Botany Final

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Plant growth and development
relies on the interplay of internal and external signals
All plants cells are
-Totipotent
-Capable of becoming any other plant cell
Internal Factors: Hormones
1. Auxins
2. Cytokinins
3. Gibberellins
4. Ethylene
5. Abscisic Acid
6. Jasmonic Acid
Jasmonic Acid
-Signaling hormone active in almost all plant defenses
-Tells all other cells to turn on chemical defense (repel, kill, secrete antifungals, warn other plants)
-Secondary metabolites mobilized by Jasmonic Acid
Auxins: Charles Darwin
-Charles Darwin and his son were the first to study Auxins.
-Studied phototropism (movement in response to light) in coleoptiles of grass. Reasoned that there was something in the tips causing phototropism
Auxins: Frits
-1926 Frits Went discovered the chemical basis of phototropism
-Determined there was a chemical involved
Function of Auxins
1.Phototopism
2. Promotion of cell differentiation
3. Fruit growth
4. Apical Dominance
5. Prevent Abcission
6. Weed control
Indole-Acetic-Acid (IAA)
-Dominant Auxin found in almost all plants
-Derived from Tryptophan
Auxins are formed in
-Meristem (tips) of stems
-meristems, coleoptiles, and young leaves
-Move downward through the cortex
Auxins
-Not accumulated in tissues by continually degraded by enzyme action and typically work by enhancing the release of enzymes that loosen cell walls.
-Break links between hemicelluloses - internal pressure of plant cells are typically high causing them to expand.
-Auxins are why you have a zone of elongation
Auxins: Phototropism
-Plant bends toward light
-Produced in meristem
-Auxins are diverted to the shady side causing cell elongation on the shady side causing it to bend.
-Differential diversion of Auxins causes differential cell growth.
Auxins: Promotion of cell differentiation
-Auxins in leaf primordia signals the differentiation of procambium into xylem and phloem.
-Buds have lots of Auxins and create own vascular tissue
-Differentiation of root tissue
NAA
Synthetic auxin; stick stem into it and it causes differentiation into root tissue.
Auxins: Fruit growth
-In developing seed - released into ovary walls to promote development of fruit
-How we have developed seedless fruits. (Artificial auxins) produce Parthenocarpic (seedless) fuit
Auxins: Apical Dominance
-Suppress lateral buds: favor lengthwise growth rather than lateral growth.
-Countered by another hormone (interplay)
Auxins: Prevent Abcission
-Prevent leaves and fruit from falling off
-Spray orchards with artificial auxins to prevent fruit from falling
Auxins: Weed control
-Many herbicides are artificial auxins
-Often times we add chlorine or bromine to create environmentally stable synthetic auxins.
-Agent Orange (2,4-D; 2,4,5-T; Dioxin): herbicide used heavily during vietnam war that killed everything in the forest. Thought to be safe to humans but it was not. Infantry soldiers have had degenerative diseases and cancer; birth defects;
Cytokinins
-Found especially in actively dividing cells (seeds, fruits, leaves and root tips)
-travel in the xylem
-Derived from Adenine
-First discovered in culturing using coconut milk.
Functions of Cytokinins
1. Promote Cytokinesis
2. Promote Lateral bud activity
3. Prevent leaf Senescence
Cytokinins: Promote Cytokinisis
-Cytokininsis = last step of cell division including formation of cell wall
-Do not really do anything alone
-Ration with Auxins determines what they do.
-1:1 - A/C = Shoot development
-high C: low A = undifferentiated tissue (thallus)
-low C: high A = root development
Cytokinins: Prevent leaf senescence
-Anti-aging hormone
-prevent DNA from turning off
Ethylene
-Ethylene gas
-Gas product of metabolism that is passed into the atmosphere but also serves as a signaling hormone
-often formed in fruits, seedlings, stems and in response to mechanical stress
Ethylene Function
1. Promotes fruit ripening
2. Promotes Abscission
3. Promotes Femaleness in monecious cucurbits
4. Promotion of stem thickening
5. Promotes Production of Aerenchyma in wetland plants
Ethylene: Promotes fruit ripening
-Favors concentration of sugar, anthocynins, the loosening of cell walls and sweet
-Production of a lot of volatile organic molecules (smell good and taste good).
-Over ripe fruits are losing ethylene - can promote the ripening of others
-Can pick things early and gas them with ethylene gas to promote ripening
Ethylene: Promotes Abscission
-Of fruits and leaves
-creates abscission layer
Ethylene: Promotes femaleness in monecious cucurbits
-Monecious - both sexes of flowers
-Cucurbits = cucumbers
-Treat cucumber vines with ethylene to convert male flowers to female flowers
Ethylene: Promotion of stem thickening
-In response to mechanical damage
-In seedlings and woody plants
-Inhibits stem elongation and promotes swelling
-Consistent high winds increases ethylene production thickening stem for better wind resistance
Ethylene: Promotes production of Aerenchyma in wetland plants
-In response to mechanical damage
-Response to Hypoxia (plant cells need oxygen)
-Ethylene release ends with production of cellulase: enzyme that breaks down cellulose (cell walls) - leads to production of large air spaces in the tissue to facilitate oxygen exchange.
Abscisic Acid (ABA)
-Discovered in 1965
-Main plant tranquilizer - puts plants to sleep
-Inhibition of development and growth
-reaches highest concentrations in dormant tissues such as winter buds and seeds (maintains dormancy).
Abscisic Acid Functions
1. Promotes bud and seed dormancy
2. Along with Auxins and ethylene it promotes fruit/leaf drop
3. Transpiration
Abscisic Acid: Promotes bud and seed Dormancy
-Changes vegetative buds to winter buds
-Reduces seed sprout
-Loss of ABA associated with break in dormancy (can be washed out; freeze/thaw cycle in winter)
Abscisic Acid: Promotes fruit/leaf drop
-Promotion of Abscission layer
Abscisic acid: Transpiration
-Shuts guard cells
-Closes stomata during water stress
Gibberellins (GA)
-Discovered in Japan in the 1920s.
-Discoverd that some rice seedlings would grow taller than others and their stems would become weak and they would fall over and die (foolish seedling disease). They were infected by a fungus from the family gibberelum that secreted gibberelic acid.
-Found that plants produced some small amounts of gibberelic acid to promote growth (young seeds, fruits, embryos and leaves)
-Function: to stimulated cell division and elongation
Gibberellins Function
1. Promotes stem growth at the internodes
2. Promotes parthenocarpic fruit development
3. Promotes the mobilization of food in seeds
Gibberellins: Promote stem growth at the internodes
-at intercallary meristems at the base of internodes
Gibberellins: Promote parthenocarpic fruit development
-Often used with auxins to produce seedless fruit
-main way they make seedless grapes and mandarin oranges
Gibberellins: Promote the mobilization of food in seeds
-Embryo - Ga - Aleurone layer - Alpha-Amylase - endosperm - sugars - malt
**Alpha-Amylase breaks down carbs to sugar
External Stimuli:
1. Tropisms (Growth responses)
2. Phytochrome-Mediated Response
3. Temperature Mediated Response
Tropisms
-Growth response toward or away from a particular stimulus
-Usually mediated by Auxins but caused by an external stimuli
Phototropism
-Growth towards or away from a light stimulus
-Auxins diverted to shady side triggers production of expansions (loosens cell walls causing them to expand).
-Causes them to bend
Gravitropism (Geotropism)
-Shoots have negative gravitropism and roots have positive gravitropism
-Statocytes: specialized amyloplasts
Hydrotropism
Roots grow towards water (weaker than gravitropism)
Thigmotropism
-Response to touch
-Bean vine winding around a pole
-Cells shorten on side where stimulus is or widen on opposite to twine around.
-requires auxins
Phytochrome-Mediated Response
-Involve a receptor known as phytochrome that serves as a detector of light (in particular, a couple of wavelengths of red light).
-Phytochrom is a membrane pigment found in 2 forms:
1. Pr: Response to red light
2. Pfr: Response to far red light
**Both are interconvertable
**(IR)(FR)ROYGBIV
-Phytochromes are found in all land plants (also fungi, cyanobacteria, and bacteria)
Pr
-Continuously synthesized from precursor molecule
-Biologically inactive form
Pfr
-Continuously degraded by enzymes
-Biologically active form
Dark reversion
-In the dark, Pfr is slowly converted back to Pr.
Phytochrome-Mediated Response is important in a lot of light dependent mechanisms
1. Seed germination
2. Shoot growth
3. Photoperiodism and flowering
Phytochrome-Mediated Response: Seed germination
-Especially is some weedy plants
-Seeds need exposure to light to break dormancy (Pfr)
Phytochrome-Mediated Response: Shoot growth
-Etiolation response: germination in the dark. Evolutionary adaptation to break through the soil and grow long to find light
-Activated by Pr; when it finds light, converts to Pfr. Triggers de-etiolation. Stem thickens and turns into a normal plant.
-Small leaves on outside of trees with larger in shade. Large ones have an etiole repsonse: longer petioles etc. until they find light and become normal leaves.
Phytochrome-Mediated Response: Photoperiodism and flowering
-Photoperiodism: plants response to length of day; triggers certain plants to bloom.
-Night length is critical.
-Timing mechanism to ensure they can put out enough vegetation for photosynthesis and reproductive output.
-Phytochromes trigger end of day. Pfr either destroyed or converted to Pr.
3 types in regards to flowering:
1. Day Neutral
2. Long day plants
3. Short day plants
Day Neutral
-No effect by light stimulus
-Mainly in tropics
-Reproductive output triggered by maturity in plant
Long Day plants
-Short night plants
-Summer plants
-Flower when night is shorter than a critical set-point
Short day plants
-Long night
-Spring and fall plants
Florigen
Hypothetical name for what converts buds/leaves into flowering buds
Temperature Mediated Responses
Many have to do with the dormancy of seeds and buds. Often need an extended period of cold.
Temperature Mediated Responses: seeds
-Stratification: extended exposure of seeds to cold
-Scarification - water uptake
Temperature Mediated Responses: Buds
-decrease in day length in fall increases dormancy (causes abcisic acid to accumulate in the buds)
-Needs prolonged exposure to cold to break dormancy (AKA - vernalization)
--Often seen in fruit trees to produce maximum amount of fruit (peaches, cherries, apples, plums)
--also explains phenomenon of winter annuals: wheat, rye, and barley - planted in fall, germinate and develop seed over the winter
--Must be exposed to the proper number of cold hours for a good crop
-Also explains biennial plants (flower every other year)