1. Flower Production, Osmolarity Balance in Plants and Animals

2. Flower Production
Four genetically regulated pathways to flowering have been identified
The light-dependent pathway
The temperature-dependent pathway
The gibberellin-dependent pathway
The autonomous pathway
Plants can rely primarily on one pathway, but all four pathways can be present

3. 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 length

5. Light-Dependent Pathway
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

6. Light-Dependent Pathway
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

7. Light-Dependent Pathway
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

8. Light-Dependent Pathway
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

9. 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

10. 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

11. 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

12. 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

13. 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

14. Addition of roots, and not the loss of leaves, delays flowering

15. 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

17. ABC Model
Explains how 3 classes of floral organ identity genes can specify 4 distinct organ types
Class A genes alone – Sepals
Class A and B genes together – Petals
Class B and C genes together – Stamens
Class C genes alone – Carpels
When any one class is missing, aberrant floral organs occur in predictable positions

20. 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 proposed

22. 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

23. Osmolarity and Osmotic Balance
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

24. Extracellular compartment
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External environment
Animal body
H2O (Sweat)
Solutes and H2O
Intracellular
compartment
Intracellular
compartment
Solutes and H2O
Extracellular compartment
(including blood)
CO2 and H2O
CO2 and H2O
Solutes and H2O O2
Solutes and H2O O2
Food and H2O
Solutes and H2O
Solutes and H2O
Urine (excess H2O)
Waste

25. Osmolarity and Osmotic Balance
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

26. Osmolarity and Osmotic Balance
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

27. Osmolarity and Osmotic Balance
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

28. Osmolarity and Osmotic Balance
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

29. 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

30. Osmoregulatory Organs
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

33. Osmoregulatory Organs
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

35. Osmoregulatory Organs
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

36. 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

38. Evolution of the Vertebrate Kidney
Kidneys are thought to have evolved among the freshwater teleosts, or bony fishes
Body fluids are hypertonic with respect to surrounding water, causing two problems
Water enters body from environment
Fishes do not drink water and excrete large amounts of dilute urine
Solutes tend to leave the body
Reabsorb ions across nephrons
Actively transport ions across gills into blood

39. Evolution of the Vertebrate Kidney
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

40. Evolution of the Vertebrate Kidney

41. Evolution of the Vertebrate Kidney
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

42. Evolution of the Vertebrate Kidney
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

43. Evolution of the Vertebrate Kidney
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

44. Evolution of the Vertebrate Kidney