Flower Production, Osmolarity Balance in Plants and Animals

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Evolution of the Vertebrate Kidney

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

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

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

Evolution of the Vertebrate Kidney