Armed and Delicious.

Go to your spice rack, make a kale smoothie, have a cup of coffee. Just about everything we use to add flavor to our lives comes from a co-evolutionary battle between an herbivore and a plant protecting its leaves.

That bite on your tongue from an arugula salad? The sulfur-containing cyanide molecules you taste are the result of glucosinolates, a characteristic defense of the mustard plant family. Mustard plants—e.g. horseradish, wasabi, mustard—all use this metabolic pathway because the burning sensation, which many people enjoy with oysters, actually works as an effective anti-herbivore toxic defense. When bugs break open cells, the enzyme myrosinase cuts a precursor to release nitriles, isothiocyanates, and other various bioactive toxic compounds. Plants in a population that are slightly more toxic survive the constant herbivore attack better and can pass on their genes to the next generation. Ah, bittersweet natural selection.

How do we know it is the bugs that put the pressure on? This paper (also summarized in a great article here) swapped mustard plants from Colorado and Montana, and found that not only did the unique spice of each plant stay consistent, but bugs preferred the visiting treat—plants that did not adapt to the local suite of herbivores. The difference in plant survival in this case is an example of local adaptation, all starting from bugs preferring the new mustard spice.

Just like these bugs and everything else in nature, we choose what we eat based on the flavors we like and what won’t kill us.

So why do we intentionally eat so many compounds plants use to make feeding difficult? Often these same toxins are essential for nutrition. The darkest green vegetables, pungent garlic, soothing mint—all play a health benefit role because of the energy plants put into making defense compounds. Bioactive toxins in low doses continue to do their toxic, bad-self thing: the alkaloid caffeine in your coffee stimulates the nervous system, the indole-3-carbinole in your kale salad degrades excess hormones that can lead to cancer, and the terpenes in oak barrel-aged wine rich in phenolic tannins can prevent carcinogens from binding to DNA and reduce the risk of harmful blood clots. The underlying theme here is that many toxins are reactive, for better or for worse.

Disclaimer: Some plant defenses are toxic to humans in high concentrations, and some plants are just plain poisonous at any dose. Don’t start eating everything toxic. Instead, appreciate the nutrients plants invest in creating highly reactive compounds in order to protect themselves, as well as the coevolutionary arms race that made plants with these exciting products succeed.

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Wings on Fire

I had always dreamed of going to the Amazon. The rainforest was a dream that breathed new life into me the way it breathes oxygen into the planet. As indescribable as the awe that comes from being amongst such biodiversity may be, the rainforest enchanted me in the movement of a toucan awkwardly wielding its beak, and in the flapping of wings on fire in the sunset as the faithful scarlet macaw streaks across the sky in its squeaky calling out to all below. Some moments brought forth vulnerabilities- the surprise of a snake’s delicate flicker of its tongue interrupting our gaze, and some brought forth grace, such as meeting the long-dreaded tarantula and experiencing not fear, but its majesty. Walls of rain sing in the rainforest upon giant sheets of leaves and dance on the surface of the turbid river. There is promise in the sunrise through the canopy mist, and in the sigh of achievement in the last orange glows on emergent branches, awaiting the stars and tree frog songs. I believe in this feeling of awe, and in its power.  Because of this, I live with more connection to the earth, more affinity for discovery, and more passion for sharing this awe with the global community. Dr. Kelly Swing, the director of the research station, quoted an Amazonian tribe leader in a presentation to visiting students, “We only know what we see; we only love what we know; we only care about what we love”. This has become my personal mission statement.

Some days I feel powerless. Numbers can be defeating—statistics on global carbon emissions, deforestation rates, biodiversity loss, increased poverty, inequality in human rights, and school shootings lead me to question whether I can have a positive impact. Science gives me hope; through science more people have a reason to care. My goal is to create research to discover new results, my findings will allow deeper understanding, and through understanding my work will facilitate an affinity for ecology. Through practice I know I have the power to connect as a science instructor and I have the power to connect across cultural boundaries. Of course I am still refining the art of connecting people from many different backgrounds to science, but luckily science inquiry is a task that always brings me joy.

-A

Blowin’ in the wind

The toxic and tough parts of plants can sometimes get more toxic or tough when induced. (Remember- inducible defenses are a cost-saving strategy for plants to “turn on” defenses when an attacker is present.) This paper by Dr. Don Cipollini provides a thinking-outside-the-box style of experiment by considering: what else* might turn on secondary metabolism? Turn on the fans!

That’s right, plant defenses can be turned on by wind!

Even further, plants from the fan treatments (wind simulation) were better protected against mites and fungi growth, which means the plant’s stress response to being blown around doubles as a protection against bugs** and pathogens! The defenses that increased in this case is called lignin, which is known for making cell walls tough (think stringy stuff in celery). Two enzymes involved in lignin accumulation also increased in windy treatments: peroxidase and cinnamyl alcohol-dehydrogenase (pro tip: usually if a biology word ends in -ase, it is probably an enzyme, which is a kind of protein (you know, the stuff our DNA tells our cells how to make).

So why does the way a plant responds to bugs and fungi after it sat in front of a fan matter? Wind is ubiquitous (=everywhere)! If wind patterns affect the way crops interact with bugs during a particularly windy year or how trees interact with diseases depending on how close they are to the other trees, it is important to incorporate wind into understanding pest resistance.

-A

*Other environmental stimuli affect the phenylpropanoid pathway that produces the enzymes and phenolics measured in this study, but the novel piece here is that wind stress led to pest resistance.

**Again, technically “bugs” belong to the phylogenetic order Hemiptera, but the term has a mainstream alias which effectively describes insect herbivores.

Santa Claus is Comin’ to Town!

In the spirit of Christmas, a time for giving, I find it fascinating that one concept perplexing many scientists is the number of species that help each other. This is called a mutualism in science speak, and is confusing to many scientists because the underlying rules of natural selection (Darwin, 1859) intuitively work against spending energy or valuable resources helping others. Remember, fitness= grow and reproduce (although this kind of fitness is also fun). Any trait involved in spending the currency on an unrelated organism that would otherwise go towards kids theoretically would not last for many generations (because it takes having kids to make a new generation with those traits). But mutualistic traits do last, even when other organisms evolve ways to cheat and take more than they provide, further baffling evolutionary ecologists.

Which is why I think this paper is so cool.

The scientists used a creative strategy to assess how plants deal with the extra loss of sugars without any return of nitrogen when the symbiotic bacteria in their root nodules cheat. The creative part is forcing bacteria to cheat: rhizobia, which take the inaccessible, triple-bonded nitrogen from the air and turn it into a useful, organic molecule for the plant are not able to provide this service when there is no nitrogen in the air! Dr. E. Toby Kiers and her team kicked out all of the nitrogen by flooding chambers with roots and nodules with oxygen, 20% (rhizobia need oxygen too), and argon, 80% – a stable gas that is heavier than nitrogen.

Plants with “cheating” rhizobia on their roots cut off the oxygen supply to those nodules!

This concept of punishing cheaters (aka host sanctioning) to maintain a fitness benefit from the relationship is a huge help in solving the evolutionary questions about two-way beneficial relationships (mutualisms!) in nature.

It’s kind of like how Santa keeps you on your best behavior so you don’t get coal in your stocking.

Merry Christmas (or whatever you celebrate)!

-A

What is plant defense?

Let’s start with plant defense in general. To follow this blog, here are some key things to know:

Plants can’t run away from their attackers. Instead, over evolutionary time, plants have developed secondary metabolism (aka, not photosynthesis- that is primary metabolism) to make toxic, tough, unpalatable, or otherwise unpleasant experiences for the bugs that try to eat the plants (I use the term bug here loosely to include all herbivorous arthropods, fully knowing that technically a true bug belongs to the phylogenetic order Hemiptera). Plant defense is when plants resist being eaten by bugs.

Plant defense has many flavors:

Chemical defense: the toxic and bitter stuff. We like to consume many of these things that plants designed in order to kill their opponents. (Small apology- I will only minimally anthropomorphize plants throughout my blog.)

Mechanical defense: the tough stuff. Usually this comes in the form of thicker and rougher leaves, or leaves covered in trichomes (fancy word for hair, and very effective: picture walking on velcro as an aphid… difficult right?).

Direct defense: the plant makes a toxic or tough compound that deters bugs.

Indirect defense: the plant relies on predators to provide the defensive service. (This is my personal favorite!) Here is how it works: plants under attack send out a cry for help either as a sugar reward or a signal in the air to attract predator bugs (ants, wasps, or spiders) that fill a hit-man role and kill or evict the herbivore from the plant. Badass! Can you attract ants to be your bodyguards? Didn’t think so.

Inducible defense: This is the on/off switch for plant defense. Usually it behooves the plant to use energy defending themselves when herbivores are around, and to save that energy when they are not under attack.

Constitutive defense: Leaving the secondary metabolite lights on all the time, even when there is nobody home (aka no herbivore attack)

That’s all for vocabulary today. I know, plant biology can get pretty crazy exciting. Get ready for the underlying evolutionary tradeoffs and hypotheses explaining why, how, and when plants defend themselves.