Allergies! Type I Hypersensitivity: When More Isn’t Better

Our last article discussed various hay fever inducing allergens encountered throughout the year. We learned that even for some of the most allergenic pollens, like birch and ragweed, only certain antigens derived from the pollen actually induce an allergic response. While the differences in the structure of these primary antigens can partially explain why some are allergenic and others are not, it really boils down to how the antigen interacts with an individual’s immune system. Some molecules make better allergens than others because they interact with the major player in Type I hypersensitivity, immunoglobulin E (IgE).

Interestingly, IgE earned its name based on the fact that it reacted with the ragweed pollen antigen E, now known as the primary ragweed antigen “Amb a 1”. In 1921, scientists K. Prausnitz and H. Kustner identified a serum component that was responsible for allergic reaction. It wasn’t until 1966 that T. and K. Ishikawa identified IgE as the serum component. Everyone has a small amount of this potent antibody circulating the blood; IgE accounts for less than 0.05-0.2% (0.1-0.4 μg/mL) of the circulating antibodies in non-atopic individuals. Some, but not all, atopic individuals have higher levels of circulating IgE, up to 0.79%.

Even Sabrina Fairchild knew that “More isn’t always better…sometimes it’s just more.”

In individuals without allergies, an IgE-mediated immune response occurs as a defense against parasitic infections. In this case, the resulting physiological changes clear the parasite and protect the body against further damage caused by the parasite. However, in individuals with allergies, the IgE-mediated response is classified as a Type I hypersensitivity.

Let’s follow a pollen grain on its first journey in an allergic individual. The first encounter of an allergen sensitizes the individual to that specific allergen, but symptoms are not experienced. Initially, the pollen particle encounters the peripheral defenses, nasal hairs, eyelids, and beating cilia in the throat. These hairs prevent most particles from entering the airway or sinuses. Pollen particles must be extremely tiny (about 1x10^-6 meters) to pass through this initial barrier. Upon reaching the nasal mucosa, enzymes in mucous secretions break down the tough outer shell of the pollen (the exine), releasing the allergenic substance.

 

Antigen presenting cells engulf the allergenic substance, process it with enzymes, and display the antigen on the cell surface within a cradle-like protein called the class II major histocompatibility complex (MHC). Another type of immune cell, called T-helper, or Th, cells bind the presented antigen. Th2 cells release molecules called cytokines, which communicate to naive B cells to begin dividing and maturing. Some B cells differentiate into plasma cells, which produce and secrete a specific class of antibodies, or immunoglobulin (Ig). Humans produce 5 circulating antibody isotypes:  IgG, IgM, IgA, IgD and IgE. Particularly, Th2 cells produce the cytokines interleukin (IL)-4 and IL-13, which stimulate B cells to produce IgE. The allergic response appears to be localized, as plasma cells secreting IgE are 1000 times greater in nasal mucosa than in circulation.

In addition to producing the correct isotype, the plasma cells also produce highly specific antibodies that will bind the antigen tightly. Through the process of clonal selection and clonal expansion, a specific IgE molecule with high affinity for the antigen is produced en masse, creating an army like the clone troopers.

Although the army of IgE clones may not be as large as the clone troopers, it's every bit as powerful in wreaking immune havoc.

The circulating IgE has a specific receptor that allows it to bind tissue mast cells and blood basophils. At this point, the body is considered “sensitized” to the allergen. Additionally, memory B cells are formed in preparation for the second encounter of the antigen.

Nothing happens yet, but the body essentially lays in wait to encounter the allergen again. Upon second exposure, the allergenic antigen binds two IgE molecules that are already situated on the mast cells and basophils. These crosslinked IgE molecules are much more stable and can continue sending signal for weeks. The signal, as allergy sufferers know all too well, is a massive inflammatory response mediated by various pharmacologically active molecules contained within and produced by mast cells and basophils. These cells store the inflammatory molecules, like histamine, in granules or inner pockets. When the antigen binds IgE, mast cells and basophils undergo degranulation, releasing large amounts of chemical mediators like the histamine targeted by most antihistamine allergy medications.

The antigen acts like Wile E. Coyote, detonating the IgE fuse, causing the mast cell bomb to explode and release clouds of histamine. Histamine, in turn, damages only our tissues, never touching the elusive (and harmless) Roadrunner allergen.

Mast cells quickly synthesize additional mediators, including leukotriene and prostaglandin. These mediators signal certain physiological changes, including vasodilation (nasal blockage), smooth muscle contraction (coughing), increased mucus secretion (runny nose), and increased vascular permeability (inflammation). Sensory nerves are stimulated, resulting in sneezing and itching. This early phase, or immediate hypersensitivity reaction, happens so rapidly that symptoms are noticed within minutes of exposure to the allergen.

Although histamine is probably the most well-known pharmacologically active molecule, it is actually not the most potent or the longest acting player. Rather, it is the first molecule released in the allergic reaction. Following degranulation, mast cells and basophils produce and release other mediators called prostaglandins and leukotrienes. Initially, contraction of bronchial and tracheal muscles is mediated by histamine, but shortly after, further contraction occurs as a result of prostaglandin and leukotrienes. Leukotrienes are 10 times more potent than histamine at causing bronchoconstriction than histamine.

How an antigen, say pollen, triggers an allergic response.

About 50% of the time, 4 to 8 hours after the early phase reaction, the late phase begins. Other cytokines, particularly IL-5, attract other inflammatory cells, including eosinophils. The symptoms of the late phase reaction resemble those of the early phase, but tend to be characterized by less sneezing and itching and more congestion and mucus production. The inflammatory response from the late phase can damage tissues and last for days.

So why do some people endure the suffering of hay fever and others do not? Tune in next time to find out the genetic and environmental factors that contribute to allergic rhinitis.

 

 

Contributed by Julia van Rensburg, PhD
Follow Julia on Twitter.

Ishizaka K, Ishizaka T, & Hornbrook MM (1966). Physico-chemical properties of human reaginic antibody. IV. Presence of a unique immunoglobulin as a carrier of reaginic activity. Journal of immunology (Baltimore, Md. : 1950), 97 (1), 75-85 PMID: 4162440

Kasaian MT, Meyer CH, Nault AK, & Bond JF (1995). An increased frequency of IgE-producing B cell precursors contributes to the elevated levels of plasma IgE in atopic subjects. Clinical and experimental allergy : journal of the British Society for Allergy and Clinical Immunology, 25 (8), 749-55 PMID: 7584687

Verstraelen, S., Bloemen, K., Nelissen, I., Witters, H., Schoeters, G., & Heuvel, R. (2008). Cell types involved in allergic asthma and their use in in vitro models to assess respiratory sensitization Toxicology in Vitro, 22 (6), 1419-1431 DOI: 10.1016/j.tiv.2008.05.008

Takhar P, Smurthwaite L, Coker HA, Fear DJ, Banfield GK, Carr VA, Durham SR, & Gould HJ (2005). Allergen drives class switching to IgE in the nasal mucosa in allergic rhinitis. Journal of immunology (Baltimore, Md. : 1950), 174 (8), 5024-32 PMID: 15814733

Hay Fever: Maladies, Melodies And Remedies

In addition to kicking off the barbeque, swimming and vacation seasons, spring also marks the beginning of that pesky and sometimes debilitating seasonal woe, hay fever. Much like Noel Coward’s 1924 play Hay Fever, the colloquial designation has really nothing to do with hay or fever. Clinically known as allergic rhinitis, hay fever describes the hypersensitivity to airborne allergens and the onslaught of bothersome symptoms they provoke. Approximately 20% of the world’s population suffers from seasonal or perennial hay fever. Even Paul Simon wasn’t spared from the suffocating spiral that is allergies.

 

With the blooming of spring flowers and sprouting of fresh green leaves and grasses, we are reminded that life is all around us. Quite literally, too, as windborne plant pollen is small enough to enter our eyes, nose, and mouth. Pollen, the primary cause of seasonal allergies, contains the male gametophytes of seed-bearing grasses and trees. Each pollen grain contains a generative cell, or sperm, which fertilizes the egg of the female plant, and a vegetative cell, which develops into a pollen tube and delivers the sperm to the ovule. Many trees and grasses rely on wind to spread their pollen and fertilize the female plant. So although your college roommate may have been discrete while attempting to procreate, wind-pollinated plants uphold no such personal boundaries.



Next time you smell a flower, realize you are sniffing a plant’s “naughty bits”.

So which plants are responsible for producing the powdered cheese-like substance that coats our houses, bicycles, and cars? Although thousands of plant species produce pollen that makes the Holderness family cough and gag, only a handful are responsible for their allergic wheezing and sneezing.

The exact timing of seasonal allergies can vary depending on region and climate. You can blame your early spring allergies on tree pollen, particularly that from birch trees. From March to May, many other trees including beech, ash, pine, box elder, cottonwood, oak, mulberry, elm, alder, cedar, hazel, willow, poplar, linden, olive, hornbeam, and plane contribute to early spring allergies. By June, grass pollen becomes predominant, especially timothy and ryegrass. Other grasses such as Bermuda, Johnson, Kentucky bluegrass, orchard, redtop, sweet vernal, and rye contribute to allergies. As the heat and humidity rises during July and August, your hair and electricity bill aren’t the only things that grow. Molds can thrive in grass, grains and leaves; airborne spores can cause hay fever.

The warm days and cool nights of late summer and autumn are perfect conditions for weeds, particularly ragweed, which is the primary cause of autumn-onset seasonal allergies. Ragweed can produce metric tons of pollen per square mile of plant. Other weeds that produce allergenic pollen are cocklebur, burning bush, lamb’s quarters, pigweed, plaintain, Russian thistle, sagebrush, mugwort, and sheep sorrel.

The heavy vegetation towards the end of the growing season provides a perfect breeding ground for additional outdoor mold. Mold grows in fallen autumn leaves, hay, and straw, and can be stirred up during raking or baling. In general, mold spores are considered perennial allergens because mold has the potential to grow outdoors and indoors, especially in kitchens, bathrooms, and basements throughout the entire year. However, growth conditions are optimal during different seasons, potentially resulting in a seasonal effect with mold allergies.

Even in winter, mold spores on indoor live pine trees can cause an allergic reaction. So even though the pine tree isn’t releasing pollen, it can still aggravate hay fever symptoms.

Other perennial allergens besides mold spores include dust mites, pet hair dander, and cockroach droppings. Dust mites are always present, but have been shown to increase with installation and use of central heating and insulated windows in apartment buildings. And although we may not even know cockroaches are present, the proteins in their droppings can cause hay fever. Cat dander is the most common cause of pet allergies, but thankfully The Big Bang Theory writers conveniently overlooked Sheldon’s alleged cat dander allergy so he could adopt this zazzy guy.

What is it about allergens that trigger the allergic response? Although scientists have worked to understand the molecular details of allergens and how they interact with components of our immune system, there is no clear answer as to what specifically makes something allergenic. However, within many of the most common allergens, the primary antigen has been identified. The antigen is the specific molecule that is recognized by our immune system. Antigens can be different components of a bacterial cell or viral particle; in the case of allergens, it is a protein derived from pollen, dander, mold, etc.

Pollen from birch trees is one of the largest contributors to hay fever in spring and early summer in North America The primary antigen from birch tree pollen, Betula verrucosa is called Bet v 1. The Bet v 1 antigen exists as a mixture of 14 isoforms that share ≥ 96.5% sequence identity; these isoforms possess different binding capabilities for the antibody immunoglobulin E (IgE). In fact, only 1 of the 14 isoforms, Bet v 1.0101, induces an immune response in an individual with birch tree allergy, and the two other isoforms tested, Bet v 1.0401 and Bet v 1.1001 induced no response (PMID:  20005001). Immune cells isolated from patients with no birch tree allergy did not react to any of the isoforms. The difference in the antigens is their affinity for the IgE, but precisely what makes one antigen more reactive than the other is unclear. On a basic level, the protein sequence and structure influence the binding to antibodies.


Birch pollen primary antigen Bet v 1 (wikipedia.org)

One complication with diagnosing and treating allergies is the potential for cross-reactivity between different antigens. In some parts of the world, allergic patients are double-sensitized to ragweed and mugwort, Artemisia vulgaris. The flowering season of these two plants overlaps, making it difficult to diagnose the primary sensitizer. In addition to increasing the number of allergies a patient may have, cross-reactivity also complicates prescription of the correct immunotherapy to combat the primary allergy. The primary antigen of mugwort is Art v 1 and up to 95% of people are sensitized to Art v 1. However, a minor mugwort antigen, Art v 6 shares high homology with and commonly cross-reacts with the primary ragweed antigen, Amb a 1. At least 90% of ragweed-allergen sufferers are sensitized to Amb a 1. This means that patients who are allergic to ragweed may be sensitive to mugwort and vice versa. New proteomic technologies allow for more accurate diagnoses of the primary sensitizer so that the proper immunotherapy can be prescribed. Treatment of allergies will be discussed in article 4 of this series.

With so many potential allergens bombarding us more or less year-round, it’s almost surprising that more of us don’t suffer from hay fever. As mentioned above, 1 in 5 people are afflicted and, unfortunately, that number is increasing, particularly in suburban areas of North America. Perhaps the reason allergies are not more common is because they are not hardwired into us, as is the immune response to infectious agents such as bacteria, viruses, and parasites. Hay fever is considered an atopy, a genetic predisposition to mount inappropriate immune responses to harmless environmental allergens. The immune response mounted against allergens will be described in detail in article 2 of this series.

The tendency to have seasonal allergies is hereditary, but does not follow Mendelian principles, like inheritance of eye or hair color. In addition to genes, the environment contributes to allergy susceptibility. Understanding the genetic and environmental factors involved in allergy development is complex and requires sound knowledge of the actual allergic response. A more complete discussion of genetic and environmental factors that influence allergy susceptibility will be presented in the third article of this series. We hope you’ll tune in for the remaining articles in this ongoing series.

Contributed by:  Julia van Rensburg
Follow Julia on Twitter.
 

Hirsch T, Hering M, Bürkner K, Hirsch D, Leupold W, Kerkmann ML, Kuhlisch E, & Jatzwauk L (2000). House-dust-mite allergen concentrations (Der f 1) and mold spores in apartment bedrooms before and after installation of insulated windows and central heating systems. Allergy, 55 (1), 79-83 PMID: 10696861

Leb VM, Jahn-Schmid B, Schmetterer KG, Kueng HJ, Haiderer D, Neunkirchner A, Fischer GF, Nissler K, Hartl A, Thalhamer J, Bohle B, Seed B, & Pickl WF (2008). Molecular and functional analysis of the antigen receptor of Art v 1-specific helper T lymphocytes. The Journal of allergy and clinical immunology, 121 (1), 64-71 PMID: 18037161

Jahn-Schmid B, Hauser M, Wopfner N, Briza P, Berger UE, Asero R, Ebner C, Ferreira F, & Bohle B (2012). Humoral and cellular cross-reactivity between Amb a 1, the major ragweed pollen allergen, and its mugwort homolog Art v 6. Journal of immunology (Baltimore, Md. : 1950), 188 (3), 1559-67 PMID: 22205029

Wopfner N, Bauer R, Thalhamer J, Ferreira F, & Chapman M (2008). Immunologic analysis of monoclonal and immunoglobulin E antibody epitopes on natural and recombinant Amb a 1. Clinical and experimental allergy : journal of the British Society for Allergy and Clinical Immunology, 38 (1), 219-26 PMID: 18028463

Asero R, Bellotto E, Ghiani A, Aina R, Villalta D, & Citterio S (2014). Concomitant sensitization to ragweed and mugwort pollen: who is who in clinical allergy? Annals of allergy, asthma & immunology : official publication of the American College of Allergy, Asthma, & Immunology, 113 (3), 307-13 PMID: 25053399

The Friday Five

In this special edition of Friday Five, we’ve collected 5 of the most popular articles from THE ‘SCOPE for 2014!

5. Is homosexuality "natural"?

4. State Fairs and Stiff Beers: Why We Can't Stop Drinking

3. Is It Really Possible For Someone To Turn Into THE HULK? Don’t Make Me Angry.

2. Science Identifies The Catchiest Songs Ever – Did Your Favorite Make The List?

1. Where Do All Those Leaves Come From?!

Contributed by:  Bill Sullivan

Follow Bill on Twitter: @wjsullivan

O Christmas Tree: It’s Not Easy Being Green

Evergreens are a remarkable mainstay in the evolution of plants. Evidence suggests that they have existed more or less in their present form for the past 300 million years. In other words, the evergreens are so resilient and exquisitely adapted to their environment that nature has not tweaked with their genetic recipe since the Permian. The evergreens can survive just about anything nature can throw at them, except humans. Nearly 40 million of these stoic conifers are chopped down each Christmas season in North America alone.

"Christmas Tree" farms cultivate a variety of evergreens that will grace one of 40 million homes each season. This makes it a lot easier than hiking into the forest to cut one down yourself.

Humans have long been fascinated by the evergreens because these trees and shrubs do not lose their leaves (needles) in autumn like the broadleaf trees. Seemingly in defiance to the harsh winter, the aptly named evergreens stay full and green all year long. Impressed with this act of endurance, early humans thought that evergreens must hold special powers. The ancient Pagans would place evergreen branches over their doors and windows to ward off evil spirits, especially during the winter solstice when the days were at their shortest and the nights at their coldest. Evergreens served as a reminder that the days would lengthen and the crops would grow once again in the spring.

A decorated evergreen is now synonymous with “Christmas Tree”, but this ritual has its “roots” in Paganism. Interestingly, it has even been argued that this passage from the Bible forbids emulating this Pagan practice.

So how do evergreens stay green year round? In winter, shorter days mean less sunlight. As sunlight is required for photosynthesis, plants face a dramatic reduction in energy during winter. To cope with this, broadleaf plants stop making chlorophyll, the molecule that drives photosynthesis and reflects green light. Consequently, the leaves change color and eventually fall off as the tree goes dormant.

By way of comparison, evergreen “leaves” do not have a lot of surface area; they are more resistant to lower temperatures and decreased moisture. Chlorophyll in these needle-like leaves is retained and photosynthesis can still generate energy from light, albeit at a much slower rate than spring or summer.

In addition to keeping chlorophyll, retaining moisture is equally important:  trees cannot extract water from frozen ground, and occasional sunlight in the winter can draw out precious moisture. Evergreen needles have a thick coating of wax and a slender shape, characteristics that help them hold water in and prevent evaporation, respectively.

A recent study has shown that the conifer’s ability to survive arid times involves the coordinated evolution of tissues regulating water supply (xylem) and water loss (stomatal pores) in the needle leaves. A plant hormone called abscisic acid helps keep the leaf’s pores sealed when water isn’t available. Another mechanism allows leaves to dehydrate and resist damage via a water transport system.

Close up image of pine needle – the small pores are stomata, which open and close to regulate gas exchange. When open, water vapor can escape.

Conifers have thousands of needle leaves, which help maximize energy production while not losing water to dehydration. Of course, evergreen needles do not last forever. They do need to be replaced, but conifers do this intermittently and a green appearance is always observed.

Ever since ancient times, the evergreens have been admired for their stamina and hardiness through the winter. They are a source of inspiration reminding us that better times are ahead. In this light, the ritual chopping down of the tree for decoration seems a most bizarre way to honor the mighty evergreen. Consider, instead, a Festivus Pole.
 

Contributed by:  Bill Sullivan

Follow Bill on Twitter.

Brodribb TJ, McAdam SA, Jordan GJ, & Martins SC (2014). Conifer species adapt to low-rainfall climates by following one of two divergent pathways. Proceedings of the National Academy of Sciences of the United States of America, 111 (40), 14489-93 PMID: 25246559

I Am Groot! Plants Are More “Alive” Than We Think

So who saw Guardians of the Galaxy this summer? Awesome, wasn’t it? How could a movie with a talking raccoon and a 1970s-based soundtrack go wrong? Oh, and then there’s Groot, the beloved walking, talking tree-like creature who sprouted the catch phrase, “I AM GROOT!” Until Groot, the only sentient plants children probably knew of were the Evil Trees hurling apples at Dorothy in The Wizard of Oz. But Groot was a “Giving Tree” incarnate.    



Groot shatters the evil living plant stereotype and saves the day in Guardians of the Galaxy.

 

Groot was so endearing, he has given my kids a new reason not to eat their veggies – they don’t like the idea of slaughtering plants. When I reminded the kids that plants are only sentient in the movies, they gave me the look of skepticism. I know it well…I taught it to them. This inspired me to leaf through some botany literature. Much to my surprise, my kids might have a point! Plants are more “alive” than we ever imagined.

Exhibit A.  Plants know when they are being eaten alive! And they fight back…

There is a plant that scientists typically use as a model to study in the lab called Arabidopsis (thale cress, similar to broccoli but tastes even worse). Researchers recently discovered that when they expose these plants to vibrations that mimic those produced by a hungry caterpillar, the plants increase production of glucosinolate and anthocyanin defenses. These are mustard oils that don’t sit well with caterpillars. In other words, the plants can tell when they are being chewed on and release oils to deter the predator.

Another sophisticated defense system used by plants comes from studies on tobacco. When caterpillars attack, these tobacco plants produce “green leaf volatiles”, compounds that act as a distress call by attracting insects that devour caterpillars!

The Hungry Caterpillar: Adorable children's tale or a horror story of gruesome predation?

 

Exhibit B. Plants have a memory and can be trained.

Everyone is familiar with the story of Pavlov’s dog, the famous experiment that demonstrated classical conditioning. Dogs salivate when presented with meat. If you ring a bell before presenting the meat, the dogs become conditioned to salivate at just the sound of a bell. Plants do a similar thing when exposed to light. Researchers have shown that when a plant is exposed to a certain wavelength of light, and then infected with a plant pathogen, the plant “learns” to build up resistance to that pathogen when it “sees” that particular wavelength of light once again. Plants that were infected and then exposed to the light developed no protective response. Plants must possess some sort of biochemical nervous system and memory in order to execute this kind of behavior.

Plants don’t have a brain, but they do behave as if they can think.

 

Exhibit C. Plants protect their young.

Seeds are the equivalent of a plant’s babies and plants have evolved a variety of fascinating ways to take care of their young. Consider serotinous plants, which keep some of their seeds inside the plant body instead of releasing them into the environment where they can be eaten or destroyed by weather. These plants can hold onto their seeds and release them when the time is most favorable for them to survive.

In the 1986 film, “Little Shop of Horrors”, the carnivorous plant Audrey II demonstrated a terrifying new way plants could protect their kin.

 

In another striking example, scientists studying a plant called sea rocket (Cakile edentula) noticed that when grown in a pot with a different member of its species, its roots grew wildly so to soak up more water and nutrients from its competitor. However, if the sea rocket was put into the same pot as its offspring, this competition did not take place!

Still not convinced that plants are more alive than we give them credit for? Check out this video by Michael Pollan.
 

 

While the evidence above isn’t sufficient to conclude that plants on Earth are like Groot, it is clear they are capable of some level of feeling and response. But don’t tell your vegetarian friends…what else would they eat?

Contributed by:  Bill Sullivan

Follow Bill on Twitter.

 

 

Appel HM, & Cocroft RB (2014). Plants respond to leaf vibrations caused by insect herbivore chewing. Oecologia, 175 (4), 1257-66 PMID: 24985883

Allmann, S., & Baldwin, I. (2010). Insects Betray Themselves in Nature to Predators by Rapid Isomerization of Green Leaf Volatiles Science, 329 (5995), 1075-1078 DOI: 10.1126/science.1191634

Karpiński S, & Szechyńska-Hebda M (2010). Secret life of plants: from memory to intelligence. Plant signaling & behavior, 5 (11), 1391-4 PMID: 21051941

Dudley SA, & File AL (2007). Kin recognition in an annual plant. Biology letters, 3 (4), 435-8 PMID: 17567552

The Friday Five

Highlighting some of the coolest science news we’ve seen lately.

1. People living with type I diabetes may have something to celebrate as scientists have successfully used human embryonic stem cells to generate beta cells. These insulin-producing cells could one day be transplanted into humans.

2. How do you ward off the obnoxious guy who won’t leave you alone in a bar? Female squid of the species Doryteuthis opalescens can steer males away by turning on fake testes. Interestingly, when the females turn on the testes, they also get a pay raise at work.

Since human females can’t turn on testes like a squid, a fake moustache might be sufficient to keep unwanted men away.

3. Blinded by the light? Neuroscientists have successfully erased specific memories in mice…using light. But they are not using this knowledge for evil, they used it to demonstrate how different parts of the brain - the hippocampus and cortex - work together to retrieve memories.



Bono remembers everything about the ‘90s because he adequately shielded his eyes from the light.

4. Still “cleaning” your ears with Q-tip swabs? Learn more about your ear wax and why you should not interfere with it.

5. Check out these amazing photos of flowers that look like other things.

A kiss from a rose...

Science quote of the week:

“Science moves with the spirit of an adventure characterized both by youthful arrogance and by the belief that the truth, once found, would be simple as well as pretty.” – James D. Watson

Contributed by:  Bill Sullivan

Follow Bill on Twitter: @wjsullivan

Tanaka, K., Pevzner, A., Hamidi, A., Nakazawa, Y., Graham, J., & Wiltgen, B. (2014). Cortical Representations Are Reinstated by the Hippocampus during Memory Retrieval Neuron DOI: 10.1016/j.neuron.2014.09.037

DeMartini DG, Ghoshal A, Pandolfi E, Weaver AT, Baum M, & Morse DE (2013). Dynamic biophotonics: female squid exhibit sexually dimorphic tunable leucophores and iridocytes. The Journal of experimental biology, 216 (Pt 19), 3733-41 PMID: 24006348

Pagliuca, F., Millman, J., Gürtler, M., Segel, M., Van Dervort, A., Ryu, J., Peterson, Q., Greiner, D., & Melton, D. (2014). Generation of Functional Human Pancreatic β Cells In Vitro Cell, 159 (2), 428-439 DOI: 10.1016/j.cell.2014.09.040

Autumn Leaves: More Than Just Pretty Colors

This post originally appeared on the Telegram and Gazette 9/2012

The green tree leaves of summer are already starting to give way to the bright yellows and reds of autumn. We should have a brilliant display of colors throughout the fall.

 

As you may remember from your high school science class, what gives green leaves (and green plants in general) their color is a compound called chlorophyll, which absorbs light energy from the sun. As the summer comes to an end, the change in temperature and change in the amount of daylight trigger processes in the leaves that cause chlorophyll to break down. As this inducer of green color disappears, the color effects of other compounds, carotenoids and anthocyanins specifically, are unmasked. Carotenoids are the compounds that give carrots their orange color and bananas their yellow color. Anthocyanins can give plants bluish, purplish, or reddish tints. Red cabbage, cranberries, and red raspberries are just a few examples of produce that have high levels of anthocyanins.

For the most part, scientists thought that the changing of leaf color in autumn was simply an effect of the disappearance of chlorophyll and signaled that the leaves were about to fall.  Over the past several years, however, researchers have found that the appearance of yellow, orange, and red leaves may have additional ecological impacts.

In 2005, researchers Martin Schaefer and Gregor Rolshausen proposed that the changing leaf color actually acts as a defensive signal against consumption by herbivores (plant-eating organisms).  The "Defense Indication hypothesis," as they termed it, is based on their own work as well as on observations that support their ideas, but were made by other researchers.  Their hypothesis (or, idea that will be tested through experiments and observations) is based on the fact that the signaling pathway that causes the production of anthocyanins also causes the production of defensive compounds to which herbivores have an aversion. After enough time, it is thought that herbivores learn to associate the defensive compounds with the colored leaves and avoid them altogether.

The very hungry caterpillar ate lots of stuff, but not orange, red, or yellow leaves.

Further, the biochemical pathways that cause chlorophyll to break down become active along with pathways that cause the production of compounds called anti-feedants, which make the leaves difficult to digest. If herbivores repeatedly consume autumn-colored leaves and then become sick due to the anti-feedants, they learn to associate the red, yellow, and orange colors with a negative eating experience and avoid those colored leaves in the future.

While the primary cause of autumn leaf colors is the loss of chlorophyll, this paper discusses just one example of how the color change has a significant impact on other organisms. Like so many things in nature, one change often has the potential to ripple through the environment and bring about widespread ecological effects.

 

Contributed by:  Kelly Hallstrom

Visit Kelly’s blog, You Don’t Have To Be A Rocket Scientist
Follow Kelly on Twitter.

Schaefer HM, & Rolshausen G (2006). Plants on red alert: do insects pay attention? BioEssays : news and reviews in molecular, cellular and developmental biology, 28 (1), 65-71 PMID: 16369938

The Friday Five

Highlighting some of the coolest science news we’ve seen lately.

1. Summer is the time for BBQ. Learn the science of BBQ in this video by It’s Okay To Be Smart.

2. Ever wonder how they get the caffeine out of coffee? Ever wonder what they do with it once it is extracted?

3. Want to know how Tylenol works? Well, so do scientists!

4. The hills might have eyes, but plants have ears! Scientists claim that plants can even hear themselves being eaten alive…much like a graduate student at a thesis defense.

5. How do you turn someone on (or off)? Stimulate their claustrum. Their what?!

 

Science quote of the week:

"The future belongs to science and those who make friends with science." --Jawaharlal Nehru 

Contributed by:  Bill Sullivan

Follow Bill on Twitter: @wjsullivan


Koubeissi, M., Bartolomei, F., Beltagy, A., & Picard, F. (2014). Electrical stimulation of a small brain area reversibly disrupts consciousness Epilepsy & Behavior, 37, 32-35 DOI: 10.1016/j.yebeh.2014.05.027