Biology 213 Regulation of Plant Growth (Ch. 37 ...

Biology 213 Regulation of Plant Growth (Ch. 37)
Reproduction in Flowering Plants (Ch.38, pgs. 802-809)

Growth is regulated by:
Genome
Environmental Cues—light, gravity, nutrient levels, etc.
Perception by receptors (photo and chemical)
Response- mediated by hormones

Signal Transduction Pathways—(same as in animals)

Plant Growth Hormones:
Abscisic acid
Auxins
Brassinosteroids
Cytokinins
Ethylene
Gibberellins

A Brief Overview of Regulation of Plant Development:

Seed Dormancy and Germination:
Causes of Seed Dormancy

Exclusion of water
Tough Seed Coat
Chemical Inhibitors- ABA
Lack of Light

Advantages of Seed Dormancy:

To Break Seed Dormancy:

Once the coat is broken:
Imbibition—uptake of water

Mobilize food reserves- Main hormone involved Gibberelin (GA)

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Germinating seedling is growing…Hormones involved in growth and development, etc.
Gibberelins-growth promoting hormone

Mobilize food reserves in seeds
Break dormancy in buds
Induce stem growth and bolting
Seedless fruit ripening and growth

Auxin is involved in many responses:
Phototropism
Gravitropism
Stem Cell Elongation and Root Cell Growth Inhibition?
Seedless fruit ripening and growth
Gene expression

Auxin: The first plant hormone discovered
History of discovery and phototropism
Darwin and son and Frits Went

Conclusions of Darwins: light was perceived at one place, response was at another, some signal was
traveling in the plant

Went- captured the chemical in gelatin blocks and showed its affects, later hormone was isolated

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Auxin and Cell Expansion:
Cellulose microfibrils in the cell wall determine which way a cell can expand
Cells take in water
Microfibrils must loosen move farther apart from each other
New microfibrils are deposited

Auxin loosens the cell wall by releasing a “wall-loosening factor” --- H+
Signal (light, gravity)
Auxin redistribution
Activates proton pump
Activates “expansins”
Cell wall can expand

Cytokinins:
Inhibit stem elongation; produce swelling of stems

Ratio of Auxin and Cytokins- Controls Cell Differentiation and Apical Dominance
Auxin is produced in buds and young leaves
Cytokinin is produced mainly in roots

“Clumsy Cow” model--- Balance determines if roots or shoots will be produced
Apical Dominance

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Ethylene- usually considered a general growth inhibitor, senescence hormone
Inhibit stem elongation; produce swelling of stems
Ripens many fruits
Breaks down chlorophyll

Ratio of Ethlyene and Auxin- controls leaf abscision

Abscicic Acid—Stress Hormone (another growth inhibitor)
Helps control stomatal opening in response to water stress
Promotes and maintains seed and bud dormancy
Inhibits stem elongation

Brassinosteroids—“newest” discovered hormone
Promotes cell elongation in stems and pollen tubes
Inhbits cell elongation in roots
Vascular tissue differentiation
Mediates many responses to light- plants that lack this hormone don’t always perceive light

Hormones are often involved in regulating gene expression:

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Plants and Light:
Plants use light for energy and information
for photobiological response- light must be absorbed
pigments or photoreceptors only absorb certain wavelengths of light

Plant physiologists want to know
What light is involved?
make an action spectrum
What pigment (photoreceptor) is involved?
make an absorption spectrum and compare to action spectrum

Blue Light Photoreceptors:
Phototropins--- Phototropism:
Cryptochromes--- Seedling development and flowering:
Zeaxanthin --- Stomata opening:

Discovery of mutants that lack a response are very helpful in determining photoreceptors!!!

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Red Light Photoreceptors: Phytochromes
The Phytochromes:

photoreversible pigments
Switch between Pr and Pfr
When in the Pfr position the pigment initiates responses

Phytochromes involved in:
Seed Germination
Deetiolation of plants
plants grown in the dark are etiolated—lack chlorophyll, spindly, “hook” etc.
Photoperiodism—detecting the season
Flowering
Breaking bud dormancy, etc.
Sensing Shade

Flowering:
Vegetative bud becomes a flower bud in response to correct age and or environmental signals, controlled
by Gene Expression
Apical meristem→ Inflorescence meristem→ Floral meristem

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Discovery of phytochromes involvement in flowering;
Maryland Mammouth Tobacco-did not flower at normal time of the year
determined that it needed a short day to flower

For Flowering: (really long-night plants)
Short-day- SDP (really short-night plants)
Long-day- LDP
Day-neutral

Evidence for the Perception of light:

Florigen: Flowering Hormone
Small protein
Produced in phloem companion cells in the leaf
Travels in the sieve elements to the apical meristems
“Necessary” for the gene expression that converts apical meristem into floral meristem

Flowering is really not that simple:
Some plants need a series of different dark lengths

Some plants need cold treatment followed by long days
Cold treatment—vernaliztion

Circadian rhythms and light perception interact to get a response

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