AP Biology, Chapter 39

SUMMARY
INTRODUCTION

PLANT HORMONES
	Introduction
	Research on how plants grow toward light led to the discovery of plant hormones: 		science as a process
		1. For each of the following scientists, describe their hypothesis, experiments, and 
		conclusions about the mechanism of phototropism: Charles Darwin, Francis Darwin, 
		Peter Boysen-Jensen, F.W. Went.
			a. Charles Darwin and Francis Darwin
				i. Hypothesis: shoot tip mediates phototropism
				ii. Experiments
					.Remove or opaquely shield tip, no bending
					.Transparent tip shield or base shield, bending
				iii. Conclusion: tip senses light, required for response
			b. Boysen-Jensen
				i. Hypothesis: tip produces substance that moves downward
				ii. Experiment: gelatin block vs. mica insert
				iii. Conclusion: chemical substance moves downward
			c. Went
				i. Hypothesis: secretion of substance is asymmetrical
				ii. Experiments
					.Agar block captures substance
					.Asymmetric application mimics phototropism
					.Purified the substance auxin
				ii. Conclusion: auxin made on the shady side causes cell enlargement, 
				bending [currently in disfavor]
	Plant hormones help coordinate growth, development, and responses to 
	environmental stimuli
		2. List five major classes of plant hormones, describe their major functions, and note 
		where they are produced in the plant.
			a. Auxin
				i. Functions
					.Stimulates cell elongation
					.Root growth, differentiation, branching 
					.Fruit development
					.Maintains apical dominance
				ii. Produced in embryo, meristems, young leaves
			b. Cytokinins
				i. Functions
					.Stimulates cell division
					.Stimulates germination
					.Delays senescence
					.Counters apical dominance
				ii. Produced in roots
			c. Gibberellins
				i. Functions
					.Stimulate cell elongation and cell division
					.Stimulates fruit development
					.Released by embryo to promote germination
				ii. Produced in apical buds, young leaves, embryos
			d. Abscisic acid
				i. Functions
					.Slows growth, inhibits vascular cambium
					.Maintains seed dormancy
					.Causes rapid closing of stomata during wilting 
				ii. Produced in terminal buds
			e. Ethylene
				i. Functions
					.Promotes fruit ripening
					.Stimulates development of the abscission layer in the fall
				ii. Produced in ripening fruits, nodes, aging leaves and flowers
		3. Explain how a hormone may cause its effect on plant growth and development.
			a. Stimulate division, elongation and differentiation
		4. According to the acid-growth hypothesis, explain how auxin can initiate cell elongation.
			a. Auxin stimulates proton pumps
			b. Protons in the cell wall stimulate enzymes that loosen it
			c. Auxin also stimulates growth of the cytoplasm and wall
		5. Explain why 2,4-D is widely used as a weed killer.
			a. 2,4-D = 2,4-dichlorophenoxyacetic acid, a synthetic auxin
			b. Application is a hormonal overdose 
		6. Explain how the ratio of cytokinin to auxin affects cell division and cell differentiation.
			a. More cytokinin  shoot buds develop
			b. More auxin  roots develop
			c. Equal amounts  undifferentiated callus grows
	Analysis of mutant plants is extending the list of hormones and their functions
	Signal-transduction pathways link cellular responses to plant hormonal signals and 
	environmental stimuli
		7. Describe how signal-transduction pathways can affect multiple responses to plant 
		hormones.
			a. Hormone binds to cell surface receptor
			b. Stimulates the formation or release of a second messenger
			c. Second messenger effects
				.Turn on proton pumps
				.Stimulate secretion of wall materials
				.Activate transcription factors
PLANT MOVEMENTS AS MODELS FOR STUDYING CONTROL SYSTEMS
	Introduction
	Tropisms orient the growth of plant organs toward or away from stimuli
		8. Distinguish tropisms and turgor movements.
			a. Tropisms
				i. Irreversible cell elongation
				ii. Bends a shoot or root toward or away from a stimulus
			b. Turgor movements
				i. Reversible due to movements of K+
				ii. Coordinated by action potentials like those in nervous tissue
				iii. Ex. Mimosa folds its leaves, beans lower their leaves at night
		9. Describe the common plant tropisms.
			a. Phototropism
				i. Shoot bends toward light
				ii. An inhibitor of elongation accumulates on the lighted side
			b. Gravitropism
				i. Shoots move away (negative), roots move towards (positive)
				ii. May involve statoliths 
			c. Thigmostropism
				i. Climbing plant tendrils coil when they contact something to climb on
				ii. Non-climbers may be stunted by touch
	Turgor movements are relatively rapid, reversible plant responses

CONTROL OF DAILY AND SEASONAL RESPONSES
	Biological clocks control circadian rhythms in plants and other eukaryotes
		10. Define circadian rhythm and explain what happens when an organism is artificially 
		maintained in a constant environment.
			a. Circadian rhythms are physiological responses on a 24-hour schedule
			b. In a constant environment:
				i. Bean plants maintain sleep movement cycles
				ii. Stomata open and close
				iii. Humans maintain sleep-wake cycle
	Photoperiodism synchronizes many plant responses to changes of season
		11. Define photoperiodism.
			a. Physiological response to the length of the day
			b. Usually timing seasonal events like flowering
		12. Distinguish between short-day, long-day, and day-neutral plants. Explain why these 
		names are misleading.
			a. Types of resonses
				i. Short-day
					.Triggered by day length shorter than a critical time
					.Ex.: poinsettias, soybeans
				ii. Long-day
					.Triggered by day length longer than a critical time
					.Spinach, radish, iris
				iii. Day-neutral
					.Unaffected by day length
					.Tomatoes, rice, dandelions
			b. Plants actually respond to night length 

PHYTOCHROMES
	Introduction
	Phytochromes function as photoreceptors in many plant responses to light and 
	photoperiod
		13. Describe the role of phytochromes in mediating photoperiodic responses.
			a. Phytochromes are kinases with light-absorbing chromophores
				i. Absorb red light and then autophosphorylate
				ii. May then activate transcription factors
			b. Ratios of the various forms can trigger physiological responses
	Phytochromes may entrain the biological clock

PLANT RESPONSES TO ENVIRONMENTAL STRESS
	Introduction
	Plants cope with environmental stress through a combination of developmental and 
	physiological responses
		14. Explain how plants deter herbivores with physical and chemical defenses.
			a. Physical: like tough coverings, thorns, hairs
			b. Chemical
				i. Toxic amino acid analogues like canavanine
				ii. When wounded may release attractant of parasitic wasp

DEFENSE AGAINST PATHOGENS
	Introduction
	Resistance to disease depends on a gene-for-gene recognition between plant and 
	pathogen
		15. Describe the multiple ways that plants defend against pathogens.
			a. Gene-for-gene resistance requires specific host and pathogen genes
			b. Hypersensitive response
				i. Local chemical response
				ii. Release phytoalexins and pathogenesis-related proteins
				iii. Antimicrobial and stimulate thickened walls
				iv. Enhanced in gene-for-gene match  
			c. Systemic acquired resistance
				i. Non-specific whole-plant hormone response
				ii. May involve salicylic acid
	The hypersensitive response (HR) contains an infection
	Systemic acquired resistance (SAR) helps prevent infection throughout the plant