The Crazy Things Plants do for Nitrogen

What’s the big deal with nitrogen, you ask? Nitrogen is the key ingredient in proteins. In biology, if our DNA does the talkin’, proteins do the walkin’. Proteins can be enzymes that help reactions happen. In plants, the sugar-making part of photosynthesis, (aka the dark cycle) is run by an enzyme called RuBisCO, which also happens to be the most abundant enzyme on earth. The remarkable thing about this super important enzyme, responsible for turning CO2 into sugars, is that it is a super slow, clunky enzyme. Plants often compensate for its slowness by making more of it. More enzyme=more protein. Plants need nitrogen to make proteins.

I thought about naming this post: “Nodules, photosynthetic pathways, and carnivorous plants, oh my!” But the number of syllables got in my way. Still, these three evolutionary wonders all help plants deal with limited nitrogen, and deserve to show up in a blog post together.

But why are plants so nitrogen-limited in nature?

The answer includes a terrifying word from Intro to Chemistry: stoichiometry. (Which I recently learned from this fantastic paper: Nitrogen and Nature.)

The number of nitrogen molecules in plants is small among all of the carbon molecules, which drives the ratio of carbon to nitrogen molecules the leaf litter, overwhelming the soil with carbon and making nitrogen hard to come by. Further, nitrogen is typically stuck directly to carbon in a covalent bond, which is harder to break off than ionic bonds that work like magnets. Even when nitrogen is in this “magnetic” form, these ions are negatively charged—just like soil particles—and easily leech from the soil with rainwater.

Yes, that is why nitrogen is limited in the soil, but most of the nitrogen in the world is in the air; isn’t there plenty of it in the air that plants could use? The atmosphere is made up of roughly 79% nitrogen. Unfortunately, that nitrogen is not accessible to plants. Each nitrogen is tightly bound to a second nitrogen atom like this: N≡N, and triple bonds are especially tough to break, even for a plant.

But some bacteria can break all three bonds!! Microbes have crazy metabolic pathways, many of which are crucial to the chemical balance of the world existing as we know it. By turning N≡N into fertilizer in a process called nitrogen fixation, the nitrogen cycle is driven by microbes.

Evolutionary wonder #1: Some plants figured this out (over evolutionary time), and formed a symbiotic relationship with nitrogen-fixing bacteria. These groups of bacteria: rhizobia, Frankia, and cyanobacteria, live in nodules on their plant hosts’ roots and provide a source of house-made fertilizer in exchange for sugars. Plants, which do photosynthesis, that have nitrogen-fixing bacteria in their roots represent the interface of two important geochemical cycles: the nitrogen and carbon cycles.

Not only are plants solar-powered sugar factories, but some plants can make sugars through several different metabolic pathways: C3, C4, and CAM. For perspective- we don’t even have one carbon assimilating pathway, but the plant kingdom has three.

Evolutionary Wonder #2: In “normal photosynthesis”, or C3 photosynthesis, plants lose water through pores called stomata when they take in carbon to turn into sugars. Some plants reduce the amount of water they lose by either opening their stomata at night when the air is cooler (Crassulacean Acid Metabolism or CAM), or by concentrating the CO2 in a separate compartment with those super slow enzymes (C4). Why are these nitrogen-limited adaptations? Because making enzymes more efficient helps plants with the initial problem of needing nitrogen to churn out tons of slow, clunky enzymes.

Yet some plants carry out old fashioned C3 photosynthesis without any N-fixing bacteria in root nodules and are able to live in extremely nitrogen-poor soil. Their secret? Eat the tiny packets of nitrogen buzzing and crawling everywhere around them.

Evolutionary wonder #3: Carnivorous plants are exciting because they deviate from our basic understanding of what plants do. When my mom let me buy a $4.99 Venus fly trap at City Market, my mind was blown. I wrote songs and recorded one too many home videos about the plant that eats flies!

Even Darwin himself was excited by this menacing behavior and laid some groundwork for research on how Venus flytraps close in on their prey.

Despite the diversity of strategy, ranging from pitcher plants that collect bugs that slip into the plants’ digestive juices to sticky plants that catch bugs with glandular hairs, carnivorous plants evolved to deal with stressful environments lacking that not-so-secret ingredient in all proteins:

Nitrogen.

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How to Catch a Bee

During my days as a summer camp unit director, one of my campers brought me a flower. When I looked closer, I realized this 11-year-old was not interested in the flower, but the bee and spider on top of the flower, head to head. I assumed they were fighting, but they were both pretty still, and it dawned on me that the bee was a goner, locked in the “jaws” of the spider, whom I figured had won the fight. I was still wrong, and the reality of this interaction is even cooler than I had imagined:

The spider hunts by hiding in flowers that attract their prey, the bees.

The unsuspecting pollinator. #entomology #ecology #biology #tritrophicinteractions #pollination…almost.

A post shared by Adrienne Godschalx (@agodschalx) on

I posted my spider-bee-flower pic on Instagram recently, which received more interest than I had expected. I even had comments asking about the evolution of this flower-sitting ambush spider, so I looked it up, and I stumbled upon a cool chemical ecology story.

The first thing I learned is that, when given a choice, both the bees and this group of spiders, crab spiders, pick the same flowers. By covering the flower with saran-wrap and watching the spiders’ flower choice change, scientists were able to figure out that spiders are choosing their flowers by following chemical signals in the air, aka smells. This means these spiders are adapted to smelling out flowers that are more likely to attract their dinner.

Do bees fall for this trap? Bees not only fall for this ambush, but are more attracted to flowers with spiders than to safe, spider-free flowers.  Why would a bee fly closer to a purple flower that has a white, hungry spider in the middle?

Clearly, bees see differently from us. As I was perfecting my Instagram post, I was struck by my black and white Instagram filter, which showed the flower and spider as same shade of white (Figure 1). Even without extensive training in bee sensory biology, I figured there must be some sort of visual trick at play.

Figure 1. Screenshot of my Instagram post with 100% of the color saturation removed.

Too bad there is no UV filter on insta, because bees can see ultraviolet. Flowers take advantage of the bees’ visible spectrum in UV and often attract bees with target-like patterns, using dark UV spots in the middle. Darker UV target patterns can mean more pollination, so this trait is selected for in both flowers and bees.

So why are bees attracted to the crab spider flowers? Crab spiders have a layer of transparent cells covering spider skin cells that can change color! There are a few types of this pigment- the ommochrome pigment, which either allows spiders to or yellowish to red, or allows white spiders to have UV fluorescent patterns. With a UV pattern on their backs, flowers with spiders look like an extra dark flower target and attract bees more effectively than flowers without spiders.

Of course, natural selection goes both ways. Native bees in Australia fly close to, but can recognize and veer away from native crab spiders, whereas introduced honeybees have not adapted to recognize this danger.

Plants interact with insects and their predators. Scientists use the term “tritrophic interactions” to describe three trophic levels, or links in the food chain, interacting and affecting one another. As my thrilled camper and curious Instagram “fans” could pick up on, tritrophic interactions are fascinating! From an applied science perspective, knowing the intricacies of tritrophic interactions is essential to fully understand the side effects of potential global solutions in food security, and pest management, and conserving biodiversity.

-A

Note: All the papers I cite in this post are by Dr. Astrid M. Heiling, who has many other fantastic papers. Check her work out!

Feliz día: International Women’s Day

Many creative and brave women shaped the way we understand the world, including many women scientists.

Recently I bought a new computer, and I decided to name it after a groundbreaking woman scientist. Marie Curie, who pioneered radioactivity came to mind, as did Rosalind Franklin, who, independent of Watson and Crick, determined the structure of DNA using x-ray crystallography. But then… I was stumped.

Even as a scientist I could only name two famous women scientists who shaped my understanding of science. I was astonished I could not name more without a google search. I admire many women currently working on cutting-edge science, but I knew many lady scientists significantly contributed to developing the ideas that we teach in intro-level courses.

Luckily, as a teaching assistant, I get to learn introductory biology all over again. One day in principles of biology lecture, my advisor described the concept of how mitochondria and chloroplasts came to exists as organelles inside of a cell- and that Lynn Margulis, despite grief from her male colleagues, solidified the endosymbiotic theory—a way of thinking that is essential to our current understanding of biology.

Here is why endosymbiosis is so cool:

Remember learning about all of the organelles inside of a cell? And about how the mitochondria is the “powerhouse of the cell”? Plants have a “make-your-own-food-from-sunlight” organelle: chloroplasts. Both mitochondria and chloroplasts are surrounded by a second membrane and contain their own circular DNA, which led Lynn Margulis to propose the idea that these organelles were originally prokaryotes, like bacteria, which were engulfed by another cell! While inside the cell, this prokaryote/ organelle ancestor paid enough “rent” by producing ATP or creating sugars from sunlight, so the landlord cell kept its tenants around.

An entire domain of life began- eukaryotes. (We belong to this domain…so does your cat.)

Yet, as I hinted earlier, this theory explaining the origin of eukaryotes and the organelles providing the energy was not accepted with open arms. I am not sure if the theory would be equally as contested if a man proposed it, but as a woman, Lynn’s publication On the Origin of Mitosing Cells was rejected 15 times, and even after it was accepted and printed in 1967, she still was not widely accepted by her male colleagues who thought her idea was ridiculous.

But with sheer courage she persisted, and now overwhelming evidence supports the endosymbiotic origin for the mitochondria and chloroplast. First, if you take all of the mitochondria or chloroplasts out of a cell, the cell cannot make more, which implies the ancestor cell took it in to begin with. Not only do both organelles have their own DNA resembling that of bacteria, but when scientists look at the DNA sequences, the base pairs line up well with current prokaryotes. Cyanobacteria is the chloroplast’s closest relative, even though chloroplasts are found in plants!

I can’t even imagine what we would teach students about how eukaryotes gained extra organelles, which gave this domain of life access to energy that enabled the crazy diversity we see today.

Lynn continued in her career by showing how symbiotic interactions—organisms interacting with one another—can act as a major evolutionary force.  My own research depends on this concept, and involves a form of endosymbiosis: bean plants take up bacteria into their roots that turn atmospheric nitrogen into a form the plant can use. This cooperation between plants and bacteria enables both to flourish.

This weekend I was honored to participate as a mentor for a Women in Science day event: girls 12-18 years old toured Genentech, a leading biotech company and talked with a wide variety of scientists to learn about possible careers. I was struck by the momentum behind encouraging girls to pursue science, and the confidence that there is power in women’s perspectives in creating innovative science.

“Life did not take over the globe by combat, but by networking”- Lynn Margulis (and Dorion Sagan)

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.

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