Plant Hormones Flashcards
Signal transduction
- Reception: hormone or environmental stimulus binds to receptor in plasma membrane
- Transduction: signal passes through relay protein and 2nd messengers in cytoplasm
- Response: activation of cellular responses
Plant response to light (de-etiolation mechanism)
Phytochrome (photo receptor) receives light
Phytochrome activates 2nd messenger cGMP and Ca+2 channel, causing it to open
cGMP and Ca+2 activate kinases
Kinases activate transcription factors
Transcription and then translation of genes creates de-etiolation (greening) response proteins
Hormones
Organic substances produced in small amounts in 1 part of the organism that are then transported to another part where they have a specific effect
Seed germination
- Cold, fire, or digestion breaks dormancy
- H2O uptake
- Embryo secretes GA (Gibberellins) which travels to the Aleurone (outer layer of endosperm)
- Aleurone secretes alpha-amylase, which breaks down starch
- Sugar produced by starch breakdown gives plant food to fuel germination
Tropisms
Growth responses and movements in plants due to specific stimuli
Phototropism
Growth response and movement due to light
Gravitropism
Growth response and movement due to gravity
Thigmotropism
Growth response and movement due to touch
Mimosa plant
Rapid loss of cell turgor in plant “joints” due to touch
Hormone effects depend on…
Site of action
Developmental stage
Concentration
Ratio (2 or more hormones)
Auxin
Increases cell division in shoots
Increases cell expansion in shoots
Produced in embryo and meristems
Auxin and phototropism
Polar transport: unidirectional from shoot toward base of plant (faster than diffusion)
Activates proton pumps
Acid growth hypothesis
- Auxin increases H+ pumping
- Decrease in pH activates expansins
- Expansins cause cell wall loosening
- Solute intake lowers water potential
- Osmosis and cell expansion
Auxin and gravitropism
Auxin in shoots is laterally transported in roots and polar transported up roots
Gravity affects lateral auxin transport: plant is laid on side- auxin accumulates on that side, stimulating the plant to grow away from the ground
Low auxin stimulates growth
Statoliths (starch-containing cells) settle to the lowest sides of the root cap cells, telling the root which side is down
Cytokinin
Increases cell division
Neither increases nor decreases cell expansion
Produced in roots and other actively growing tissues
Causes apical dominance (apical meristem suppresses other areas of meristem from growing), anti-aging of flowers, and interact with auxin to cause plant growth
Gibberellins (GA)
Increases cell division in stems and leaves
Increases cell expansion in stems and leaves
Produced in roots and shoots, but affects shoots
Used in fruit development (seedless grapes); increases size of fruit
Causes bolting (plant producing flowering stalk)
Abscisic acid (ABA)
Decreases cell division
Decreases cell expansion
Produced in roots and leaves
Inhibits germination in seeds (imbibation step in germination washes out this hormone)
Precocious germination: lack of this
Stress hormone: when roots sense drying soil, this causes stomatal closure
Wilting: opens K+ channels
Ethylene
Decreases cell division
Decreases cell expansion
Produced in almost all plant tissues, but especially in fruits
Causes apoptosis and senescence (irreversible aging of plant that leads to death)
Fruit ripening and leaf abscission (dropping leaves)
Triple seedling response
Caused by ethylene
Slowed vertical growth
Increased horizontal growth
Thickening of stem
Brassinosteroids
Similar to auxin
Increases cell division in shoots
Increases cell expansion in shoots
Produced in seeds, fruit, shoots, and buds
Inhibits root growth, retards leaf abscission, and promotes xylem differentiation
Photoperiodism
Relative effects of night and day
Phytochromes sense light
Effects: reproduction and senescence
2 forms of phytochromes: Pr (absorbs red) and Pfr (absorbs far red)
Pr is default state; absorption of red photon causes Pr to change into Pfr
Pfr is “active” state: seed germination, control of flowering, etc.
Pfr slowly converts back to Pr in darkness
Lettuce seeds and light
Lettuce seeds germinate in red light
Far red light resets plant (like it never germinated)
Short day plants
Low amounts of Pfr stimulate flowering
Long day plants
High amounts of Pfr stimulate flowering
