, red, and blue light; far-red light can reverse red light effects (Pfr → Pr)

What is the effect of UV light on apple skin?

UV light activates genes that produce red pigments on the sun-exposed side

• Stimulates the biosynthesis of anthocyanins (water-soluble red, blue, purple pigments)

  • They protect cells from UV damage (like sunscreen)

    • Thus they would be considered as flavonoids

What are flavonoids?

UV-blocking pigments made by plants that also act as antioxidants (i.e. anthocyanins)

What are anthocyanins?

Red/blue/purple pigments in flowers and fruits produced by UV light to attract pollinators

• UV light triggers the production of red pigments in apples by stimulating the biosynthesis of arthocyanins.

  • Specific photoreceptor gets activated and starts a signaling cascade

What is photoperiodism?

A plant's response to changes in day/night length (plant’s ability to detect day length)

• Controls:

  • Flowering

  • Leaf drop (senescence)

  • Bud dormancy or break

  • Tuber or bulb formation

What happens in deciduous plants as days shorten?

They stop making chlorophyll, recycle proteins, and shed leaves

• Proteins like Rubisco (used in photosynthesis) contain a lot of nitrogen so it gets broken down into amino acids + nitrogen and and goes back into stems/roots/buds

What are day neutral plants?

Plants that flower regardless of day/night length

• Much more geographically adaptable and flexible in flowering time

What are short day plants?

Plants that bloom when days get shorter

• This ensures seed development occurs under favorable conditions and avoids competition

What are long day plants?

Plants that bloom when days get longer

• Flowering during more sunlight

What are long day short day plants (Sequential Photoperiodic Plants) ?

Plants like chrysanthemums that bloom only after long days followed by short days

• Allows them to flower in a very specific seasonal window

What is phytochrome?

A plant pigment sensitive to red and far-red light that controls light-related growth responses (Pr and Pfr)

What happens when Pr absorbs red light (~660 nm)?

It becomes Pfr

What happens when Pfr absorbs far-red (~730 nm) light?

It becomes Pr

How does phytochrome respond to night length?

Long nights produce more Pr; short nights produce less

• In the absence of light, Pfr slowly reverts back to Pr

  • Short-day plants flower when enough Pfr has dissapeared (long nights)

  • Long-day plants flower when enough Pfr is still present (short nights)

What is etiolation?

A pale, elongated growth pattern seen in plants grown in darkness

• It’s a survival strategy: plants are trying to reach light quickly

• Caused by active auxins (growth hormones) + lack of light signals to stop stem elongation

What changes when etiolated plants are exposed to white light?

They become green (produce chlorophyll), leaves expand, internodes shorten, and tips become erect

• This is known as de-etiolation

What is cryptochrome?

A blue light receptor that starts signal transduction in plants and is found in all eukaryotes

• helps regulate growth, circadian rhythms, and development

What is thigmomorphogenesis?

A plant's altered growth response to physical touch

• Can increase lignin in tissues to add rigidity and support

What is gravitropism?

A plant’s growth response to gravity

1. Roots = positive gravitropism (grow downward)

2. Shoots = negative gravitropism (grow upwards)

What plant doesn't respond to gravity?

Cycads

What is auxin?

A plant growth regulator that stimulates cell growth and division, mainly in shoot and root tips

• Central role in phototropism and gravitropism

What is the natural form of auxin?

Indole-3-acetic acid (IAA)

• Produced in shoot apical meristems, young leaves, and seeds

What is the artificial form of auxin?

naphthaleneacetic acid (NAA)

• Promotes root formation in cuttings

• Prevents fruit drop

Where is auxin made?

In shoot tips, root tips, young leaves, shoot apical meristem, stems, buds, seeds, seedlings, and fruits

• Auxin is transported top (shoot tip) → bottom (roots)

How is auxin used in plant culture?

To stimulate root formation and combined with cytokinin to grow tissues

What are gibberellins?

Growth regulators that promote cell division and elongation

• Mostly produced in developing seeds and shoot apical meristems

• Travel via xylem AND phloem

What is GA3 (Gibberellic Acid 3)?

The most famous gibberellin used to stimulate plant growth

• Stimulates cell division, elongation, and enzyme production

What is foolish seedling disease?

A fungal disease where a fungus secrets GA₃ which make rice grow too tall and fall over

What effect do gibberellins (GA₃) have on seedless grapes?

They increase size and weight, making grapes bigger and rounder

• Also loosens the grape cluster making it easier to harvest

How do gibberellins affect citrus fruit?

They preserve fruit on the plant longer and allow flowering and fruiting at the same time

• GA₃ helps maintain green color and delays rind aging

• Also increases the fruit size

What are cytokinins?

Plant hormones that stimulate cell division

• Produced in the roots and transported upward through the xylem to the shoots

• Also promote:

  • Shoot development

  • Delay of leaf aging (senescence)

What is zeatin?

A cytokinin found in corn (Zea mays)

What determines plant tissue outcome in culture?

More auxin = roots, more cytokinin = shoots, equal amounts = callus

• Callus is a mass of undifferentiated plants cells

What is abscisic acid (ABA)?

A stress hormone that helps plants conserve water and enter dormancy

• Closes the stomata to reduce water loss

• Produced in mature leaves, roots, and seeds

• Travels through the xylem and phloem

How does ABA affect stomata?

It causes stomata to close to prevent water loss

What does high ABA indicate?

Dormant state in seeds and buds during winter

• Also indicates that the plant is under stress, specially from drought, salt, etc…

What is ethylene?

A gaseous hormone involved in stress response and fruit ripening

• Triggers softening, color change, aroma, and sugar production

What does ethylene do in fruit?

It converts starches to sugars, softens fruit, and enhances aroma to attract animals

• Ethylene acts as a hormonal signal that activates ripening-related genes

What happens when one wounded apple produces ethylene?

It causes nearby apples to ripen and then spoil

• Ethylene is a gas, so it easily spreads through the air to nearby fruits.

What are eicosanoids in plants?

Signaling molecules made from phospholipids in response to stress

What is the function of salicylic acid in plants?

It helps activate defenses against viruses and prevents their spread

• Regulates senescence (leaf aging)

• Inhibits auxin transport (to redirect energy)

• Cross-talk with other hormones

What compound is derived from salicylic acid and used as a medication?

Aspirin (acetylsalicylic acid)

• a platelet aggregate inhibitor

What are the main Plant Growth Regulators (PGRs):

1. Auxins

2. Cytokinins

3. Gibberellins

4. Abscisic Acid

5. Ethylene

6. Salicylic Acid

7. Eicosanoids