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.

Advertisements

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)