2e3 flower production, osmolarity balance in plants and animals

2E3

Flower Production, Osmolarity Balance in Plants and Animals

Flower Production

• Four genetically regulated pathways to flowering have been identified1. The light-dependent pathway2. The temperature-dependent pathway3. The gibberellin-dependent pathway4. The autonomous pathway

• Plants can rely primarily on one pathway, but all four pathways can be present

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Light-Dependent Pathway

• Also termed the photoperiodic pathway• Keyed to amount of dark in the daily 24-hr

cycle (day length)• Short-day plants flower when daylight

becomes shorter than a critical length• Long-day plants flower when daylight

becomes longer• Day-neutral plants flower when mature

regardless of day length3


• In obligate long- or short-day plants there is a sharp distinction between short and long nights, respectively

• In facultative long- or short-day plants, the photoperiodic requirement is not absolute– Flowering occurs more rapidly or slowly

depending on the length of day

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• Using light as a cue allows plants to flower when abiotic conditions are optimal

• Manipulation of photoperiod in greenhouses ensures that short-day poinsettias flower in time for the winter holidays

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• Conformational change in a phytochrome (red-light sensitive) or cryptochrome (blue-light sensitive) light-receptor molecule triggers a cascade of events that leads to the production of a flower

• In Arabidopsis, regulate via the gene CONSTANS (CO)

• Phytochrome regulates the transcription of CO

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• CO protein is produced day and night– Levels of CO are lower at night because of targeted protein

degradation by ubiquitin– Blue light acting via cryptochrome stabilizes CO during the

day and protects it from ubiquitination

• CO is a transcription factor that turns on other genes– Results in the expression of LFY– LFY is one of the key genes that “tells” a meristem to

switch over to flowering

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Temperature-Dependent Pathway

• Some plants require a period of chilling before flowering – vernalization – Necessary for some seeds or plants in later stages

of development• Analysis of plant mutants reveals that

vernalization is a separate flowering pathway

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Autonomous Pathway

• Does not depend on external cues except for basic nutrition

• Allows day-neutral plants to “count” and “remember”

• Tobacco plants produce a uniform number of nodes before flowering

• Upper axillary buds of flowering tobacco remember their position if rooted or grafted

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• Plants can “count”• If the shoots of these plants are removed at different

positions, axillary buds will grow out and produce the same number of nodes as the removed portion of the shoot

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• Plants can “remember”• Upper axillary buds of flowering tobacco will remember their

position when rooted or grafted• Terminal shoot tip becomes committed, or determined, to

flower about four nodes before it actually initiates a flower

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Autonomous Pathway

• How do shoots “count” and “remember”?• Experiments using bottomless pots have

shown that it is the addition of roots, and not the loss of leaves, that inhibits flowering

• Clear that inhibitory signals are sent from the roots

• A balance between floral promoting and inhibiting signals may regulate flowering

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• Addition of roots, and not the loss of leaves, delays flowering

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Model for Flowering

• 4 flowering pathways lead to an adult meristem becoming a floral meristem– Activate or repress the inhibition of floral

meristem identity genes• 2 key genes: LFY and AP1

– Turn on floral organ identity genes– Define the four concentric whorls

• Sepal, petal, stamen, and carpel

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ABC Model

• Explains how 3 classes of floral organ identity genes can specify 4 distinct organ types1. Class A genes alone – Sepals2. Class A and B genes together – Petals 3. Class B and C genes together – Stamens4. Class C genes alone – Carpels

• When any one class is missing, aberrant floral organs occur in predictable positions

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Modifications to ABC Model

• ABC model cannot fully explain specification of floral meristem identity

• Class D genes are essential for carpel formation

• Class E genes SEPALATA (SEP)– SEP proteins interact with class A, B, and C

proteins that are needed for the development of floral organs

• Modified ABC model was proposed20

Osmolarity and Osmotic Balance

• Water in a multicellular body distributed between– Intracellular compartment– Extracellular compartment

• Most vertebrates maintain homeostasis for– Total solute concentration of their extracellular

fluids– Concentration of specific inorganic ions

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• Important ions– Sodium (Na+) is the major cation in extracellular

fluids– Chloride (Cl–) is the major anion– Divalent cations, calcium (Ca2+) and magnesium

(Mg2+), the monovalent cation K+, as well as other ions, also have important functions and are maintained at constant levels

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Animal body

H2O(Sweat)

CO2 and H2O

O2 Solutesand H2O

Solutesand H2O

Solutesand H2O

Solutesand H2O

Solutesand H2O

Solutesand H2O

CO2 and H2O

FoodandH2O

External environment

Urine (excess H2O) Waste

Extracellular compartment(including blood)

O2

IntracellularcompartmentIntracellular

compartment

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.


• Osmotic pressure– Measure of a solution’s tendency to take in water by

osmosis

• Osmolarity – Number of osmotically active moles of solute per liter of

solution

• Tonicity – Measure of a solution’s ability to change the volume of a

cell by osmosis– Solutions may be hypertonic, hypotonic, or isotonic

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• Osmoconformers– Organisms that are in osmotic equilibrium with their

environment– Among the vertebrates, only the primitive hagfish are

strict osmoconformers– Sharks and relatives (cartilaginous fish) are also isotonic

• All other vertebrates are osmoregulators– Maintain a relatively constant blood osmolarity despite

different concentrations in their environment

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• Freshwater vertebrates– Hypertonic to their environment – Have adapted to prevent water from entering

their bodies, and to actively transport ions back into their bodies

• Marine vertebrates– Hypotonic to their environment– Have adapted to retain water by drinking seawater

and eliminating the excess ions through kidneys and gills

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• Terrestrial vertebrates– Higher concentration of water than surrounding

air– Tend to lose water by evaporation from skin and

lungs– Urinary/osmoregulatory systems have evolved in

these vertebrates that help them retain water

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Osmoregulatory Organs

• In many animals, removal of water or salts is coupled with removal of metabolic wastes through the excretory system

• A variety of mechanisms have evolved to accomplish this– Single-celled protists and sponges use contractile

vacuoles– Other multicellular animals have a system of

excretory tubules to expel fluid and wastes

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• Invertebrates– Flatworms

• Use protonephridia which branch into bulblike flame cells

• Open to the outside of the body, but not to the inside

– Earthworms• Use nephridia• Open both to the inside and outside of the body

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• Insects– Use Malpighian tubules

• Extensions of the digestive tract

– Waste molecules and K+ are secreted into tubules by active transport

– Create an osmotic gradient that draws water into the tubules by osmosis

– Most of the water and K+ is then reabsorbed into the open circulatory system through hindgut epithelium

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• Vertebrate kidneys– Create a tubular fluid by filtering the blood under

pressure through the glomerulus– Filtrate contains many small molecules, in addition

to water and waste products– Most of these molecules and water are

reabsorbed into the blood• Selective reabsorption provides great flexibility

– Waste products are eliminated from the body in the form of urine

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Evolution of the Vertebrate Kidney

• Made up of thousands of repeating units – nephrons

• Although the same basic design has been retained in all vertebrate kidneys, a few modifications have occurred

• All vertebrates can produce a urine that is isotonic or hypotonic to blood

• Only birds and mammals can make a hypertonic urine

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• Kidneys are thought to have evolved among the freshwater teleosts, or bony fishes

• Body fluids are hypertonic with respect to surrounding water, causing two problems1. Water enters body from environment

• Fishes do not drink water and excrete large amounts of dilute urine

2. Solutes tend to leave the body• Reabsorb ions across nephrons• Actively transport ions across gills into blood

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• In contrast, marine bony fishes have body fluids that are hypotonic to seawater

• Water tends to leave their bodies by osmosis across their gills

• Drink large amounts of seawater• Eliminate ions through gill surfaces and urine• Excrete urine isotonic to body fluids

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• Cartilaginous fish, including sharks and rays, reabsorb urea from the nephron tubules

• Maintain a blood urea concentration that is 100 times higher than that of mammals

• Makes blood isotonic to surrounding sea• These fishes do not need to drink seawater or

remove large amounts of ions from their bodies

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• Amphibian kidney is identical to that of freshwater fish

• Kidneys of reptiles are very diverse– Marine reptiles drink seawater and excrete an

isotonic urine• Eliminate excess salt via salt glands

– Terrestrial reptiles reabsorb much of the salt and water in their nephron tubules• Don’t excrete urine, but empty it into cloaca

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• Mammals and birds are the only vertebrates that can produce urine that is hypertonic to body fluids

• Accomplished by the loop of Henle • Birds have relatively few or no nephrons with

long loops, and so cannot produce urine as concentrated as that of mammals

• Marine birds excrete excess salt from salt glands near the eyes

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2e3 flower production, osmolarity balance in plants and animals

Documents

autonomous pathway plants

tobacco plants

temperaturedependent

gibberellindependent

separate flowering pathway

length of day

photoperiodic pathway

shortday poinsettias